The Sorting and Deposition of 

Allochthonous Plant Material in a

Modern Environment at Silwood 

Silwood Park, Berkshire, England

(GEOLOGICAL SURVEY PROFESSIONAL PAPER 114S

The Sorting and Deposition of 

Allochthonous Plant Material in a 

Modern Environment at Silwood Lake, 

Silwood Park, Berkshire, England

By ROBERT A. SPICER

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1143

A study of how a potential plant-fossil deposit 

was formed in a fluviolacustrine environment

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Spicer, Robert A

The sorting and deposition of allochthonous plant 

material in a modern environment at Silwood Lake, 

Silwood Park, Berkshire, England.

(Geological Survey professional paper ;

Includes bibliographical references.

Supt. of Docs, no.: I 19.16:11^3

1. Paleobotany- -Recent. 2. Paleobotany- -England 

— Silwood Lake. 3- Paleolimnology — England — Silwood 

Lake, k. Sedimentation and deposition. I. Title* 

II. Series: United States. Geological Survey. 

Professional paper ; 11^3 • 

QE931.3.S6U 560 I .1 I 78 80-60785^

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

Page

Abstract __________________________________________________ 1

Introduction ______________________________________________ 1

Early work_______________________________ 2

Analysis of quantitative data __________________________ 2

Quantification in palynological studies_________________ 3

Quantitative megafossil studies _________--_____----____ 4

Krasilov's classification of paleosuccessions __-__-____---- 5

Acknowledgments ______________________________________ 6

Field site ________________________________ 6

Materials and methods -__-_-_--_____-_-_--_--___-_--_-____ 9

Analysis of vegetation surrounding Silwood Lake ________ 9

Delta leaf traps ______________________________________ 9

Wind direction and speed _____________________________ 10

Measurement of leaf floating time ______________________ 10

Leaf-degradation experiments __________________________ 12

Rolling apparatus ________________________________ 13

Leaf distributions in delta surface sediment ___________ 13

Analysis of leaf remains ______________________________ 14

Coring procedure ______________________________________ 14

Vegetation analysis----------------.--------------.-------- 17

Nature of the trees _-------------------_------------._- 18 .

Environment __________________________________________ 18

Climate _________________________ 18

Altitude and exposure ___________________________ 18

Soil factors____________________________________________ 18

Tree density __________________________________________ 18

Time ___________________________„____________________ 19

Relation between projected crown areas and leaf deposition

on the delta __________________________ 20

Fluid dynamics in relation to leaf transport ________________ 21

Wind transport____________________________________________ 22

Shape _______________________________ 23

Wind transport—continued Page

Size _____________-______ — — — -- — ------ 23

Weight ________- — ---_ — -_--- — --_ — -- — - 24

Leaf size sorting ______________-__--____--------------- 24

Dispersal through the trunk space __ — ______-_-----___- 27

Effect of the deciduous habit _________________----__--- 28

Water transport ______________-__-__-_--___--_------_----- 28

Discussion of leaf-floating experiments __________----_--- 28

Leaf degradation, biological, chemical, and mechanical __ 30

Relative deposition rates of leaves and sediment ____________ 40

Three-dimensional structure of the Silwood delta ______-----_ 41

Plant remains ______________--_----_-----------_---__- 41

Inorganic sediment _________________________----------_ 41

Interpretation of cores _______________________________ 44

A general model of leaf deposition in a fluviolacustrine environment ______-_________________-----___-_----------___- 44

Sampling fossil assemblages-------------------------------- 48

Stand size ________-___-_______-----___-_------_----_-- 49

Three-dimensional sampling __________________________ 49

Qualitative and quantitative assessments of abundance __ 50

Principal components ordination and correspondence analysis 53

Standardization ______________________________________ 53

Principal components analysis ordination of the Silwood

delta surface data _______________________------_---_ 53

Correspondence analysis ___________________----__---_- 55

Correspondence analysis of the Silwood delta surfacesediment data __________________________________ 56

Density measures _________-________-___---_-_--__ 56

Cover abundance measures _______________----_-__- 60

Conclusions ______________________________________________ 61

References cited __________________________________________ 66

Data tables—Silwood delta fragment density and whole-leaf

density and area data-________________________------_--_- 73

ILLUSTRATIONS

FIGURE

Page

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

Map of Silwood Lake field area __________________________________________________ 7

Map showing distribution of tree species around Silwood Lake-_________________________________ 8

Photograph of Silwood stream-__________________________________________________________________ 9

Map showing positions of delta leaf traps ________________________________________________________ 11

Sketch showing construction of delta leaf traps _______________________________________ 12

Diagram of apparatus for recording leaf sinking events _________________________________ 13

Diagram of leaf-rolling apparatus _________________________________________________ 14

Map showing positions of leaf samples taken from delta sediment_____________________________ 15

Diagram of frame used for taking leaf samples from delta sediment____________________________ 16

Sketch showing delta core locations-_____________________________________________________ 16

Diagram of core-barrel assembly _______________________________________________________ 17

Photograph showing coring of delta_-____-_______________________________________ 17

Plot of annual total litter production relative to latitude ______________________________________ 18

Diagram of stream lines around a rigid flat plate held normal to a fluid flow at various Reynolds' numbers ______ 21

Graph of relation between time taken for a leaf to fall a vertical distance of 3.18 m in still air and its weight per unit

area _______________________________________________________ 23

16. Histogram of lateral blowing of artificial leaves along a plane surface __ _ __ _ _ _ _____ __ _ _____ _ _ _ 23

III
IV CONTENTS

Page

FIGURE 17. Histogram of lateral blowing of folded paper leaves and flat and curled Fagus leaves ______________ ___ _ 24

18. Histogram showing size distribution for Fagus sylvatica leaves recovered from both delta and stream traps ________ 24

19. Diagram showing mean wind directions between July and December 1973 _______________________ 25

20. Histogram showing size distribution for Alnus glutinosa leaves recovered from both delta and stream traps ______ 26

21. Plot of distribution of leaves as a function of distance from lake shore_-__-_____________________________ 27

22-26. Graphs showing the distribution of floating times:

22. Fagus sylvatica leaves with and without water turbulence ______________________________________________ 29

23. Alnus glutinosa leaves with agitation_____________________________________________________ 29

24. Alnus glutinosa leaves in still water ______________________________________________________________ 29

25. Ilex aquifolium leaves with agitation ____________________________________________________________ 29

26. Rhododendron sp. leaves with agitation______________________________________________________ 29

27-36. Leaf degradation outlines:

27. Fagus sylvatica, Silwood stream ________________________________________________________________ 31

28. Fagus sylvatica, Silwood Lake __________________________________________________________________ 31

29. Quercus robur, Silwood stream ______________________________________________________________________ 32

30. Quercus robur, Silwood Lake ________________________________________________________________ 32

31. Alnus glutinosa, Silwood stream _________________________________________________________ 33

32. Alnus glutinosa, Silwood Lake _______________________________________________________________ 33

33. Betula pubescens, Silwood stream____________________________________________________ 34

34. Betula pubescens,Silwood Lake __________________________________________________________________ 34

35. Salix cinerea, Silwood stream ____________________________________________________________________ 35

36. Salix cinerea, Silwood Lake ________________________________________________________ 35

37-39. Histograms showing size distribution whole leaves and fragments before and after rolling:

37. Fagus sylvatica before and after rolling following 1 and 2 months exposure in Silwood Lake ____________ 37

38. Quercus robur after 1 month on lake bottom __________________________________________________ 37

39. Quercus robur after 2 months on lake bottom ____________________________________________ 38

40-43. Graphs showing loss of mechanical strength of leaves as a function of time after exposure to both lake and stream 

environments:

40. Fagus sylvatica _____________________________________________________________________________ 39

41. Quercus robur _____________________________________________________________________ 39

42. Betula pubescens ___________________________________________________________________ 39

43. Salix cinerea ______________________________________________________________________ 39

44. Cores from Silwood delta __________________________________________________________________________________ 42

45. Cores from delta front and near delta Typha bed ________________________________________________________ __ 43

46. Hypothetical core summarizing structure of delta _________________________________________________ 43

47. Diagram illustrating deposition of leaf beds in a fluviolacustrine delta as described by a generalized model of leaf

deposition in such an environment _____ __________________-_________________________ 45

48. Schematic drawing of leaf-bed formation in a lateral lake ______________________________________ — _________ 47

49. Idealized longitudinal vertical sections through a completely infilled lake basin ____________________________ 48

50. Diagram illustrating potential errors involved in estimating abundance of plant remains in a fossil deposit _______ 52

51. Axes 1 and 2 of principal components ordination of unstandardized leaf-fragment density data from Silwood delta

surface sediments _________________________________________________________________ 54

52. Axes 1 and 2 of principal components ordination of standardized leaf-fragment density data from Silwood delta surface

sediments____________________________________________________________________________________ 54

53-56. Correspondence analysis plots:

53. Species ordination of leaf-fragment density data from Silwood delta surface sediments _______________— 56

54. Companion stand plot to figure 53 _________________________________________________ 56

55. Species ordination of Silwood leaf fragment density data withNuphar, Acer, and Typha data removed ____ 57

56. Companion stand plot to figure 55 ____________________________________________________________ 57

57. Computer map showing sampling position on Silwood delta of Fagus /Quercus group as presented by correspondence

analysis ordination of leaf-fragment density data ____________________________________—_________________ 58

58. Correspondence analysis plot for species ordination of Silwood whole-leaf data __________________________________ 59

59. Companion stand plot to figure 58 _______________________________________________—_____ — — ____ 59

60. Correspondence analysis plot for species ordination of Silwood fragment---________________________ — ______ 60

61. Companion stand plot to figure 60 ______________________________________________________ 61

62. Computer map showing sampling position on Silwood delta of Fagus /Quercus group as presented by correspondence

analysis ordination of cover data _______________________________________________________ 62

63-68. Correspondence analysis plots:

63. Species ordination of Silwood fragment area data after removal ofNuphar, Acer, and Typha ______ —— ____ 63

64. Companion stand plot to figure 63 _____________________________-__ — ___ — _-__ ——— --- 63

65. Species ordination of Silwood delta whole-leaf area data _________ —— _______ ————— ______ — ___________ 64

66. Companion stand plot to figure 65 ___________________________—___ — ——— __ — __ — — ___ — ___ 64

67. Species ordination of Silwood delta total area of all leaf material data.________—_-_ ————— — ——— — 65

68. Companion stand plot to figure 67 ________________ ——— ____ ——— ——— ——— _ — ——— ————— -_ 65
CONTENTS V

TABLES

Page

TABLE 1. Percentage crown cover of trees occurring within circular areas of increasing radius centered upon the delta, Silwood

Lake ____________________________________________________________________________ 10

2. Numbers of leaves deposited in the delta traps between September 12, 1973, and November 14, 1973 ______________ 12

3. Summary statistics of plant remains collected from within the top 2 cm of delta sediment _________________ 19

4. Mean weekly wind directions for July to December 1973 ____________________________________________ 26

5. Water content, ferric iron concentration, pH, and Eh of the Silwood delta sediment as sampled from a core taken in the

Typha bed ________________________________________________________________ 43

6. Information partition of fragment density per quadrat ________________^______________________--________ 51

7. Information partition of whole-leaf density per quadrat ________________________________________ 51

THE SORTING AND DEPOSITION OF ALLOCHTHONOUS

PLANT MATERIAL IN A MODERN ENVIRONMENT AT 

SILWOOD LAKE, SILWOOD PARK, BERKSHIRE, ENGLAND

B\ ROBERT A. SPICER

ABSTRACT

Quantitative statistical sampling of plant debris entombed within 

deltaic sediments of a Holocene fluviolacustrine environment at Silwood Lake, Berkshire, demonstrates that patterns of deposition exist 

that can be used to reconstruct the relative spatial distributions of 

species within the source vegetation.

Samples of unconsolidated sediment, each 0.5 x 0.5 x 0.02 m in 

volume, were collected from subaerial deltaic deposits, using a regular sampling matrix, and examined for their content of plant material. Density and cover measurements were made of both whole and 

fragmented recovered leaves and the data subsequently analyzed 

using principal components analysis and correspondence analysis 

ordination methods. Both theoretically and practically, correspondence analysis proved the more useful technique and revealed that the 

pattern of fragment deposition, unlike that of whole leaves, is associated with stream distributaries. The various species compositions of 

the whole-leaf and fragmented-leaf distribution over a single bedding surface can be used to reconstruct the relative positions of the 

source species within the vegetation.

Sorting by long-distance wind transport favors small sun leaves; in 

open-lake environments the deposited leaf size population can depart 

significantly from that exhibited by the growing source vegetation.

The vertical pattern of plant-debris deposition was revealed by 

coring. A lower leaf bed represents lake-bottom deposition, an upper 

leaf bed was formed by direct aerial transport of leaves, fruits, seeds, 

and other material to the subaerial deltaic sediments as well as by 

collective settling at the top of the foreset slope of stream-transported 

material. Theoretically, these two distinct leaf beds differ in species 

composition such that the lower leaf bed reflects the vegetation immediately surrounding the basin of deposition, and the upper a deposit enriched by plant material derived from more distantly growing upstream vegetation. Preferential feeding by invertebrate organisms and differential microbiological breakdown may seriously 

distort this pattern, but the effect may be compensated for by examining leaves preserved in relation to rapidly deposited sediments. 

Rapid deposition limits the biological degradation that can take 

place and that may totally destroy susceptible species under more 

normal conditions.

Most information of paleoecological value is in the fragmented 

material rather than the more complete "museum specimens." Vertical differences in the species composition of plant-fossil beds may 

reflect special species distributions in the source vegetation rather 

than floristic changes with time.

INTRODUCTION

The development of any science follows a process of 

gradual change from a qualitative to a quantitative 

approach, and in this respect paleobotany is no exception. Early collectors were mainly concerned with obtaining only the most perfect specimens, and interest 

was largely esthetic rather than scientific. To this end, 

professional collectors were employed solely to prospect 

for such fossils. As time went by, the extensive range of 

morphologies exhibited by fossil plants became evident, and gradually a more botanical interest evolved, 

leading to the development of techniques that enable 

us to examine specimens in finer detail. As a result of 

this, the paleobotanist began to do his own field collecting, although a considerable selective element remained, for only the best preserved, the atypical, or the 

most conspicuous (usually the largest) specimens 

were collected. This museum-specimen concept still 

survives, and probably always will, but in the process 

of finding and collecting such a fossil, much valuable 

information is typically either disregarded or destroyed. Any paleoecological deductions based on 

specimens yielded by this selective collecting must 

necessarily be subject to the distortions inherent in the 

selective sampling process and may well lead to grossly 

inaccurate conclusions. Unless more systematic sampling methods are adopted, such a state of affairs is 

likely to continue.

Only recently has there been a growing awareness of 

the need to investigate ways of extracting paleoecologically useful data from fossil deposits. A prerequisite of 

such an approach is the use of systematic sampling 

techniques and a change of emphasis from an analysis 

of the individual to an analysis of the population of 

fossils within an assemblage. The paleobotanist's 

traditional interest in anatomical and morphological 

detail for taxonomic and evolutionary schemes will continue to be critical because it is fundamental to the 

science.

No integrated study of the deposition of plant material with respect to the formation of megafossil assemblages has ever been carried out. Such a study will 

not only enable us to more accurately interpret 

paleovegetation and paleoclimates but also indicate 

where and how to sample plant remains so that more 

reliable statigraphic work can be done.
SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

Traditionally, such reconstructions of paleovegetation as have been attempted have been inferred by 

analogy with living vegetation with a focus on the 

ecological tolerance of supposed living relatives of extinct forms. Where the fossil record is considered incomplete, the associational method of determining the 

flora has sometimes been used. This method relies on 

the application of the principal of uniformitarianism 

taken to its extreme and attempts to "correct" the 

paleoflora, as represented by fossil assemblages, in 

terms of the species that ought to occur together rather 

than those that actually do. Wolfe (1969) has discussed 

the inherent errors of such an approach by using actual 

examples from the literature. It is reasonable to suppose, indeed it is to be expected, that the species constituting a certain type of community have changed 

with time: that the floristic composition of communities has evolved as well as individual taxa. 

Clearly, if valid reconstructions of paleovegetation are 

to be made, then the criteria upon which such reconstructions are based should be as independent as possible of time-dependent biological variables and must be 

largely restricted to the physical and chemical processes that lead to the formation of a fossil assemblage. 

Little attention has been paid to the relations between 

fossil plant remains and their entombing matrix or 

those sedimentary characteristics that could furnish 

information on the depositional environment. Objective sampling of fossil remains and associated 

lithofacies may well provide a more stable basis for 

reconstruction of paleovegetation.

EARLY WORK

The first systematic paleobotanical sampling followed by a statistical analysis of the resulting data was 

carried out by Chaney (1924), although earlier attempts of quantitative sampling had been made by 

Davies (1908, 1920, 1921, 1929). Chaney investigated 

an allochthonous flora of Oligocene age in central Oregon, the Bridge Creek flora, and quantitatively compared the fossil assemblage with a modern redwood 

forest community that he considered to resemble the 

Oligocene vegetation.

The fossil deposit was sampled by excavating an area 

of rock some 4.5 m long and 1.2 m wide down to a depth 

of 0.3m and splitting the excavated slabs parallel to the 

bedding in order to expose the fossil leaves, which were 

then counted. Further excavations were made at two 

other sites, and, in all, 2.65 m3 was excavated, yielding 

a total of 20,611 specimens. Even though he obtained 

this large sample size, Chaney believed that perhaps 

no more than 75 percent recovery was being achieved. 

Chaney then asked the question: "Can it be assumed 

that the numbers of leaves and fruiting parts are an 

accurate indication of the relative abundance of the

species they represent in the Bridge Creek forest?"

Such a question is clearly fundamental to any 

paleoecological investigation. Chaney attempted to 

supply the needed information by investigating what 

he considered to be a modern-day equivalent of the 

fossil flora. Leaves on stream deposits within a redwood forest were sampled using 0.3-nr quadrats, and 

leaf counts of the taxa occurring within these quadrats 

were then compared with the number of trees of those 

taxa growing within a radius of 15.2 m from each sampling station. Correlation values between the deposited 

leaves and the numbers of source plants were then calculated and these data used to estimate the number of 

source plants in the Bridge Creek forest that once grew 

within a radius of 15.2 m from the depositional site.

Unfortunately, this promising, though not flawless, 

(Wolfe, 1969) early work of Chaney was not until recently seriously followed up. Part of the reason for this 

was the lack of suitable statistical techniques with 

which to analyze the complexities of allochthonous assemblages where extensive predepositional sorting has 

taken place. Chaney did not investigate any floristic 

pattern that might have existed in the distribution of 

plant remains within the volume of rock he excavated 

but rather regarded each of the excavations as a single 

sample. Part of the aim of this paper is to show that 

patterns may exist, and that under certain circumstances they can be paleoecologically useful.

ANALYSIS OF QUANTITATIVE DATA

The development of the quantitative approach in 

plant ecology has run parallel to a similar change of 

emphasis in branches of both earth and social sciences, 

and similar statisical methods are frequently employed. All are concerned with the analysis of situations where no single variable may be readily isolated, 

and the effect of this variable on the sample population 

studied. The causal factors of patterns within a set of 

data cannot always be readily identified. In such complex situations Lambert and Dale reported (1964) 

"there may be so many variables that the whole pattern cannot be intuitively grasped; if, however, the 

data can be so simplified that their internal interrelationships can be economically displayed, the investigator may, with greater confidence, suggest an 

hypothesis concerning the causal factors involved."

The aim of multivariate statistical analysis, then, is 

to simplify a set of data in the most efficient way. A 

requirement for the successful use of an statistical 

technique is that it be performed on a suitable set of 

data and that if any valid conclusions concerning the 

sampled population are to be made, the data be appropriately collected. Special consideration to this problem is given in the section "Sampling Fossil As-
INTRODUCTION

semblages."

In plant ecology, there has arisen a dichotomy of approach to classification and ordination that has been 

the subject of extensive discussion in the literature (for 

example, Greig-Smith, 1964; Lambert and Dale, 1964).

Classification involves arranging stands, or observations, into groups, the members of which have in common a number of characteristics setting them apart 

from the members of other groups. Classification is to 

some extent an arbitrary process. Such an approach is 

ideal for the taxonomist whose objective is to categorize 

a population in the most efficient and meaningful way; 

the desirability of applying classificatory techniques 

for the identification of lateral patterns within a single 

fossil assemblage is arguable.

The apparent distributional homogeneity of plant 

remains within a fossil plant assemblage is likely to be 

marked, owing to complex interaction of physical, 

chemical, and biological variables during transport, 

deposition, and diagenesis. Blackith and Reyment 

(1971) noted that sedimentary data contain a strong, 

apparently random element, and recent work by C. R. 

Hill (1974) has shown that large variation in species 

density between adjacent stands within an allochthonous plant bed is to be expected. The use of classificatory techniques where data are forced into discontinuous groupings under these circumstances may lead to 

highly erroneous results, particularly if incorrectly interpreted.

The alternative, multivariate ordination techniques, 

such as principal components analysis, does not assume discontinuities within the data set. Rather, in an 

ordination of stands, an attempt is made to order the 

stands relative to one or more axes in such a way that 

the relative position of a stand to the axes conveys the 

maximum amount of information about the composition of that stand (Greig-Smith, 1964). A continuum is 

implied, but this does not preclude the existence of discontinuities within the sample population, and if these 

are present, they will be displayed by the technique.

The application of two ordination methods, principal 

components analysis (P.C.A.) and correspondence 

analysis (C.A) (M. O. Hill, 1974; called reciprocal averaging in M. O. Hill, 1973), will be tested on data 

relating to the deposition of plant remains in a recent 

sedimentary environment. From this, it is hoped that 

their suitability for detecting paleoecologically 

significant structure in data derived from allochthonous plant fossil populations can be assessed.

QUANTIFICATION IN PALYNOLOGICAL STUDIES

Although a quantitative approach has long been 

routine in the field of stratigraphic palynology, 

analysis of the resulting data has remained at the

simplest level. Summary presentation has mainly been 

in the form of histograms or pollen profiles, and considerable expertise in subjectively interpreting such diagrams has developed.

While this approach was adequate for stratigraphic 

correlations, those workers interested in Quaternary 

paleoecology recognized that meaningful paleoecological conclusions could only be obtained if adequate information were available regarding the different pollen productivities, dispersal, and deposition of the 

component species of the source vegetation. Perhaps 

the most important piece of work in this respect was 

that of Muller (1959), who investigated the pollen dispersal and deposition in and around the Orinoco Delta. 

This stimulating work was followed by a number of 

similar investigations of various depositional environments, some studies concentrating mainly on pollen 

deposition relative to sedimentation (see, for example, 

Traverse and Ginsberg, 1966), some on deposition relative to the source vegetation (see, for example, Peck, 

1973).

It was soon realized that if reconstruction of past 

vegetational communities was ever to be achieved, 

generalized relations had to be derived linking the 

number of pollen grains of a particular taxon deposited 

to some measure of the abundance of that taxon in the 

vegetation surrounding the site of deposition. One attempt to do this was by means of the correlation or R 

value proposed by Davis (1963). By measuring the 

ratio of the pollen percentage of a species in a Holocene 

depositional environment to the vegetational percentage of that species in the surrounding extant plant 

communities, fossil pollen percentages could be "corrected" to give an approximation of the species abundance in the ancient vegetation.

A number of serious flaws in the use of the R value 

have been discussed in the literature (Davis, 1969; 

Faegri, 1966). H. J. B. Birks (1973) stated that the 

most serious constraint is that the lateral extent of the 

vegetation contributing pollen to a medium- or largesized basin of deposition is generally not known with 

any certainty; it is therefore impossible to accurately 

delimit the size of plot to be sampled in order to make 

valid comparisons between the fossil and modern 

source vegetation. Only in deposits within closed 

canopy forests or small basins can this problem be 

overcome (Andersen, 1970, 1973). Another variable is 

the structure of the source communities themselves; 

Janssen (1967) and Comanor (1968) have independently demonstrated significant variations in R values 

for the same tree taxa occurring in different forest 

types (H. J. B. Birks, 1973).

Provided that reasonably accurate data concerning 

differential productivity of the various taxa is available (Anderson, 1970, 1973), a more fundamental ap-
SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

proach to the problem of representation may be 

adopted by investigating the variables of dispersion. 

Tauber (1965) suggested a generalized model for pollen 

transfer in a forested area wherein he divided the pollen spectrum into a trunk-space component, a canopy 

component, and a rainout component. By considering 

the aerodynamic properties of the pollen grains in relation to the air flow near the ground and the position of 

the source relative to the depositional environment, 

the potential bias in the representation of the various 

taxa deposited could, in theory, be determined and 

suitable allowances made when considering the reconstruction of paleocommunities.

Analysis of modern pollen dispersal and deposition 

has become more sophisticated and now constitutes a 

major branch of Quaternary studies. Multivariate 

statistical methods have also been used to analyze lateral distributions of pollen that (for example, O'Sullivan and Riley, 1974), and there is every reason to 

suppose that such studies will continue. Oldfield (1970) 

introduces a cautionary note: "Elegance of technique or 

consistancy of results are secondary, for within a 

palaeoecological context, there is little point in studying pollen production or dispersal at the present day 

purely as an end in itself."

QUANTITATIVE MEGAFOSSIL STUDIES

Quantitative assessments of megafossil abundance 

have sometimes been used to supplement pollen data, 

as it is generally considered that megafossils represent 

only the local components of the flora.

The most notable of such studies was carried out by 

Watts and Winter (1966), who examined the megafossil content of cores taken for pollen analysis from 

Kirchner Marsh, Minn. The results were presented in 

the form of seed diagrams similar to those traditionally 

used for displaying the pollen spectrum. These workers 

found that many of the concepts of pollen analysis 

could be equally well applied to seed analysis, although 

they pointed out that seeds are not so suitable as pollen 

for statistical studies because they are less efficiently 

mixed in the seed rain. Such considerations are critical 

for both stratigraphic and paleoecological studies, for 

conclusions based on a restricted sample from only a 

small number of cores could lead to erroneous conclusions. Yet, this heterogeneity in the megafossil distribution (not only with seeds but also leaves and other 

remains) may well prove to be the strong point of 

megafossil analysis. The high degree of mixing that 

pollen and spores undergo during dispersal tends to 

destroy any pattern that might be used to determine 

the spatial relation of the various sources. This, to a 

large extent, is not the case with the dispersal of macroscopic remains.

The application of multivariate statistical techniques to the analysis of megafossil distributions, as in 

pollen studies, has met with some success. H. H. Birks 

(1973) sampled some 32 lakes occurring in a number of 

different vegetational types. Surface samples of peat or 

mud were taken by hand or dredge. A known volume of 

sediment, usually 50 cm8 , was then analyzed for its 

megafossil content, and the numbers of each taxon 

were expressed as numbers per 100 cm3 of sediment. 

Megafossil and vegetational data were then analyzed 

using correspondence analysis (referred to in Birks' 

paper by its French title "Analyse factorielle des correspondences''); the resulting ordinations showed a direct 

relation between the megafossil content of the lake 

bottom muds and the surrounding vegetation. The pattern of megafossil distributions within the lakes was 

observed; most species were concentrated close to the 

shoreline.

The distribution of megaremains (including leaves) 

in a lake environment was investigated by McQueen 

(1969) at nine stations in and around a New Zealand 

lake. From simple tabulation of the data, McQueen 

concluded that in lake sediments the plant remains 

represent only the dominant members of the surrounding vegetation, as no remains were found that had been 

transported any appreciable distance. In unimpeded 

rivers, however, remains may be carried considerable 

distances and still be recognizable. For instance, it was 

concluded that a leaf of Nothofagus menziessii Oerst. 

from a swamp site bordering the lake must have 

traveled at least 2 km downstream; most of the distant 

taxa are filtered out by reed swamps and therefore 

never reach the lake-bottom sediments.

Interest in transport, deposition, and analysis of 

megafossil assemblages has also come from 

paleobotanists working on specific fossil plant assemblages. Observations by Mosely (1892) on plant debris 

transported out to sea by the River Ambernoh in New 

Guinea were quoted by Reid and Chandler (1933), who 

proposed a similar mechanism to explain the occurrence of leaves, fruits, and seeds in the London Clay. 

Ferguson (1971), studying the Kreuzau Miocene flora 

of Germany, investigated the possible distances leaves 

could be transported by rivers before becoming so damaged that they were unrecognizable. By means of laboratory experiments in which leaf discs were rolled together with water and sand, it was concluded that 

abrasion alone was unlikely to cause serious destruction of fresh material.

C. R. Hill (1974) investigated the practicability of 

quantitatively sampling fossil plant remains of the 

Yorkshire Jurassic flora exposed at Hasty Bank. Three 

vertical sections of the deposit were recovered by excavating known volume of rocks, and recovery of the
INTRODUCTION

plant material was achieved by careful splitting along 

the bedding planes. Contiguous samples were taken 

from top to bottom of the sections that cut across two 

lithologies, a siltstone and a mudstone. In one section, 

accurate counts were made of the number of plant 

fragments of the different species that were recovered 

as well as visual assessments of abundance using a 

pseudologarithmic scale of 1-10. In the other two sections, only the visual abundance estimate was used. 

Hill presented his data in histogram form, from which 

he was able to conclude that there were a number of 

different assemblages present within the rock that he 

sampled.

Part of Hill's data was then analyzed using both 

principal components analysis (P.C.A.) and correspondence analysis (C.A.) (Spicer and Hill, 1979). 

Using C.A., all assemblages identified by Hill could be 

detected but only when the count data were either 

logarithmically transformed or reduced to an arbitrary 

pseudologarithmic scale. When data from a group such 

as the Bennettitales (Cycadeoides) was ordinated, C.A. 

was able to detect associations between the vegetative 

and reproductive parts of fragmented plants. It was 

known from other evidence that the detached organs so 

grouped did in fact represent parts of the same type of 

plants.

R. Pfeiffer (oral commun., 1974) has investigated the 

lateral variation in plant distributions in shales above 

coal seams exposed in Indiana strip mines. As mining 

operations proceeded, 0.3-m3 samples were cut from the 

rock by means of a chain saw, and analysis of the 

species content of the samples made in the laboratory.

Scott (1977, 1978) has attempted to investigate the 

distribution of Pennsylvania plants in roof shales 

using quantitative estimates of "cover."

KRASILOV'S CLASSIFICATION OF PALEOSUCCESSIONS

Krasilov (1969) proposed a scheme of classification of 

plant-fossil deposits based on those processes supposed 

to have determined the floristic composition of the assemblage found within them. He observed that "every 

layer Lof rockY containing plant remains is succeeded 

in the geologic section by a layer which (a) does not 

contain plants, or (b) contains mainly other plants, or 

(c) contains mainly the same plants but in a different 

state of preservation, or with a different frequency of 

occurrence, or a different orientation in relation to the 

strike and dip." Krasilov then asserted that it is necessary to distinguish between changes due to real successions of the ancient vegetation and changes due to 

transport and burial.

Krasilov's classification of paleosuccessions can be 

summarized in this way: 

Taphogenic paleosuccessions—resulting from the

selective characters of burial, transportability, 

and strength of plant remains.

1. Selective taphogenic—resulting from sorting during transport.

2. Lithification paleosuccessions—resulting from 

changes in the composition of the buried vegetation assemblage in the course of diagenesis. 

Cenogenic paleosuccessions—produced by the evolution and migration of floras.

1. Mutational cenogenic—as yet to be recognized in 

the field but formed by the alteration of vegetation brought about by genetic changes.

2. Phenological—expression of seasonal variations.

3. Migrational—brought about by the migration of

floras due to:

Topogenic factors—change in topography and relief.

Edaphogenic factors—changes in the soil characteristics. 

Climatogenic factors—climatic changes.

While such a classification has its uses, its rigid application invites oversimplification of the processes involved in determining the species composition of an 

assemblage. More than one type of paleosuccession 

may occur in the same deposit, and a merging of on 

type with another, both vertically and horizontally, is 

probably more common in nature than is at first 

realized. Indeed, the logical consequence of the leafdeposition model presented in the section "A General 

Model of Leaf Deposition in a Fluviolacustrine Environment" is just such a situation where, at deposition, 

taphogenic and topographic/edaphic paleosuccessions 

merge laterally, whereas subsequent diagenetic 

changes introduce the possibility of a lithification 

paleosuccession being formed. Such a classification 

may be useful when describing large-scale depositional 

regimes, but its application to individual deposits is 

undocumented and may prove to be of doubtful value.

This background to the present study portrays many 

workers experimentally investigating isolated aspects 

of sorting and deposition as an aid to interpreting fossil 

assemblages. The deposition of plant megaremains is, 

however, a process that involves many variables and 

cannot be understood unless an integrated study of the 

various interrelated aspects is made. The quantitative 

analysis of fossil plant assemblages to give 

paleoecologically significant information is clearly in 

its infancy. Before going too far forward, particularly 

in the development and application of sophisticated 

numerical techniques, it is necessary to ask some basic 

questions: What is it we are trying to measure? What 

are the factors involved in distorting the information 

we collect from the information that was potentially 

available in the living community? How can we obtain
SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

the most objective and efficient sampling of a fossil 

deposit?

Answers to these and similar questions involve a 

wide range of considerations from the measurement of 

extant vegetation, the mechanics of transport and deposition, through the fossilization process to the sampling of the fossil population and subsequent mathematical analysis. The aims of this paper, then, are to 

evaluate the effect on paleoecological information of 

these processes using a small modern fluviolacustrine 

depositional environment as a working system, to erect 

a generalized model of deposition of megaremains in 

such an environment, and to show how existing mathematical techniques can be used to reveal significant 

patterns of distribution of deposited plant remains.

ACKNOWLEDGMENTS

The interdisciplinary nature of this work has necessitated that I consult a large number of people in a 

wide variety of subjects. To individually acknowledge 

all those who have given me advice or assistance would 

be impossible, but to all of them I extend my sincere 

thanks.

Part of this work was carried out under the terms of 

a Natural Environment Research Council studentship, 

part as a Lindemann Fellow working at the U.S. 

Geological Survey in Menlo Park, Calif.

I would like, however, to especially thank J. A. 

Wolfe, of the U.S. Geological Survey; M. D. Muir, K. L. 

Alvin, T. R. E. Southwood, and F. H. (and Mrs. Ludlam, of Imperial College London University; and D. K. 

Ferguson, of the Rijksuniversitair Centrum, Antwerpen, Belgium. Thanks are also due to P. R. Grant, 

Richard Giddens, Eric Denman, Frank Chatfield, J. B. 

Steyn, and Phillip Giles, who, at the time this work 

was done, were associates at Imperial College.

FIELD SITE

The modern depositional environment chosen for 

this study was an actively advancing freshwater lacustrine delta in the grounds of the Imperial College Field 

Station at Silwood Park (Grid ref. SU 941691), Berkshire, England. Rocques' Survey of Berkshire (1761) 

(Berkshire County Records office) does not figure a lake 

in the area of what is now Silwood Park, only a stream. 

An enclosure map of 1815, printed by Benjamin 

Badcock of Oxford (Berkshire County Records office), 

shows a lake, much larger in extent than at present, 

extending southwestward as far as Cascade Bridge (fig. 

1); on a geologic map of 1819 by William Smith (Imperial College Geology Department, London), no lake is 

shown. It is likely that an existing, possibly slightly 

out-of-date, map was used as the base for the geologic 

map; the date of the construction of the lake may be

reasonably supposed to have been just prior to 1815.

Silwood Lake was artificially constructed by damming the northeastern end of the existing valley. It 

was thought that the lake had been "puddled" to prevent 

drainage through the underlying porous Bagshot 

sands, but cores taken in the delta area show a continuous clay lining to be absent. A spring line at the 

base of the surrounding hills indicates that the water 

table in the Bagshot sands is locally higher than the 

water level in the lake, obviating the need for an impervious clay lining.

From the extent of the open water as delimited in the 

1815 enclosure map, it appears that most of the southwestern arm of the lake has since filled with sediment 

brought down by the stream (fig. 1). The present delta 

front, approximately 300 m northeast from Cascade 

Bridge, is some 60 m or so wide. If the average sediment depth is 2 m, then the approximately 18,000 m3 of 

sediment has been deposited since about 1800, an average rate of 100 m:! per year. Although no accurate 

measurements were made, the delta was seen to be 

actively laterally advancing at the rate of about 1.0 m 

per year during the course of this study. Most of this 

delta deposit now supports a mature Alnus glutinosa 

(L.) Gaertn. thicket (figs. 2 and 3).

The existing weir was constructed in 1958 and now 

maintains the lake level constant to within a few centimeters; during construction of the weir, the lake was 

partially drained for some time.

It is known that the stream supplying sediment to 

the southwest arm of the lake has been artificially 

straightened (E. Green, Imperial College, oral commun., 1974), although no accurate date for this event is 

available. It appears that before this construction, the 

stream emptied into the lake to the south of its present 

inflow, during the period of this study (October 1972- 

October 1975); it was seen to revert to this position 

after periods of heavy rain. The stream frequently 

breaches the partly natural levees along the terminal 

300 m of its extent, resulting in periodic flooding of 

large areas of the compacting delta sediment. No accurate streamflow data are available, but during the 

course of our observation, the flow rate rarely exceeded 

1.0 meter per second.

Silwood Lake, small by comparison with many 

freshwater bodies (surface area approximately 13,000 

m2 ), is about the same size as many fossil-leaf deposits: 

for example, the Tennessee clay pits described by 

Dilcher (1971) and the Kreuzau deposits described by 

Ferguson (1971). Both of these deposits are supposed to 

have been laid down in an oxbow type of lake. Such an 

environment may well be conducive to the fine preservation exhibited by fossils from both localities. Unfortunately, it is unlikely that this type of deposit will 

yield significant information concerning distant vege-
FIELD

tation, because these basins are isolated from flowing 

water, except during flood periods, and are restricted to 

the flat topography of flood plains, where vegetational 

diversity is likely to be small.

Although the natural processes of lake-basin formation are many and varied, lateral lakes of a large river 

system provide an environment similar to that of the 

Silwood environment. These lakes form when sediments of a large river are deposited as levees and back

SITE 7 

up water in a tributary stream. Such lakes are Lake 

Tung-ting and other lakes of the Yang-tse-kiang, lakes 

of the lower Danube, and lakes of the Red River in the 

Mississippi drainage basin (Hutchinson, 1957). The 

deposits of such lakes may well yield fossil leaves derived from both riparian and slope vegetation. Despite 

the artificial nature of the Silwood lake and the present 

stream course, natural processes of transport and 

sedimentation are considered to be operating.

50 100 METERS

Delta deposits

Area of delta 

examined for 

deposited leaf 

material

0 50 100 150 200 250 300 FEET 

CONTOUR INTERVAL 5 FEET

EXPLANATION

— — — — — — — Course of inflowing

stream prior to 

channel straightening

— — — — Fences

Bridge

Weir

Landing stage 

Building

FIGURE 1.—Silwood Lake and immediate area showing extent of deltaic deposits.
Alnus glutinosa (L.) Gaertn. 

Betula pubescens Ehrh. 

Fagus sylvatica L.

Quercus robur L. Salix cinerea L. 

Ilex aquifolium L.

EXPLANATION

Aesculus hippocastanum L. 

Acer pseudoplatanus L. 

Crataegus monogyna Jacq.

7>p/za latifolia L. Nuphar lutea (L.) Sm. Castanea sativa Mill.

Salix alba L. 

Fraxinus excelsior L. 

Pinus sylvestris. L.

oo

O 

GO

r 

o nffi ^ffi 

o2 O

£2Fo 

o

oI-1K2 O2O

FIGURE 2.—Distribution of tree species constituting the canopy around Silwood Lake, as seen from the air. Except where the canopy is continuous, approximate sizes of the

crowns are indicated.
MATERIALS AND METHODS 9

MATERIALS AND METHODS 

ANALYSIS OF VEGETATION SURROUNDING SILWOOD LAKE

It is clear from the literature (see, for example, Newbould, 1967; Hughes, 1971) that to obtain a representative estimate of the total litter produced by an area of 

vegetation as extensive as that surrounding Silwood 

Lake would be a major task and take a number of 

years. To undertake such a project in the course of this 

study was not feasible, but it was possible to map the 

position of every tree species that constitutes part of 

the canopy and in this way estimate the relative potential source areas for the different species of deposited leaves. An aerial photograph (Aerofilms 

HSLUK:71173) taken on September 8, 1971, was used 

as the basis for a vegetation map, and the species of 

every tree seen from the air then determined and plotted by ground survey (fig. 2).

FIGURE 3.—Stream flowing toward Silwood Lake as viewed from Cascade Bridge.

The vegetation map was divided into regions of 

interest at various distances from the delta by scribing 

circles of 2, 4, 6, 8, 10, 12, 14, and 16-cm radius (equivalent to 50, 100, 150, 200, 250, 300, 350, and 400 m full 

scale) from the delta front center and measuring (using 

a 1-mm grid) the areas of the canopies of the various 

species lying between successive circles. The percentage area of each species within each ring was then 

calculated. The areas of each species in all the regions 

of interest were progressively summed and successive 

percentage areas for the species within the complete 

circles of increasing radii calculated. The percentage 

species composition for the 400-m-radius circle was 

taken as representative of a summary of the vegetation 

in the field area. The results are shown in table 1.

DELTA LEAF TRAPS

To determine the proportions of leaves of the various 

species deposited on the delta surface by direct wind 

transport, 15 water-filled leaf traps were positioned on 

the delta (fig. 4). The trap frames (fig. 5) were constructed of wood covered with a polyethylene sheet 

("1,000" grade) pinned to form a 0.5-m2 shallow watertight tray 5 cm deep. A small hole was made in one 

corner, close to the top of the wooden frame, to allow 

excess rainwater to overflow without washing out any 

leaves. The traps were then staked in position on the 

delta (fig. 4) at coordinates obtained from a random 

number table (Fisher and Yates, 1963) and filled with 

water. At frequent intervals between mid-September 

1973 and mid-November 1973, leaves were removed 

from the traps by means of a small net, placed in plastic bags, and stored in 5 percent aqueous formaldehyde 

until such time as they could be counted. The water 

level in the traps was topped up as required.

Leaf traps of similar design were staked at 15-m 

intervals along the inflow stream and the leaves collected on a weekly basis. It was hoped that in this way 

an estimate of the litter fall, as intercepted by the 

stream, could be achieved. It was soon found that at 

periods of bankfull discharge, many traps, laden with 

leaves, became swamped, and the trapped leaves either 

washed out or the samples were contaminated with 

waterborne leaves. Because of this problem, the experiment to estimate litter input to the stream was abandoned, but some of the leaves collected in undisturbed 

traps were retained for size comparison with the lake 

samples.

The numbers of leaves of different species falling 

into the delta traps were determined and the results 

given in table 2. The areas ofFagus andAlnus leaves 

that had been caught in the delta traps were measured 

using a Paton electronic planimeter and compared 

with samples of leaves caught in the stream traps. The
10 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

TABLE 1.—Percentage crown cover of trees occurring within circular areas of increasing radius centered upon the delta, Silwood Lake

Circle

No.

1

2. _ ___

3

4_ __ __ ____

5 ___ .________.

6— _ — ___ -__.

7

8 — —

1

2

3 _ __ __ _

4 _________ .

5— — __ __ -_.

6-__— ________

7

8_ ———— __—

Radius 

(ml

__ __ 50

. ____ 100

. ___ 150

----200

.--—250

——300

_ ___ 350

- ___ 400

————50

— — 100

-__ — 150

— ___200

. ___ 250

300

— —350

— —400

Alnus

51

34

9^

24

18

12

11

8

51

37

30

28

25

23

22

21

Betula

8

18

Q4

34

25

28

51

18

8

16

26

30

28

28

31

28

Fagus

2

Q

6

17

9

3

31

2

3

4

7

7

9

n Salix Salix, UuercUK AH. ^ cinerea Alba

Percent total area

9 32 1

13 32 ____

10 24

90 R

26 6 __ _

O'J B

17 15

22 ____

Cumulative percent

Q Q9 1

13 32

11 27

15 18

17 1 ^

19 14

18 14

19 13 — _

Ilex

Q

Q

2

1

1

1

1

Aesculus

1

4

2

6

1

2

1

2

2

2

Crataegus

1

2

4

2

1

1

2

2

1

2

2

Acer

1

4

Q

2

9

1

2

2

2

2

2

Fraxinus

1

1

1

1

1

Castanea

r.

1

6

1

1

1

results are presented by histogram, figures 19 and 20, 

and tested using a Fisher-Behrens test (as described in 

Campbell, 1967).

WIND DIRECTION AND SPEED

Surface-wind direction and strength for July to December 1973, as measured at Heathrow Airport (12.9 

km northeast of Silwood Lake), were obtained from the 

British Daily Weather Reports. Although local topography may slightly modify the wind patterns around 

the lake, it was assumed that these readings would 

serve as an estimate of the wind conditions at Silwood 

Park. The wind direction and speed was recorded four 

times in every 24 hours.

Each wind-direction reading was weighted by the 

appropriate wind speed (measured in knots) and from 

this data the mean wind direction for every week calculated (fig. 18). Weighting by wind speed was employed 

because it may be argued that a strong wind exerts 

more influence on wind dispersal than a gentle breeze. 

No corrections were made for periods of rain that could 

have reduced wind dispersal. The weighted winddirection data was processed using the program ORIPAL as published in Reyment (1971).

MEASUREMENT OF LEAF FLOATING TIMES

In order to estimate the relative leaf dispersal that 

could be attributed to differences in leaf floating times, 

the following experiments were carried out.

Samples of freshly fallen leaves from trees and 

shrubs growing around Silwood Lake were collected, 

returned to the laboratory, and dried at room temperature. A known number of leaves of each species 

(usually 50) were taken and put into buckets that were 

three-quarters full of chlorinated tap water. At various 

times during the course of the experiment, the leaves

were stirred round in the water and left to settle; the 

number that remained floating were counted. It was 

soon realized that with those species where most leaves 

sank within 2 days, large errors were incurred by not 

having any readings during the night. To overcome 

this problem, a light-plane recording apparatus (diagrammatically shown in fig. 6) was constructed.

A 12-volt automobile headlamp bulb was positioned 

at the focus of a concave parabolic mirror and powered 

by an optically stabilized rectified power supply. By 

means of collimation slit in front of the bulb, a parallel 

plane of light was made to pass down the length of a 

rectangular glass tank in such a way that any sinking 

object would interrupt part of the beam. A line of optical fibers was then positioned normal to the glass, 

across the width of the tank, at the end opposite the 

light source. The free ends of the fibers were then 

formed into a bundle and abutted against the sensitive 

surface of a selenium light-sensitive resistor, which in 

turn was connected to an amplifier and pen recorder. In 

this way, the time that any sinking leaf cut the plane of 

light was recorded.

To compensate for any discoloration of the water 

brought about by leaching of compounds from the 

leaves, a reference beam was set up to pass through the 

water down one side of the tank, and the light cell 

receiving this reference beam was electronically balanced with the cell receiving the light plane.

It was noted that unless the leaves were intermittently agitated, they were prevented from sinking, 

even though they were waterlogged, by the formation 

of gas bubbles on the external epidermal surfaces. A 

mechanical agitator was therefore constructed to 

periodically generate ripples on the surface of the 

water. In practice it was found that even minimal agitation of the water cause previously sunken leaves, 

with a buoyancy only slightly in excess of neutral, to be
Silwood Lake

0 10 METERS

0 10 20 30 FEET 

CONTOUR INTERVAL 5 CENTIMETERS

FIGURE 4.—Locations of leaf traps on delta superimposed on a contour map of delta surface. Distributaries are numbered 1 through 4. Relat

visually assessed, expressed in terms of percentage of total stream flow during period of sampling. Distributary 1: 50 percent; 2: 30 percent; 

not directly connected to the inflow stream and carried only what was considered to be runoff and seepage from the south bank. Contour map 

bathymetric matrix.
12 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

stirred up and intercept the light plane. For this reason, the use of the apparatus was unfortunately limited. A deeper tank would no doubt have overcome this 

problem, but this remains to be tested.

LEAF-DEGRADATION EXPERIMENTS

The relative degradation rates of leaves of Quercus 

robur L., Fagus sylvatica L., Betula pubescens Ehrh., 

Salix cinerea L., and Alnus glutinosa (L.) Gaertn. were 

investigated by the following experiments.

Three hundred and sixty freshly fallen intact leaves 

of Quercus, Fagus, Salix, Betula, and Alnus trees, all 

common in the field area, were collected in the autumn 

of 1973 and returned to the laboratory. Here they were 

dried at room temperature for 21 days. Seventy-five

15.0 cm

2.5cm

1000' grade polythene

Wooden frame

FIGURE 5. Construction of leaf traps. Wooden frame constructed to dimensions shown and lined with polythene 

sheet. Shallow tray so formed is filled with water. Buoyancy 

of wooden frame insures that even when trap is positioned 

in open water, edges are clear of water, preventing leaf exchange in and out of trap.

TABLE 2.—Numbers of leaves deposited in the delta traps between 

_ September 12, 1973, and November 14, 1973

Trap No.

I fig 48)

1 _______I__

2 ________

3 _________

4 __

5 __________

6 ___ _ _

7 _______ ___

9

10

11

12

13

14

15

Total ___ 542

Alnus

6(T

40

49

36

13

10

3

6

8

9

13

23

65

86

121

542

79

Betula

~~9~~

4

3

4

1

1

1

1

7

"Y"

4

6

42

6

Fagus

3

5

1

1

_____

4

2

7

24

3

Quercus

~~5~~

5

2

1

1

2

3

1

2

1

5

1

18

47

7

fTM ^ar

'8 !_"_"_

2

4 __

"2 IIII

4 '2

6 ____

1 ____

~3~~ IIII

1 __

____ ____

31 2

Percent total____ 79 5 _.__

'Probably transported by water fowl since both pieces bore beak marks and the trap was 

apparently undisturbed.

leaves of each of the taxa were placed in each of four 

rectangular nylon cages (25 x 25 x 6.5 cm) constructed 

of 5-mm- mesh Netlon. No metal was used in the construction, and all seams were made using nylon thread. 

Two cages were weighted (with iron ballast sealed in a 

"l,000"-gauge polythene bag so that the leaves could 

not be contaminated by the ballast) and deposited on 

the bottom of the lake, and two cages were tied to a 

wooden stake in the stream that had not been treated 

with preservative or any other "foreign" compound.

After periods of approximately 1, 2, 4, 6, and 8 

months, samples of 25 leaves of each of the species were 

removed from the cages and returned to the laboratory, 

where they were processed as rapidly as possible. The 

first three samples (months 1, 2, and 4) were taken 

from the same cages in each environment, the remaining samples from the other.

Five leaves of each species were taken for X-ray microanalysis (a full account of the X-ray microanalysis 

studies, which are not discussed here, is given in 

Spicer, 1975), the remaining 20 leaves used for leafdegradation studies. The leaves were carefully washed 

to remove loose sediment and the extent of natural leaf 

degradation recorded, either by photocopying or by 

tracing the outline. In this way, the extent of lamina 

loss, primarily due to biological agencies in the relatively protected environment of the nylon cages, could 

be determined. The mesh of the cages was such as to 

afford free access to most aquatic invertebrates, yet 

retain all but the smallest leaf fragments. The degree 

of breakdown of the various species is shown in figures 

27-36.

The washed leaves were loaded into the drum of a 

rolling apparatus (described below) together with 2.5 

liters of tap water and rolled for 90 minutes. From 

preliminary experiments, using both fresh leaves and 

leaves collected from the streambed, it was found that 

in this treatment fresh leaves were only minimally 

damaged, whereas leaves from the stream were highly 

fragmented. It was therefore considered that such 

treatment would yield a range of fragmentation representative of the leaf-degradation state.

All species from the first two sampling periods were 

rolled together; the last three could only be treated 

separately because a large number of fragments were 

generated and subsequent identification of the fragments became extremely tedious. The resulting fragments were then stored in 5-percent aqueous formaldehyde solution until such time as they could be 

counted and, in some cases, their areas determined 

using a Paton electronic planimeter. Because a drop in 

accuracy was found when measuring large number of 

small areas, only the areas of fragments from the 

leaves receiving the shortest exposure to the lake or 

stream environments were measured.
MATERIALS AND METHODS 13

ROLLING APPARATUS

The rolling apparatus (fig. 7) consisted of a plastic 

drum 22 cm in diameter and 36 cm long with a 1.2-cmthick removable transparent plastic ends. The drum 

was fitted with cylindrical wooden paddles as shown in 

the diagram and supported on two rubber-clad rollers 

(external diameter 17 mm), one driven at constant 

speed by a :/4-horsepower electric motor. The drum rotated at a rate of 70 revolutions per minute. A watertight seal between the drum and the end plates was 

achieved using "Sealastic" sealing compound.

The wooden paddles provided turbulence and obstacles against which the leaves would knock, similar to 

branches and other static objects likely to be found in a 

small stream. It was not intended, however, that the 

experiment should simulate any particular stream, but 

rather that it would expose the leaves to some of the 

forces likely to be found in nature. The water energies 

acting on the leaves within the drum were fairly high, 

but this insured that fragmentation was as rapid as 

possible and that the effect of continued, indeed accelerated, microbiological activity under the relatively 

warm laboratory conditions was minimized during the 

rolling period.

The frequency distributions of the various fragment 

areas produced by rolling were plotted as histograms, 

(figs. 37-39). At first it was thought that an adequate 

representation of the relative fragmentation might be 

assessed in terms of the percentage shift in the mode or 

median of the distributions. Examination of the histograms (figs. 37-39) reveals that the distributions are in 

fact bimodal. It appears that the peak at low size

classes represents large numbers of small fragments 

produced by attrition. The remnants of the whole 

leaves form the peak at the larger size classes. In view 

of the bimodal distribution, a more suitable way of expressing fragmentation was considered to be average 

fragment area, after rolling, divided by the original 

average whole-leaf area. This fraction was then expressed as a percentage. The fragmentation of leaves so 

expressed is shown by the figures 40-43.

LEAF DISTRIBUTIONS IN THE DELTA SURFACE SEDIMENT

The lateral distribution patterns of leaves deposited 

within the surface sediment of the Silwood delta were 

investigated in the following way. A grid was laid out 

on the delta by means of parallel strings 2 m apart, 

each marked off into 2-m lengths. In this way accurate 

sample location was achieved. Access to the delta was 

by a flat-bottomed dinghy, from which samples were 

collected. Water depths were measured using a 

weighted measuring tape and the water-depth data 

used to draw a contour map (figs. 4, 8, 57, 62) of the 

delta surface.

Samples of the leaves entombed in known volumes of 

sediment were taken at the positions indicated in 

figure 8. Species/area curves were constructed using 

rectangular wire quadrats, and from this data a sample 

area of 0.5 mxO.5 m was chosen as being most suitable. 

An arbitrary sediment depth limit of 2 cm was then 

chosen for each sample. Replication of the sampling 

volume was achieved by using the apparatus shown in 

figure 9. A sheet-metal frame (A) was constructed 0.5 

m square with external lateral "wings" (5 cm wide)

Mirror

FIGURE 6.—Diagram of light-plane apparatus for recording leaf-sinking events. See text for explanation. Tank is approximately 1 m long.
14 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

that prevented its sinking into the sediment during 

sampling. Two centimeters below these wings, on the 

inside of the frame, were two ledges along which the 

cutting edge of the sieve quadrat (B) could be pushed. 

After positioning the frame in the sediment, the sieve 

quadrat was pushed into the frame, and then carefully 

lifted out, so that a sample of sediment 0.5x0.5x0.02 

m was retained on the sieve quadrat. The sediment was 

then carefully washed through the sieve (0.5-cnr 

mesh), which retained the larger plant remains. These 

remains were then removed from the sieve, returned to 

the laboratory in polythene bags, and stored in 

5-percent aqueous formaldehyde until needed for examination.

ANALYSIS OF LEAF REMAINS

The leaf remains, washed free of formaldehyde solution, were separated into whole leaves and fragments. 

A leaf was considered to be whole if it was thought to 

have at least 80 percent of the area of its lamina present and not less than 80 percent of its margin intact.

A count was then made of the whole leaves and 

fragments of each species, and the area of all individuals measured to the nearest 0.25 cm2 using a gridded 

perspex overlay. If any fragment proved difficult to 

identify, the cuticle was prepared by oxidizing the tissues in 20-percent aqueous chromium trioxide for 12 

hours at room temperature. The cuticle was washed 

with water, stained in 1-percent aqueous safranin, and

mounted in glycerine jelly before being identified by 

comparison with a reference collection of cuticles. The 

resulting data were subsequently analyzed using multivariate statistical techniques.

CORING PROCEDURE

The three-dimensional nature of the Silwood deposit 

was investigated by taking 22 cores, each 7.5 cm in 

diameter, in the positions shown in figure 10. The corer 

used was modified from the design published by 

Livingstone (1955). Various workers have suggested a 

variety of modifications to Livingstone's original design (Vallentyne, 1955; Rowley and Dahl, 1956; 

Brown, 1956; Gushing and Wright, 1965), but all use 

the piston to extract the core. The apparatus described 

here utilizes a flexible polythene liner to facilitate removal of the core from the core barrel.

A 2-m length of plastic pipe served as the core barrel. 

This material, light weight and durable, proved ideal 

for this type of coring. An externally threaded brass 

ferrule was fixed to the outer surface of the pipe, 5 cm 

from the end, with an epoxy resin glue. A tubular 

"l,000"-grade polythene liner was tasselated, bent 

back over the end of the core barrel, and taped to the 

external surface of the plastic pipe below the brass ferrule. A brass cutting head screwed onto the ferrule so 

that an internal brass lip closed onto the end of the 

liner and pipe, forming a flush seal (fig. 11). The cutting surface of the bit was serrated to aid in penetratPlastic drum

FIGURE 7.—Diagram of the leaf-rolling apparatus (36 cm long). See text for explanation.
MATERIALS AND METHODS 15

C/l 

CC 

111

o -J- o

UJ 

U

o u> I- N

GC 

' O UJ

GC

o 

o

cd 

03
16 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

ing organic debris.

To facilitate safe working on the delta, a floating 

drilling "rig" was constructed (fig. 12). After positioning the rig on the sediment, the core barrel, with its 

previously inserted liner, and bit were assembled. The 

operation of the corer is similar to that described by 

Livingstone (1955). The rubber piston (fig. 11) was positioned at the cutting edge of the bit. The piston was 

then held stationary relative to, and in contact with, 

the sediment surface by means of a rope that passed 

over a pulley at the top of the rig. The core barrel was 

pushed manually into the sediment around the piston 

such that when the core barrel was full of sediment, the 

piston was at the top end of the barrel and still level 

with the sediment surface. The piston was prevented 

from passing back down the barrel, during extraction 

of the pipe from the sediment, by clamping the piston 

rope at the top end of the barrel. The barrel was then 

extracted from the sediment by means of a yoke, which 

clamped onto the external surface of the plastic pipe 

and was attached to a rope passing over a pulley at the 

top of the rig. As soon as the barrel was extracted from 

the delta, it was laid horizontal, the bit and extracting 

yoke removed, and the ends of the barrel sealed with 

rubber bungs. The core was returned to the laboratory 

before being examined. In this way, not less than 95 

percent recovery of each of the 2-m lengths of cored 

sediment was obtained.

The core was extracted by pulling on the plastic liner 

after it had been untaped from the end of the barrel. 

This part of the procedure is fundamentally different 

from the piston extrusion method described by 

Livingstone. When numerous gas vesicles are present 

within an unconsolidated core, his procedure may introduce what may be termed "secondary compaction" 

during the extrusion process. By extracting the core 

within the liner, this problem was avoided. After the 

core was exposed by cutting the liner with a razor

Cutting edge

B

Wing

Internal ledge

FIGURE 9.—Sampling frame for collecting leaf samples from delta surface sediments. Frame A was positioned in sediment such that 

"wings" were level with sediment surface. Sieve quadrat B fits into 

frame A so that cutting edge runs along internal ledge of A. When B 

is lifted out of A, sediment is carefully washed through sieve; retained leaves are removed for laboratory analysis.

blade, a full description of the core was made and samples of sediment were removed for X-ray diffraction 

studies. The positions of included leaves were noted, 

and where the leaf material appeared concentrated, 

sections of the core were carefully removed for further 

study. No attempt was made to quantitatively sample 

the leaf concentrations within the core sections; the 

leaves and fragments were carefully removed from the 

sediment by washing, and a subjective examination 

was undertaken.

Eh and pH measurements were made by Dr. P. Bush, 

of Imperial College, on freshly extracted cores in the 

field. After removal of the core from the barrel, the core 

was retained in the polythene liner in which holes were 

cut, and the pH/Eh electrode was rapidly inserted into 

the sediment. The results are shown in table 5.

Sediment samples, approximately 10 g each, were 

taken at 0.2-m depth intervals and weighed in aluminum boats of known weight. The sediment was then 

dried at 80°C to constant weight. The dried sediment

Typha bed

O 

T1C4

O 

T1C3

O 

T2C4

O 

T2C3

O 

T3C4

O 

T3C3

Subaerial 

delta 

deposits

Alnus crown

IT4C4

O \ 

T4C3

FIGURE 10.—Position of cores relative to delta. (See figs. 44 and 45 

for diagrams of cores.)
VEGETATION ANALYSIS 17

was subsequently digested in concentrated nitric acid 

and the iron content of the digests measured using 

atomic absorption spectrophotometry. After obtaining 

the iron content of the sediment, the weight loss on 

drying was corrected for oxidation of the iron by assuming that no ferric iron was present at the start of 

the drying and that total oxidation took place. The corrected water loss on drying, as well as the iron content, 

is shown in table 5.

VEGETATION ANALYSIS

Before attempting any comparison between a deposPolythene 

~ liner

Plastic

- core

barrel

-Brass cutting bit

CENTIMETERS

FIGURE 11. — Corer assembly. See text for description.

ited assemblage of plant remains and the source vegetation, it is important to carefully consider in what 

terms the source vegetation should be described.

The concept of vegetational communities imparts the 

idea of vegetation being heterogeneous, each community displaying a greater degree of homogeneity within 

itself than is exhibited in the whole. In natural vegetation, boundaries between communities are rarely well 

denned, however, and ecotones may occupy areas 

larger in extent than the communities themselves. Yet 

workers studying pollen dispersal frequently treat 

vegetation as though it were made up of blocks of uniform composition and label the blocks according to the 

apparently dominant species. Each block then tends to 

be regarded as a distinct pollen source. As delimiting 

these vegetational areas has usually been a subjective 

process, the actual parameters likely to control pollen 

production have not been recorded.

Although it is dangerous to oversimplify vegetation 

into "blocked" source areas, such a concept is useful

FIGURE 12.—Coring delta deposit. Unconsolidated nature of sediment allowed core barrel to be driven by hand. For full description 

of coring procedure, see text.
18 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

when attempting to reconstruct ancient vegetation. 

The criteria on which divisions of the source vegetation 

are made must be related to the type of information 

likely to be preserved in the potential fossil deposit.

In megafossil analysis, factors affecting the litter 

production by the vegetation are critical because a 

megafossil assemblage may be thought of as a distorted 

sample of the capacity of the vegetation to produce 

leaves, fruits, seeds, or other organs. Bray and Gorham 

(1964) review a series of factors likely to influence the 

production of litter by a mature stand of forest vegetation, and these parameters may be summarized and 

discussed as follows, bearing in mind that the observations refer to the dry weight of material, not the 

number of leaves, fruits, and seeds.

NATURE OF THE TREES

Evergreen gymnosperms yield approximately onesixth more total litter annually than angiosperms. The 

difference in leaf litter alone is 8 percent. Such differences are noted only when a wide range of sites are 

considered.

14

3 13

O 12 

— 11

fc tr 10 5 <

? > 9 

ll 8

DC UJ

T r i r i i

10 20 30 40 50 60 

NORTH OR SOUTH LATITUDE, IN DEGREES

70

* Equatorial

A Warm temperate

EXPLANATION

o European cool temperate 

• Arctic/Alpine

• North American cool 

temperate

FIGURE 13.—Annual total forest litter production in relation to 

latitude. After Bray and Gorham, 1964.

ENVIRONMENT

CLIMATE

The effect of latitude on litter is plotted in figure 13. 

If such a pattern existed in the past, it is not surprising 

to find that many fossil assemblages represent tropical, 

subtropical, or warm temperate floras, even allowing 

for high rates of biological degradation. The relevance 

of the plot (fig. 13) to the litter production of ancient 

vegetation systems is difficult to evaluate. Clearly, the 

total productivity of a plant community is dependent 

on the solar energy input and therefore might be expected to be related to latitude, but this relation may 

be radically affected by climatic conditions, which are 

known to have varied markedly in the past.

AL III L'DE AND EXPOSURE

Bray and Gorham suggest that in mountainous 

areas, rainfall and temperature tend to be optimum for 

forest growth at intermediate elevations; the evidence 

presented is not conclusive. They imply that litter production would be at a maximum under these conditions. As the chances that a mountain flora will be 

preserved in the geologic record are small, effects of 

this kind on litter production are of limited interest to 

the paleoecologist. It is possible that increasing exposure adversely affects litter production, but the evidence is inconclusive.

SOIL FACTORS

The data reviewed by Bray and Gorham suggest that 

litter production tends to decrease when the soil factors 

are different from those required for optimum growth 

of a particular species. Krasilov (1969) presents examples of how edaphic factors can powerfully influence 

the floristic composition of a fossil plant assemblage. 

He was mainly concerned with the effects of major 

changes in soil type, but minor soil differences could 

significantly affect litter production and hence the 

likelihood of some species being represented in the fossil record.

TREE DENSITY

There is no evidence to suggest that tree litter is 

affected by the density of individuals, provided the 

canopy is complete. But there does appear to be a distinct correlation between the annual fall of litter and 

stand basal area in both gymnosperms and angiosperms. In this respect, Bray and Gorham quote the 

work of Bonnevie-Svendsen and Gjems (1957). It appears that quantitative analysis of closed-canopy vegetation using tree-density measures may give rise to 

misleading community classifications with respect to 

any derived megafossil assemblages.
VEGETATION ANALYSIS 19

TIME

As would be expected, the pattern of litter fall varies 

with seasonality of the climate. In equatorial forests, 

fall is more or less continuous throughout the year 

with slight variations due to such climatic fluctuations 

as wet and dry seasons. In temperate latitudes, most 

species show a peak in litter production at certain 

times of the year, most coinciding in the fall.

The total litter fall may differ greatly from year to 

year; if any useful litter estimates are to be achieved, 

measurements must be taken over a period of several 

years. Although, as stated, such a study is necessary 

outside the scope of this present work, litter falling 

directly onto the lake was intercepted by water traps in 

order to estimate the relative proportions of species 

entering the lake by direct aerial transport.

Variations in any of the factors that affect some 

species more than others within a community will 

necessarily affect the chances of those species entering 

a fossil deposit. In a sense, the deposit "sees" the source 

vegetation in terms of the litter productivity of the various component species. Note, however, that major 

components of the source vegetation, namely, the 

herbaceous plants of the understory, are rarely represented in the deposited assemblage (table 3). The impracticality of undertaking long-term litter measurements from the vegetation of the Silwood drainage 

basin forced the employment of alternative techniques.

In vegetation such as surrounds Silwood Lake, 

where there are areas of both open and closed canopies 

and trees of mixed age, tree density measures are

clearly inappropriate. Cover, defined as the proportion 

of ground occupines by a perpendicular projection onto 

it of the aerial parts of individuals of the species under 

consideration (Greig-Smith, 1964, p. 5), would be a 

more suitable measure, since it is more directly related 

to litter production, but estimating cover would be difficult on the scale required.

The measurement of crown cover, that is, the measurement of the area of horizontal ground surface occupied by a vertical projection onto it of the crown of a 

tree, was adopted as a compromise that offered a rapid 

estimate of species abundance that was somewhat related to litter productivity. The only part of the total 

litter production that has a high probability of being 

transported by wind any distance to the depositional 

site is that produced by the topmost parts of the crown. 

Here leaves are exposed to winds sufficient to transport 

them significant horizontal distances. In the Silwood 

environment the only place where the greater part of 

the total litter production from a group of trees could 

form the input to a depositional environment is at the 

streamside or lakeside. Leaves falling into the stream 

water are likely to represent a mostly unbiased sample 

of the woodland litter.

If, however, the stream is of such width that the 

canopy is broken, the resulting increase in light intensity along the stream course will stimulate the growth 

of streamside understory plants. Such a phenomenon is 

clearly seen in riverside vegetation in tropical rain 

forests (Richards, 1966) and is common around lake 

margins. Under such conditions, the taxa represented 

in the litter fall into a stream or lake will not be repreTABLE 3.—Summary statistics of the plant remains collected from within the top 2 cm of delta sediment at the positions shown in figure 8

__-

Alnus Betula Fagus Quercus Ilex Aesculus Acer Crataegus Typha Nuphar Salix, Alba Totals

cinerea

Number of

Number of

Total number

Fragments/

Fragments/

Total

Total

Total

Percent fragments/

Percent fragments/

Mean wholeStandard deviation wholeMean

Percent total number of

Percent total

Percent total

1 <2 percent unidentified. 2 cm2 .

— __ 1,315

722

___ - 7,561.5

_____ 11,573.5

___ _ 19,135.0

— - 15.94

_____ 45

______ 63

272

623.5

1,940.5

47

32

8.10

9

6

175

495

i ft**

65

768.5

2,181.5

2,950.0

10

559

69

628

8.10

89

826.5

1,879.5

78

15.40

8.78

1.48

4

13

6

482

413

895

54

882.0

1,800.5

2,682.5

49

33

4.36

4.37

26

18

9

165

170

97

218.0

248.5

715

88

6.10

3

1

23

9

32

2.56

59

232.5

542.0

75

43

25.79

20.71

1

1

2

7

7

100

10.5

10.5

100

1

1

4 185 31

4 _ _ ____

8 185 31

1.00

50 100 100

10.0 321.5 235.0

15.0 ____

25.0 321.5 235.0

67

40 100 100

3.75 ____ ____

3.01 ____ -_-

1 ___ ....

141

111

20

14

34

1.43

59

84.5

143.5

228.0

59

37

10.25

5.38

4.23

1

1

1

3,383

1,598

4,981

2.12

68

11,774

18,424

30,198

64

39
20 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

sentative of the species composition of the surrounding 

vegetation.

In a closed canopy situation, only the very tops of the 

trees, those parts of the crowns visible from the air, are 

exposed to high-wind energies and hence have less 

leaves transported over long distances by wind. In open 

vegetation, all parts of the crown are exposed to wind 

energies sufficient to transport leaf litter, but here 

again leaves that are most likely to travel significant 

distances will probably come from the upper parts of 

the crown.

The crown area, as projected perpendicularly onto 

the ground, does not represent the actual area of the 

crown's surface exposed to the wind. If the shape of the 

crown approximates a sphere, then the projected crown 

area is roughly one quarter of the actual crown surface 

area. By the above argument, the litter source may be 

restricted to the upper hemisphere of the crown so that 

the projected area now approximates half the actual 

area. In closed canopies, this difference between the 

two areas becomes even less since only the topmost 

part of each crown is exposed and the crown shape is 

modified by the close proximity of other crowns. The 

leaf density varies throughout each crown, the distribution being dependent on species, habit, and proximity to adjacent trees.

It has been assumed that those leaves transported by 

wind originate from the outer surface of the crown. 

Whether this assumption is justified is open to question 

because no relevant data are available. What is clear is 

that on the basis of crown cover as defined, the error in 

estimating the size of the source for wind transport is 

likely to be greater for open vegetation than for woodlands that have closed canopy.

- One way of correcting the estimate of crown areas to 

approximate more closely what the leaf litter production might be to weight the crown cover areas by the 

appropriate leaf-area indices. The leaf-area index is 

defined as the total surface area of leaf material of the 

species under consideration occurring above a unit 

area of ground surface. If the mean leaf area is known 

for a species, then an estimate of the number of leaves 

produced by a unit of vegetation may be deduced from 

known ground coverage. The dry weight, the form in 

which most litter studies are presented, may be similarly calculated. As pointed out, however, in nonriparian 

vegetation only litter produced in the upper parts of the 

crowns has a chance of being transported to the depositional site by wind.

RELATION BETWEEN PROJECTED CROWN 

AREAS AND LEAF DEPOSITION ON THE DELTA

By referring to percentage crown cover of trees with

distance from the delta, table 1, and numbers of leaves 

of these genera caught in traps, a direct comparison 

between the vegetation distribution and the leaves 

caught can be made. It is clear that the local species are 

grossly overrepresented. Alnus and Salix, with some 

Quercus and Betula, form the local species; of these, 

Alnus is the most abundant bordering the lake, hence 

its extremely high leaf count from the delta traps. 

Salix, by comparison, is underrepresented in the traps, 

undoubtedly because of the filtering action of the 

Typha bed. On the delta, Salix cinerea rarely attains a 

height greater than 4 m, and abundant Salix leaf litter 

was found among the Typha reeds. As expected, the 

more distant taxa, Fagus, Aesculus, and Crataegus, are 

underrepresented. Quercus, though more abundant 

some distance away from the delta, is a component of 

the local vegetation that, like Betula, occurs mostly 

behind the Alnus on the southeast lake shore. Once 

again, considerable filtering by lakeside vegetation reduces the probability that non-lakeside trees will be 

represented in the delta sediments.

When percentage crown cover (table 1) is compared 

with the summary statistics of leaves recovered from 

the delta sediments (table 3), a similar picture 

emerges. The species percentage, based on the number 

of whole leaves of the different taxa collected from the 

sediment, shows a closer resemblance to the proportion 

of the species in the source vegetation, but the overrepresentation of Alnus is still present. Species that are 

more distinct, but grow close to the stream, for example 

Fagus and Quercus, are now much less underrepresented than species of wider distribution, for example, 

Betula. The proportion of Salix whole leaves in the 

delta sediments is considerably higher than of those 

leaves arriving on the delta by wind transport alone 

(table 2); from this, it may be concluded that most of 

the Salix leaves represented in the delta were 

transported by the stream.

No clear quantitative relation exists between the 

number of leaves deposited on the delta and the vegetation as measured by the projected crown area. It is 

most unlikely, when one considers the large numbers 

of variables involved, that a simple direct relation between the deposited remains and the source vegetation 

can ever be found. The use of proportions to assess the 

relative representations of the taxa creates its own 

problems, because, being a closed system, a departure 

from a direct relation exhibited by one species necessarily affects the others. Failure to derive relations between the source vegetation and the deposited assemblage in terms of species proportionality does not 

preclude the use of alternative approaches to the problem. If it is not possible to deduce how common a plant 

was in the past, it may yet be possible to determine
FLUID DYNAMICS IN RELATION TO LEAF TRANSPORT 21

where it grew relative to the depositional site and 

thereby discover paleocommunity relations.

FLUID DYNAMICS IN RELATION 

TO LEAF TRANSPORT

The movement of sediment grains by flowing water 

has received much attention from sedimentologists. 

Many hydrodynamic concepts, developed in other 

branches of science, have been employed by making 

certain assumptions about the sedimentary particles, 

for example, that the grains approximate spheres and 

do not interfere with each other. And it would be desirable to determine the hydrodynamic properties of 

leaf material in order to isolate those characteristics 

that may lead to sorting during transport.

Mantz (1973) has shown that for solid shapes that 

depart only slightly from a spherical form (but which 

may still be described as having a single geometric 

length dimension), the shape effect on entrainment is 

minimal. For shapes that are essentially two dimensional, the orientation of grains in a sedimentary bed 

has considerable effect. Although there is no information available concerning the entrainment of leaves, 

Schoklitsch (1914), Shields (1936), and Pang (1939) 

have all investigated the entrainment of flaky solids. 

Schoklitsch and Shields consider that flaky solids move 

at fluid stresses higher than those necessary to entrain 

granular solids, whereas Pang (1939) comes to the opposite conclusion. The cause of their disagreement may 

well be in the different initial orientations of the plates 

in their sediment beds, which were artificially laid (P. 

Mantz, oral commun., 1975). Mantz (1973, 1977) carried out similar experiments, but using a naturally

laid mixed sediment bed, and found that the entrainment of fine-grained flaky solids occurred at fluid stresses less than those required to move granular solids 

with the same fall diameter. If leaves behave similarly, 

then considerable sorting would result, solely on the 

basis of shape, between, for example, leaves and fruits. 

It should be noted, however, that all workers have used 

plane sedimentary beds (defined as plane to within one 

solid particle diameter), a condition that rarely, if ever, 

exists in nature.

The effect of leaf orientation and aspect on fluid flow 

clearly determines the critical fluid stress at which the 

leaf will begin to move. Elongate objects such as twigs 

tend to remain stable at fluid flows of greater magnitude when they are alined parallel to the direction of 

flow, and this effect is sometimes observed in fossil deposits. Since the object presents a smaller area to the 

fluid flow, the fluid stresses on the object are low and 

movement begins at correspondingly greater fluid 

velocity.

Many fluid-dynamics equations require that an effective geometric dimension be ascribed to the 

sedimentary particle under investigation. For a 

sphere, this dimension is the diameter. It is difficult to 

directly ascribe such a length dimension to an essentially two-dimensional object such as a leaf, because 

the effective value of this parameter will vary with the 

orientation of the object to the fluid flow. The fluid flow 

(fig. 14) will affect the orientation of the object because 

the turbulent wake will impart instability to an unfixed plate (such as a leaf), with the result that its 

orientation with respect to the fluid flow will continually change. Such a condition may be observed in 

leaves falling in still air. If the terminal velocity of a

Van Karman's 

Vortex Street

Re =o=10 Re =2=250

FIGURE 14.—Stream lines around rigid plate held normal to fluid flow. As Reynolds number is increased, here achieved by increasing flow past 

plate, eddies form in lee of plate. At first these eddies are small and symmetrical, but as fluid flow is further increased, they become unstable 

so that at high Reynolds numbers, a series of unequal eddies stretch along wake behind plate. After Batchelor, 1967, plate 4. The Reynolds 

number is a dimensionless number that may be interpreted as being proportional to the ratio between inertial and viscous forces given by

Re = ULp/n 

Where U is velocity, L reference length, p = density, and fj. = dynamic viscosity.
22 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

leaf is such that a turbulent wake is produced, then the 

leaf will flutter or spin to the ground, whatever the 

attitude at which it begins to fall or turbulence in the 

surrounding air.

Mantz (oral commun., 1975) has suggested that for 

flaky solids, the nominal diameter might be used as an 

approximation to a suitable characteristic length. This 

measurement is the diameter of a sphere of equal 

volume to that of the flake. Mantz used this to examine 

the entrainment of dry autumn leaves by wind:

"An estimate was made of the critical wind stress (T,.) which produces initial transport of surface leaves from a bed of natural autumn leaves. An anemometer placed at 40 cm above the leaf bed 

surface was used to record the wind velocity on 20 occasions when 

surface leaves were first transported. The Karman-Prandtl velocity 

distribution law was then used to estimate the associated average 

wind stress as 0.3 N/rrr. The average leaf statistics were 0.13 m 

length, 0.00017 m thickness, and 450 kg/m3 density (cr). These were 

used to estimate the Shields (1936) critical stress parameter (0f. = 

r,./crgZ) N , where Z)\ is the nominal diameter for a leaf) as 0.004. Such 

a value is an order of magnitude below that for the incipient 

transport of natural flakey minerals under water (Mantz, 1977). The 

result is however to be expected for the main reason that a natural 

leaf bed packs more loosely than a natural mineral bed." (P. Mantz, 

Imperial College, written commun., 1980).

Berthois (1962) found little similarity in hydraulic behavior of approximately spherical quartz particles and 

mica grains of the same nominal diameter. Experiments on mineral solids that do approximate spheres 

indicate that trnasport behavior often cannot be predicted better than to within one order of magnitude; 

thus similar predictions for leaf transport may be 

greatly in error.

The settling of leaves in water is further complicated 

by the fact that as leaves become saturated, the density 

of the leaf will alter not only with time but also from 

place to place within the leaf. The deposition of sediment particles on the surface of the leaf and invasion of 

the tissues by microorganisms will also affect the density. These factors, together with the natural variation 

in size and shape and the flexible nature of leaves, 

make assigning a characteristic length impossible; 

even under ideal conditions, where fluid turbulence is 

minimal, the behavior of leaves cannot be described by 

simple equations of fluid dynamics.

Despite the difficulties of theoretically describing individual leaf/fluid interactions, the effect of hydraulic 

sorting of leaf populations may be strong enough to 

produce characteristic patterns of assemblage structure that may prove useful in determining the relative 

source areas of the component species.

The concept of hydraulic equivalence may be employed to determine the predominant transport 

medium of various plant species in relation to the entombing sediment and thereby distinguish source 

areas. Two particles are said to have the same hydraulic equivalence if they react similarly to a given set of 

fluid stresses. If two sediment grains come to rest side 

by side in a sediment bed that is being deposited under 

a field of gradually decreasing fluid stresses, then they 

are said to be hydraulically equivalent. If it is 

supposed, for the moment, that stream waters, on entering a lake, flow over the delta slope with a gradually 

decreasing velocity, leaves will be deposited along with 

sedimentary grains of the same hydraulic equivalence 

at positions on the delta front that reflect their hydraulic characteristics. This will result in considerable sorting. On the lake bottom, where the fluid stresses are 

small, the sunken leaves are likely to exhibit a wide 

range of hydraulic characteristics.

If a population of fossil-leaf remains is judged to have 

the same hydraulic equivalence as the sediment grains 

composing the surrounding matrix, then it is probable 

that both were deposited in response to the same fluid 

medium. Under certain circumstances, it may be possible to determine the transport medium (air or water) 

for a particular group of remains and thereby gain a 

clue as to the species distribution within the source of 

vegetation.

It is unlikely that the hydraulic characteristics of 

plant remains can be theoretically deduced: consequently, an empirical approach to the investigation 

of sorting is required.

WIND TRANSPORT

The role of wind transport in the dispersal of fruits 

and seeds has long been recognized (Ridley, 1930); in 

recent years attempts have been made to investigate 

differential pollen dispersal as an aid to the interpretation of pollen diagrams (Tauber, 1965, 1967). While a 

number of the concepts described for pollen dispersal 

may be equally well applied to the transport of megascopic plant remains, the dispersal of leaves as such 

has attracted little attention.

Although it cannot be expected that leaves will be 

transported long distances by wind, populations of 

leaves from different species may be carried 

significantly different distances, and a distorted picture of the source vegetation may be presented by an 

allochthonous assemblage. The preliminary investigations into the possible causes of differential leaf dispersal described here were carried out in conjunction with 

Dr. O.K. Ferguson at the Rijksuniversitair Centrum 

Antwerpen.

Wind transport can be divided into a vertical and a 

horizontal component, both of which operate simultaneously in nature. The experiments to investigate the 

effect of leaf size, shape, and weight on dispersal were
WIND TRANSPORT 23

carried out using artificial leaves, the parameters of 

which could be controlled within narrow limits. 

"Leaves" made of air-mail paper (0.0030 g/cm2 ), cartridge paper (0.0123 g/cm2 ), and card (0.0346 g/cm2 ) 

were used, cut into circles, equilateral triangles, and 

two rhombic shapes of length to breadth ratios of 1.45:1 

and 6.90:1. In most experiments, two sizes of each 

shape were used, 11 cm2 and 44 cm2 ; some additional 

measurement were made using units of 22 cm2 . The 

time taken for the units (which were released with 

their flat surfaces horizontal) to fall an arbitrary vertical distance of 3.18 m in still air was recorded and the 

results plotted as figure 15.

Once on the ground leaves may be blown laterally. 

An attempt to investigate this aspect of dispersal was 

made by blowing leaves, both artificial and natural, 

along a plane flat surface using a ducted fan, moved 

from side to side over a distance of 3 m for a period of 1 

minute so that all the units were blown evenly. Although the air flow from this fan was extremely turbulent, and by no means uniform, the relative dispersal of 

the units could be determined; the results are shown in 

figures 16 and 17.

The results suggest the following generalizations.

SHAPE

It was observed that while falling through still air, 

shapes with axes of unequal length tended to rotate 

about the longest axis. This affected the flight so as 

generally to increase the length of time a leaf remained

8.0

6.0

w 4.0

O

2.0

0.0

0.0 0.01 0.02 0.03 0.04

WEIGHT PER UNIT AREA, IN GRAMS 

PER SQUARE CENTIMETER

o Paper leaves

o Natural leaves

r Rhododendron

I Lauras nobilis L.

EXPLANATION

p Platanus X hispanica Muench 

f Fagus sylvatica L. (flat) 

f* F. sylvatica (curled)

FIGURE 15.—Time taken for leaves (both paper and natural) to 

fall a distance of 3.18 m in still air plotted relative to their 

weight per unit area.

in the air as well as to broaden the ground dispersion. 

These differences are insignificant, however, when 

compared with those resulting from variations in 

weight.

Flat shapes blown horizontally along the ground 

were distributed laterally by a combination of saltation 

and rolling; the greatest lateral dispersion was found 

with circular shapes that tended to roll. When the flat 

shapes were folded such that when viewed along the 

axis of the fold a V shape was formed, the distance they 

were transported increased for a given windspeed but 

imbrication caused a marked positive skewness in the 

histogram (fig. 17). A similar effect was noticed for 

Fagus leaves that had curled during drying.

SIZE

The rate of vertical fall in still air appears to be little 

affected by size alone, except that the small shapes of 

the heavier materials had a tendency to be more stable 

in the air and generally remained horizontal while fall30

20

10

30

20

10

30

20 -

10 -

Wind 

source7

wt. 0.003 g/cm2

wt. 0.0123 g/cm2

11 cm2 paper leaves 

/44cm2 paper leaves

wt. 0.0346 g/cm2

6 8 10 12 14 

DISTANCE, RELATIVE

16 18 20

FIGURE 16.—Lateral blowing of flat rhombic paper leaves 

(length-to-breadth ratio 1.45:1) along a plane flat surface. 

Horizontal distance leaves will travel will be dependent on 

wind strength; distance classes arbitrary.
24 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

ing after being released in that position. This behavior 

was observed with the larger size of the heaviest material. The effect of this phenomenon was to reduce 

ground dispersion.

No significant difference in lateral ground dispersion 

could be attributed to size.

WEIGHT

Variations in falling rates in still air appear to be 

determined by differences in weight per unit area more

30

20

10

30

20

10

0

,44 cm 2 paper leaves 

,11 cm2 paper leaves

Flat

Curled

0 2 4 6 8 10 12 14 16 18 20

DISTANCE, RELATIVE

FIGURE 17.—Horizontal distance traveled by folded paper 

(0.0123 g/cm-) as compared with both flat and curled 

Fagus leaves under influence of a given wind speed. A, 

Folded paper; B, Fagus leaves. Curled Fagus leaves appeared to be blown fractionally farther than flat leaves, 

but imbrication caused marked positive skewness.

than any other single factor. Shapes made from card, 

the heaviest material, showed little variation between 

sizes, shapes of air-mail paper more, possibly because 

of relative increase in importance of size at low-weight 

levels. As would be expected, shapes with a greater 

weight per unit area fell faster, while the lightest 

shapes took correspondingly longer. The relation does 

not appear to be linear (fig. 15).

Variations in lateral movements of dispersed units 

under the influence of horizontal winds also appear to 

be largely determined by weight per unit area. At all 

three weight levels used in the experiment, the lateral 

dispersion of each group was normally distributed over 

the same number of distance classes irrespective of distance traveled. Increasing weight caused a progressive 

shift in the mean of the distributions toward the wind 

source (fig. 16), which, because the wind energy in this 

experiment was inversely proportional to the distance 

from the source, means that the wind velocity that is 

necessary to bring about incipient movement is proportional to the weight per unit area of the dispersal 

unit.

All artificial paper leaves used in the above experiments lacked a petiole, but experiments carried out 

with dried natural leaves indicate that the effect of the 

petiole is negligible except where it accounts for a substantial part of the total weight of the leaf or is exceptionally long.

LEAF SIZE SORTING

Considerable differences may exist in leaf weight per 

unit area and size between sun and shade leaves 

within a single species, for example, Fagus sylvatica. 

The leaves at the top of the crown of many trees tend to 

be smaller and have a greater weight per unit area than 

those lower down, particularly in a dense forest

EXPLANATION 

Delta trap

Stream trap

249 499 749 999 1249 1499 1749 1999 2249 2499 2749 2999 3249 3499 3749 3999 4249 4499 4749 

AREA, IN SQUARE MILLIMETERS

FIGURE 18.—Size-frequency histogram forFagus sylvatica leaves recovered from both stream and delta traps. Stream trap was positioned in a 

part of stream that flowed past a stand of mature trees forming a closed canopy over stream; sample may be considered to be representative of litter on woodland floor. Following Fisher-Behrens test for two sample comparisons with unequal variances (as described in 

Campbell, 1967), the null hypothesis that the two populations were equal as regards their means had to be rejected because the observed 

statistic d (= 6.94) exceeded the published value at the 1-percent level of significance.
WIND TRANSPORT 25

canopy. Despite these differences, the top leaves have a 

greater chance of wide dispersal because they are exposed to higher wind energies than those within the 

canopy or trunk space. Most leaves of distant species, 

such as Fagus sylvatica, caught in the delta surface 

traps were small sun leaves, not, as might be expected 

from the laboratory experiments, leaves of a smaller 

weight per unit area (fig. 18).

The difference in mean leaf area between leaves 

trapped on the delta and those from the stream in the 

mixed woodland above Cascade Bridge was extremely 

large for Fagus sylvatica and was shown to be 

significant at the 1-percent probability level (fig. 18). It 

cannot be argued that the leaves in the delta traps 

represented a sample from a group of Fagus trees that 

might bear only small leaves, because the absence of 

any strong prevailing wind direction during the sampling period suggests that the sample was representative of the surrounding vegetation (fig. 19 and table 4). 

Alnus leaves from the delta traps are compared in 

figure 20 with a sample of Alnus leaves from a stream 

leaf trap. There is no significant difference in the 

means, although a slight skewness to the left is seen in 

the delta sample. This suggests that even with leaves

from local elements of the flora, a larger proportion of 

small leaves enters the lake than is representative of 

the litter from the surrounding vegetation, although 

the difference may not be significant.

The representation of at least part of the flora by 

only the small dense leaves has critical consequences 

for paleoclimatic interpretations based on fossil deposits. Dilcher (1973) discusses the correlation of leaf size 

with climate and demonstrates how both temperature 

and moisture influence the relation. The wet tropics 

are characterized by a high percentage of large leaves, 

whereas a change to a drier or cooler climate is accompanied by an increase in percentage of small leaves. 

Wolfe (1971) recognized the problems by differential 

selectivity during transport and deposition; "large 

leaves obviously will, in many environments, tend to 

be fragmented by turbulent currents and hence be 

under represented. Another problem is that fossil assemblages may contain an over-representation of 

streamside plants (MacGinitie, 1953, p. 46) [such as] 

stenophyllous plans that typically fall into the low 

size-classes (Richards, 1966)***. Such overrepresentation would yield an analysis indicating a 

cooler climate***."

270

/ Dec. 12-18^ Oct. 26> 

July 18-24

Dec. 28-Jan. 4

Nov. 14-20 

July 11-17 \ Sept. 12-18 \

\

\

\

\

180

FIGURE 19. — Mean weekly wind directions for July to December 1973 as computed 

from data collected at Heathrow Airport, London. Actual values, together with mean 

angles of deviation, presented in table 4.
26 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

TABLE 4.—Mean weekly wind directions for July to December 1973 as 

measured at Heathrow Airport, London

\R is related to the closeness of clustering of the component angles of wind direction (weighted for speed (knots) and is denned as the distance from the center of a unit circle (with a 

radius of D]

July

Aug

Sept

Ort

Nov

Dec

(1973)

4—10

11-17

18- 24 _________

25-31 _________

1-7 ________

15-21 _________

22-28 _____ ___

29-4 _ __.__

5-11 _________

19-25 _________

26-2 _______

3-9 ________

10-16 _________

17- 23 _______ _

24- 30 _________

31-6 _____ ___

14- 20 _________

21- 27 _________

28-4 _________

5-11 _________

19-25 ________

Mean 

direction 

(degrees)

282

184

275

165

7

221

31

110

329

93

179

129

333

30

172

335

78

64

60

199

66

221

82

287

127

a

(fig. 18)

0.10

.14

99

09

.32

.03

.05

.36

.22

49

.20

.07

.05

.10

19

.31

.15

.12

.14

.14

.32

.11

.07

.22

.43

Mean angle 

of deviation 

(degrees)

76 9

7^ 0

68.0

77 9

66.5

79 6

78.8

64.6

71.5

57.8

72.4

77 9

78.9

76.8

72.7

67.1

74.4

75.7

74.8

74.8

66.8

76.0

77.8

71.2

60.9

Mean 82 0.03 79.4

Dilcher (1973) points out that in the clay deposits of 

Kentucky and Tennessee leaves of greatly different 

size classes occur together and that Wolfe's assumption 

that the linear or lanceolate leaves, typical of stream 

sides, are smaller in total area than those in the forest 

is open to question. Dilcher's observation that both 

large and small leaves occur in the Tennessee deposits 

in no way invalidates Wolfe's argument, since the 

Tennessee material was undoubtedly laid down under 

tranquil conditions, whereas Wolfe was considering a 

higher energy, more fluviatile environment in which 

the larger leaves would tend to be destroyed. In any 

case, it is impossible to prove, from a fossil flora, that 

overrepresentation has, or has not, taken place.

From observations on Fagus sylvatica leaves deposited in the Silwood stream trap, it is clear that leaves 

entering a small woodland stream are nearly representative of the forest litter. In more open waters, distant 

components of the surrounding flora are represented 

only by the smaller leaves, as a result of differential 

aerial transport. While it is difficult, in the absence of 

any positive evidence, to endorse Wolfe's assumption 

that overrepresentation of small leaves results from 

overrepresentation of streamside plants, differential 

aerial transport of leaves from the more distant species

EXPLANATION 

Delta trap 

Straam trap

25 249 499 749 999 1249 1499 1749 1999 2249 2499 2749 2999 3249 3499 3749 3999 4249 4499 47494999

AREA, IN SQUARE MILLIMETERS

FIGURE 20.—Size-frequency histogram for Alnus glutinosa leaves recovered from both stream and delta traps. Distributions are similar with 

perhaps a slight positive skewness exhibited by delta sample. Fisher-Behrens test showed no significant difference in the means (d =0.56).
WATER TRANSPORT 27

can bring about a similar effect, with the result that 

the fossil flora may be interpreted as apparently representing a cooler or dryer climate than actually existed.

Trees growing around a lake margin can contribute 

leaves to the lake sediments by direct leaf fall and minimal lateral transport. The leaf population derived 

from such trees will closely resemble that found on the 

forest floor and, as such, will consist of both "sun" and 

"shade" leaves. Lacustrine sediments deposited in 

small basins are consequently likely to contain a population of fossil leaves that reflects the full size range of 

leaves, at abscission, in the original forest. Swamp deposits will similarly contain a minimally biased sample of the leaves from the source plants. Conversely, 

deposits laid down in basins where the minimum lake 

width is large relative to the canopy height will be 

relatively enriched with sun leaves from both the locally and distantly growing source taxa.

If the local community is restricted in area or leaf 

productivity, then its contribution to the fossil assemblage may dilute out, an effect analogous to that 

described by Neves (1958), Chaloner (1958), and 

Chaloner and Muir (1968) for spores.

Climate will affect the dispersal of leaves because 

conditions of high humidity will increase the weight 

per unit area of the leaves. If liquid water is present, 

their ability to be transported by aerial saltation and 

rolling will be seriously diminished because they will 

tend to cling together and to any substrate as a result 

of surface tension forces. This aspect of climatic control 

over dispersal has been discussed by Ferguson (1971). 

He has also noted that high humidity will tend to promote a well-developed river system that will take over 

a certain part of the role played by wind action in dispersal.

DISPERSAL THROUGH THE TRUNK SPACE

Tauber (1967) considered that objects such as pollen 

passing through the trunk space in a forest will tend to 

be caught on shrubs next to a clearing. Work carried 

out by the author with D. K. Ferguson (unpub. data 

1973) has shown that leaves ofLaurus nobilis L. will be 

trapped only on the windward side of a diffuse obstacle, 

such as a bush, provided that the windspeed is low (10 

km/hr). Speeds exceeding this rate create a buildup of 

leaves behind the obstacle (provided that the leaves 

can pass through the "mesh" of the obstacle), and at 

higher speeds this leeward tail increases in length. 

This is because wind energy is dissipated in turbulence 

behind the obstacle leading to a deposition of the "suspended" load. Pollen, being so much smaller, has a 

much slower settling velocity and is likely to pass 

through mostly unaffected.

Some workers have used a negative exponential 

decay model to describe the decrease in deposition of 

windborne plant material as a function of distance 

from the source (Richerson and others, 1970; Edmonds and Benninghoff, 1973; Rau, 1976). The form of 

the equation used by Rau (1976), who studied litter 

deposition in a lake, is

Zx =Zr exp (-k[r-x])

where x = distance from the lake center, Z r = deposition occurring at distance x, r - distance from the lake 

center to the shoreline, Z, = deposition at the 

shoreline, and k — (r—jc)~'ln(ZirZ,."').

The curve of figure 21 shows the relationship between the observed windborne leaf deposition on the 

delta at Silwood and the expected deposition as predicted by a least-squares best fit of the negative exponential model. Although the fit was fair (r2 = 0.91), 

variations in the observed data could well have been 

produced by the close proximity of the hydrosere vegetation on the delta.

225

200

Observed numbers of leaves

Curve fitted by calculation 

xy 3

r2 = 0.91 

Zr = 208.52 leaves

5 10 15 20 25 

DISTANCE FROM SHORE, IN METERS

FIGURE 21.—Deposition of leaves onto delta surface by wind as a 

function of distance from north shore of Silwood Lake. Observed 

leaf deposition compared with that predicted by a negative exponential model (Rau, 1976). Coefficient of determination, r-, is an 

estimate of how closely equation fits experimental data and varies between 0 and 1. The closer r2 is to 1, the better the fit.
28' SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

This model describes only the pattern of leaf deposition onto a water or land surface and does not necessarily relate to the distribution of plant material on the 

lake bottom. Subsequent redistribution of floating 

leaves by wind or water currents and degradation on 

the lake bottom may invalidate the application of the 

negative exponential model to the description of the 

distribution of organic matter on the lake bed; further 

redistribution of the more finely divided plant material 

may be brought about by water currents set up during 

thermal overturn.

EFFECT OF THE DECIDUOUS HABIT

None of the leaves examined from the Silwood deposits could be assigned with certainty to any herbaceous 

species. The importance of leaf-shedding habits of 

plants in affecting their chances of representation in 

the fossil record has been noted by other workers (for 

example, Chaney, 1924, and Ferguson, 1971). The lack 

of deciduous habit results in leaves of herbaceous 

plants withering and disintegrating while still attached to the stem; hence, the extreme rarity of herbaceous plants in sedimentary environments. Many 

deciduous trees also fail to shed leaves, particularly 

when juvenile (for example, Fagus sylvatica and Quercus robur], which must in turn distort the fossil record.

The evergreen species do of course shed their leaves, 

usually not in the autumn but when new growth begins. Ilex aquifolium L., for example, commonly sheds 

its leaves in the spring. It has been noted by Bray and 

Gorham (1964) that evergreen gymnosperms, as well 

as being generally more productive, yield approximately one sixth more total litter annually than angiosperms, not necessarily in the form of leaves. Seasonal shedding of leaves probably greatly affects the 

dispersal of plant remains, especially if the leaf fall 

from one component of the flora coincides with a rainy 

season while the remainder shed their leaves during 

the dryer part of the year. The spring fall of Ilex may 

well lead to a different dispersal pattern than exhibited 

by leaves shed in the autumn months.

WATER TRANSPORT

Botanists have long recognized the role of water in 

the transport and dispersal of plant material. Berry 

(1906) observed large "rafts" of leaves being 

transported downstream by river waters, and Ridley 

(1930) made numerous observations on the transport of 

plant debris by floods following tropical storms. A 

number of such reports are brought together in a vivid 

discussion on the mode of formation of fossil floras by 

Chandler (1964, p. 4-8).

DISCUSSION OF LEAF-FLOATING EXPERIMENTS

Moseley (1892), while on board H.M.S. Challenger, 

observed the transport of plant debris offshore from the 

mouth of the river Ambernoh, New Guinea, and made 

the following report:

The leaves evidently first drop to the bottom, whilst vegetable drift is 

floating from a shore. Thus as the debris sinks in the sea-water a 

deposit abounding with leaves, but with few fruits and little or no 

wood will be formed near shore, whilst the wood and fruits will sink 

to the bottom further off land.

The powerful effect of differential sorting brought 

about by differences in the floating times of various 

plant organs is therefore well established.

Preliminary experiments with Fagus sylvatica 

leaves have shown that if only one side of a leaf is 

wetted and there is little wave action, such leaves may 

remain afloat almost indefinitely, buoyed up by surface 

tension. Conditions such as these are rare in nature, 

being found only in small pools within forests with a 

closed canopy, in which case such transport as there 

might be is minimal and confined to the limits of the 

pool. Slow-flowing rivers might also have comparatively calm surface waters where a leaf might remain 

afloat in this manner for some time.

If both surfaces of a leaf are wetted and the leaf is in 

only slightly turbulent gas-saturated water, 

significant increases in floating time may result from 

the formation of gas bubbles on the leaf. Using the 

light-beam apparatus (fig. 6), it was found that this 

phenomenon caused previously sunken leaves to refloat. In order to achieve consistent results and thereby 

make valid the comparisons of floating times between 

species, it was necessary to agitate the water surface 

periodically. Unfortunately, even minimal agitation 

caused flow currents to be set up in the water of sufficient energy to stir up leaves that had already sunk but 

had buoyancies only slightly in excess of neutral. The 

stirred-up leaves frequently interrupted the light beam 

and gave false readings. For these reasons, the lightbeam apparatus was found to be of only limited use. 

The effect of turbulence and air bubble formation is 

demonstrated in figures 22, 23, and 24.

In the turbulent surface waters of a lake or river, 

bubbles might well be knocked off as soon as they had 

formed, and the sinking of a population of leaves is 

likely to follow the pattern observed in most laboratory 

bucket experiments. From the results of such experiments and the range of floating times as shown in 

figures 22, 23, 25, and 26, the sinking pattern commonly resembles the cumulative log-normal distribution curves typical of dosage response.

At first it was considered suitable to analyze such 

curves using probit analysis. However, the design of 

the experiments was such that this was inappropriate
WATER TRANSPORT 29

(Finney, 1971, p. 27). If the experiments were redesigned to conform to the form required for probit 

analysis, that is, based on successive batches of test 

leaves rather than a single batch tested during a continually increasing dosage, such an analysis would be 

ideal for expressing the differences between species. 

The concept of the E.D. 50, in this case the length of 

time taken for 50 percent of the leaf population to sink, 

is eminently suitable for summarizing the sinking of a 

population of leaves, and by converting the log-normal 

distribution to a straight line, direct comparisons with 

other species may be made.

It can be readily seen that the vastly different floating times exhibited by the various species could result 

in a high degree of species sorting. The wide variations 

in floating times caused by air bubble formation 

means, however, that little precision in paleocommunity reconstruction is likely to be achieved by studying 

this aspect of dispersal. Analyses of floating times are 

critical in determining the likelihood of a fossil being 

represented far from its growing point and therefore

No agitation of surface water

Frequent agitation of surface water

40 30 120 

TIME, IN HOURS

160

FIGURE 22.—Effect of water turbulence on floating time of air-dried 

Fagus sylvatica leaves as determined from bucket experiments.

3456 

TIME. IN HOURS

24

FIGURE 23.—Floating time of Alnus glutinosa leaves in experiment 

carried out in a bucket with frequent agitation.

the possibility that it may represent a distant source 

community. It should be born in mind, however, that

i i i i i i i i i i i i i i i i i i in i

2 4 6 3 10 12 14 16 13 23 

TIME, IN HOURS

FIGURE 24.—Floating time of Alnus glutinosa leaves in experiment 

carried out in light-plane apparatus with no agitation. Line fitted 

by eye.

10 20 30 40 50 60 70 80 215 

TIME, IN HOURS

FIGURE 25.—Floating time of freshly fallen//ejr aquifolium leaves in 

experiments carried out by bucket method with frequent agitation. 

Line fitted by eye.

2 4 6 8 10 12 14 16 13 20 22 

TIME, IN DAYS

FIGURE 26.—Floating time of freshly fallen Rhododendron 

sp. leaves as recorded from a bucket experiment with 

frequent agitation.
30 SORTING AND DEPOSITION OF AUTOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

perhaps the greater part of water transport occurs during that period of time when the leaf is either in 

suspension or is part of the bedload. Casual observation showed that at any given time, even during the 

autumn, far more leaves were present in the Silwood 

stream as part of the bedload than were floating.

LEAF DEGRADATION, BIOLOGICAL, 

CHEMICAL, MECHANICAL

Leaf fragmentation and ultimate destruction in the 

course of water transport is a major factor influencing 

the representation of a species in the fossil record. It 

has long been recognized that certain species are more 

resistant to breakdown than others, but most of the 

work that has been done has expressed degradation in 

terms of weight loss rather than in measures directly 

applicable to paleobotanical problems. Weight losses 

may occur owing to leaching of soluble substances, for 

instance, which may not directly affect the chances of 

that leaf being represented in the fossil record. In the 

present study, therefore, degradation is presented as a 

series of figures showing the extent of lamina loss. The 

amount of leaf material escaping destruction will 

clearly determine whether or not the species is recognizable in the fossil record.

Petersen and Cummins (1974) have studied leaf degradation by constructing 10-g artificial leaf packs of 

single species (held together with a nylon fastener) and 

measuring the weight loss of the packs over known 

periods of time in stream environments. The packs 

were constructed to be similar to the "stacks" of leaves 

often observed in streams on the upstream side of obstacles during the autumnal leaf fall. An alternative 

experimental method is to expose leaves to the degrading environment enclosed within nylon bags (for 

example, Mathews and Kawalczewski, 1969). This approach has the advantage that access by degrading organisms can be controlled by the mesh size of the bags. 

The use of single species, whether in the form of leaf 

packs or enclosed within nylon bags, may be criticized 

in that the ecology of the organisms affecting breakdown may be altered by restriction of the food source to 

one species, and hence degradation may not proceed as 

it would under natural conditions, where a number of 

different species are likely to be present.

In the Silwood study, it was decided to use samples of 

leaves of mixed species. Nearly rigid nylon cages (0.5- 

cm2 mesh size) were used to allow all but the largest 

invertebrate detritivores access, at the same time retaining all the identifiable leaf fragments. The 

semirigidity of the cages insured that the leaves were, 

to some extent, protected from mechanical breakdown 

(except that caused by abrasion between themselves). 

This protection was desirable because estimates of the 

mechanical strength of the leaves were to be determined at various times during the experiment. Instead 

of estimating dry weight loss as a measure of degradation, the outlines of the leaves were recorded. This 

measured only breakdown in terms of the visible leaflamina loss, not internal tissue destruction as might be 

caused by mircoorganisms. The amount of the lamina, 

and, more especially, the area of intact cuticle would 

ultimately be the criterion that would determine 

whether species identification in the fossil state would 

be possible. The degree of breakdown as affecting the 

mechanical strength of the leaf, which in turn determines the susceptibility to fragmentation, was also investigated.

Biological

The dominant food energy source of a small woodland stream is the allochthonous input (see Egglishaw, 

1964; Cummins and others, 1972, 1973), and it has 

been demonstrated that the aquatic insect community 

becomes synchronized to the autumnal input of leaf 

material (Hynes, 1961), emergence, oviposition, and 

eclosion occurring just prior to leaf abscission. The type 

of leaf degradation caused by invertebrate organisms 

may be classified as that brought about by the activities of large particle feeders and that produced by 

small particle feeders (Yonge, 1928; Petersen and 

Cummins, 1974). The activity of both these groups of 

organisms may be seen in figures 27-36. The large 

rounded holes are typical of the large particle feeders, 

while the removal of predominantly intercostal tissue 

only is the result of the activities of the small particle 

feeders. Sometimes attack appears to begin at the 

margins (for example, fig. 32, lake 1 month), but extensive damage to the lamina rapidly takes place; by the 

fourth month in the lake environment, some leaves are 

extensively degraded. The midrib and large secondary 

veins are seldom damaged, even by the large particle 

feeders, except when the rest of the lamina is almost 

completely destroyed (for example, fig. 31, stream 4 

months).

It can be seen that the degree of degradation that 

takes place over a given time varies from species to 

species. Petersen and Cummins (1974) suggest that differential invertebrate destruction of leaves is a function of microbial colonization and conditioning. In this 

respect, they quote the work of Kaushik (1969), who 

proposed that selection of leaves for food by invertebrates was probably determined by differential rates of 

microbial colonization. Kaushik and Hynes (1971) 

demonstrated that stream invertebrates such as 

Gammarus exhibit a preference for certain species that 

reflects the susceptibility of those species to colonization by fungi, which are more damaging than bacteria 

during the initial stages of breakdown. Barlocher and 

Kendrick (1974) have confirmed these findings, show-
WATER TRANSPORT 31

ing that those areas of leaves with a high hyphal content are preferentially consumed by invertebrates. If 

leaves are fragmented, the initial rate of decay is increased, but this effect does not extend beyond a few 

weeks (Kaushik and Hynes, 1971).

FRESH

STREAM 

TREATMENT /"\

Month

CENTIMETERS

FIGURE 27.—Outline of Fagus sylvatica leaves showing area of 

lamina loss after various periods of exposure to stream environment.

Early work by soil ecologists indicated that the rate 

of breakdown of organic matter was related to the type 

of organism involved in the processing as well as the 

chemical nature of the material, especially the nitrogen content (Waksman and Tenny, 1927; Waksman

FRESH

LAKE 

TREATMENT

Month

CENTIMETERS

FIGURE 28.—Outline of Fagus sylvatica leaves showing area of 

lamina loss from exposure to lake environment.
32

FRESH

SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

FRESH

STREAM 

TREATMENT

Month

CENTIMETERS

FIGURE 29.—Outline of Quercus robur leaves showing area of 

lamina loss after various periods of exposure to stream environment.

LAKE 

TREATMENT

Month

CENTIMETERS

FIGURE 30.—Outline of Quercus robur leaves showing lamina loss 

after various periods of exposure to lake environment.
WATER TRANSPORT 33

FRESH

STREAM 

TREATMENT

Month

and others, 1928; Melin, 1930). Melin (1930) found that 

within a species there was a direct relation between 

the amount of nitrogen in the material and its rate of 

breakdown, but between species this relation was not 

so clear, as other factors, such as the amount of lignin

FRESH

LAKE 

TREATMENT

Month

CENTIMETERS

CENTIMETERS

FIGURE 32.—Outlines of Alnus glutinosa leaves showing extent of 

degradation after various periods of exposure to lake environment 

conditions. After 8 months, only fragments were left.

FIGURE 31.—Outline of Alnus glutinosa leaves showing extent of 

degradation after exposure to stream environment for various 

lengths of time.
34 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

or antifungal compounds present, affected microbial 

colonization. Mathews and Kawalczewski (1969), using 

bags with different mesh sizes, found that although 

bags with a coarse mesh size (3 mm) gave access to a 

greater invertebrate fauna, the leaves did not break 

down any more rapidly than in bags with a fine mesh 

size (0.27 mm). They concluded that most losses (dry 

weight) are caused by microbial activity. They also 

noted significant increases in the nitrogen content of 

the leaves, presumably by uptake of nitrogen by increasingly large numbers of microorganisms. If nitrogen is added to a system in which leaves are decaying, 

the rate of breakdown is increased to a maximum rate 

that is in part species dependent. This effect is further 

enhanced when phosphorus is added to the system 

(Kaushik and Hynes, 1971).

FRESH

STREAM 

TREATMENT

Month

CENTIMETERS

FIGURE 33.—Outline of Betula pubescens leaves showing progressive lamina loss in stream environment. After 8 months, only 

fragments were left.

Chemical

The rate of invertebrate leaf degradation is controlled by microbial conditioning, and the rate of microbial colonization seems to be dependent on the chemical composition of the leaf. As Nykvist (1962) has 

shown, however, the chemical composition of leaves 

can alter markedly within a few hours after they enter 

a stream. Nykvist anaerobically leached samples of 

Alnus glutinosa, Fagus sylvatica, and Quercus robur 

leaves and found that after 24 hours' leaching Alnus

FRESH

LAKE 

TREATMENT

Month

CENTIMETERS

FIGURE 34.—Outline of Betula pubescens leaves showing progressive lamina loss in lake-bottom environment. After 8 months, only 

fragments were left.
WATER TRANSPORT 35

had lost 12 percent of its dry weight as soluble organic 

substances. In contrast, Fagus had lost only 3.8 percent 

and Quercus 1 percent. Water-soluble inorganic substances lost after 1 day of leaching amounted to 1.3 

percent dry weight for Alnus, 1.1 percent for Fagus, 

and 0.9 percent for Quercus. Most of the inorganic subFRESH

STREAM 

TREATMENT

Month

0

CENTIMETERS

FIGURE 35.—Outline ofSalix cinerea leaves showing gradual lamina 

loss under conditions of stream environment.

stances were lost after 1 day, since continued leaching 

did not bring about any significant further losses. 

Grinding Fagus litter produced an increased loss of 2.4

FRESH

LAKE 

TREATMENT

Month

CENTIMETERS

FIGURE 36.—Outline ofSalix cinerea leaves showing area of lamina 

loss from exposure to lake-bottom environment.
36 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

percent of the dry weight in the form of water-soluble 

organic substances after 1 day's leaching. For Quercus 

the increase was 6 percent; for Alnus only 0.2 percent. 

For inorganic water-soluble substances, grinding gave 

an increased loss of 0.3 percent for Fagus, 0.2 percent 

for Quercus, but no increase for Alnus. The amounts of 

water-soluble organic substances obtained by aerobic 

leaching were slightly less than those from anaerobic 

leaching.

Grinding appears to lead to very little increase in the 

loss of soluble organic substances from Alnus leaves, 

which indicates that all the leachable substances are 

removed from the intact leaf, and therefore leaching is 

not inhibited by the structure of the leaf. Quercus and 

Fagus lose less than Alnus, partly because the watersoluble organic substances are bound by the leaf structure and are released only when the leaf is extensively 

damaged. It is known that tannins, in the insoluble 

condensed form, occur in large amounts in Fagus and 

Quercus and that they have antifungal properties (Benoit and others, 1968; Williams, 1963). If tannins compose a substantial part of the organic substances that 

are leached from Alnus, then it would appear that this 

species is predisposed to breakdown. The precise role of 

the cuticle in restricting losses of substances by leaching is not known, but it is presumed to be important.

Mechanical

Microbiological activity clearly affects the mechanical strength of leaves. Although it cannot be denied 

that with most species the loss of mechanical strength 

is the result of both invertebrate feeding and microbiological breakdown, the example of Fagus sylvatica illustrates loss without any sign of invertebrate 

attack being evident. Fagus shows little sign of invertebrate attack even after being exposed for 8 months in 

either lake or stream environment (fig. 27-28), but results of the rolling fragmentation experiment (figs. 

37-43) show that loss of mechanical strength, even in 

this apparently resistant species, is considerable (fig. 

40).

Most species appear to exhibit a higher rate of degradation in the stream environment than in the lake. 

This is perhaps most clearly shown in the case of Alnus 

(figs. 31 and 32). Examination of the type of destruction 

suggests that the cause for the greater loss of material 

in the stream-processed leaves is a combination of both 

biological and mechanical degradation. The leaves exhibit an angular fragmentation, which is characteristic 

of mechanical damage, rather than the rounded holes 

typical of the attack by large particle feeders or the 

skeletal vein pattern produced by microbial degradation and small particle feeders. The contrast may be 

clearly seen in the case of Quercus (figs. 29 and 30). As

the leaves become less strong, the fluid forces acting on 

the stream nylon cages caused successively greater 

fragmentation.

Mechanical fragmentation, like biological breakdown, is to some extent influenced by leaf structure 

and form. Salix cinerea, for example, has a strong midrib; typically, the lamina breaks up, but the pieces remain attached by means of the midrib. Fagus seems to 

be destroyed by random attrition, and in Quercus the 

lobes tend to break off, and the remaining lamina is 

fragmented, although as in Salix, the midrib tends to 

hold the pieces together. Alnus tends to tear rather 

than break clean like Fagus and, though much less 

brittle than Fagus, is easily destroyed once the tissues 

are weakened by microbial attack.

In most species, the mechanical breakdown will take 

the form of gradual attrition at the margins. If the 

energy of the environment is sufficiently high, then 

brittle species such as Fagus sylvatica (paticularly the 

"sun" leaves) may be broken across the midrib. This 

rarely happens with more pliable leaves. The result of 

attrition is that where the areas of the whole leaves 

follow a normal distribution, a bimodal distribution is 

produced, one peak composed mainly of the fragmented 

pieces, the other of the larger remains. This pattern 

can be seen in both Fagus and Quercus, illustrated in 

figures 37-39, despite the slightly different forms of 

their fragmentation. The complex nature of the curves 

limits the use of such statistics as the median and 

mode; the measures of fragmentation presented in 

figures 40-43 are based on average fragment area. It 

should be pointed out that these estimates are probably 

higher than the actual numbers because, for practical 

purposes, a lower limit had to be put on the size of 

fragments to be counted after rolling. This lower limit 

was chosen to be 25 mm2 . By observation, however, the 

number of fragments falling below this size for the 

conditions of the experiment was found to be low. Apparently the forces exerted on a piece of leaf smaller 

than 50 mm2 were insufficient to break it, but further 

microbiological attack would probably lead to greater 

destruction.

Chaney and Sanborn (1933) considered a thin leaf to 

have only half the chance of a thick one for entering 

the fossil record, all other things being equal. To test 

this theory, Ferguson (1971) cut discs of fresh leaves 

from 11 species (which covered a range of different leaf 

thicknesses) and rotated them with sand and water in 

a revolving drum. Each run lasted 100 hours and was 

repeated three times, but even after this treatment, all 

species showed little sign of wear. It therefore seems 

unlikely that leaf thickness alone, without substantial 

microbial preconditioning, contributes much to the 

probability that a leaf will enter a depositional envi-
WATER TRANSPORT 37

ronment in a recognizable form. The results of experiments with whole fresh leaves show that they are 

more resistant to fragmentation than leaves exposed to 

microbiological attack. Unless the leaf is exceptionally

large (or small), leaf size and thickness are not likely to

be as critical in determining the preservation potential

of a leaf type as the rate of microbiological degradation.

Cuticle thickness is commonly proposed as a major

5 r

Lake: Month 1 before rolling

0 i i i i i i i i i | i i i i I I i i i | i i i i I I i i I I i i i i I i i II | i I i I i I I II IMII fl ft15

10

After rolling

nuMnrhLl.MpM.nn.i.n

10

Lake: Month 2 before rolling

o —i i i i i i i i i i i i i i i i i i i

15r

10

After rolling

0 '—i i i i i i i i i i i i i i I i i 

25 500 1000

i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i | 

1500 2000 2500 3000 3500 4000 

SIZE CLASSES, IN SQUARE MILLIMETERS

10

FIGURE 37.—Size distributions of whole leaves ofFagus sylvatica before rolling and fragments after rolling.

Lake: Month 1 before rolling

fl——FR- 41

10

After rolling

*> 111111111—m 1111 n 1111 n n 1111111111111111111111 n

25 500 1000 1500 2000 2500 3000 

SIZE CLASSES, IN SQUARE MILLIMETERS

i i i i i i i i i i i i i i I i i i I I i i i i 

3500 4000 4500

FIGURE 38.—Size distributions of whole leaves ofQuercus robur before rolling but after 1 month in lake and fragments after rolling.
38 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

factor in the preservation of a leaf. It is certainly true 

that the cuticle is far more resistant to breakdown 

than, for example, cellulose or lignin (White, 1933), but 

it is not necessarily such an effective barrier to the 

microbiological colonization of the leaf tissues as is 

sometimes assumed. Where the cuticle is thick, it is 

commonly left intact even after the rest of the leaf has 

been destroyed. An extreme example of this is Ilex 

aquifolium, where frequently all that remains of the 

leaf is an intact "bag" of cuticle. Such a "bag" is surprisingly robust; all "whole" Ilex leaves recovered from 

the Silwood deposit were in this form. The cuticle of the 

upper surface ofQuercus leaves was commonly left intact after an 8-month exposure to the lake environment. DeVries and others (1967) have reported that, in 

the process of cuticle degradation, attack begins on the 

surface originally in contact with epidermal cells, and 

the cutin of the anticlinal walls is the first to be affected. Destruction of the leaf material may take place 

from the inside and not externally. More extensive 

cuticle degradation may be prevented by the external 

coatings of extremely resistant waxes (White, 1933).

It was noted that many leaves present in the Silwood 

stream become coated with a layer of sediment within 

1 or 2 weeks of entering that environment. This encrustation, fully discussed in another paper (Spicer, 

1977), may limit external microbiological attack.

Because most fragmentation occurs after some mi10

Lake: Month 2 before rolling

10

^TTTTTT i i rr i i I I i 

00 1 500

i i i i ii 

2000

-rfl-r MI Plfl ——

2500

III i i i I 

3000

I

3500 4000

-i —— R-^ ———— 

4500

—— p ————

5000 55

SIZE CLASSES, IN SQUARE MILLIMETERS

50

40

30

After rolling

500 1000 1500 2000 2500 3000 

SIZE CLASSES, IN SQUARE MILLIMETERS

i i i i i i M i | 

3500 4000 4500

FIGURE 39.—Size distribution ofQuercus robur whole leaves that had been on lake bottom for 2 months and fragments produced by rolling the

leaves.
WATER TRANSPORT 39

crobiological breakdown of the tissues has taken place, 

fragmentation during the floating period will be minimal. Consequently, it is not until after the leaf becomes 

saturated and is transported in suspension or as part of 

the bedload that the main process of fragmentation begins. Floating times of as little as 2 or 3 days may mean 

that some leaves are transported great distances before 

they begin to be fragmented. These leaves may, depending on the magnitude of the source, be frequent, or 

even common, in their eventual depositional environment.

The upper limit to the distance a leaf may be 

transported in suspension, or as part of the bedload, 

and still be recognizeable is determined by the fragmentability of the leaf. The five species investigated 

from the Silwood environment can be placed in an increasing order (from top to bottom) of resistance to 

biological breakdown as follows:-

UJ

3< ioc z

UJ UJ 

>0

DC

100

90

80

70

60

50

u. 40

"J 30

"j 20

i 10

5 0

EXPLANATION

——c—— Lake

——«—— Stream

456 

MONTHS

10

FIGURE 40.—Degradation ofFagus sylvatica leaves in rolling experi " 

ments after exposure to lake and stream environments for various 

periods of time.

EXPLANATION 

—— *——Lake 

Stream

123456789

FIGURE 41.—Degradation ofQuercus robur in rolling experiments 

after exposure to lake and stream environments for various 

periods of time.

Alnus glutinosa 

Salix cinerea 

Betula pubescens 

Quercus robur 

Fagus sylvatica

It is not possible at this time to extrapolate these 

findings to other taxa, even to species of the same genera. More work is required to identify the characteristics of a leaf that determine its susceptibility to breakdown. These characteristics, whether physical or chemical, would, we hope, be those that may be identified in 

the fossil record.

The differential destruction of leaves during water 

transport depends on an interplay between potentially 

destructive environmental conditions and the speciesdependent characteristics of the leaves that affect the 

rate of degradation.

< UJ 

UJ FT

Z til h- UJ -JZ

100

90

80

70

60

50

40

30

20

10

0

EXPLANATION

——*•—Lake

——•—— Stream

Total destruction

1 3456 

MONTHS

89

FIGURE 42. — Degradation of Betula pubescens in rolling experiment after exposure to lake and stream environment for various 

periods of time.

< uj "Joe

Z UJ i_ "J-'z

2| uj UJ

O _i o 

<O DC OCX UJ

U. » Q.

o m -

if

100

90

80

70

60

50

40

30

20

10

1 i l I 

EXPLANATION

345 

MONTHS

FIGURE 43.—Degradation of Salix cinerea in rolling experiment 

after exposure to lake and stream environments for various 

periods of time.
40 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

RELATIVE DEPOSITION RATES 

OF LEAVES AND SEDIMENT

In all fossil-plant deposits (except those resulting 

from peat accumulations), the plant remains are 

closely associated with inorganic sediment grains to 

some degree. Variations in the concentration of leaves 

relative to sediment are used as a basis for distinguishing between assemblages or paleosuccessions 

(Krasilov, 1969). These fluctuations in plant-fossil concentration may not necessarily be a function of the 

source vegetation, and therefore the restriction of an 

investigation to well-defined plant beds may give only 

limited paleoecological information. As a preface to 

considerations of sampling in fossil-plant deposits, it is 

pertinent to examine the plant/matrix depositional relations in more detail.

The rate of leaf deposition, Rh at any given point on a 

sedimentary surface can be expressed as

RI = l/8t, (1)

where I represents some measure of the amount of leaf 

material expressed as numbers, volume, or mass deposited in a small time interval t. If t is small enough, the 

rate of leaf deposition can be regarded as constant. 

Similarly, the sediment deposition rate, Rs , can be expressed as

Rs = s/8t, (2)

where s is a measure of the amount of sediment deposited during time t. If s and I are measured in the same 

units, an index of leaf concentration, K, within the sediment can be expressed as the leafrsediment ratio:

K =7 (3)

Expanded to include the leaves from the various component species (a, b, c . . . n) of the source vegetation, 

this expression is

Rt tc . . . Rhl )/Rg — (4)

Assuming no postburial losses, the fossil leaves on a 

rock surface exposed by splitting along the bedding 

plane represent those leaves deposited in a small time 

period, and any lateral variation in component species 

or leaf abundance is indicative (.assuming adequate 

sampling) of differing sedimentation rates of either 

leaves or sediment over that depositional surface.

A vertical section through horizontally bedded sediments containing fossil leaves presents a series of bedding planes with varying leaf densities. Depending on 

the thickness of the rock investigated, these vertical 

densities represent deposition over considerably longer 

time intervals than have so far been considered. At the 

time interval, t, increases, so does the probability that 

the deposition rates of either leaves or sediment will 

not remain constant. Under these conditions, expression (1) becomes

tz

to

(5)

or

l=f. R,8t

*s n

and expression (2) becomes

s =f * Rs 8t

t/i

(6)

by integrating between (time) t0 and (time) tz , the beginning and end of the depositional period.

Clearly, the smaller the interval t~-tlt is, the better 

the resolution obtainable in the analysis of the deposit. 

Although practical considerations dictate t~—t0 cannot 

be infinitely small, loss of resolution and sensitivity in 

sampling can be minimized by employing the concept 

of the sedimentation unit proposed by Otto (1938). The 

sedimentation unit is defined as "that thickness of sediment which was deposited under essentially constant 

physical conditions." By definition and under ideal 

conditions, then, if such a thickness of rock is sampled, 

the relative rates of leaf and sediment deposition have 

remained nearly constant during the period of deposition, and no loss of resolution will be incurred during 

sampling. Samples including part of one sedimentation 

unit and part of another should be avoided. The words 

"essentially constant" do not preclude the existence of 

linear or cyclic trends of sedimentation, which, if present, result in changing rates of deposition. Such trends 

can be identified and allowances made for them during 

sampling.

The internal structure of a sedimentation unit (individual laminae) is considered by Otto to be the result of 

erratic perturbations in the transport system and related to "nonassignable" causes. Jopling (1964) pointed 

out that little is known of how laminae form, but it 

may be under a wide range of flow conditions, including uniform and steady-state regimes. Clearly the term 

"essentially constant" must be interpreted somewhat 

loosely until the mechanisms of lamina formation are 

better understood. Nevertheless, the concept of the
THREE-DIMENSIONAL STRUCTURE OF THE SILWOOD DELTA 41

sedimentation unit is a useful one in the context of 

defining the framework for quantitatively sampling 

plant fossils.

Two possible causes for the variation of leaf concentrations, K, between samples are physical conditions 

and source of leaf material. The primary souce of variation must come from the differing physical conditions 

(hydrodynamic variables and variables of sediment 

type and availability) that define the sedimentation 

unit. For leaves being transported by the same medium 

as the inorganic sediment grains, these conditions will 

determine sorting, which will inherently affect the 

value of K.

The second cause of variation, the source of the leaf 

material, is dependent on the distribution of species 

within the source vegetation. This may lead to a fluctuating value of K, even though the physical conditions 

of deposition of the inorganic sediment matrix may be 

constant.

The problem of distinguishing between these two 

components of variation is a major obstacle in the 

analysis of past vegetational regimes. Yet by careful 

analysis of the three dimensional patterns of deposition in a deposit, this problem can sometimes be overcome.

THREE-DIMENSIONAL STRUCTURE 

OF THE SILWOOD DELTA

In order to determine the three-dimensional structure of the Silwood delta, 22 7.5-cm-diameter cores 

were taken at the positions shown in figure 10 using 

the assembly (fig. 11) and procedures described under 

"Coring Procedure." Diagrams of the cores so obtained 

are shown in figures 44-46. All cores were taken in 

that part of the delta formed subsequent to its partial 

drainage in 1957 to construct the present outlet weir. 

Alder overgrowth of older parts made coring impossible.

PLANT REMAINS

The uppermost part of all cores (with one exception), 

though semiliquid, exhibited high concentrations of 

leaf material (leaf bed 1), in some cores admixed with 

small quantities of coarse silt. Many of the leaves 

showed signs of both mechanical and biological degradation. Leaf bed 1, though of variable thickness, never 

extended more than 0.2 m below the sediment surface. 

The core taken in 0.45 m of water (core DF1, fig. 45) 

lacked a concentration of leaf material at the top of the 

core, indicating that at this water depth on the delta 

foreset slope leaf deposition was minimal. Below 0.2 m, 

the occurrence of leaves in the cores became rare to 

0.7-0.8 m, where another concentration of leaves (leaf 

bed 2) was encountered. The leaves of bed 2, in contrast

to bed 1, typically showed signs of extensive biological 

degradation but little evidence of mechanical damage. 

In leaf bed 2, some 0.2-m-thick, pellets of inorganic 

sediment were interspersed with leaf remains. These 

pellets, though widespread laterally, occurred only at 

this depth (see section entitled "Inorganic Sediment'').

In many cores, rootlets were found in growth positions associated with, and below, leaf bed 2, commonly extending down to a dense layer of wellpreserved (predominantly Alnus glutinosa] leaves 

overlying, in some cores, a layer of sand. Although 

leaves were usually not common in the sand layer itself, twigs and Fagus cupules were occasionally found. 

Immediately below the sand layer was a fourth concentration of leaves, somewhat poorly preserved compared 

with those overlying the sand. Below this leaf bed, leaf 

remains became rare; the only macroscopic evidence of 

megaplant remains was layers (not more than 0.1 m 

thick) of unidentified fibrous plant material.

It was not possible to analyze quantitatively the 

species distribution of leaves between the cores, but 

analysis of leaf remains washed from the sediment 

showed that the species composition of leaf beds 1 and 2 

was very similar: both were rich in Fagus and Quercus 

leaves, with some Ilex cuticles.

INORGANIC SEDIMENT

The uppermost centimeter of sediment on the delta, 

orange brown in color, consisted of flocculent ferric 

hydroxide admixed with a small silt-size quartz fraction. Much of the silica of this surface sediment was in 

the form of diatom frustules, commonly Navicula sp. 

The flocculent nature of the sediment precluded any 

meaningful measurement of settling velocity or grain 

size, but the bulk of the flocculated sediment had a 

settling velocity much less than that of most of the 

plant material.

Large areas of shallow-water delta sediment were 

stabilized by a prolific growth of blue-green algae belonging to the Oscillatoriales. The ability of these 

filamentous algae to migrate toward light prevented 

the burial of the colonies by subsequent sediment deposition. During periods of rapid photosynthetic activity, 

large gas bubbles formed that became trapped among 

the filaments. Consequently, areas of the algal mats 

became buoyant and floated off into deeper water, 

where agitation by wind-generated turbulence of the 

lake waters shook the bubbles free, leading eventually 

to the sinking of the algal/sediment pads. Any sorting 

of sediments that might have occurred as a result of 

physical processes of deposition tended to be destroyed.

The upper oxidized layer of the sediment had a pH of 

6.8 (table 5), and no crystalline iron minerals were detected by X-ray diffraction studies. Most stream and
42 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

0.0

T1C5 T1C4 T1C3 T1C2 T1C1 T2C5 T2C4 T2C3 T2C2 T2C1

0.2- 

0.4- 

0.6- 

0.8- 

1.0- 

1.2- 

1.4- 

1.6-

w . , oc I-' 

ui

in5 2.0-1

Z T3C5

~ 0.0

X

0.2 H

0.4

0.6-

0.8-

1.0-

1.2-

1.4-

1.6-

1.8-

2.0-

»^

T3C4 T3C3 T3C2 T3C1 T4C5 T4C4 T4C3 T4C2 T4C1

Sand with gravel 

Iron-rich mud

FIGURE 44.—Cores taken from delta in locations shown in figure 10.
THREE-DIMENSIONAL STRUCTURE OF THE SILWOOD DELTA 43

delta sediment is probably similar in composition to 

that described by Coey and Readman (1973), Fe(OH)3 - 

/iH2 O. Sheaths of the iron bacterium Sphaerotilus sp. 

were abundant in both stream and delta surface sediment; they are figured in Muir, Hamilton, Grant, and 

Spicer (1974).

Approximately 1.0 cm below the surface of the sediment, the color changed from orange brown to black, 

indicating a change from ferric to ferrous iron. No 

Sphaerotilus or diatom frustules were found in the 

black sediment of the cores, and in only a few cases 

were the degraded remains of blue-green algal sheaths

DF1

0.2-

0.4 -

0.6-

0.8 H

z 1.0 H

S 1.2 H

1.4 -

1.8

2.0 -1

0.2-

0.4-

0.6 -

0.8

1.0-

1.2 -

1.4 -

EXPLANATION

Clay pellets 

Sand with gravel 

Iron-rich mud

FIGURE 45.—Two cores taken from Silwood delta. Core RB1 was 

taken close to Typha bed, core DF1 on prodelta slope, at locations shown in figure 10.

present. The sediment was extremely fluid to the depth 

of the second leaf bed (approximately 0.8 m), where it 

became gelatinous. This change in consistency was not 

associated with any visible differences in sediment

TABLE 5.—Water content, ferric ion concentration, pH, and Eh of the 

Silwood delta sediment as sampled from a core taken in the Typha 

bed

Depth

(meters below 

sediment surface I

Percent water Percent Fe***

loss on by weight of

drying at 80C C dried samples

PH Eh (mvixlOO

0.0 ______

2

.4

.6 ______

.8 ______

1.0 ______

1.2 ______

1.4 ______

1.6 ______

1.8 ______

2.0 ______

__ _ 6.8 (stream)+1.25

83

85

87

88

81

84

79

79

81

65

'26.9

'28.1

36.1

'32.5

33.4

32.0

31.8

34.8

21.4

11.8

7.00

7.4±0.2

7.00

7.30

7.80

7.90

8.05

8.05

7.90

-4.80

-4.95

-6.00

-5.00

-6.10

-5.98

-5.90

-5.90 -5.2

'Incomplete digestion.

DEPTH,

ORGANIC IN 

REMAINS METERS 

0.0^

Leaf Bed 1

Few leaves 

dispersed 

through 

sediment

0.2 -

0.4 -

0.6-

0.8-

Leaf Bed 2

Roots in growth 1.0 

position

Well-preserved

leaves 1>2

Poorly preserved 

leaves 1.4

Occasional leaf 

layers 1.6 -

Fibrous layer 

remeins of 

aquatic 

plants 1.8

Roots in growth

position 2.0

INORGANIC 

SEDIMENT

Top 1.0 cm Fe"H"l"hydroxide/ 

oxide floes plus some 

coarse silt sized quartz 

grains

Fe^iron-rich sediment as 

a black semiliquid mud. 

Clay minerals more or 

less absent throughout 

the core

Angular pellets 0.5 cm 

diameter composed of 

same sediments 

as mud

Black more or less 

gelatinous mud

Sand layer of reworked 

Bagshot sands

Black gelatinous, ironrich mud

More or less undisturbed 

Bagshot sands

FIGURE 46.—Hypothetical core that summarizes vertical structure of

delta.
44 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

composition or any large drop in the water content (table 5). X-ray diffraction analysis shows no change in 

mineral composition.

The pellets dispersed in the upper part of leaf bed 2 

were apparently of the same composition as the surrounding sediment. The origin of these pellets is in 

doubt, but they may be derived from the subaerial deltaic sediments. When the lake was partially drained 

for some time during the reconstruction of the weir 

overflow in 1957, the delta surface may have partially 

dried out. The sediment, once dry, was observed to remain as hard angular pellets that would not redisperse 

on wetting. It is therefore likely that partial drying of 

the delta surface may be the origin of the pellets. When 

the lake level rose again and the distributaries wandered laterally over the delta (continually changing 

their courses as parts of the delta subsided or channels 

became choked), the pellets were eroded and deposited 

on the unconsolidated sediment of the delta slope, 

where they sank under their own weight until 

obstructed by the lower leaf bed.

At a depth of approximately 1.2 m, many cores 

passed through a sand body of variable thickness. 

X-ray diffraction and optical examination indicate that 

its mineral composition is similar to that of the 

Bagshot sands; it seems likely, then, that the sand 

body was derived from erosion of the streambanks during flood conditions. The Bagshot sands were usually 

encountered at a depth of about 2.0 m and in some 

cases contained the remains of small vertical roots.

INTERPRETATION OF CORES

The sediments lying above leaf bed 2 (including the 

pellets) are interpreted as being deltaic sediments. 

Leaf bed 1 represents deposition of leaves on the delta 

surface by direct aerial transport from the local trees 

as well as deposition of leaves transported by the 

stream distributaries, commonly mechanically fragmented.

Leaf bed 2 probably represents deposition of organic 

material on the lake bottom. The biological degradation of this material in the area from which the cores 

were taken was probably almost total under normal 

conditions, but the comparatively rapid deposition of 

the deltaic sediments arrested this process before it 

had gone to completion and consequently some of the 

leaves were preserved. The roots among leaf bed 2, and 

extending below it, are probably the remnants of aquatic plants that were growing on the lake bottom. The 

leaves constituting leaf bed 2 were unlikely to have 

been derived from leaf bed 1 by sinking down through 

the unconsolidated sediment, as leaves placed on deltaic sediments in the laboratory and left for periods as 

long as 2 months did not show any tendency to sink.

The sand body undoubtedly represents a period of 

abnormally rapid deposition. As the sand was being 

deposited, litter from the surrounding vegetation must 

have been washed into the lake, where it became 

waterlogged and sank on top of the sand layer. A considerable amount of fine material must still have been 

in suspension, and as this settled out, it rapidly buried 

the newly deposited leaves and other organs. Wellpreserved Alnus leaves are found in the cores only in 

this position, underlain by coarse sediments and overlain by fine.

Below the sand layer is another leaf bed that, like 

leaf bed 2, represents organic matter on the lake bottom that was preserved, after only partial degradation, 

by the rapid deposition of the sand body.

At about 2.0 m, the lake sediments are found to directly overlie the Bagshot sands. In a few cases, the 

remains of a soil horizon occasionally containing vertical roots overlie the Bagshot sands. These roots are 

interpreted as belonging to the original vegetation that 

grew in the valley before it was flooded.

A GENERAL MODEL OF LEAF DEPOSITION 

IN A FLUVIOLACUSTRINE ENVIRONMENT

The information presented here is of limited value if 

it cannot be related in principle to processes that are 

likely to have operated in the past. While biological 

factors may change significantly with time, the physical and chemical processes of transport and deposition 

are likely to be less time dependent. To be valid over 

geologically significant periods in the past, generalizations derived from the Silwood study should be 

as independent as possible of biological variables.

From the core data, it is evident that the deltaic sediments are vertically bounded by two leaf beds. In 

order to explain such a phenomenon, a dynamic model 

of leaf deposition in a freshwater deltaic sedimentary 

environment is now proposed that has critical 

paleoecological implications.

The relative rates of leaf and sediment deposition 

were used (p. 40-41) to show how variations in leaf 

concentration within a volume of sediment might well 

reflect both the hydrodynamic properties of the plant 

material in relation to the inorganic sediment grains 

and more significantly the distribution of the source 

vegetation. Utilizing this approach, it is now possible 

to give an example of how such parameters affecting 

the variations of the leaf: sediment concentration ratio 

(K) may be analyzed.

Consider the representation of figure 47. A stream, 

laden with sediment, empties into the relatively static 

waters of a lake. If the sediment load is high, then the 

stream water will have a higher specific gravity than
MODEL OF LEAF DEPOSITION IN A FLUVIOLACUSTRINE ENVIRONMENT 45

that of the lake, and a bed density current (Smith, 

1975) will be formed. In many lakes, such a condition 

may not be set up and turbulent mixing of the stream 

and lake water will take place at the top of the prodelta 

slope. Whether or not a bed density current is set up, 

the bulk of the coarse sediment will be deposited as 

soon as the energy of the transporting water falls below 

a critical value. This usually occurs close to the stream 

mouth.

In an open lake situation, the leaves from the surrounding vegetation either fall, or are blown, onto the 

surface of the lake, where they may float for various 

periods of time. During this time, if the leaves are free 

from the influence of any inflowing streams, they will 

be distributed directly by wind or wind-generated 

water currents. Most leaves, fruits, and seeds will 

enter the water close to the banks and may be trapped 

by aquatic vegetation such as reeds orNuphar sp. pads. 

As the plant material sinks to the bottom, it will 

gradually form a lake bed, rich in organic material, 

and provided organic input exceeds degradation, a potential plant megafossil bed will form that reflects the 

composition of the local vegetation (H. H. Birks, 1973).

When there is no prevailing wind direction, the deposition along any line running parallel to the 

shoreline of leaf material by means of direct aerial 

transport from the local vegetation is likely to be 

nearly uniform, because all points along the line are

Lake water

equidistant from the source. The schematic (fig. 47) 

depicts a vertical section through the delta parallel to 

the shoreline and normal to the delta front. The relative sedimentation rates are represented by the length 

of the two vectors normal to the sediment surface. As in 

the Silwood situation, it is assumed that most of the 

inorganic sediment has a settling velocity less than 

that of waterlogged plant remains.

The rate of inorganic sedimentation in the lake is 

very low relative to that of the "rain" of local plant 

material. If we consider sedimentation at a point A on 

the lake bed, the rate of leaf deposition, Rh clearly 

exceeds that of the inorganic sediment component Rs 

such that the ratio Ri/Rs (K) is greater than 1. As the 

delta advances across the lake bed, the lake deposits 

become covered by deltaic sediments. At point B, at the 

base of the prodelta slope, Rs has begun to increase, 

while R,, dependent on the local vegetation, remains 

essentially constant such that K~l. At point C, on the 

prodelta slope, Rx far exceeds R; and K becomes much 

less than 1. At the top of the delta slope, at point D, the 

water energy is sufficiently high to keep most of the 

finer inorganic particles in suspension and R8 begins to 

fall. However, leaf fragments transported as bedload 

material and both organic and inorganic sedimentary 

particles with high settling velocities collectively settle 

out in the even higher water energies acting at the top 

of the foreset slope. At Point-E, ihen,Ri>Rs , and again

F _ Direction of

streamflow

surface

•—_"~__ .•* _JJpper leaf bed"

c /

s

—— *r

B

i \

~~t *^i?

-_-

-— Lower leaf bed

Vector representing rate of 

sediment deposition

EXPLANATION

<- - - - —— Vector representing rate of 

leaf deposition

Bedding surfaces

FIGURE 47.—A diagrammatic representation of leaf deposition in a freshwater delta environment. See text for explanation. See text for

discussion of points A-F.
46' SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

a concentration of leaves is produced. At point F, 

within the stream channel, deposition and erosion are 

nearly balanced, although leaf input may become so 

great during autumn that the channel becomes temporarily choked.

Seasonal variation in leaf deposition may impose 

further pattern on this basic model. If the advance of 

the delta slope is sufficiently rapid, seasonal fluctuations in leaf input to the system may be preserved as 

variations in the thicknesses of the two leaf beds. The 

rapid input of leaves at this time may lead to preferential preservation; although there is evidence to suggest 

that the breeding cycles of many aquatic invertebrates 

is linked to periods of rapid organic input (Petersen 

and Cummins, 1974), the population of leaf-degrading 

organisms present in the depositional environment 

may not be large enough to destroy all the material 

before a substantial amount becomes buried.

The two leaf beds clearly have separate origins, a 

difference the species compositions might be expected 

to reflect. The local elements of the flora will be represented predominantly in the lower leaf bed, the species 

growing at more distant locations in the upper bed. Not 

only will the species composition differ but so also the 

type and extent of degradation. The transport of the 

local species to the surface of the lake water will be 

mainly by wind and mechanical damage therefore minimal. In the lake-bottom waters, rich in organic material, biological degradation could be considerable and 

would be detectable by the characteristic rounded holes 

of invertebrate attack and the loss of intercostal tissue 

caused by microbiological activity. In contrast, the 

leaves of the upper bed would exhibit the angular 

breaks and tears associated with mechanical fragmentation during water transport as well as evidence of 

biological degradation. Repeated reworking of this 

upper-leaf bed as the distributaries wander laterally 

over the delta surface will further mechanically degrade the leaf remains. Because not all the leaves 

coming down the stream will be at the same state of 

waterlogging on reaching the stream mouth, any sorting effects due to the various hydrodynamic properties 

dependent solely on fragment size or thickness are not 

likely to be observed.

Observation in the Silwood stream showed that a 

small fraction of the leaf population (most likely to be 

whole for the reasons discussed in the section "Leaf 

Degradation") will still be floating on reaching the 

delta slope and will be carried off into the lake to be 

deposited along with the local species on the lake bed. 

This distinct-species component of the lower-leaf bed, 

although small, may become extremely significant. A 

subjective assessment of the abundance of species 

within the leaf-bed samples in the cores suggested

there was very little difference in species composition 

between the upper and lower leaf beds. Such a condition is explained by considering the relative degradation rates of the different species as shown by experiment. The Alnus leaves were found to be very easily 

destroyed relative to leaves of Fagus and Quercus. In 

this situation, it appears that one local species was 

quickly removed from the potential fossil record, resulting in an overrepresentation of the distant component in the lower leaf bed. Such conditions need not, of 

course, apply to all situations. There is no evidence to 

suggest that susceptibility to breakdown is a general 

feature of all riparian plants; different components of a 

flora may be either underrepresented or preferentially 

preserved in a fossil deposit.

Clues as to which floristic components were easily 

degraded in the past can be obtained by examining the 

leaf/sediment relations. It has long been recognized 

that the quality of preservation of organic matter is 

directly proportional to the speed of burial: breakdown 

is retarded in the anaerobic environment that rapidly 

develops when oxygen diffusion is limited by overlying 

sediment. Such conditions prevailed at Silwood when 

the sand body penetrated by the cores (figs. 44-46) was 

formed. After deposition of the sand, a great amount of 

silt-and-clay-sized particles must have still been in 

suspension for some time. During this period, leaves 

sank on top of the sand and were rapidly buried by the 

settling fines. Only under these conditions were wellpreserved Alnus leaves found in the cores.

Although rapid burial produces locally anaerobic 

conditions conducive to preservation, so does the shape 

and size of the lake in relation to input of organic matter. The exchange of oxygen through a moderately disturbed water surface into water of uniform oxygen concentration is given by the equation

dO

dt

= A <x (P-pt),

where A is the area of the interface, P the partial pressure of oxygen in the atmosphere, pt the pressure at 

which the concentration of gas at time t in the water 

would be in equilibrium, and <* a coefficient termed the 

entrance coefficient (Hutchinson, 1957, p. 588). It is 

evident, therefore, that the rate of movement of oxygen across the air:water interface is dependent on the 

area of that interface. As the area of the lake surface 

diminishes before the advancing delta, the total oxygen exchange between the atmosphere and the lake 

drops. At the same time, the ratio between the perimeter length and the lake area increases, giving rise to an 

effective increase in organic input per unit area of the
MODEL OF LEAF DEPOSITION IN A FLUVIOLACUSTRINE ENVIRONMENT 47

lake surface (Rau, 1976). A combination of these factors leads to a thickening of the layer of organic matter 

on the lake bed. It is unlikely, however, that there 

would be a proportionate increase in the more rapidly 

degraded species until the rate of organic breakdown 

was limited to such an extent that it became less than 

the rate of input of those species (Kaushik and Hynes, 

1971).

Such a treatment is, of course, very simplistic in that 

it does not take into account the oxygen production by 

aquatic photosynthetic organisms or the distribution of 

dissolved oxygen throughout the lake waters by wind 

or thermally induced circulation. It does indicate that 

the possible effects of the shape and size of a depositional basin on floral representation must be considered.

In the deposition model (fig. 47), the upper leaf bed 

was seen to originate from differences in the hydrodynamic characteristics of leaves and sediment. 

Clearly, the magnitude of this difference will determine, to a large extent, the discrete nature and position of the upper leaf bed. The range in hydrodynamic 

characteristics displayed by a population of leaves, 

apart from any other megaremains, has already been 

shown to be wide and unpredictable, but most of the 

population will fall between certain limits and be deposited in a position on the prodelta slope where the 

water energies are sufficently low so as to allow settling. An upward-coarsening sequence, typical of many 

large fluviomarine deltas (Selley, 1971), is commonly 

less well developed in freshwater deposits owing to the 

rapid dissipation of energy in the turbulence caused

when river waters meet static lake waters of similar 

density. Where there is a more gradual fall off in 

stream competence (such as in a bed density flow 

(Smith, 1975)), leaf material will be deposited along 

with clastic particles of similar hydraulic equivalence.

In order to examine the consequences of the leafdeposition model just described, let us consider the infilling of a lateral lake by the progressive advance of 

deltaic sediments (fig. 48). The bottommost lake deposits might reflect the sediment load of the major river 

during flood, but subsequently the basin would steadily 

fill with megaremains from the local flora together 

with a little inorganic sediment. Evidence of intermittent flood events might be preserved. As the delta advances across the lake, the upper and lower leaf beds 

are formed as described. Initially, the upper lake is 

maximal in area and well oxygenated. Under such 

aerobic conditions, the organic input to the lake is 

rapidly broken down and the only local leaves preserved are those that become covered by the deltaic 

deposits before they are destroyed. At the very beginning of deltaic sedimentation, immediately after the 

formation of the lake, there would be only one deposit 

rich in organic material, but as the delta begins to 

form, so will the upper leaf bed. Initially, the lower leaf 

bed might be very thin, whereas the upper leaf bed 

would be, by comparison, rich in species growing 

upstream.

As the delta advances, its surface becomes successively colonized by various plants, and a hydrosere develops. Some of the leaves from the distant species are 

now filtered out by new vegetation, which contributes

APPROXIMATE SCALE

FIGURE 48.—Schematic drawing of leaf-bed formation in a lateral lake.
48 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

its own remains. The developing delta reduces the area 

of the lake, and the water becomes less aerobic, resulting in a gradual thickening of the lower leaf bed. The 

upper leaf bed may also thicken by secondary contributions of the hydrosere vegetation, which, in species 

composition, may resemble the lakeside communities.

The final condition of the basin is shown diagrammatically in figure 49. A vertical section through 

the completely infilled basin parallel to the direction of 

infilling will reveal two leaf beds. If a distance measure, D, is used to represent the species diversity of the 

assemblages, it is clear that as infilling proceeds, the 

species composition of the assemblages converge (fig. 

49B).

If the basin is then vertically sectioned normal to the 

infill direction (fig. 49A), the bottom leaf bed may exhibit a thickening toward the basin margins (although 

the bulk of the finely divided, unidentifiable organic 

matter may accumulate in the bottom of the basin; 

Rickett, 1921), while the upper leaf bed will vary in 

composition laterally depending on the pattern of the 

distributaries. The upper leaf bed will undoubtedly be 

riddled with roots from the hydrosere community and 

the lower leaf bed, at least initially, mixed with some 

remains of the vegetation that originally grew in the 

now-drowned valley.

Direction of infilling

N D»NL Upper leaf bed N. »Nr

N L »N

Lower leaf bed

B

Upper leaf bed

Lower leaf bed

FIGURE 49—Longitudinal vertical sections through a completely infilled lake basin. A, Species composition of upper leaf bed changes 

from one dominated by a large distant component N,, in early 

stages of infill to an assemblage dominated by local components of 

flora A^ as a hydrosere develops on compacted delta sediments. B, 

If a distance measure of species diversity D is used to describe the 

two leaf beds, the situation described in A may be represented by 

two converging lines.

Such a situation is likely to be only rarely preserved 

intact and recognized in consolidated deposits. The pattern of two leaf beds, separated by comparatively unfossiliferous fine-grained sediment, is common, and 

this model illustrates one way by which such a structure may form. Knowledge of other ways, or their 

paleoecological implications, may come from work carried out on other recent environments. Decay and 

diagenesis subsequent to burial may, of course, radically alter the species pattern from what it was at deposition, but if three-dimensional structure of the deposit and the relation between the two leaf beds and the 

surrounding inorganic sediments are determined, then 

useful paleoecological information will be derived. 

Above all, it should not be assumed that a vertical 

change in species composition necessarily means a 

change of floristic composition with time. It may, in 

fact, reflect the distribution of vegetation types over 

the land surface surrounding the basin of deposition. 

Conversely, lateral changes in species composition 

within a single continuous leaf bed may reflect a 

change in vegetation with time as, for instance, in the 

development of a hydrosere.

SAMPLING FOSSIL ASSEMBLAGES

The main objective of any sampling is to derive as 

much information as possible about the sampled population in the most economical way. When sampling fossil plant assemblages, the sampled population is the 

complete population of fossils within a rock unit, and 

using the nomenclature adopted by C. R. Hill (1974), 

this fossil population may be termed the "Target population I." By describing the parameters of this population as accurately as possible, it is hoped that some 

information about the original source vegetation communities, "Target population II," will be derived.

The degree of accuracy achieved in describing the 

Target population I will clearly depend on the nature of 

the sampling methods, and any conclusions arrived at 

concerning the Target population II are dependent on 

the quality of the estimate of the Target population I 

parameters.

In the course of the development of quantitative 

plant ecology, numerous sampling regimes and 

methods have been devised to efficiently estimate factors of floristic importance from vegetational communities. The nature of the rock matrix, combined 

with the three-dimensional nature of a sedimentary 

deposit, imposes certain constraints on the sampling 

methods that have been developed in connection with 

phytosociological studies.

The condition that a set of quadrats (or sample
SAMPLING FOSSIL ASSEMBLAGES 49

stands) should be distributed within a sample area (or 

volume) in such a way that each quadrat is independent of all the other quadrats, as well as any structure 

(in the sense of Orloci, 1968a) within the sampled 

community, cannot always be met when sampling fossil plant assemblages for the following reasons:

(1) Restrictions on the amount of material available 

for sampling—The fact that any deposit is exposed for 

sampling by the paleoecologist is usually the result of 

partial removal of the original fossil population. The 

initial exposure is often made by natural erosion but 

may also be the result of activities of man, for example, 

quarries or road cuts. Some of the deposit, moreover, 

may be inaccessible because it is covered by large 

amounts of other rock in the form of scree, fallen boulders, or slumping.

(2) Weathering.—Of the accessible deposit, large 

volumes may be differentially weathered, and this may 

lead to the selective destruction of some of the component fossil species.

(3) The splitting properties of the rock matrix.— 

Clearly any sedimentary rock that cleaves parallel to 

the bedding will favor the exposure of entombed fossils, 

since most remains are deposited, or distorted by compaction; consequently, their plane of largest area is 

parallel to the bedding surface. A large number of factors, including the presence of organic remains, affect 

the splitting properties of rocks.

In all but the most homogeneous deposits, the factors 

given will largely determine the sampling regime, and 

because pattern (which is what we are trying to detect) 

is by definition the result of heterogeneity, true random sampling of a three-dimensional deposit can 

rarely be achieved. The statistical methods employed 

to determine pattern must be either robust or nonparametric. Fortunately, robustness is a quality of 

many multivariate techniques such as principal components analysis and correspondence analysis.

As regards sample size, the criteria used in 

phytosociological studies are equally applicable to the 

fossil remains.

STAND SIZE

In the plant ecology, the size of stand is primarily 

determined by scale within the vegetation, and this is 

equally critical in the analysis of fossil-plant assemblages. The construction of a species-area (or 

species-volume) curve to determine a suitable .stand 

size (see Greig-Smith, 1964, p. 151-155, for relevant 

discussion) has to be based on the portion of the sample 

population immediately available at the start of sampling. This estimate of stand size may not be suitable 

for sampling all parts of the fossil population.

The effect of pattern within fossil deposits may 

greatly influence the choice of size for the minimal or 

representative quadrat area (Greig-Smith, 1964, p. 

155):

"Both the scale and intensity of pattern of the different species will 

affect the size of minimal area found, if, indeed, any definite area can 

be determined. A species will appear at a relatively small size of 

quadrat if the only pattern it exhibits is small scale. If large-scale 

pattern is present, the size at which it appears will depend on its 

intensity: a species with dense clumps, separated by spaces in which 

it is absent *** will tend to appear only in large quadrats. Conversely, a species with large-scale pattern of mozaic of patches with 

higher and lower density *** will appear at a smaller quadrat size."

Large variations in fossil density between adjacent 

quadrats have been noted by C. R. Hill (1974). Such a 

situation is likely to obtain in most, if not all, fossil 

plant beds laid down under any but the most uniform 

conditions. Work at Hasty Bank, England, carried out 

by C. R. Hill, has shown that the choice of stand area, 

or volume, may in fact have to change with differing 

lithologies, fossil density, and fragment size. (C. R. 

Hill, 1974; Spicer and Hill, 1979).

THREE-DIMENSIONAL SAMPLING

The analysis of the distribution of plant remains 

over a single bedding plane is analogous to investigating vegetation pattern over a land surface, as both may 

be assessed by using a two-dimensional sampling regime. It may be supposed therefore that subsequent 

three-dimensional information about a deposit may be 

obtained by sampling successive bedding planes.

Once again, however, practical considerations limit 

the approach.

(1) The concept of a bedding plane is not immediately 

conducive to this type of sampling, because it may not 

have any physical expression. Sampling over large lateral distances would inherently involve the possibility 

of sampling across different bedding surfaces.

(2) The orientation of plant remains may not be parallel to the bedding; consequently, the stand must become three dimensional in order to adequately recover 

these remains for identification. The required excavation will then encroach upon underlying bedding 

planes, resulting in the breakdown of the sampling regime.

(3) The splitting properties of the matrix may preclude 

the exposure of planar stands of sufficiently large area; 

consequently, the excavation of rock of a finite thickness is inevitable.

Sampling volume rather than area introduces yet 

further problems. The extension of the boundaries of a 

stand (sample) in a direction normal to the bedding 

introduces the likelihood that the rock, including individuals that constitute the stand, was laid down under
50 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

changing depositional conditions. As stated, to 

minimize this problem it is necessary to utilize the concept of the sedimentation unit proposed by Otto (1938). 

Otto's concept permits heterogeneity in the distribution and abundance of the various species of plant remains without prejudicing the delineation of the unit, 

because it allows for chance deviations of all sedimentary characteristics about a mean value. Nevertheless, 

it should be borne in mind that, because we are considering dispersed plant units as sedimentary particles, 

the gross abundance of plant remains may be used as 

one of the criteria for defining the sedimentation unit. 

Other criteria for identifying these boundaries are 

given in Otto's paper (1938).

After deposition, sediments will become compacted, 

and this will increase the abundance of plant remains 

per unit volume of rock. If compaction is uniform, it has 

little consequence, but such a condition is not always 

fulfilled. The degree of compaction is likelier to differ 

from place to place throughout the deposit such that 

the relation of fossil abundances between stands becomes distorted. Under these circumstances, any 

statistical procedure that is susceptible to varying 

interstand abundances in the derivation of floristic ordinations, as in principal components analysis, should 

not be used.

QUALITATIVE AND QUANTITATIVE 

ASSESSMENTS OF ABUNDANCE

There has been much discussion in the literature 

about the use of quantitative and presence/absence 

(P/A) scores in phytosociological studies. Lambert and 

Dale (1964) argue that P/A records form a selfcontained logical system, whereas quantitative measures can only record the extent to which a species is 

present, not the extent to which it is absent. Quantification of a set of stands in which not all the species are 

present in all stands must therefore be a truncated 

quantification. This differs .from Greig-Smith's (1964) 

view that absence is an extreme of a continuous variable that describes the amount of a species that is present. Orloci (1968b) considers P/A scores as primarily 

relating to establishment and survival of individuals, 

whereas quantitative measures of abundance relate 

more to relative performance. Such concepts are of 

course meaningless when one is dealing with paleobotanical data derived from allochthonous assemblages, 

but it is nevertheless necessary to consider from a practical standpoint how the stands should be scored. 

Whatever assumptions are made regarding the relative merits of quantitative scores, the amount of information contained in P/A scores of a particular species 

can be measured and compared with that contained in 

quantitative scores. Subsequent decisions may then be

made on the basis of a pilot study as the the type of 

scoring system to be adopted in the main investigation. 

The term information in this context is used in the 

strictly technical sense and describes a physical property of the data that is related to the likelihood of an 

observation rather than any intrinsic "meaning." Orloci (1968b) presents a method by which the information content of a suite of observations on a species may 

be partitioned into two independent components, a 

presence/absence component and a quantitative component. The loss of information incurred by assessing 

P/A instead of making quantitative measures of abundance can be measured and its significance tested.

The total information in the jth species of a data matrix (X) (in which rows represent species and c columns 

represent sites), with xu representing the value of the 

jth species in the 7 th site, may be expressed as:

fij (1)

where the c observations have been summarized in an 

s- celled frequency table for each species and the class 

frequencies and corresponding proportions are given 

by fu or pu such that

7 = 1, 2 Pi. = f--/c ^ in' L > —i

J = 1

fu = c, and p,j = 1

The presence/absence component is then given as 

In = -Mj = 0) lnp(*;; = 0>-A*y>0)lnpUu>0). (2)

The quantitative component is then given as

!,*=!, -In- (3)

Because /, consistently underestimates the population 

value, a correction factor /, + A,- is required where

A, = (s-l)/2-(2-3s+ l/pj)/6c.

j = 1

The sampling variance for / is then

V(I { ) =c( 1 Puln'pu -HI).

(4)

{5)

This last expression differs in two respects from that 

originally given in Orloci (1968b). The last term o is 

omitted to correct an error in the paper, and H-, is defined as Ii/c rather than (/, + A,)/c to avoid negative 

values (Orloci, written commun., 1976); the confidence 

limits are defined by

Conf = 1.96 W(/,). (6)
SAMPLING FOSSIL ASSEMBLAGES 51

Using this method, it was possible to assess the information content of both the whole-leaf and fragment 

density data from Silwood delta; the results are presented in tables 6 and 7.

It can be seen that in both cases more information 

resides in the quantitative component and that in all 

but the rarer species, scoring on the basis of P/A rather 

than density would lead to significant losses of information.

It is difficult to make sound generalizations concerning P/A scoring versus quantitative scoring in 

paleoecology owing to the lack of relevant information. 

It would not be surprising in many circumstances, 

however, to find that over single bedding planes, most 

of the information resides in the quantitative component. The so-called random variation in sedimentary 

data (Blackith and Reyment, 1971) brought about by 

the innumerable variables of deposition will tend to 

make the distribution of any population of sedimentary 

particles lying on a single bedding surface appear almost homogeneous. On the other hand, a vertical series 

of samples that cut across different sedimentary units 

is likely to exhibit more heterogeneity, and therein a 

larger component of the total information will reside in 

P/A scores. These intuitive comments remain to be 

tested.

Unless presence or absence data are to be used, some 

measure of abundance of individuals within stands 

must be attempted. The two measures used in 

phytosociology that are applicable to fossil plant assemblages are density and cover, and these may be 

defined thus: 

Density—A count of the number of individual fossils,

whether representing whole or fragmented organs or

organisms, that occur within a unit sample. 

Cover—The proportion of a planar unit sample area,

alined parallel to the bedding, that is occupied by a 

normal projection onto it of the individuals representing a species form that occur within that unit 

sample.

The cover definition assumes that the dispersed leaf 

remains have their surface of greatest area lying parallel to the plane of bedding, and both refer to the potentially measurable parameters of the Target population I. The success of achieving an accurate assessment 

of species abundance in the Target population I is dependent on characteristics of the sampling method as 

well as the rock matrix.

To demonstrate this contention, consider a diagrammatic representation of a vertical section through a 

three-dimensional sample stand, figure 50. The upper 

surface represents the best exposure of the bedding 

plane A-B that could be achieved by excavation and 

splitting. Clearly recovery will depend on the splitting 

properties of the rock matrix and the influence on this 

of the incorporated fossil material. The actual number 

of leaves lying within the rock close to this plane was 

four. Fragmentation of the leaf on the extreme right, 

however, leads to the exposure of five pieces of material; in this case, then, a 25-percent overestimate of 

density might be made. If cover estimates were attempted, using, for example, point quadrats (see 

Goodall, 1952) and it was possible to measure the exposed pieces of leaves by this method without any 

errors, there would still be an underestimation of cover 

by 50 percent. The complete exposure of fossil leaves, 

necessary for accurate cover measurements, is rarely if 

ever achieved; as a result, underestimation of the 

Target population I cover abundance is inevitable. 

Density, on the other hand, may lead to a potential 

overestimation of abundance if excavation or splitting 

of the stand surface is not carefully carried out.

TABLE 6.—Fragment density per quadrat

[Information partition statistics calculated using leaf-fragment density data collected from 

top 2 cm of delta sediment]

Species

Alnus glutinosa

Betula pubescens

Fagus sylvatica

Quercus robur

Salix cinerea

Ilex aquifolium

Aesculus hippocastanum

Acer pseudoplatanus 

Crataegus monogyna 

Typha latifolia

Nuphar lutea

Salix alba

Total ________

Percent total

Total 

information

— 278.54

.__ 181.05

— 176.40

.— 216.35

— 206.91

.— 151.82

.— 53.30

— 26.39 

— 16.29 

- 157.21

.— 54.23

40.54

—1,559.03

__ 100

95- percent 

confidence 

interval 

1.96 V(J)

11.04

12.55

24.78

15.71

37.34

16.13

18.01

11.47 

51.86 

19 07

58.83

18.35

Presence/ 

absence 

component

9.75

41.64

51.86

35.81

28.51

52.95

37.39

26.39 

16.29 

53.83

37.39

28.51

420.32

27

Qv antitative 

component

268.79

139.41

124.54

180.54

178.40

98.87

'15.91 '.0 

'.0 

103.38

'16.84

'12.03

1,138.71

73

TABLE 7.—Whole-leaf density per quadrat

[Information partition statistics derived from whole-leaf density data collected from top 

2 cm of delta sediment]

Species

Betula pubescens _

Aesculus hippocastanum

Crataegus monogyna __ 

Salix alba

Total ___ _ _ _

Percent total __

Total 

information

150.14

142.06

98.61

99.27

183.32

19.10

29.73

13.20

5.70

741.13

100

95-percent 

confidence 

interval 

1.96 VCH

39.89

9.74

23.49

24.38

22.89

11.44

13.92

3.95 

8.49

Presence/ 

absence 

component

41.64

49.31

55.25

52.95

38.89

19.10

26.39

13.20

5.70

302.43

41

Quantitative 

component

108.50

92.75

43.36

46.32

144.43 '.00

'3.34

'.00 '.00

428.70

59

'Losses not significant at 95-percent level if presence-absence component replaces quantitative scores for abundance.

'Losses not significant at 95-percent level if presence-absence component replaces quantitative scores for abundance.
52 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

Another problem of measuring cover abundance is 

that many of the more practical methods of assessment, such as point quadrats, are in themselves only 

able to estimate the exposed cover, because the rationale behind them is of a statistical nature. Recording the number of individuals exposed on a rock surface is, on the other hand, a more direct measure but 

can be rather tedious when the concentration of plant 

material is high.

It is clear that although both cover and density are 

measurements of abundance, they measure different 

properties of the population, and the meaning of either 

of them, or the relation between them, is obscure. The 

presence within a stand of an individual, whether it be 

a whole leaf or a fragment, represents a discrete depositional event that took place along with the other depositional events of the inorganic sediment grains. Consequently, density is related to the processes of 

sedimentation that characterize a particular depositional environment. It is therefore likely that 

throughout a deposit, in particular vertically, variations in plant density will be associated with variations 

in lithology.

Cover is controlled mainly by morphology. A locally 

abundant plant X, producing large tough leaves, will 

tend to have a more uniform cover abundance score 

from those stands in which it occurs than, for instance, 

a similarly abundant pteridophyte Y with a large 

number of small fragile pinnules. Species X will tend to

EXPLANATION

—» Actual leaf size as would be measured by cover if all 

the leaf was exposed

—» Leaf size as measured by cover estimates

— Lines representing the orientation of bedding planes

FIGURE 50.—Schematic representation of a vertical section 

through a three-dimensional sample stand. A-B represents the 

best exposure of a bedding plane that could be achieved by splitting. Leaves, shown in section, have influenced the splitting 

along A-B but have become fragmented in the process, making 

density and cover estimates show erroneous assessments of 

abundance: density overestimates abundance, and cover underestimates abundance.

have a high cover rating from those stands in which it 

occurs. Species Y will exhibit a wide range of cover 

abundances, depending on how many pinnules are 

present within the stands. Throughout the deposit, 

species Y must be present in greater numbers than X 

even to equal the cover score for X.

It might be assumed that this characteristic of some 

species components being less variable from stand to 

stand provides more suitable data for principal components ordination. This feature merely reflects the 

morphological influence inherent in cover abundance 

estimates, and there is a danger that it would invalidate any floristic pattern derived from the ordinations.

C. R. Hill (1974) has observed, "the extent of fragmentation of fossil leaves *** varies tremendously from 

sample to sample, and for this reason, abundance 

counts (density) can scarcely be expected to reflect directly the real proportions of the whole leaves deposited in the plant bed. Even less can they be expected to 

reflect the proportions produced by the different species 

when they were alive." Hill suggests that theoretically 

at least this information might be better provided by 

estimating "cover" in the fossil assemblage. He then 

comments that the fragmentary nature of many fossil 

forms, as well as the frequent lack of extant relatives, 

prevents such parameters of the ancient community 

from being reconstituted even from cover data.

The results of the delta trap experiments show that 

long-distance aerial transport selects in favor of small 

leaves (figs. 18 and 20). The selection of small leaves by 

aerial transport, which results in the biased representation invalidates cover as an abundance measure 

because species transported to the depositional site 

mainly by wind will, overall, have a lower abundance 

score.

A similar argument may be advanced against the 

use of density measures in that fragmentation is dependent not only on the species of leaf, but also on the 

distance and length of time it is transported prior to 

deposition. Because the processes that lead to mechanical fragmentation are predominantly those found in 

fluviatile environments, however, the detection of pattern that is linked to fragment density is potentially 

useful for distinguishing those taxa that might have 

grown around the lakeside from those that occupied 

streamside habitats.

While there are clearly arguments both for and 

against the use of either measure, it would seem from a 

consideration of the large potential errors involved in 

the estimate of cover, the morphological bias, and the 

problems of relating cover estimates to parameters of 

the source vegetation that density is a more suitable 

measure of abundance.
PRINCIPAL COMPONENTS ORDINATION AND CORRESPONDENCE ANALYSIS 53

PRINCIPAL COMPONENTS ORDINATION 

AND CORRESPONDENCE ANALYSIS

Multivariate statistical techniques such as principal 

components analysis (P.C.A.) and principal coordinates 

analysis (Gower, 1966) have been used for some time in 

studies of living vegetation (for example, Orloci, 1966). 

P.C.A. has proved particularly suitable for investigating environmentally dependent patterns that might 

occur within vegetation where no pattern has previously been suspected. In this context, pattern is used 

in a broad sense and should be regarded as nearly 

synonymous with the term "structure" as used, for 

example, in phtyosociology to describe intrinsic properties of a particular data set (see Orloci, 1968b). Although normally presenting a continuum that might 

be assigned to an environmental gradient, P.C.A., and 

indeed most ordination methods, will detect discontinuities, and the data will thereby be grouped, if such 

a clustering is justified. The clustering produced by 

P.C.A. may then be used as a basis for dividing the 

vegetation sampled into communities, each exhibiting 

a more homogeneous structure. Despite a theoretical 

requirement for linearity of data structure (Orloci, 

1975), P.C.A., like many other multivariate statistical 

methods, is remarkably robust, which means that it 

will still give a fair result on data not fulfilling this 

premise. This robustness allows application of such 

methods to the analysis of plant fossil distributions, 

where sampling may be subject to constraints of exposure and lithology and where the distribution of the 

abundances of individuals may depart from normality.

Principal components analysis may be thought of in 

the following way. If only two species were present in 

the sample population, the occurrence of these species 

within a stand (quadrat) would define the position of 

that stand on a two-dimensional graph, the uses of 

which represent some quantification of the two species. 

Similarly, if three species occurred, then a threedimensional graph could be constructed. Although impossible to visualize, the same procedure can be mathematically expressed for any number of species and 

any number of stands, with the result that a swarm of 

stands (or if stands are used as the axes, species) conceptually exist in multidimensional hyperspace. 

Clearly this only begins to have understandable meaning if the distribution of stands or species can be summarized by projection onto three dimensions or less, 

but this unfortunately introduces distortion. The distortion may be minimized if the projected axes are 

alined with the principal axes of variation within the 

multidimensional scatter. P.C.A. is a method of detecting these principal axes, or major components of variation, in data matrices.

The implication of the graphic analogy is that the 

extracted axes of variation are orthogonal. This may 

not represent their true relation, but it does not invalidate the technique for ordering the data. Orthogonality in P.C.A. and other truly multivariate analyses 

plays exactly the same role as it does in univariate 

analyses, the preservation of the statistical independence of the components of variation (Blackith and 

Reyment, 1971).

STANDARDIZATION

Standardization is a widely used technique for 

transforming the data to allow comparison of variables 

that have been measured in different units. The 

transformation of the raw data matrix is achieved by 

subtracting from each observation the mean of the set 

of data and dividing by the standard deviation. In this 

way, a new matrix is derived in which the variables 

have a mean of zero and a variance of one. Such a 

procedure, provided that all raw variables had initial 

variances > 1, reduces the overall variance in a set of 

observations and gives equal weighting to all variables. Standardization "adjusts" certain components of 

the data more than others; for example, those species 

with a high variance are divided by a correspondingly 

high standard deviation. Such adjustment may not always be desirable, but when differences in abundance 

between stands, which cause fluctuation in variance, 

are brought about by factors other than those of direct 

paleoecological significance (for example, differential 

rates of sedimentary compaction), standardization 

could be accepted.

Another way of reducing the overall variance in a set 

of data is by means of a logarithmic transformation. 

The usefulness of this procedure, which does not alter 

the relation between species (or stands), is demonstrated by Spicer and Hill (1979) using data collected 

from a Jurassic plant fossil locality.

PRINCIPAL COMPONENTS ANALYSIS ORDINATION OF THE 

SILWOOD DELTA SURFACE DATA

Leaf-fragment abundance data from the surface sediment of the Silwood delta (see fig. 8 for sampling positions) were analyzed by PCA ordination using both 

unstandardized data and data that had been standardized to unit variance. The computer program 

(Q.P.C.A.), written by A. J. Morton of Imperial College, 

is based on an iterative procedure (Lawley and Maxwell, 1963) and operates on a half matrix to reduce 

computer memory requirements.

The plot of the unstandardized principal component 

ordination (fig. 51) exhibits a tight bunching of most of
54 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

the stands on the left of the diagram. Reference to the 

raw data (data table 1) shows all these stands have 

relatively low species abundances. It has been pointed 

out by Orloci (1966) that species-rich assemblages differ in more species than species-poor assemblages and 

that where there are significant numbers of speciespoor stands, these will ordinate close together with respect to the more species-rich stands. In figure 51, the 

stands plotting apart are those with high species abundances. Axis 1 then appears to reflect variation due to 

abundance. Such an observation is not new, and indeed 

is common in P.C.A. ordinations. Axis 2, however, may 

be assigned, by inspection, to reflect a trend from proportionately high Alnus abundances to proportionately 

high FaguslQuercus abundances.

The standardized data orientation (fig. 52) seems to 

differ little from that obtained from unstandardized 

data. Axis 1 is again alined along an axis of abundance; 

axis 2 reflects ordering from proportionately high 

Alnus abundances to proportionately high Fagusl 

Quercus abundances. The total variation accounted for

by the first two axes of the unstandardized data ordination was 94 percent, whereas that for the standardized 

was only 52 percent. Standardization has apparently 

reduced the effectiveness of the ordination. Both ordinations have identified, on axis 2, an axis in the data 

along which stands may be ordered in terms of the 

relative abundances of Alnus or Fagus and Quercus.

Successful as P.C.A. ordinations are in presenting 

trends such as floristic gradients, the method itself is 

constrained by a number of drawbacks. Unstandardized P.C.A. ordinations are not corrected for species 

abundance, and the results show that the rarer species 

have a smaller axis score than the more abundant 

species even though their distributions throughout the 

sampled population may be similar. This introduces 

problems when dealing with fragmented plant fossils 

that may have been only parts of a single living entity. 

Naturally occurring associations between organs 

would not be detected, because, for example, flowering 

parts would normally be considerably rarer than 

leaves.

In a stand ordination, this characteristic of P.C.A. 

has even more disturbing consequences. If the abun80-

60 -

40 -

Low

densities 2n -

76 ,

Q&jSJi W, 63

I ^gmo"

35 16 —

-20 -

—

-40-

_

PERCENT TOTAL VARIANCE

| Axis 1=61 -9 ^83

|l Axis 2 = 31 -8

>

1

.41

*76.68.7

•55 Hj9h 

.71 .25 densities

6 42

23I • I I I I I I I I 1

%! o 20 * 73 40 60 80

x o 44

332 °33

14 o 0 15 oil

° 2?

se •5:

0 13

^ m

FIGURE 51. — Axes 1 and 2 of a principal components ordination of nine

species of leaves occurring in Silwood delta sediments. Ordination 

based on unstandardized fragment-density data. Numbers refer to

quadrat numbers (see fig. 8). Dots represent those quadrats belonging to the FaguslQuercus group as identified by correspondence 

analysis (fig. 54); circles represent the A Inus -characterized group.

dance of fossils within a unit volume of rock is noted

2

8 -

6-

4-

Low 2 - 

densities °Q

-€

18 °cJ0q/ i Nt#j§i§ir'o

-2 * %!5P o3

76 43od 

77 68°

-2 -

-

-4 -

-

-6-

0 10

13

° 0 27

g 

°I133 011 High

59 1o .22 .73 o densities

• 78 •

•3&\ I I I I I I I I

.^.42 2468

21 56 .25

.41

PERCENT TOTAL VAR I ANCE

Axis 1 = 32-0

Axis 2=19-9 .53

FIGURE 52. — Ordination similar to figure 51 but based on data 

standardized to unit variance and zero mean.
PRINCIPAL COMPONENTS ORDINATION AND CORRESPONDENCE ANALYSIS 55

and these data are subsequently ordinated using 

P.C.A., spurious results may be introduced owing to 

the effect of varying abundance scores arising from differential compaction during fossilization rather than 

any vegetational variables or depositional variables 

such as those giving rise to sorting.

Another problem with P.C.A. ordinations is that 

they are not particularly suitable when it becomes desirable to use fossil forms as stratigraphic indicators. 

The variation of fossil species, both within and between 

different lithologies, may be dependent on a number of 

controlling factors, and these are clearly critical when 

considering relations between the fossil assemblage 

and the living community. It becomes important to relate any structure or pattern within a stand ordination 

to those species characterizing the various parts of that 

structure, and the absence of any direct relation between stand and species ordinations arising from 

P.C.A makes this process somewhat complicated.

CORRESPONDENCE ANALYSIS

Correspondence analysis (M. O. Hill, 1974), also 

known as L'analyse factorielle des correspondances 

(Escofier-Cordier, 1969; Benzecri, 1969, 1973), reciprocal averaging (M. O. Hill, 1973), and reciprocal ordering (Orloci, 1975), is an eigenvector method of ordination somewhat akin to P.C.A. ordination but one 

that overcomes some of the problems of P.C.A. Although it may be thought of as operating in a way 

similar to P.C.A., its rationale is developed from a different standpoint. The following is based on M. O. Hill 

(1973): A species (row) by stand (column) data matrix 

is constructed and an arbitrary set of species starting 

scores, between 0 and 100, is allocated. Preferably this 

should in practice reflect what is suspected as being the 

main gradient of change, as a good initial choice reduces the amount of calculation required. Using this 

set of starting scores, a set of stand scores is obtained 

by averaging stand data in terms of the estimated 

species scores. These scores are then rescaled between 

0 and 100. From this set of stand scores, a new set of 

species scores is again derived, by averaging, and rescaled between 0 and 100. This procedure continues 

back and forth until, after a certain number of iterations (dependent on the initial species scores), the 

species and stand scores stabilize. The resulting vectors are a unidimensional ordering of the stands and 

species derived simultaneously from the data matrix. 

The second axis may be obtained by using a set of 

scores that were fairly near the final scores of the first 

axis. These scores are then adjusted by subtracting a 

multiple of the first axis and the iterations continued 

until a new set of scores stabilize. These scores become

the second axis. The third and subsequent axes are 

derived in this way.

Strictly speaking, correspondence analysis should be 

used with contingency table data only, but continuous 

data can be analyzed by dividing the ranges of variates 

into a number of discrete pieces. If the pieces are small 

enough, an approximation to the continuous case is 

achieved (Naouri, 1970; M. O. Hill, 1974). David and 

others (1974) and Jbreskog and others (1976) suggest 

dividing the elements of the data matrix by the sum of 

all the elements in the matrix. The resulting proportions may be interpreted as probability values, and 

the matrix becomes a contingency table in the sense of 

Fisher (1940).

The simultaneous derivation of stand and species 

scores results in directly comparable ordinations. The 

species score is equal to the average score for those 

stands in which the species occurs (but rescaling so 

that the total range is 0 to 100), and the stand score is 

equal to the average species score for those species that 

occur in the stand (but again rescaling between 0 and 

100). This duality leads to the interpretation of a stand 

plot, in terms of the abundance of species within the 

stands, by simply overlaying the species plot onto the 

stand plot. Although there is a risk of circularity being 

introduced into the argument at this point, the species 

would not be used to interpret the stand plot in terms of 

causes of variation but only to characterize clusters, if 

they arise, from the ordination.

Hill makes the point that where there is a long floristic gradient, it will always be presented linearly along 

the first axis of an ordination using C.A., whereas with 

P.C.A., "where there is a long and strong floristic gradient, stands which are extreme on the first axis of the 

ordination need not be extreme on the floristic gradient, and vice versa." It should be stressed that the 

species scores derived by C.A. are corrected for species 

abundance, which, as pointed out, is not the case with 

unstandardized P.C.A., even though they may have the 

same distribution.

Hill notes that with both P.C.A. and C.A., the second 

axis displays a quadratic dependence on the first axis, 

and with C.A., this relation applies to both stand and 

species ordinations. With P.C.A. the axes are orthogonal, whereas with C.A. the axes are not restricted to 

this condition even though they are so plotted for clarity.

It should be noted that the eigenvalues quoted for 

the axes on the C.A. ordination are not measures of the 

variance extracted by the axes as given for the P.C.A. 

ordinations but rather a measure of the relation between the stand and the attribute ordinations (M. O. 

Hill, 1974). Orloci (1975) considers it "an indication of 

the conceptual difficulty with which the quadrats can 

be ordered based on the species scores."
56 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

CORRESPONDENCE ANALYSIS OF THE SILWOOD DELTA 

SURFACE-SEDIMENT DATA

The unconsolidated nature of the Silwood delta surface sediment facilitated the separation of whole leaves 

from fragments. Fragments and whole leaves were 

therefore separately treated in order to determine the 

contribution made by each to any paleoecological picture that might emerge.

DENSITY MEASURES

The most straightforward estimate of leaf abundance 

is clearly a density measure, and this will be considered first. From the attribute (species) plot obtained by 

C.A. of the fragment density per quadrat of the twelve 

taxa determined from the delta deposits (fig. 53), it can 

be readily seen that the majority of species lies along 

the top of the cube face representing the plot of axis 1 

against axis 2. This depicts an ordination along axis 1 

with all attributes having similar high scores on axis 2, 

except Nuphar, which occurs in the bottom right-hand 

corner. Apparently Nuphar leaves have a very differSPECIES PLOT

Typha\ 

'Acer

Crataegus Salix cinerea

Aesculus

.Salix ^cinerea Hex' KT

Crataegus* ^\ 

Aesculus• * \5fl//.x alba 

Betula" *Typha

Nuphar

NUMBER OF FRAGMENTS PER QUADRAT

FIGURE 53.—Correspondence analysis ordination of fragment densities per quadrat as sampled from the delta surface sediment of Silwood Lake. Twelve species ordinated. View depicts three internal 

faces of a cube onto which the positions of the species in threedimensional space have been projected. Thus there are three times 

as many points in total plot as there are, in this case, species. Numbers 1, 2, and 3 at corners of plot denote three axes originating from 

center of diagram.

ent distribution pattern than other species. Such a result is perhaps to be expected because, being an obligate aquatic occurring only in the lake, the leaves of 

Nuphar are subject to dispersal processes different 

from leaves of all the other taxa. Indeed, the presence 

of Nuphar pads bearing beak marks in an apparently 

undisturbed delta leaf trap (table 2) indicates that 

water fowl, as well as movements of the lake surface 

waters, may be responsible for some of its distribution.

Examination of the stand plot of the same ordination 

(fig. 54) shows a similar crowding of stands with high 

axis 2 scores, the only exceptions being those stands 

containing Nuphar.

It is apparent that the ordination of stands along 

axis 1 is not continuous: a minor break is indicated by 

an arrow. By examining the species plot (fig. 53), we 

find that the group of stands to the left of the arrow in 

figure 54 (stands 6, 7, 8, 21, 22, 25, 26, 41, 42, 43, 45, 

46, 56, 57, 58, 59, 60, 62, 70, 71, 72, 73, 74, 76, and 77) 

is likely to be characterized by proportionately high 

densities of Acer, Quercus, and Fagus with some Ilex 

and Salix cinerea. In contrast, the group to the right of

STAND PLOT

NUMBER OF FRAGMENTS PER QUADRAT

ESTIMATE OF EIGENVALUE 

Axis 1 (ED =0-25 

Axis 2 (E2) = 0-09 

Axis 3 (E3) =0-07

FIGURE 54.—Stand plot from same ordination as in figure 53. 

Arrow marks discontinuity in ordination along axis 1.
PRINCIPAL COMPONENTS ORDINATION AND CORRESPONDENCE ANALYSIS 57

the arrow (that is, with higher axis 1 scores) is likely to 

contain proportionately high densities of Alnus, Betula, Typha, and Salix alba with some Crataegus and 

Aesculus. By reference to the original data (data table 

1), it is apparent that this is so. (Typha in this case 

probably includes a small number of fragments of other 

monocotyledonous plants, for example, grasses.)

Not all of the 12 species ordinated here occur as 

whole leaves. In order to compare legitimately the ordinations of the fragments and whole leaf data, the 

fragment density ordination was again carried out 

using only those species that also occur as whole 

leaves. The resulting plots (figs. 55 and 56) indicate 

that the ordination along axis 1 is essentially unchanged. With the removal of Nuphar, however, the 

axis 2 scores are completely altered. Removal of the 

"constraint" imposed by Nuphar on the other species 

has led to a "spreading out" down axis 2, and now the 

Betula distribution is shown to differ from the others. 

This dissimilarity of the Betula distribution was previously depicted on axis 3 of figure 53 along with that 

of Typha. It therefore follows that those species accounting for the greatest variation within the "Alnus 

group" of figure 54 are dominantly Nuphar, then Betula and Typha. The separation of Aesculus on axis 3 of

SPECIES PLOT

Salix

Crataegus 

.Ilex 

•Alnus

'Fagus 

'Quercus

Salix cinerea

Fagus,

Aesculus ,

'flex 

Quercus

Crataegus

Alnus*

Salix cinerea Salix alba

Betula^Fagus

Salix alba 

NUMBER OF FRAGMENTS PER QUADRAT

FIGURE 55.—Fragment-density plot from Silwood delta surface sediment. Nuphar, Typha, and Acer densities were not included in 

ordination (see text).

figure 55 suggests that further variation is accounted 

for by the distribution of this species.

The discontinuity in the ordination on axis 1 of 

figure 56 is still preserved, even after the removal of 

Nuphar, Typha, and Acer, suggesting that such a discontinuity is a strong characteristic of the data. The 

resultant separation of the stands into two groups 

facilitates mapping the delta surface in terms of 

species distributions. Figure 57 shows the position on 

the delta of those stands occurring in the "Fagus/ 

Quercus" group. It is to be noted that they all occur 

around the stream distributaries 1 and 2, which, during the period of time sampling was carried out, carried 

an estimated 80 percent of the streamflow (fig. 48). Distributary 3, however, passed through the Typha bed 

before crossing the exposed delta surface; most of the 

leaves it may have been transporting were filtered out 

(McQueen, 1969). Distributary 4 was little more than 

seepage from the nearby bank.

The pattern of leaf-fragment distribution on the 

delta surface evidently is primarily controlled by the 

pattern and energies of the distributaries. It was earSTAND PLOT

NUMBER OF FRAGMENTS PER QUADRAT

ESTIMATE OF EIGENVALUE 

Axisl (ED =0-25 

Axis2(E2) = 0-09 

Axis 3 (E3) = 0-07

FIGURE 56.—Stand plot of fragment density based on data not including densities "of Nuphar, Typha, and Acer.
Silwood Lake

SS) (59) (6QK..S

0 10 METERS

0 10 20 30 FEET 

CONTOUR INTERVAL 5 CENTIMETERS

FIGURE 57.—Position of those stands (quadrats) constitutingFagus/Quercus group of figures 54 and 56 with respect to delta contours. Ordinatio

density data.
PRINCIPAL COMPONENTS ORDINATION AND CORRESPONDENCE ANALYSIS 59

Her pointed out that most leaf fragmentation was by 

biological and mechanical degradation during water 

transport; it is not surprising, then, that fragment pattern is associated with stream-channel courses. It follows that any lateral species separation between the 

area of the distributaries and the rest of the delta surface must indicate spatial separation of species in the 

source vegetation. The Fagus/Quercus group revealed 

in figures 54 and 56 is clearly the result of streamtransported fragments from the mixed woodland area 

upstream from Cascade Bridge (fig. 1), whereas the 

remaining group of stands, constituting the "Alnus 

group," represents the more local flora.

From the arguments presented in the section entitled "Leaf Degradation," the distribution of whole 

leaves on the other hand might be expected to reflect 

more strongly the effect of wind transport in determining dispersal patterns. The most striking feature about 

the C.A. ordination plot of the whole-leaf densities in 

the quadrats from the Silwood delta, figure 58, is the 

separation, on axis 1, of Fagus, which suggests a 

marked difference in distribution from the other 

species. The stand plot, figure 59, shows no obvious 

discontinuities in the data, but there is possibly a gap 

halfway along axis 1. If stands with an axis 1 score of 

less than 50 are plotted back onto the sample location

SPECIES PLOT

Salix alba

Aesculu

Salix cinerea 

Salix cinerea

NUMBER OF WHOLE LEAVES PER QUADRAT

FIGURE 58.—Correspondence analysis species ordination plot 

based on whole-leaf density data from surface sediment of Silwood delta.

map, no clear pattern emerges, and certainly no pattern ascribable to stream-channel effects. The diffuse 

nature of the Fagus group, if indeed it may be called a 

group, suggests that the variation in Fagus content 

between stands is high.

In the fragment density ordinations, Fagus and 

Quercus plot close together, which suggests that their 

distributions are similar. The whole-leaf density distributions, however, apparently are not related, as 

Fagus and Quercus plot apart in figure 58.

The delta water-trap data (table 2) indicate that approximately twice the number of Quercus leaves land 

directly on the delta surface by wind transport as 

Fagus leaves. On the other hand, the results of the 

leaf-degradation experiments (figs. 27-43) indicate 

that Fagus is more resistant to both mechanical and 

biological degradation, resulting in a higher proportion 

of whole Fagus leaves surviving stream transport. This 

effect must have contributed to the large number of 

whole Fagus leaves recovered from the delta sediment 

quadrats: 175 as compared with only 69 whole Quercus 

leaves (data table 2).

On axis 1 in figure 58, Quercus plots close to the local

STAND PLOT 2

NUMBER OF WHOLE LEAVES PER QUADRAT

ESTIMATE OF EIGENVALUE

Axisl (ED = 0-31

'Axis2(E2) = 0-17

Axis3(E3) = 0"IO

FIGURE 59.—Companidn stand plot to figure 58.
60 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

species such asAlnus and Betula. The vegetation map 

(fig. 2) shows that Quercus occurs in some abundance 

behind the Alnus trees bordering the lake and, in this 

respect, may be regarded as local. The only way that 

leaves of these trees can enter the lake deposits is by 

direct wind transport, hence the Quercus plotting, 

along with Alnus and others, on axis 1 of the whole-leaf 

density ordination (fig. 58). Axis 2, however, separates 

Quercus and Salix cinerea from the other species, suggesting that they differ in distribution. In the case of 

Quercus, the difference is more clearly shown by axis 3. 

The small numbers of whole Quercus leaves that do 

survive stream transport may account for this.

The position of Salix cinerea on the plots is worthy of 

some discussion. Although clearly a local species, the 

contribution of leaves directly to the delta by wind 

transport is small (data table 2), probably because <S. 

cinerea rarely attains a height greater than 3 m on the 

delta and the Typha bed effectively traps any airborne 

leaves before they reach the lake. Observations made 

during sampling indicate that this was so because the 

leaf litter trapped on the Typha bed was rich in Salix 

leaves. Most of the leaves must therefore enter the deposit by the stream, and, as a result, S. cinerea consistently plots near Quercus, which exhibits a "mixed" 

origin.

It is possible to reconstruct the Silwood vegetation by 

examining the distribution patterns of fragments and 

whole leaves separately as consisting of a local element, including such species as Alnus, Betula, and 

Salix alba, and a more distant element rich in Fagus. 

Quercus appears to be a component of both local and 

distant floras, as apparently is Salix cinerea. The positions of Ilex, Aesculus, and Crataegus appear to be 

intermediate.

The special position of Nuphar may at first present 

some problems, but anatomical studies would soon reveal the likelihood of its being an aquatic plant. Some 

measure of paleocommunity reconstruction may be 

possible using only density measures of fragments and 

whole leaves.

COVER ABUNDANCE MEASURES

If the area of each individual fragment is measured, 

and if it is assumed that most leaves are lying parallel 

to the bedding planes, the total area per quadrat of that 

species may be considered comparable to a fragment 

cover estimate. It is pointless to calculate the percentage cover, since all the quadrat volumes are the same.

On a plot of the total fragment areas per quadrat of 

all the leaves of the 12 taxa found in the Silwood data 

sediments, figure 60, axis 1 clearly separates out 

Nuphar from the other species. Reference to the raw 

data (data table 3) suggests a possible reason. In addition to the difference in distribution, the morphology of 

the large Nuphar pads is totally different from that of 

any of the other species: consequently, their fragments 

can exhibit a much greater size range than any other 

species. The total area of Nuphar can be seen to fluctuate greatly from quadrat to quadrat. The data concerning Aesculus, which has comparatively large 

leaflets, also show this pattern, again reflected in the 

ordination of figure 60, because Aesculus is separated 

from the other species on both axes 2 and 3.

The stand plot (fig. 61) reveals an apparent break in 

the ordination along axis 2, the lower group coinciding 

with the positions of the taxa Salix cinerea, Crataegus, 

Ilex, Quercus, Fagus, and Acer on figure 60. The position of these stands on the delta surface is shown in 

figure 62. Clearly the pattern derived from the density 

measures (figs. 53 and 54) is being repeated, which 

suggests that the cover abundance measures, despite 

the powerful morphological effect, are retaining 

sedimentary-pattern information.

The ordination would not be so easily interpreted if 

one of the species determining the lower group was 

morphologically very different from the others. This 

effect can be seen to some extent in figures 63 and 64. 

Here the species not occurring as whole leaves (Nuphar, Typha, and Acer) have been excluded. The

SPECIES PLOT

Salix alba,

.Typha

^Alnus % Betula 

Nuphar* . Sdixcinerea 

Crataegus *

* Quercus

jix alba

TOTAL FRAGMENT AREA PER QUADRAT

FIGURE 60.—Correspondence analysis ordination of leaf fragments 

from Silwood delta surface sediment based on "cover" rather 

than density.
CONCLUSIONS 61

morphological effect, linked with the occurrence of 

Aesculus, now dominates the ordination resulting in 

this species plotting away from the others. As Aesculus 

was, to some extent, contributing toward the separation of the Fagus/Quercus group on axis 2 of figure 61, 

the separation of the stands into two groups is not so 

well defined. Those stands occurring to the left of the 

apparent discontinuity (arrow) on axis 1 (fig. 64) are in 

fact, the same as those plotted in figure 62.

When the species are ordinated on the basis of the 

whole-leaf total area per quadrat (data table 4; fig. 65), 

Fagus is separated from the other species on axis 1 as 

in ordinating the whole-leaf density data (fig. 58). Aesculus is further separated from the main group, and its 

difference picked out strongly on axis 3. Once again, 

this may be interpreted as primarily a morphological 

effect. Except for this difference, the distribution of the 

species on axes 1 and 2 is very similar to that resulting 

from the density data. Similarly, the stand plot (fig. 66) 

exhibits the same pattern on axes 1 and 2 as in figure

STAND PLOT 2

TOTAL FRAGMENT AREA PER QUADRAT 

ESTIMATE OF EIGENVALUE

Axisl (ED =0-63 

Axis 2 (E2) = 0-28 

Axis 3 (E3) =0-25

FIGURE 61.—Correspondence analysis ordination of fragment 

areas per quadrat. Apparent discontinuity in stand ordination on 

axis 2 marked by arrow. Points below arrow represent those 

stands relatively rich in Fagus and associated species.

59. The variation on axis 3, however, now separates 

those stands rich in Aesculus rather than Qwercws. The 

separation ofQuercus from the "local" group as a result 

of its mixed origin now becomes overriden by the powerful morphological influence associated with Aesculus.

If the whole-leaf and fragment areas are combined to 

give the total area for each species per quadrat (data 

table 5), the resultant ordination has the appearance of 

figures 67 and 68. This type of data approximates a 

cover abundance measure for each species.

The magnitude of the data scores for species that are 

local to the site and those with leaves resistant to degradation will tend to be largely determined by the 

number of size of the whole leaves. For other species, 

fragment size and number will be critical. Clearly 

those taxa producing large leaves will amass large 

cover scores even if only a few individuals are present 

in each quadrat. As expected, the morphological influence on the ordinations is very strong, with the result 

that Aesculus plots away from the main group on all 

three axes. Fagus is separated on axis 1. The cause for 

this is probably not so much its morphology as its resistance to degradation. As indicated in table 3, Fagus 

fragments constitute only some 26 percent of the total 

area of Fagus occurring in the sampled delta deposits. 

Where Fagus occurs, then, the cover will commonly be 

determined by the number of whole leaves. Table 2 

shows that the contribution of whole leaves to the delta 

by direct aerial transport will not add significantly to 

the total Fagus cover scores. Therefore, whole leaves 

transported by the stream will largely determine the 

total Fagus area per quadrat, but, unfortunately the 

high cover scores of the local species swamp the 

stream-controlled Fagus pattern. Under conditions 

where all stream-transported taxa are easily degraded, 

any pattern caused by sedimentary processes is lost. 

The prospects of detecting such patterns are much better if density measures are used, because, with increased degradation, the fragment density in the distributaries increases and better contrast between the 

stream distributaries and the rest of the delta surface 

is obtained.

CONCLUSIONS

The large number of simultaneously operating processes that result in the formation of an allochthonous 

plant fossil deposit require that any study of the sorting and deposition of plant material be of a multidisciplinary nature. A restricted study of the kind described 

here cannot be definitive, but certain conclusions can 

be drawn, some of which may seem obvious on reflection, others not so. Much has been taken for granted in
Silwood Lake

0 10 METERS

0 10 20 30 FEET 

CONTOUR INTERVAL 5 CENTIMETERS

FIGURE 62.—Positions of those stands (quadrats) constituting lower group on axis 2 of figure 61. This group, again rich inFagus and Quercus, is th

on cover data.
CONCLUSIONS 63

the past concerning the mechanisms of sorting and 

deposition, but if paleobotany is ever to gain from a 

more quantitative approach, then even the most basic 

assumptions must be tested.

From the results of the present study and a consideration of the number of variables involved, it is not 

likely that any direct relation will ever be found to link 

the abundances of the source vegetation component 

species and their representation in an allochthonous 

fossil assemblage. The factors affecting litter production alone are sufficiently numerous to introduce 

significant fluctuations in the source material, which, 

when mechanisms of transport, deposition, and 

diagenesis have disproportionately sorted the plant 

remains, produce such a wide range of possible abundances within the fossil deposit that direct interpretation becomes meaningless. The use of a closed system 

of expressing species proportions (for example, percent 

scales) also means that overrepresentation of riparian 

plants seriously biases the proportions of the other 

taxa. Even species that are clearly local to the depositional environment will be seriously underrepresented where growing behind a screen of other vegetation.

Although proportional representation of the source 

taxa is not to be expected, information relating to relaSPECIES PLOT

esculus

Salix alba,

Salix cinerea .

Betula

TOTAL FRAGMENT AREA PER QUADRAT

FIGURE 63.—Correspondence analysis ordination based on 

"cover" abundance of nine taxa: Fagus, Quercus, Alnus, Betula, 

Salix cinerea, Ilex, Aesculus, Salix alba, and Crataegus.

tive positions of the source species with respect to the 

depositional site might be preserved in the fossil assemblage.

A theoretical approach to the study of fluid transport 

and deposition of plant material is hampered by parameters of the plant material itself. The twodimensional nature of most plant debris, progressive 

waterlogging, colonization by microorganisms, and 

fragmentation during transport make impossible the 

use of fluid dynamics equations for predicting 

transport and deposition phenomena. An empirical approach was therefore adopted for this study.

Experiments with both artificial and natural leaves 

showed that weight per unit area is the single most 

critical factor affecting the transportability of a leaf by 

wind. Yet only the small dense sun leaves growing at 

the top of a tree crown are exposed to wind energies 

sufficient to insure a chance of long-distance transport. 

This factor alone results in significant biases in the 

leaf-size distribution within a laterally extensive 

lacustrine fossil-plant assemblage. In fossil-leaf asSTAND PLOT 2

TOTAL FRAGMENT AREA PER QUADRAT

ESTIMATE OF EIGENVALUE 

Axisl (ED = 0-29 

Axis2{E2) = 0-25 

Axis3(E3)

FIGURE 64. — Stand ordination based on "cover" abundance and a 

companion plot to figure 63. Arrow marks apparent discontinuity 

of ordination on axis 1. »
64 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

semblages deposited in an open-lake environment, a 

large part of the source vegetation will be represented 

only by small sun leaves, and paleoclimate interpretation based on such an assemblage could be wrong. 

Swamp and small lake basins, where the lake width is 

minimal relative to the forest canopy height, are 

likelier to contain leaf assemblages that accurately reflect the leaf-size populations within the source vegetation, because both sun and shade leaves will be represented throughout the deposit.

Unlike wind transport, aqueous transport usually 

involves extensive degradation of the plant material. 

Fragmentation may be categorized according to the 

casual agency, mechanical or biological. Biological 

degradation, which can take place in both stream and 

lake bottom environment, may be divided into that 

produced by invertebrate detritivores, both largeparticle feeders and small-particle feeders, and that 

brought about by microorganisms.

Fresh material is comparatively robust, but microbial colonization predisposes leaves to invertebrate attack and mechanical fragmentation within a few 

weeks of entering an aqueous environment. Chemical 

rather than morphological leaf characteristics affect 

the rate of microbial breakdown, which in turn affects 

subsequent degradation.

Because mechanical fragmentation occurs predominantly as a result of water transport, examination of

SPECIES PLOT

Fagus

 Salix alba

. Crataegus 

flex 

' 'Betula

• Salix cinerea

Alnus 

Salix alba 

Crataegus ^

Betula !

• Hex Salix cinerea •

Aesculus

Quercus

Salix cinerea 

* • Betula

.Crataegus 

flex~

Salix alba

WHOLE LEAVES TOTAL AREA PER QUADRAT

FIGURE 65.—Species plot of whole-leaf "cover" data.

the fragmentary condition of a fossil can be paleoecologically useful. By examining the type of degradation, particularly in a fluviolacustrine environment, it 

should be possible, even in the absence of inorganic 

sedimentary structures, to distinguish those leaves 

transported from upstream from those that grew locally. Leaves representing upstream plants will exhibit the rounded holes indicative of invertebrate 

attack, the loss of intercostal tissue produced by microorganisms, and the angular breaks brought about by 

mechanical damage. The local leaves deposited in the 

lake will lack extensive mechanical fragmentation.

From the cores taken throughout the Silwood delta, 

it is evident that two physically distinct leaf beds were 

formed, one related to the lake-bottom deposits and one 

at the top of the stream-derived deltaic sediments. 

Their separation is the result of the leaves and sediment with different hydrodynamic properties being 

acted upon by the fluid stresses at the stream/lake 

interface.

At Silwood, no obvious differences in species composition were noted between the upper leaf bed (1) and

STAND PLOT 2

WHOLE LEAVES TOTAL AREA PER QUADRAT 

ESTIMATE OF EIGENVALUE

Axis! (E1) =0-45 

Axis2(E2)=0'22 

Axis 3 (E3) =0-14

FIGURE 66.—Companion stand plot to figure 65.
CONCLUSIONS 65

the lower leaf bed (2); this is attributed to the rapid 

biological destruction ofAlnus glutinosa leaves on the 

lake bottom. Only the more resistant Quercus and 

Fagus leaves survived to be incorporated in the lower 

leaf bed, thereby making the species composition of the 

two leaf beds appear similar.

On the evidence available, generalizations cannot be 

made concerning the susceptibility of riparian plants 

to biological destruction. If, under different circumstances, the leaves from trees bordering the lake were 

resistant to breakdown, a considerable difference in 

species composition between the upper and lower leaf 

beds would exist to reflect the relative growth positions 

of the source taxa. Leaf bed I may represent those 

species growing some distance upstream from the depositional environment admixed with leaves from the 

local streamside plants, leaf bed 2 the local lakeside 

vegetation.

The physical separation of the beds is dependent 

upon the relative settling velocities of the leaves and 

sediment. In the Silwood environment, where the leaf 

material had a settling velocity greatly exceeding that 

of the sediment, the bulk of the plant debris that was 

transported as bedload collectively settled out at the 

top of the foreset slope. In higher energy environments, 

where the inorganic sediment settling velocity would

SPECIES PLOT

Aesculus

Crataegus Salix alba

t c ,. Salix cinerea

Quercus

Fagus Salix cinerea 

.Crataegus 

lie.

TOTAL AREA OF ALL LEAF MATERIAL PER QUADRAT

FIGURE 67.—Correspondence analysis ordination of combined 

whole-leaf and fragment "cover" abundance data.

approach or exceed that of the plant debris, the separation of the leaf beds might be lost.

If two physically different leaf beds separated by 

fine-grained sediment devoid of any concentration of 

plant material are identified in a fossil deposit and can 

be related to the type of deposition described here, the 

patterns of deposition of the plant fossils could be used 

to reconstruct the source vegetation in some detail. The 

upper deltaic leaf bed will contain mechanically fragmented plant remains from vegetation growing 

upstream and may represent slope plants as well as 

those growing on consolidated deltaic deposits. The 

lower leaf bed will contain fossils representing the 

local lakeside vegetation together with some leaves 

blown in by long-distance wind transport and some 

stream-transported leaves that were still floating on 

arrival at the lake. Evidence of mechanical degradation will be minimal.

As infilling of the lake basin progresses, the species 

composition of the upper leaf bed is likely to change as 

leaves from distantly growing species are filtered out 

by the developing delta vegetation and leaf input from

STAND PLOT 2

TOTAL AREA OF ALL LEAF MATERIAL PER QUADRAT 

ESTIMATE OF EIGENVALUE

Axisl (ED = 0-27 

Axis2(E2) =0-13 

Axis 3 (E3) = 0-08

FIGURE 68.—Companion stand plot to figure 67.
66 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

the delta vegetation increases. A lateral change in 

species composition, then, may reflect a short-term 

temporal change in species distribution, whereas a difference in species composition between the upper and 

lower leaf beds (assuming no disproportionate degradation of leaves from one component of the vegetation 

has taken place) at any point might reflect a spatial 

separation of taxa within the source vegetation.

Any heterogeneity in species distributions that 

might originally be present within a body of sediment 

at the time of deposition could be seriously distorted by 

postdepositional differential degradation. In the Silwood deposit, we find an almost complete loss of the 

locally abundant Alnus glutinosa leaves from the lake 

bottom. It is only when deposition is abnormally rapid 

that such species are preserved. Because preservation 

is evidently linked to speed of burial, analysis of fossil 

remains should include specimens from all parts of a 

deposit, not just those occurring within concentrated 

leaf beds, and evidence of rapid burial should not be 

ignored. Detailed analysis of those plant remains that 

are consistently found only in association with coarse 

or poorly sorted sediments may provide critical clues as 

to the nature of those parts of the flora normally missing from the fossil record owing to a lack of resistance 

to biological degradation. For this reason, highly fragmented remains as well as well-preserved "museum" 

specimens should be collected, for it is the fragmented 

material that is likely to contain more paleoecologically useful information. Species abundance and type 

will commonly vary as a function of lithologic type 

even though all remains represent the same vegetation.

When quantitatively sampling fossil plant deposits, 

a large number of factors, primarily lithologic, prevent 

the sampling regime's being ideal; consequently, 

robust statistical methods have to be used to analyze 

the resulting data. The usefulness and ability of both 

principal components analysis (P.C.A.) and correspondence analysis (C.A.) to detect and display trends 

and sometimes discontinuities within the data is self 

evident; on both theoretical and practical grounds, 

C.A. is more suitable for the analysis of paleobotanical 

data. The duality of the stand and species plots make 

C.A. particularly useful for detecting the relations between assemblage structure and lithology, and C.A. 

ordinations are not affected by overall species abundance. There can be no doubt that future investigations 

will confirm a correspondence analysis as a powerful 

paleoecological tool.

Work described here represents an investigation of 

only a single depositional environment; consequently, 

any extrapolation of the findings to situations other 

than a fluviolacustrine delta system must be done with

care. Nevertheless, attempts have been made to generalize whenever possible, and by studying a number of 

aspects of what, after all, is a multivariate system, it 

has been possible to assess the relative importance of 

some factors that determine structure in a plant-fossil 

deposit.

The following generalizations can be made regarding 

the investigation of a fossil-plant deposit with a view to 

reconstructing the positions of various source taxa relative to the depositional site:

(1) Detailed study is required not only of the fossil 

material but also of the lithology and sedimentary structure of the entombing matrix.

(2) The sediments above and below the fossil material 

should be studied to ascertain relative deposition 

rates (and thereby the likelihood of preservation 

of species normally susceptible to rapid degradation) and, if possible, basin geometry.

(3) Sampling should be carried out in an objective 

manner and include comparatively unfossiliferous sediments.

(4) Fragmentary as well as complete specimens should 

be collected.

(5) The type, biological or mechanical, and degree of 

fragmentation should be determined within each 

sample.

(6) Where possible, sampling should be carried out 

with the aim of determining the threedimensional nature of the fossil deposit.

(7) Individual samples should not cross boundaries of 

sedimentary units.

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the sub-recent vegetation in four major vegetation types in Minnesota: Rev. Palaeobotany and Palynology, v. 2, p. 331-342.

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68 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

Kaushik, N. K., and Hynes, H. B. N., 1971, The fate of dead leaves 

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Rev., v. 3, p. 20-76.

DATA TABLES 1-5

DATA TABLES 73

Silwood delta fragment density and whole-leaf density and area data

[Material collected from top 2 cm of delta sediment]

DATA TABLE 1.—Silwood delta fragment density per quadrat

Stand

1 ________

2 _____ __

3 _ ______

4 _ _

5 _ _ _ 

6 _________

7 _________ 

8 _____ _

9 _________

10 _

11 _________

12 _________ 

13 ___

14 _________

15 _________

16 _______

17 _________

18 _________

19 _________ 

20 _______ _

21 _________

23 _________

24 ___ ____

25 __-._

26 _________

27 ___---.__

28 __ __

29 ________

30 _ _______

31 ___

32 _____

33 ________

34 _________

35 _________ 

36 _________

37 ____

38 _________

39 _________

40 ____

41 _________

42 _________

43 _ ____ 

44 _____

45 _________

46 _________

47 ______

48 _________ 

49 _________

50 _______

51 _________

52 _________

53 _________

54 _________

55 _________

56 _________

57 _________

58 __ _ __

59 ________

60 _________

61 _________

62 _________

63 _________

64 _________

65 _________

66 ______

67 _________

68 _________ 

69 _________

70 _________

71 _________

72 _________

73 _____ __

74 _________

75 _________

76 _________

77 _______ _

78

79 _________

Alnus

_____ 1

_____ 6

_____ 9

"""""29

__ 23

55

____ 0

___ 43

__ -22

___ 12

____ 3

_____ 4 

_____ 9

_____ 7 

_ _ 11

_____ 5

13

_ ___23

28

__. _ 5

_____ 3

____ 31

"_"_^ 6

_____ 7

_____ 8

12

j^

_____ 0

____ 11

_____ 6

3

_____ 0

3

__ 8

9

____ 24

_____ 2

___ 14

_____ 6

__ _ 4

7

Betula

7

4

2

4

0

7

11

4

2

0

2

0

0

9

0

0

2

10

5

2

3

2

4

2

6

2

0

3

6

0

0

2

11

3

Fagus

0

1

1

10

3

0

0

0

0

4

0

7 

7

6

2

9

3

0

0

0

0

0

0

8

7

0

0

2

0

0

0

54

3

6

2

0

0

8

2

16

3

Quercus

0

3

1

21

2

3

7

0 

2

10

0

22

0

on

2

2

2

0

1 

2

rq

17

2

4

q

0

100

16

6

6

7

0

q

10

3

13

6

12

Salix cinerea

4 

0

6 

0

12

2

5

q

1

9

7

38

q

10

2

6

3

6

0

2

q

q

2

1

0

26

9

5

2

6

Ilex

0 

2

0

2

3

0

0

0

0 

2

2

7

2

q

15

0

0

2

2

q

0

6

6

0

2

1

0

0

0

0

q

0

0

0

2

2

0

Aesculus

0 

0

0

0 

0

2

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

2

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Acer

0 

0

0

0

1

0

0 

0

0

0 

0

0

0

0

0

0

0

0 

0

0

0

0

0 

0

0

0 

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

9

1

Crataegus

0

0

1

0

0 

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0 

0

0

0

0 

0

0

0 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Typha

0

0

0

1

1 

9

19

8

6

3

0

0

1

0

0

0

2

2

0

2

0

0

0

0

1

2

0

0

0

0

0

0

0

0

0

0

7

q

0

0

0

0

Nuphar

7 

0

0

0

0 

0

0 

0

0

0

0

0

0

0

0

1

7

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

2

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

Salix, Alba

0 

0

0

0

0 

0

0

0

0

0

2

0

0

0 

0

0

2

0

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0
74 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND

DATA TABLE 2.—Silwood delta whole-leaf density per quadrat

Stand

1

2

3 _ _

4

5 ___

6

7 ___

8

9 ___

10

11 ___

12 

13 ___

14 ___

15 ___

16 _ _

17 ___

18 ___

19 _._

20 _ _

21 ___

22 ___

23 __

24 ___

25

26 ___

27 ___

28 ___

29

30

31 __

32

33

34 ...

35 ___

36 ...

37

38 ___

39

40 

41 ___

42 _ _

43

44

45 __

46

47 ___

48

49 ___ 

50

51 ___

52 ___

53 .._

54 _ .

55 ___

56 _..

57 ___

58 ___ 

59 ___

60 __

61 ___

62 ___

63 __

64 _._

65

66 __

67 ...

68 _ _

69 .__

70 ...

71 __

72

73

74 __

75 ___

76 __

77 ___

78

79 .._

Alnus

__ _______39

4

__ ________ 6

______ 4

9

___________!!

_ _ ____ ..11

23

_------_27

________87 

___________27

___________18

_____ __ _57

___________37

____ _____55

47

___________16

___________ 3

___________ 4

_ ___ __ 1

___________ 1

__________ 1

2

2

___________12

___________ 6

_ _______ 46

1

2

__ 1

___________ 5

___________!!

___________16

___________16

__ _ ___ __10

___________ 3

__ ______ 8

___________12

3

___________ 0

____ 7

_____ ___ 0

0

___________ 0

14

___________ 1

___ ___ __ 1

1

___________ 1 

___________ 2

3

___________ 5

_____ _ __ 5

1

___________ 0 

___________ 0

___________ 2 

___________ 0

_ _ _ __ 0

___________ 2

_______ _ 0

0

____ _ ____ 1

___________ 1

0

___ __ _ 3

1

1 

___________ 2

0

1

o

o

___________ 7

___________15

_____ _ __ 0

o

_ 0

Betula

31

3

1

2

10

0

19 

2

4

3

12

7

2

1

0

1

1

0

1

0

1

2

2

2

1

4

0

0 

2

0

1

4

0

3

3

1

2

.6

2

0 

0

2

1

0

0

0

3

0

2

0

2

0

2

Fagus

22

5

14

9

0

1

1

1

0

10

6

3

16

0

0

7

1 

5

0

g

2

0

2

0 

4

4

2

0

2

0

0

2

3

14

Quercus

4

1

0

0

1

0

1

2

4 

3

3

3

7

0

1

0

0

0

4

2

6

0

1

0

1

0

0

2

0 

2

0

1

2

0

0

0

0

0

3

0

0 

3

1

0

0

0

0

0

2

0

0

Salix cmerea

52

7

5

4

26

6

24 

15

11

18

4

6

3

12

1

0

2

3

2

8

2

1

2

0 

27

3

4

1

0

3

4

2

1 

13

7

2

2

4

1

7

7

2

Ilex

0

0

0

0

0

0

0

0 

0

0

1

1

0

0

0

1

0

0

0

0

0

1

0

0

0

0 

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

Aesculus

1

1

1

0

0

1

1

0

0 

0

0 

0

0

0

0

2

0

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0

Acer

0

0

0

0

0

0

0

0

0

0 

0

0 

0

0

0

0

0

u

0

u

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Crataegus

0

0

0

0

1

0

0

0

0

1

0

0 

0

0

0

1

0

0

0

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0 

0

0

0 

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

Typha

0

0

0

0

0

0

0

0

0

0 

0

0 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0 

0

0

0 

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

Nuphar

0

0

0

0

0

0

0

0

0

- 0 

0

0 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0 

0

0

0 

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

Salve, Alba

0

0

0

0

0

0

0

0

0

0 

0

0 

0

0

0

0

0

1

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0 

0

0

0 

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0
DATA TABLES 

DATA TABLE 3.—Silwood delta fragment areas per quadrat

75

Stand

1 _ ___

2 _____

3 _____

4 ____

5 _____

7 ____

8 _____

9 __ _

10 ____

11 _____

12 _____

13 ____

14 ______ 

15 ____

16 _____

17 _ ___

18 _____

19 _____

20 ______

21 ______

22 -___

23 ____

24 ___._

25 _____

26 ____

27 _____

28 _ _

29 __ _

31 

32 _____

33 _____

34 ___

35 ____

36 _____

37 _____

38

40 _ __

41 _

42 _____ 

43 ___

44 ___

45 ___

46 _____

47 ____ 

48 ___ _

50 _____ 

51 ___ _

52 ____ 

53 ___ _

54 ___

55 _____

57 ___

58 _____

60 IT"

61 _____ 

62 __

63 _____

64 ___

65 _____ 

66 _____

68 _____

69 _____

70 ___ _

71 _ _

72___-_

73 ____

74 ____

75 _ _

77 _ 

78 _____

79 ______

Alnus

_______ 30.5

___ _ 28.0

_______ 68.5

___ 30.0

______ 92.5

__ ___ 91.0

__ ..110.5

__ __ 299.0

-628.0

______ 173.5

___ __ 80.5

____ 351.0

_ ___ 215.5

_______ 43.0

_______ 9.0

_______ 38.0

______ 97.5

14.0

________ 40.5

_ ____ 210.5

______ 22.0

___ _ 206.0

____ 22.5

_______ 448.5

_______ 66.0

_______ 60.5

___ _ 40.5

IIIITllS.S

___ ..180.0

--.-170.5

__ ___ 137.5

_______ 75.0

___ __ 81.0

_ ___ 45.5 

___ _ 21.0

_______ 68.0

_______ 27.0 

_ 21.0

_____ 17.5

_____ 195.0

___ __ 21.0

IT 22^5

_ __ __ 81.5

_______ 22.5 

__ __ 42.5

~TII~105l6~

____ 74.5

_______ 4.0

75.0

_____ 28.5

_______ 60.5 

_______ 28.5 

____ 16.0

_______ 54.0 

__ 9.0

__ _ _ 43.5

_____ 6.0

___ __ 12.5

92 0

___ _ 31.5

_______ 11.0

_ _ __ 4.0

_____ 23.0

_____ 2.5

3.5

_______ 27.5

____ 32.0

_ __ _ 186.5

_______ 32.0 

_ ___ 8.5

_______ 6.5

Betula

22.5

3.5

1.0

3.5

20.0

6.5 

43.0

24.5

19.0

32.0

12.5 

3.5

50.5

2.0

1.5

2.0

45.0

16.0 

11.0

13.0

4.5

8.0

6.0

1.0

10.0

3.0 

90

11.5

22.0

0.5

18.5 

12.0 "0.5

14.0 

5.5

16.0 

2.5

14.5

1.0

3.0

4.5 

6.5

9.0 

2.5

0.5

9.5

9 0

9.0

0.5

1.0

7.0

1.0

28.5

8.0 

7.0

4.0

Fagus

1.0

13.5

2.0

36.0

88.5

9.0

49 5

6.5

23.5

27.0

17.0

5.0

0.5

9 0

10.0

1.5

72.0

13.5

14.0

9.5

74.5

0.5

0.5 

1.5

53.0

6.0

10.0 

61.0 

1.5

1.5 

8.5

1.0

2.5

1.0

2.0

4.5

5.0

0.5

1.5

14.0

1.0

24.5

8.0 

0.5

2.0

Quercus

0.5

Q n

38.0

77.0

4.0

3.5

0.5

9 0

2 0

0.5

0.5

0.5

4.5

70.0

18.5 

9 0

3.0

0.5 

5.0

3.0

2.5 

4.5

1.0

1.5

2.5

26.0 

35.5 

2.5

3.5

3.0

4.5

1.0

0.5

3.0

20.0

3.0

8.0

16.5

2.5

10.0

3.0 

1.5

12.0

Salix cinerea

0.5

20.0

6.0

7.5

24.0

q |\

4.0

4.0

12.0

11.0

91 0

1.0

8.5

0.5

3.0

2.0

2.0

6.5

3.5

1.5 

0.5

0.5

0.5

2.0

0.5

6.5 

7.5 

1.5

2.5 

6.0

1.5

9.0 

3.0

8.0

1.0

1.5

2.5

9.0

0.5

0.5

9.0

0.5

14.5

2.0 

4.0

6.0

Ilex

2.5

2.0

20.5

1.0

3.0

1.5

5.0

3.5

8.5

7.5

4.5

1.5

1.0 

1.5

2.0

iblo

~0.5

3.5

4.0 "slo

3.0

0.5

TVS

0.5

1.0

6.0

1.0

1.0

3.5

3.5

3.0

1.5

2.5

Aesculus Acer Crataegus Typha Nuphar

__-- 185.0

0.5 0.5

4.5 0 5 3.0 19.5 _ __

51.0 _ __ 

30 _ 3.5 36.0

13.0

1.5 ____ -___ 4.0

19.0 _ __

1.0 __- 17.5 

2.5

3.5

49.5 __ — __ 3.0 4.5

_ _ ____ - — _ 2.5

540 . _ 2.5 1.5

2.0 1.0

7.5

21.5

4.0

1.5

1 5 - 18.5

2.5 ____

1.5

____ 5.5 ____ 

4.0

5.0 0.5

14.5

3.5 ____

3.0

3.0 1.0

3.0

44.5 ____ _ _ ____ __-_

__ _ 0.5 0.5 _ _ — -

~~ll5 IIII IIII T6 II- -

2.0

13.5

1.0

2.0 

1.5

1.5 _--_

IT IT IIII "elo To

____ 1.5 

5.0

5.5 0.5

0.5

1.0 --_- --__

27.5 ____ ____ __-- _--- 

5.5 1.0

0.5 ..__

2.0

~T IT IIII IIII "elo

18.5

__ _ 5.0 -_--

1.0 _-__

5.0

21.5 ..__ ____ 1.0

____ ____ _ _ 5.0 — -

____ 1.5 ____ --__ ____

Salix, Alba

- —

20.5

5.5

28.5

18.5

5.0

4.0

____

To

— -

-—

2.5

____

____

____

— -
76 SORTING AND DEPOSITION OF ALLOCHTHONOUS PLANT MATERIAL, SILWOOD LAKE, ENGLAND 

DATA TABLE 4.—Silwood delta total area of whole leaves per quadrat

Stand

1

2

3 ___________

4

5

6 ___——_ —

7 ___ ____ __

8

9

10 ___________

11 ________ __

12 ___________

13 _ __--____

14 - — — —

15

16_ — ---- -

17 --__ ______

18

19

20

21 ___________ 

22 ___________

23 —— ___ ....

24 — — — -

25—— ______

26

27

28 ___________

29

30— — ———

31 _________

32

33 ___________

34

35 -____-.____ 

36— ——— —

37 -_ __ __ _

38 ___________

39 ___________

40

41

42 __________

43 __________

44 _ ___ ____

45

46 ___________

47 ___________

48

50 ___________

51

52

53 — ———— _

54 ___________

55 _ _ _ ____

56

57

58

59 _______ __

60 ________

61 __________

62

63 __________

64 ___________

65

66 __________

67

68 ———————

69 __________

70

71

72 ___________

73 __________

74

75

76

77

78 __ _______

79 __________

Alnus

_ 37.0

_ 70.0

_ 30.5

_ 134.0

_183.5 

-180.0

-415.5

134.1

.331.5

-954.0

-263.0

_634.0

_906.5

-820.0

_259.0

_ 59.5

_ 2.0 

_ 5.0

_ 9.5

_ 32.0

92 0

_ 12.0

_ 34.0

_ 96.5

_398.5

-309.5

-267.0 

- 75.0

-119.0

- 95.0

_ 46.5

_ 86.0

_ 6.5

_ 39.5

_ 64.0

_ 70.5

57.5

19 0

53.0

_ 36.0

28.5

_ .7.5

_ 2.0

_ 32.5

_ 50.0

- 44.0

_ 1.0

_ 26.5

_ 3.5

-116.0

.251.0

Betula

190.0

5.0

6.0

9.0

7.5 

3.5

54.0

16.5

134.0

46.0

28.5

111.0

59.5

7.5

0.5

1.0

6.0

3.5

20.5

15.5

91 ^

2.0

2.0

8.0

4.0

22.5

5.0

14.0 

5.5

40.0

10.0

4.0

20.0

31.0

16.5

13.0

19.5

43.5

1.0

3.5

6.0

6.0

12.5

18.5

1.5

6.0

3.0

3.5

10.0

12.5

Fagus

58.0

167.0

8.5

44.0

5.5

7.0

7.5

121.0

37.5

179 0

"14.5

63.5

39.5

OQ A

25.5

91.5

12.0

19.0

17.0

50.0

17.5

39.5

44.0

37.5

47.0

27.0

32.0

15.0

15.5

46.0

212.5

4.5

31.0

Quercus

62.0

34.0

13.5

7.0

49 0

20.0

10.0

68.0

139.0

6.0

"2.0

22.0

77.0

4.5

____

18.5

44.0

oe e

14.5

75.5

4.0

Salix cinerea Ilex Aesculus Acer Crataegus Typha Nuphar

115 53 5

6.0 __ _ 53.0 __-_ -___ -_-- ___-

12 5

8.5

28.5 

26.5 — - 2.5

15 5

97 0 65 35

43.0 — _ _— —— ____ —— . ——

125 5 25

31.5

95 90

27 5

18 5 1.0

33.5 — _ 71.0

1 5

120

15.5 ____ ____ ____ _ _ ____ 

1 5

10 0

1 0

55.5 — . ____ — - 8.0

128 0

16.0 __-_ ____ _ _- ---- —— ——

5 0

15 0 25

47 0

120

39 5

0.5 

2 5

7 5

7 5

1700

120

6 5

9 5

1 5

17 5

12 5

7 0

05

8 5

2 0

1 0

48 5

105

4 5

7 0

4 5

8 0

9 0

4 5

1 0

7 5

0 5

05

25 0 13 5

13 0

0 5

3 5

Salix, Alba

——

9.0

31.5

16.0

53.0

11.5

19.5

1.5

1.5
DATA TABLES 

DATA TABLE 5.—Silwood delta area of all leaf material per quadrat

77

Stand

1 __ ....

2 ______

3 ______

4 __ ._

5——.

6 . — — . 7 ... ....

8 — ....

9 ______

10 ......

11 ______

12 ______

13 ______

14 ______

15 ______

16 ._.....

17 ______

18 ______

19 ______

20 ———

21 _ ___

22 _____

23

24 ______

25 _---_

26 _____

27 ______

28 ______

29 ______

30 _______

31 _______

32

33 _______

34 _______

35 _ ___-.

36 _____-.

37 _______

38 _______

39— ___.

40— — .

41 _____ .

42 _______

43 ______

44 ______

45

46 _______

47 _______

48 _-_-._.

49 _______

50

51 — — .

52 _______

53- — ..

54 _______

55———.

56 ————

57 _..___.

58 _______

60_ — _._

61 _ __ _.

63

64 _______

65

66

67

68 _______

69———.

70———.

71 .......

72

73 ._ ....

74——— _.

75

76—....

77 ___ _..

78 __ _ .

79 _______

Alnus

— — 594.5-

——— 138.5

— — 60.5

-_. -226.5

— -.274.5

——— 290.5

——— 505.0

—— __343.5

———1568.0

— —985.0

——— 1035.0

_____ 302.0

--._._ 34.5

— — 111.5

_____ 46.0

— —220.0

— —405.5

——— 54.0

——— 78.0

.-__ _ 82.0

74 <_

— — 291.0

. ——— 576.0

———437.5

——212.5

— — 194.0

——176.0

—— __ 92.0

.. __ 21.0

—— _ 27.0

______ 27.5

_____ 121.0

— — 105.0

— — 106.5

... .„ 70.5

______ 70.5

—— . 93.5

— — 89.0

9ft t\

______ 16.0

______ 71.5

9 0

.. ___ 43.5

______ 8.0

_____ _ 45.0

.. _ _102.0

— — 142.0

.——— 75.5

12 0

______ 30.5

. _ __ 23.5

_____ 6.0

— — 3.5

97 t\

———437.5

——— 8.5

._. - 6.5

Betula

212.5

8.5

6.0

13.5

1.0

11.5

3.5

103.5

6.5

197.0

41.0

65.0

166.0

41.0

1 A C

110.0

21.0

0.5

2.0

6.0

0.5

22.5

5.0

17.0

6.5

24.0

15.0

5.0

25.0

48.0

16.0

1.0

26.0

3.0

42.0

0.5

49.5

28.5

9.0

27.0

5.5

35.5

46.0

15.5

1.0

6.5

4.5

1 9 c

27.0

8.5

0.5

27.5

9 0

0.5

1.5

7.0

38.5

7.0

16.5

Fagus

216.5

1.0

180.5

10.5

80.0

24.5

9.0

7.0

1.5

7.5

11.5

170.5

76.5

91.5

23.5

40.0

0.5

0.5

9.0

93.0

12.0

13.5

19.0

26.5

124.5

0.5

0.5

19.0

39.5

97.0

57.0

65.5

28.5

33.5

8.5

16.0

2.5

2.0

5.0

0.5

17.0

14.0

oq7 f\

1 9 c

0.5

33.0

Quercus

3.0

1.0

84 0

AC) C

CO A

9C C

71 <_

171 5

11.5

9 0

10.5

119.5

16.5

51.0

30.0

0.5

1.0

0.5

86.0

OA C

48 5

3.0

0.5

3.0

1.0

75.5

1.5

2.5

1.0

3.0

19.5

0.5

16.5

9 5.

33 0

64 5

1.5

12.0

Salix cinerea

11.5

6.5

48.5

49.5

OAq c

91 t\

148.0

Q1 ft

38.0

166.5

99 5.

12.5

27 5

1 9 c

149.0

6.0

91 Q ft

17.0

15.0

82.0

7.0

10.5

7 t\

0.5

268.5

14.0

Q c

6.0

23.5

16.0

8.5

9.0

2.5

0 A

0.5

7 5

6.0

9.5

18.0

7.5

2 5

10.0

9.5

0.5

0.5

9.5

25.5

47.0

15.0

4.5

95

Ilex

0.0

2.5

20.5

4 5

3 0

10.5

1.5

1.5

q n

ft ft

q c

qq A

q A

7 t\

q c

3 5

8.5

7 ^

1.5

1.0

1.5

0.5

q c

4.0

3 0

q A

0.5

0.5

1.0

1.0

1.0

3 5

q c

q A

1.5

2 5

Aesculus

CO A

15.0

5.0

3 0

120.5

54 0

7 5

1.5

44.5

27.5

13.5

fil A

Acer Crataegus Typha

0.5 0.5

25

3.0

2.5

1.5

.... 1.5

5.5

5.0

3.0

3.0

14.5

1.0

2.0

1.5

1.5

6.0

1.5

—— 5.0

—— .... 5.5

0.5

—— 2.0

18.5

5.0

.... —— 1.0

5.0

____ .... 5.0

Nuphar

185.0

1.0

4.5

2.5

1.5

7.5

21.5

1.0

1.0

0.5

1.0

6.0

Salix, Alba

9 0

CO A

28.5

23.5

1.5

2.5

1.5
GPO 789-Q36/5C