TREE RINGS AS INDICATORS

OF HYDROLOGIC CHANGE

IN THE GREAT DISAAAL SWAMP,

VIRGINIA AND NORTH CAROLINA

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations 78 136
50272-IQI________________

REPORT DOCUMENTATION 

PAGE

1. REPORT NO.

4. Title and Subtitle

 HEE RINGS AS INDICATORS OF HYDROLOGIC CHANGE IN THE 

GREAT DISMAL SWAMP,VIRGINIA AND NORTH CAROLINA

7. Author(s)

R.L.Phipps, D.L. lerley and C.P. Baker

9. Performing Organization Name and Address

U.S. Geological Survey, Water Resources Division 

National Center, Mail Stop #461 

12201 Sunrise Valley Drive 

Reston, Virginia 22092

12. Sponsoring Organization Name and Address

U.S. Geological Survey, Water Resources Division

National Center, Mail Stop #461

12201 Sunrise Valley Drive

Reston, Virginia-22092_________________

3. Recipient's Accession No

5. Report Date

November 1978

8. Performing Organization Rept. No.

LISGS/WRI-78-136

10. Project/Task/Work Unit No.

11. Contract(C) or Grant(G) No.

(C)

(G)

13. Type of Report & Period Covered

Interim

14.

15. Supplementary Notes

i6. Abstract (Limit: 200 words) Analysis of tree rings of largej Can0 py loblolly pines (Pinus taeda L.) 

growing near a drainage ditch in the Great Dismal Swamp have indicated that the tree rinc 

are datable and hydrologically (climatically) sensitive. Climatic and prior growth 

factors in regression explained 87 and 71 percent of the variance of the preditching and 

postditching earlywood widths, respectively, and 82 and 70 percent of the latewood widths 

for the same time periods. Early summer precipitation was significantly, and positively 

correlated with preditching latewood growth. When pre- and post-ditching records were 

merged into a single record, regression analysis explained less growth variation than whe 

the two time periods were considered individually, implying a change in growth response 

following ditching. Prior to ditching, growth was most limited by dry summers which 

followed dry summers. After ditching, growth was less strongly linked with precipitation 

and more strongly linked with temperature. Regression results are compatible with the 

contention that growing season water levels in the proximity of the collection site have 

been lower since ditching.

17. Document Analysis a. Descriptors

Dendrochronolgy, swamps, loblolly pine trees, ditches, alteration of flow

b. Identifiers/Open-Ended Terms

Dendrocl imatologic calibration

c. COSATI Field/Group

18. Availability Statement

Release unlimited

19. Security Class (This Report)

Unclassified

20. Security Class (This Page)

Unclassified

21. No. of Pages

-JO

22. Price

(See ANSI-Z39.18) See Instructions on Reverse OPTIONAL FORM 272 (4-7 

(Formerly NTIS-35)
TREE RINGS AS INDICATORS 

OF HYDROLOGIC CHANGE 

IN THE GREAT DISMAL SWAMP, 

VIRGINIA AND NORTH CAROLINA

by R.L. PHIPPS, D.L IRELEY and C.P. BAKER

U.S. GEOLOGICAL SURVEY 

Water-Resources Investigations 78 136 \ 

1979
UNITED STATES DEPARTMENT OF THE INTERIOR

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY

H. William Menard, Director

For additional information write to:

Chief Hydrologist 

U.S. Geolosical Survey, WRD 

461 National Center 

Reston, Virsinia 22092
CONTENTS

Page

Abstract ................................................. 1

Introduction ............................................. 1

The Study Area ........................................... 6

Tree-Ring Data ........................................... 7

Climatic Data ............................................ 8

General Observations ..................................... 8

Regression Analysis ...................................... 11

Summary and Conclusions .................................. 23

References ............................................... 25

111
ILLUSTRATIONS

Page

Figure 1. Sketch map showing location of study area in

Great Dismal Swamp................................ 3

2. Graph of filtered ring width data................. 12

3- Graph of filtered temperature and precipitation

data for Elizabeth City, North Carolina........... 13

4. Graph of stepwise multiple regression results

between tree rings and climate.................... 17

5- Diagram showing changes in growth response to

rainfall after ditching........................... 19

IV
TABLES

Page

Table 1. Statistical characteristics of ring width,

earlywood and latewood chronologies............... 9

2. Basal area growth of 11 loblolly pines for

20-year increments before and after ditching...... 10

3- Percent variance accounted for by regression

between tree rings and climate.................... 15

4. Conditions resulting in narrow rings in

Pinus taeda....................................... 21

5. Years of small tree rings between 183G-1950....... 22

v

TREE RINGS AS INDICATORS OF HYDROLOGIC 

CHANGE IN THE GREAT DISMAL SWAMP, VIRGINIA AND NORTH CAROLINA

By R. L. Phipps, D. L. lerley, and C. P. Baker

ABSTRACT

Analyses of tree rings of large, canopy loblolly pines (Pinus 

taeda L.,) growing near a drainage ditch in the Great Dismal Swamp have 

indicated that the tree rings are datable and hydrologically (climatically) sensitive. Climatic and prior growth factors in regression 

explained 87 and 71 percent of the variance of the preditching and 

postditching earlywood widths, respectively, and 82 and 7O percent of 

the latewood widths for the same time periods. Early summer precipitation was significantly, and positively, correlated with preditching 

latewood growth. When pre and postditching records were merged into 

a single record, regression analysis explained less growth variation 

than when the two time periods were considered individually, implying 

a change in growth response following ditching. Prior to ditching, 

growth was most limited by dry summers which followed dry summers. 

After ditching, growth was less strongly linked with precipitation 

and more strongly linked with temperature. Regression results are 

compatible with the contention that growing season water levels in 

the proximity of the collection site have been lower since ditching.

INTRODUCTION

The Great Dismal Swamp is a large, forested wetland, straddling 

the Virginia-North Carolina border on the coastal plain of the eastern 

United States. Agricultural development of this region, during the 

past 3OO years, has gradually nibbled away at the swamp, particularly 

along its eastern and southern borders, and reduced it to an area of 

about 850 km . The original extent of the swamp is not precisely known, 

but it seems likely that it may have once occupied an area more than 

twice its present size.

The largest contiguous part of the Great Dismal swamp is roughly 

2O by 5O km in size with its long axis running north and south. On 

its west edge, it is bordered by a Pleistocene beach ridge, the Suffolk 

Scarp (Oaks and Coch, 1973). Although appearing topographically flat, 

the surface of the swamp slopes gently eastward from the base of the 

scarp, with an average drop in elevation of about 20 cm/km. On its 

north side, the swamp is bordered by U.S. Highway 58, between Suffolk 

and Portsmouth, Virginia; it is bordered on the east by the Dismal
Swamp Canal, and on the south by U.S. Highway 158, in North Carolina 

(Fig. l). At present, much of this area is in public ownership, under 

the administration of the Fish and Wildlife Service, U.S. Department 

of the Interior, and the State of North Carolina.

Recent work by Oaks and Coch (1973) and by Whitehead (1972) indicates 

that the Great Dismal Swamp developed on a gently sloping plain of low 

relief. Radiocarbon dating methods suggest that the accumulation of the 

organic muck deposits (referred to colloquially as peat) that underlie 

most of the swamp began approximately 90OO years ago (Whitehead, 1972). 

Prior to the formation of peat, however, the area was apparently 

drained by a stream of well-developed dendritic pattern. Accumulations 

of organic detritus obstructed stream channels and caused the ponding of 

surface water; well-defined channels eventually disappeared, and the 

drainage pattern was gradually obscured. The development of peat was 

self-perpetuating; the accumulation of organic debris created an ever 

enlarging swampy environment, in which the decomposition of the organic 

material was extremely slow. The accumulation of peat was often considerable, with thicknesses exceeding 12 feet (Oaks and Coch, 1973)-

The Great Dismal Swamp is maintained by water from several sources. 

Oaks and Whitehead (1978) state that precipitation alone is adequate to 

maintain the swamp, with annual rainfall averaging 100 to 125 cm in the 

Norfolk area. Our observations from a series of shallow wells have 

indicated that water levels in the peat change rapidly and fluctuate 

greatly in response to precipitation. In addition to direct precipitation, the swamp receives inflow from several drainage basins west of the 

Suffolk Scarp, as well as from the scarp itself. The combined discharge 

from these areas is considerable, as they provide a total catchment area 

of about 300 km2 . The major part of this discharge is in the form of 

surface flow, which, with rainfall, is an obvious source of water to 

the Great Dismal Swamp. In addition, some groundwater probably discharges from the shallow aquifer that underlies the swamp and crops out 

on the escarpment. Lichtler and Walker (l97^t) have suggested that the 

artesian flow from this aquifer may be a significant source of water; 

however, its volume has not been determined. During late winter and 

early spring, we have also observed seepage of ground water into the 

swamp from many places along the base of the scarp.

Prior to the formation of the Dismal Swamp peat, the above-mentioned 

drainage basins appear to have been western extensions of the former 

dendritic drainage pattern (Oaks and Coch, 1973)- As has already been 

pointed out, the accumulation of peat eventually obscured that part of 

the drainage pattern east of the scarp. Despite the absence of definite 

channels, however, surface flow from the streams west of the scarp 

apparently continued to move eastward across the swamp in broad, anastomosing drainageways that approximated the location of the ancient, 

pre-peat channels. Shaler (1890, p. 317) noted, "There may thus be 

within the limits of the Dismal Swamp a number of stream valleys which
__VIRGJNIA__ 

NORTH CAROLINA

Figure 1. Location of study area in Great Dismal Swamp.

Ditch-roads accompany all ditches shown except the Feeder Ditch, 

Approximate location of nearby towns indicated by circles; 

Norfolk and Virginia Beach are along seacoast.
have been so encumbered by the accumulation of vegetable matter that they 

are no longer evident to the eye."

Timber records, historical accounts, and the presence of stumps 

indicate that the Great Dismal Swamp was at one time partly dominated 

by forests of Atlantic white cedar (Chamaecyparis thyoides (L) B.S.P.), 

often occurring in almost pure stands. Tracts of bald cypress 

(Taxodium distichum (L.) Richard), black gum (Nyssa sylvatica Marshall), 

and water tupelo (Nyssa aquatica L.) were dominant in the wetter parts 

of the swamp, particularly along natural drainageways (Kearney, 1901; 

Akerman, 1923; Reid, 1952; Dean, 1969). The combined effects of lumbering, burning, and drainage, however, have produced striking changes in 

the composition of the vegetation of the swamp. Shaler (18*90, p. 337) 

wrote, "Nearly all of the original forest has been removed, culled by 

the woodmen, or destroyed by fire." He went on to say that the yearly 

occurrence of forest fires in the dry seasons eradicated not only the 

growing trees, but also their seeds, as the fires burned to considerable 

depths in the peat underlying the swamp. Kearney (1901) stated that 

cypress and white cedar had formerly been much more abundant in the 

swamp, but because of the high value of these species, they were rapidly 

logged out. He further stated (p. 423) that "extensive areas formerly 

covered with juniper (cedar) forest are now almost entirely destitute of 

trees, and are occupied by woody undergrowth, or even largely by herbaceous plants. Such lands are particularly subject to fires, which 

effectually prevent the renewal of the forest." He also noted that red 

maple (Acer rubrum L.) seemed to be increasing in the gum-cypress areas 

more rapidly than any other species. Indeed, red maple seems to have 

become the dominant tree species in much of the Dismal Swamp today.

Ditching and canal-building have caused changes in water levels in 

the Great Dismal Swamp. Long-time residents of the region State that 

the swamp is "much drier" than it was 2O or 30 years ago. The first 

major ditch was constructed circa 1770 by the Dismal Swamp Land Company 

(Brown, 1970). This ditch, which was later named for George Washington, 

a founder and major stockholder of the company, ran eastward from the 

base of the Suffolk Scarp, near the mouth of the Cypress Swamp, to Lake 

Drummond (Fig. l). The lake, located approximately in the center of 

the swamp, is a nearly circular body of water about 4 km in diameter, 

with a maximum depth of about 2 m. In 1810, Jericho Ditch was constructed from the vicinity of the town of Suffolk, at the northwest 

corner of the swamp, to Lake Drummond. Both of these ditches were 

constructed primarily as transportation arteries to provide access to 

the rich timber resources in the interior of the swamp.

The Dismal Swamp Canal has undoubtedly had the greatest impact on 

the hydrologic system of the Great Dismal Swamp. Constructed between 

1793 and 1805, it roughly bisected the swamp from north to south. The 

excavated material was piled along both sides of the canal, and formed 

an effective barrier to the eastward flow of surface water. Shaler 

(1890 p. 318) noted that "the western dike of the canal serves to retain
the waters in all that region (that is, the area west of the canal), 

and at the same time to fend these waters from the region east of the 

excavation." The effect of the canal on water levels resulted in 

much litigation between land owners and the Dismal Swamp Canal Company 

(Brown, 1970); land to the east of the canal gradually dried up, while 

there was an increase in wetness and flooding on the west side.

It had been assumed that ground water seepage would be sufficient 

to maintain the water level in the canal, but this turned out not to be 

true. Eventually, it became necessary to tap Lake Drummond as a water 

supply, and in 1812 the Feeder Ditch was constructed between the lake 

and the Dismal Swamp Canal. Prior to 1900, the canal consisted of several levels, separated by locks. The highest, or Summit Level, was 

connected to Lake Drummond via the Feeder Ditch. During reconstruction, 

from 1896 to 1899» the canal was greatly deepened and widened, and all 

locks were eliminated except one at either end. The canal bottom at 

the Summit Level was lowered by 17-5 feet (5-3 ni) at this time, and the 

Feeder Ditch was likewise deepened between the canal and Lake Drummond, 

with a new lock being installed at the Lake Drummond end.

One of the effects of these early ditches and canals was to channel 

much of the surface water from the swamp. Akerman (1923) noted that, 

prior to the construction of Washington Ditch, water entering the swamp 

from the west (that is, from Cypress and Pocosin swamps) tended to 

spread out and move slowly around to the north of Lake Drummond. With 

the construction of Washington Ditch, and the subsequent construction 

of Jericho Ditch and the Feeder Ditch, much of this surface flow was 

shunted into the Dismal Swamp Canal, via Lake Drummond.

Between 1900 and 1950, few additional ditches were constructed. 

Many of the older ditches, including Washington Ditch, had been abandoned, and were gradually being overtaken by vegetation and filling in 

with debris. Between 1951 and I960, however, approximately 95 kro of 

ditching was done in the northern half of the swamp (Union Camp, 197l)- 

New ditch construction was accompanied by reconstruction of Washington 

and Jericho ditches. The earlier ditches had been relatively narrow 

and shallow, and were bordered by low spoil piles (Akerman, 1923; 

Mason, 1952). Because of the low capacity of these ditches, and the 

ineffectiveness of the spoil piles as barriers to surface flow, considerable lateral movement of water occurred across them during periods 

of high water. The new ditches, however, constructed with modern 

machinery, are much deeper and wider than the earlier ones and are 

bordered by large spoil piles that have been graded into roads. The 

deep, wide ditches have permitted surface water to be drained rapidly 

from the swamp, whereas the parallel roads have been very effective as 

barriers to the flow of surface water across the ditches. Although the 

deep ditches may also intersect underlying aquifers, thus permitting a 

loss of groundwater to the ditch system (Lichtler and Walker, 197^)» 

the most significant effect of the ditches appears to be on the surface 

water.
At the present time, the entire flow from Cypress and Pocosin 

swamps (fig. l) is intercepted by the ditch system and channeled into 

Lake Drummond. Consequently, the broad sheet of water that once 

flowed eastward across the swamp, north of the lake, no longer exists. 

Nonetheless, the former drainageway is still readily detectable on 

color infrared aerial photographs and on satellite images, on which 

it appears as a band of wet, deciduous forest (Carter and others, 

1977). The presence of much bald cypress and water tupelo in the area 

is further evidence that surface water, often quite deep, was at one 

time present most of the year.

One of the keys to unlocking the mysteries of the Great Dismal 

Swamp, particularly with respect to its hydrologic history, may be 

the use of tree ring data. Trees contain annual records of growth 

responses to their environment, and thus are of potential value for 

the examination of past hydrologic or climatic change. This report 

presents results of an initial study to determine tree-ring sensitivity to swamp climatic and hydrologic conditions, thereby determining 

the feasibility of using tree rings to examine changes in past hydrologic conditions. If the tree rings were found to be climatically 

sensitive, then a subsequent objective was to determine if growth 

responses to climate changed after ditching.

THE STUDY AREA

The site chosen for the preliminary study is on the east side of 

Lynn Ditch, O.5 km north of Middle Ditch (fig. l), and north of the 

former drainageway described above. From early autumn until late 

spring water is rarely more than 3O cm deep and occurs in isolated 

pools rather than in a continuous sheet. Quite possibly, the average 

depth of water in this area was much greater prior to the construction 

of the extensive ditch network (including Lynn and Middle Ditches) 

during the 195O's, and there may have been an eastward flow of surface 

water. At present, any tendency toward surface flow is effectively 

blocked by ditch roads adjacent to the ditches. Consequently, surface 

water in the study area is derived mostly from rainfall and, perhaps, 

from the upward movement of ground water from the underlying aquifer. 

It was noted in February 1977? that Lynn Ditch was entirely frozen 

over except for a short reach of about 5O m adjacent to the study area. 

A possible explanation of the unfrozen reach is that groundwater, which 

was warmer than 0*C, was being discharged through the ditch bottom.

A stand of loblolly pine extends about 25O m along Lynn Ditch 

and about 5OO m in a generally northeast direction from the ditch. 

The stand is composed of large pine trees with full crowns and is 

interspersed with red maple and black gum at all vegetative levels. 

Tree-ring dating indicates that the stand became established a few 

years prior to 1830.
The organic layer at the study site is 30 to 50 cm thick. The 

charcoal at its base may be evidence of a former peat layer destroyed 

by fire during or following timbering. According to Reid (1952), 

this section of the swamp was first timbered sometime between 1830 

and 1870. Conditions after a fire may have been similar to those in 

a recently timbered and burned section of the swamp. In the latter 

area, 30 to 50 cm of the underlying peat was destroyed, leaving 

numerous tree stumps perched on their roots, high above the burned 

surface. In the study area, pine seedlings may have become established 

on stumps and roots remaining after the fire and thus were protected 

from inundation during the wet season. The pines remaining seem to 

be perched on mounds 40 to 60 cm above the adjacent ground surface. 

Periodic observations of water levels suggest that even if preditch 

water levels were no higher than at present, water was probably high 

enough during parts of the year to kill any young pine seedlings 

that had become established on the ground surface. However, seedlings 

germinating on stumps and roots would be less vulnerable to inundation 

and thus have a greater chance of survival.

TREE-RING DATA

Preliminary examination of increment cores from loblolly pines 

at the study site indicated that some trees are characterized by extremely narrow or missing rings for the period corresponding to the 

early 1950's. For these trees it is seldom possible to accurately 

date the narrow rings, there being essentially no year-to-year ring 

width variation. To obtain the final collection, nearly all pines of 

good form were sampled. If field examination of a core indicated 

extremely narrow rings for the early 1950's, the sample was discarded 

and the tree tagged accordingly. If the first was acceptable, a 

second core from the opposite side (generally north or south side) 

was taken, and the tree was tagged with a collection number. Cores 

were saved from only about one out of every six trees. The collection 

is kept on file and is available for inspection at the Tree-Ring 

Laboratory, U. S. Geological Survey, Reston, Virginia.

The cores were air-dried in paper soda straws, surfaced, crossdated, and their ring widths measured to the nearest 0.01 mm. In 

pines earlywood is composed of large, light-colored, thin-walled cells 

added as new tissue early in the growth season. Latewood is composed 

of smaller, dark-colored, thick-walled cells added as new tissue later 

in the growth season. The transition from earlywood to latewood is 

gradual in some pines and more abrupt in others. The earlywood and 

latewood of the study area samples are quite distinct, permitting 

separate earlywood and latewood measurements of each ring. The measurements of each core were standardized, or converted to indices, by 

fitting a straight line or simple curve to the data and then dividing 

ring widths by corresponding curve values. The computer program for 

this procedure, which was adapted from a program developed by Fritts 

and others (1969), not only removes general trends from the data but
also results in a more-or-less homogeneous variance with time (Fritts, 

1976). The indices for all samples were combined to form three chronologies; one each for earlywood, latewood, and total ring width.

Variance characteristics of the chronologies are included in 

Table 1. The common variance of the chronologies ranged from 4l-4? 

percent, suggesting that local climate is only moderately limiting 

to tree growth. By comparison, trees on semiarid sites in the southwest can retain as much as 80 percent common variance in the total 

chronology (Fritts, 1976). The low variance between trees and the 

greater variance between cores within each tree suggest that crowding 

and competition of crowns and root systems result in significant 

differences between different sides of the same tree, but that, as 

a whole, the study area is uniform. These results seem reasonable, 

as the study site composes a closed-canopy forest in a homogeneous 

swamp habitat.

CLIMATIC DATA

Data of daily temperature extremes and precipitation from Elizabeth City, North Carolina, were utilized for the period 1917-1975 

(U.S. Weather Bureau). Elizabeth City is southeast of that part of 

the swamp presently under public ownership, and is about 40 km from 

the study site (fig. l). Data from five other stations were available, 

but none except that from Norfolk, Virginia, were as old as 50 years. 

Norfolk, at the mouth of the Chesapeake Bay, seemed influenced by sea 

temperatures and breezes. Elizabeth City is about the same distance 

from the coast as the Great Dismal Swamp, and so was felt to be more 

representative of swamp climate than Norfolk.

GENERAL OBSERVATIONS

One objective of the study was to determine if ditching in the 

1950's might have altered water patterns enough to be reflected in 

tree growth. A comparison of growth-rate for 20-year periods before 

and after 1950 indicated no apparent change after ditching (Table 2). 

Six trees showed basal area increases greater than 10 percent after 

ditching, two showed essentially no change, and three showed decreases 

of more than 10 percent. The average change of the group was 11 +_ 29* 

indicating that for at least the 20 years after ditching, hydrologic 

change was not great enough to outweigh other environmental factors 

and to effect a distinct growth response.

As a means of further exploring pre and postditching growth responses, variations in the ring-width pattern after standardization 

were examined. It was felt that similarities in growth pattern between 

trees would indicate common response to envirpnmental factors, whereas 

dissimilarities would indicate specific responses of individual
Table 1. Statistical characteristics of ring width, earlywood, and latewood 

two cores each from eleven trees.

Characteristics

Chronology length

Mean ring width, mm

Standard deviation of indices

Mean sensitivity

First order serial correlation

Analysis of variance

Interval analyzed 

Mean ring width, mm 

Average st. dev. of core indices 

Average mean sensitivity of cores 

Average first order serial correlation 

Variance component (percent)

Common to chronology 

Between trees 

Within trees

Average cross-correlation (1917-1975)

Between all cores 

Within trees 

Between trees

Ring Width

1836-1975

1-75

0.32

0.37

0.4l

1917-1975 

1.63 

0.47 

0.40

47 

19 

34

0.50 

0.59 

0.49

Earlywo

1830-19

0.96

0.36

0.33

0.35

1917-19

0.86 

0.46 

0.34 

0.52

44

19 

40

0.43 

0.63 

0.42
Table 2. Basal area growth of 11 loblolly pines for 20-year increments 

before and after ditching. Based on raw ring width measurements of two increment cores per tree.

2 Basal Area (cm )

Diameter at

breast height

(cm) 1931-195O 1951-197O Percent change

38

57

57

65

53

49

66

62

69

84

65

231

239

256

27O

289

319

339

396

4 10

4 13

647

319

265

377

300

2O9

254

539

498

404

4 13

489

38

11

47

11

-28

-20

59

26

-1

0

-24

Avg 6O 346 369 11

10
trees to such factors as crowding and shading. To determine general 

(low frequency) variation in ring width patterns, the data were 

averaged by use of a low-pass digital filter. Year-to-year variations in tree-ring indices may be thought of as high-frequency 

variation whereas the general growth trends express very low frequency 

variation. The filter was designed to pass frequencies where the 

cycles were approximately 8 years or greater (Stockton & Fritts, 197l) 

Examination of the resultant curves for earlywood and latewood of each 

of the 22 core samples (2 cores each from 11 sample trees) revealed a 

great similarity in pattern between trees.

Plots of the filtered data of the earlywood and latewood chronologies are shown in figure 2. Included in the figure are bars with 

hatch marks indicating the average and standard deviation of each 

fifth year of the filtered data of all 22 cores from 192O to present. 

Though the standard deviations indicate considerable variations 

between cores, a general pattern is evident. A growth peak in the 

late 192O's is followed by a steady decline to about 195O, a rapid 

increase for 8 to 1O years, and then, decline to 197O.

It seemed evident from the growth-trend variations (filtered data) 

that the trees were responding to something common to all of them. 

Therefore, climatic data were subjected to the same filtering techniques 

to determine if any climatic variations matched growth-trend variations. 

Some definite trends are evident in the climatic data (figure 3) but 

correspondence with growth variation for the length of record is not 

obvious. However, when considering only parts of the time period, 

during 192O-7O, some correspondence between May to mid-June precipitation and growth may be noted. A pronounced precipitation peak in the 

late 192O's matches with a growth peak, and a smaller precipitation 

peak around 195O matches with a period of reduced growth. However, a 

third rainfall peak in the 196O's seems to correspond neither to 

increased nor decreased growth, meaning that no consistent relationships 

between trends in climate and trends in growth is apparent.

In summary, 20-year growth rates do not distinguish differences 

before and after ditching. Further, a distinct pattern of growth-trend 

variation that does not correspond to any climatic trends tested suggests a nonclimatic influence. This is not to say that year-to-year 

variations in tree growth do not respond to, or are limited by, climate, 

but simply that growth trends during 192O-7O seem independent of 

climatic trends.

REGRESSION ANALYSIS

To further explore the possibility that tree growth might have 

been influenced by ditching, additional tests were made by regression 

analysis of yearly growth indices with seasonal climate. These tests 

are described below. Stepwise multiple regressions were run, using

11
2-

o2n

0 J

EARLYWOOD o° °0ooooo,

o °o0oooo0ooo

"°0o3°00o<*>0- °oo°

LATEWOOD

0°°°0

'oov

0°00 

Q 0OOOOQOOn

o o

°OoO°

1830 1840 1850 1860 1870 1880 1890 1900

2n

0- 

2n

EARLYWOOD Ilo°-o o ( 0°t

-1

1900 1910 1920 1930 1940 1950 1960 1970

o°o LATEWOOD . 00 I 1O

o o ^ooofio 

o o

Figure 2. General trends in ring-width, 1835-1970.

Trends were determined by averaging indices with a low-pass 

digital filter. Average and standard deviation for each 

fifth year for all 22 samples is indicated for the analysis 

period after 1917.

12
MAY TO MID-JUNE SEASON

30 -1 Maximum 

25-1 ***

10-1

Minimum 

,» 

5

'........ ......'

1920 1930 1940 1950 1960 1970

|B

S

UJ 

UJ

&S E&

35 H 

30- 

25- 

20- 

15-

MID-JUNE THROUGH JULY SEASON 

Maximum

Minimum 

 " ....... 

1920 1930 1940 1950 1960 1970

Figure 3« Filtered temperature and precipitation data for Elizabeth 

City, North Carolina.

Only late spring (May to mid-June) and early summer (midJune through July) are shown. Precipitation is expressed 

as centimeters and temperature as degrees Celcius.

13
climatic data as independent variables and tree-ring indices as dependent variables. If the time series analyzed contains many years, then 

it might be justifiable to examine many independent (environmental) 

variables. However, since the time periods of interest in this study 

composed relatively few years, and since a degree ef freedom is lost 

for each variable entered into regression, it was felt desirable to 

limit the number of variables available for entry.

It was expected that, as is common in analyses between tree rings 

and climate elsewhere (Fritts, 1976), growth of the swamp loblolly 

would show responses to temperature and precipitation during the year 

prior to growth as well as to conditions concurrent with growth. Preliminary analyses suggested that some times during the year factors are 

not as influential as others, and so could be lumped together as a 

means of limiting the number of potential variables. Various combinations of months or seasons were tried. For final analysis average 

maximum temperature, average minimum temperature, and total precipitation were each used as variables for each of the following seasons:

May to mid-June prior to growth (l.5 months), assumed to be

period of earlywood growth of prior year, 

mid-June through July prior to growth (1.5 months), assumed to be

period of majority of latewood growth of prior year, 

August through October prior to growth (3 months), thought to be

period of leaf bud development and food accumulation, 

November through April prior to growth (6 months), thought to be

period of relative dormancy and soil-water replenishment. 

May to mid-June concurrent with growth (1.5 months), period of

earlywood growth, and 

mid-June through July concurrent with growth (l-5 months), period

of latewood growth.

In addition to the eighteen climatic variables thus calculated, one or 

two variables representing prior growth were also included to help 

account for serial correlation of the tree-ring data. Stepwise multiple 

regression was performed on indices of each core sample from every tree, 

as well as on chronologies of earlywood, latewood, and total ring width. 

As results of analyses with chronologies seemed reasonably representative 

of the individual samples as a whole, only results with chronologies are 

reported.

Table 3 summarizes percent variance accounted for by regression 

between chronologies and climate (including prior growth). When the 

entire length of record (191? - 1975) was considered, climate accounted 

for only 54 percent of the total ring-width variance. However, when 

the length of record was subdivided into pre and postditching segments, 

climate accounted for 72 percent and 67 percent, respectively, of the 

variance. When earlywood and latewood growth data were analyzed separately, climate accounted for 87 percent of earlywood growth and 82

14
Table 3. Percent variance (R x 1OO) accounted for by regression

between chronologies and variables for climate plus prior 

growth.

Chronology

Earlywood Latewood Ring Width

1917-1950

(preditching)

1951-1975

(postditching)

1947-1975

87

71

69

82

70

74

72

67

54

15
percent of latewood growth during the preditching era; after ditching, 

climate accounted for only 71 percent of earlywood growth and 7O percent of latewood growth. Had relationships between climate and growth 

been constant with time, splitting the time period of the analysis into 

two shorter periods would have decreased the variance explained by regression because of the loss of degrees of freedom. However, the fact 

that a much greater part of variance was explained when pre and postditching periods were analyzed separately strongly suggests that . 

climatic effects on growth were different before and after ditching.

Figure 4 contains graphic representation of results of stepwise 

multiple regression of earlywood and latewood chronologies for pre and 

postditching periods. Climatic factors considered were average daily 

maximum and minimum temperatures per season and total precipitation per 

season, where the seasons were determined as described above for Combination III. Variables for prior earlywood and latewood growth were 

also included to account for the serial correlation of growth. There 

was one variable for each of the six seasons for maximum and minimum 

temperature and precipitation, and two prior-growth variables, giving 

a total of 2O variables available for entry into regression. The bar 

graphs represent all variables entered into regression (fig. 4).

Preditching earlywood growth responded most significantly to previous August to October maximum temperatures, with significant negative 

correlations indicated for previous May through July precipitation and 

previous May to mid-June minimum temperature. Current May to mid-June 

precipitation was also picked by the stepwise process but did not seem 

highly significant. Thus, earlywood growth seemed to be influenced more 

by conditions of the previous season, especially by the previous autumn 

temperature, when growth had nearly stopped, but temperatures were 

still warm enough for photosynthetic food production and storage.

Preditching latewood growth was positively correlated with precipitation for mid-June through July, the time, it is speculated, when 

most latewood growth occurs. Excessive rainfall in swampy areas 

inundates the root systems, thereby reducing gas exchange between roots 

and "soil" and resulting in "physiological drought" and reduced growth. 

Evidently, by early summer (mid-June through July) the water levels in 

the peat are typically low enough that rains do not unduly inundate 

the root systems. Furthermore, rainfall may raise the water to a level 

closer to the shallow root systems, thereby making it more readily 

available to the trees. On the other hand, rainfall in a wet environment may be less important to growth than the effects of cloud cover 

and high humidity. During wet, rainy summer months, high humidity and 

reduced heat load on the leaves reduce transpiration stress, resulting 

in increased growth.

16
EARLYWOOD CHRONOLOGY 1917-1950 (R*=0.8690) EARLYWOOD CHRONOLOGY 1951-1975 (R2=0.7072)

MONTHS

mm mti

PRECIPrrATION

MONTHS MONTHS

0.4-

0.2-

0.0-

-0.2-

TEMWuV

Hiii&iH imti IU?I MINIMUM 

TEMPERATURE

EJ 

MONTHS TOIKI

PRECIPITATION

0.4-, 

0.2-

< -02- 

z -0.4 -

MONTHS MONTHS

M-Kdfl Mill Mltlftffi-l 1^ JlJlOlAl iJUl iJlJlOlAI I,

MAXIMUM MMMUM PRIOR

TEMPERATURE TEMPERATURE GROWTH

ILI EI

Iwlwl

LATEWOOD CHRONOLOGY 1917-1950 (R2=0.8175)

0.8- 

0.6- 

0.4- 

0.2- 

0.0-

-0.2-

-0.4- PRECIPATION 

-U-IA-IN-I

PRIOR

jij 

CURRENT

MAXIMUM 

TEMPERATURE

IjljloiAl IJIJI 

PRIOR CURRENT

PRIOR 

GROWTH

MUIA-IN-I MU| lEILl 

IjljlolAl fjljl Iwlwi 

PRIOR CURRENT

0.8-1 

0.6-

* 0.4-

i

S 0.2-

< 0.0-

z -0.2-

-0.4 -I

LATEWOOD CHRONOLOGY 1951-1975 (R*=0.7015)

PRECIPrTATION HUP*olAl 

PRIOR

Ijljl 

CURRENT

MAXIMUM 

TEMPERATURE

M-|J-|A-|> 

IjljIolA

PRIOR

1JU 

CURRENT

MINIMUM PRIOR 

TEMPERATURE GROWTH

RHlttftl I11H IwU

PRIOR CURRENT

Figure 4. Stepwise multiple regression results from analysis of ring 

widths of Pinus taeda for time periods before ditching 

(1917-1950) and after ditching (1951-1975) in the Great 

Dismal Swamp, Virginia. Variables available for entry into 

stepwise regression included total seasonal precipitation, 

average daily maximum and average daily minimum temperature 

per season for seasons from late spring prior to the season 

of growth through early summer concurrent with growth. See 

text for definition of seasons. The bars represent standardized regression coefficients for all variables selected by 

the stepwise process (selection required f_ ratios greater 

than l). Number of dots indicates significance level: 

 >0.95, ..^0.99, ..-2-0.999.

17
Of further interest is the fact that spring minimum temperatures 

are positively correlated with growth, and summer minimums are negatively correlated with growth (fig. 4). This suggests that spring 

minimum temperatures are normally low enough that any increase would 

aid in food manufacture, whereas summer minimum temperatures are 

normally high enough that any increase in them would increase respiration and transpiration sufficiently to reduce the net gains of produced 

foods.

Results of postditching regressions (fig. 4) indicate that 

increased summer rainfall causes some increase in latewood growth 

and significantly reduces earlywood growth of the following year. 

Compared with preditching responses, the effect is smaller on current 

latewood growth and greater on the following year's earlywood. Following the above reasoning, it would seem that after ditch construction, 

the trees experienced less water stress during typical summers; in 

other words, growth increased during rainy summers before and after 

ditching, but the increase was less significant after ditching.

Of particular interest was the fact that postditching earlywood 

growth was negatively correlated with both maximum and minimum summer 

temperatures (mid-June through July) even though most earlywood growth 

typically occurs during the May to mid-June period. By way of explanation, it may be that increased summer temperatures are indicative of a 

hotter, drier summer, under which conditions the transition from earlywood to latewood occurs earlier, resulting in less earlywood growth. 

In the preditching period, however, summer temperatures seemed to have 

little or no effect on earlywood growth.

Although the effect of a rainy summer was to increase latewood 

growth during the current year, in both pre and postditching periods, 

the increase was generally smaller after ditching. The result of a 

dry summer was to decrease earlywood growth, both in the current year 

and the following year, in pre and postditch periods; however, the 

decrease in earlywood growth was greater after ditching. In conclusion, 

it would seem that ditching modified the effect of increased summer 

rainfall; that is, increase in growth associated with wetter summers 

was smaller. On the other hand, after ditching, the effect of decreased 

summer rainfall was more pronounced, with a greater decrease in growth 

associated with drier summers.

A lowered water table allows a reasonable explanation of changes 

in growth responses ascribed to ditching (fig. 5)- With normal amounts 

of summer rainfall, the lower average water levels resulting from 

ditching would be expected to cause greater internal water stresses 

and reduced growth relative to preditching conditions. If water availability to the roots is further reduced by less-than-normal rainfall, 

then growth, as borne out by regression, is even further reduced. 

Increases in rainfall partially compensate for the normally reduced

18
LESS THAN NORMAL RAINFALL NORMAL RAINFALL GREATER THAN NORMAL RAINFALL

higher temperatures greater solar radiation J I

greater respiration greater transpiration

rates rates

greater decrease 

in growth

less water inflow 

from west of site

*

lower summer 

water level

I

more cloud cover

lower heat load Jower temperatures

on foliage i

I I

water less available less internal lower respiration

to roots water stress rates

greater internal 

water stress

less growth

less growth

greater growth

less increase 

in growth

Figure 5» Changes in growth response to rainfall after ditching

19
availability of water, but, as shown by regression, the growth response 

is not as marked during postditching conditions as it is during preditching ones.

The indications of greater decreases in growth during and after 

dry summers, and smaller increase during rainy summers, suggest that, 

overall, growth after ditching should be expected to be less than growth 

before ditching. This however, is not confirmed by comparisons of 

growth for 20-year periods before and after ditching (table 2). It 

seems possible that as a result of lower water levels after ditching 

there may have been growth of the root systems to greater depths. Increased root system size would tend to negate the postditching effects 

described above and result in increased growth (fig. 2: mid-1950's to 

mid-1960's). With environmental conditions in the root zone more 

favorable to growth, poorer correlations with climate would result, 

which would account for much of the unexplained variance of postditching growth (table 3)- With continued expansion of the root systems, 

competition between systems would increase; climatic factors would again 

become more limiting, and a decline in growth rates would result. Thus, 

the growth peak around I960 may simply be the result of root release 

after lowering of the water levels and it is somewhat analogous to crown 

release after lumbering.

Results of regression analysis may be summarized by describing 

conditions associated with the formation of narrow tree rings (table 4). 

Generally, before ditching, narrow rings were formed during a dry year 

that followed a wet year. After ditching, narrow rings were most commonly 

formed during a dry year that followed a dry year. Before ditching, 

narrow earlywood formation was associated with wet and cool conditions 

of the previous season and narrow latewood with current dry, warm 

summers. After ditching, there was some indication that narrow earlywood followed wet summers, and that narrow latewood was a result of 

wet summers followed by dry summers.

Years of narrow rings by total ring, earlywood, and latewood are 

listed in table 5- Also indicated are years after 1917 in which 

climatic conditions leading to reduced growth are satisfied. As indicated in the table, reduced growth occurred about twice as often as 

conditions warranted. In addition (now shown in the table) there were 

years in which conditions for reduced growth seemed to be indicated, 

but in which growth was not significantly reduced. Though entries in 

the table cannot be used for climatic reconstruction, note that of the 

8 years of reduced growth after 1917i 6 were years that seemed conducive to narrow ring formation. The exceptions, 1948 and 1949, were in 

the midst of an extended period of reduced growth. The extended periods 

of reduced growth in the 1890's and 194O's (see also fig. 2) seem to 

be exceptions to the regression results as described. These could 

represent extended droughts in the swamp, not reflected by seasonal 

averages of climatic variables used in regression.

20
Table 4. Conditions resulting in narrow rings in Pinus taeda.

prior May, June

prior June, July

prior August - October

prior November - April

current May, June

current June, July

prior May, June

prior June, July

prior August - October

prior November - April

currant May, June

current June, July

* ss significant 

** m significant

*** cs-J rm-i f -i ^an+-

Total 

Ring Width

wet* , warm*

wet***

wet* , cool

no effect

dry* , cool*

dry , warm*

Total 

Ring Width

cool

warm*

warm*

dry

no effect

dry, warm**

at 95 percent 

at 99 percent

Q-f QQ Q nciirr^can

JPreditching

Earlywood 

Only

wet , warm

wet

cool**

no effect

dry*

no effect

Postditching

Earlywood 

Only

cool*

wet

no effect

dry*

no effect

warm*

+

Latewood 

Only

cool*

cool*, warm*

wet*

no effect

dry*, cool*

dry* , warm*

Latewood 

Only

no effect

no effect

wet*, warm*

no effect

no effect

dry*

maximum temperature 

minimum temperature

21
Table 5. Years between 1830-1950 (period before extensive ditching) in 

which the growth index was less than the mean minus the standard deviation. Type of growth is identified as total ring 

width (T), earlywood (E), or latewood (L).

1831

1838

1842

1844

1846

1847

1864

1881

1890

1891

1892

1893

1894

1895

1896

1897

1900

1911

1912

1921

1930

1936

1941

1944

1947

1948

1949

T

T

T

T

T

T

T

T

T

T

T

x*

x*

T

T

x*

T

T

E

E

E

E

E

E

E

E

E

E

E*

E*

E

E

L

L

L

L

L

L

L

L

L

L

L

L

L

L*

L*

L

L

L*

L

* Years after 1917 with climatic conditions, determined by regression, 

to be conducive to reduced growth.

22
SUMMARY AND CONCLUSIONS

An analysis of tree rings of large loblolly pines growing near 

Lynn Ditch in the Great Dismal Swamp was conducted as part of a study 

to determine climatic sensitivity of tree rings from trees in wetland 

habitats. Of particular concern was whether tree growth might provide 

evidence of changes in hydrologic conditions after ditching.

Tree rings from increment core samples from 11 trees were found 

to cross-date with each other, and analysis of variance suggested that 

growth is at least moderately correlated with climate.

Basal area increments of each sample tree were examined for 20-year 

periods before and after 1950, the approximate date at which ditch construction began. No significant growth difference was noted between 

pre and postditching periods.

Tree ring indices and seasonal values of temperatures and precipitation were examined for trend variations by the use of low pass digital 

filter techniques. A similarity in patterns between trees is indicated. 

However, the pattern did not correspond significantly with trend variations in climatic factors tested. It is probable that low frequency 

growth variations are related to regional environmental conditions not 

tested for. Among these conditions cycles of low water levels in the 

swamp peat are considered most likely.

Yearly variations in growth were compared with yearly variations 

in seasonal climate by the use of stepwise multiple regression. Only 

about half of the variance of total ring width for the period of record, 

1917-1975? was explained by climate and prior growth. When the time 

period was divided into pre and postditching segments, more than twothirds of the growth variance was explained by climate and prior growth 

and the degree to which various factors limited growth was found to be 

different after ditching. It is suggested that hydrologic change as a 

consequence of ditch construction was the most likely cause of changed 

growth responses to climate. Before ditching, total ring width was 

mostly limited by rainfall, with narrow rings being associated with 

dry summers that followed wet summers. After ditching, growth seemed 

to be limited more by temperature than by rainfall.

Further examinations of growth responses were afforded by dividing 

the annual growth increments into earlywood and latewood components. 

With such division, temperature, precipitation, and prior growth 

accounted for about three-fourths of the tree-ring variance. Narrow 

earlywood widths formed before ditching were associated with cool 

temperatures of the preceding fall and warm, wet conditions of the 

previous summer. After ditching, narrow earlywood widths were strongly 

associated with wet summers of the previous year. Narrow latewood

23
growth before ditching was significantly linked with dry summers. 

After ditching, dry summers were only weakly associated with narrow 

latewood. Overall, in both pre and postditching periods, latewood 

growth seemed to be limited more by earlywood growth of the same 

year than by climate.

Changes in growth responses after ditching were explanable 

when it was assumed that water levels in the peat after ditching 

were lower than before ditching. However, it is emphasized that 

this observation is based on samples from a small area near the Lynn 

Ditch-Middle Ditch intersection. Though other areas in the swamp may 

be similar, it seems probable that the effects of ditching on water 

levels throughout the swamp are highly variable. Furthermore, this 

study has provided no evidence that species composition has responded 

to any hydrologic change consequent to ditching.

The pines at the study site were more than 12O years old at the 

time of ditching in the 195O's, probably having become established on 

stumps and roots remaining after a peat fire before 1830. Since 

establishment of the pines, peat seems to have accumulated at an 

average rate of 3O cm per 1OO years. Construction of Lynn Ditch in 

the early 195O's evidently resulted in lowered water levels in the 

study site. It is concluded that after ditching, root growth probably 

increased, with the consequence that, in compensating for lowered water 

levels, the trees have become less limited by, and growth is thus more 

poorly correlated with, droughty growth seasons. Extended periods of 

narrow tree rings were noted for the l89O's and also about 195O- It 

is speculated that these may represent extreme droughty conditions in 

the swamp, which were not indicated by climatic factors used in regression.

Tree-ring data used in this report are on file at the Tree-Ring 

Laboratory, National Center, Reston, and at the International Tree-Ring 

Data Bank, Tucson, Arizona.

24
REFERENCES

Akerman, Alfred, 1923, The white cedar of the Dismal Swamp: Virginia 

Forestry Pub. no. 3O, Virginia Geological Commission, 21 pp.

Brown, A. C., 19?O, The Dismal Swamp Canal: Norfolk County Historical 

Society, Chesapeake, Virginia, 234 p.

Carter, Virginia, Garrett, M. K., Shima, Lurie, and Gammon, Patricia,

1977j The Great Dismal Swamp: management of a hydrologic resource 

with the aid of remote sensing: Water Resources Bull., v. 13, no. 1, 

p. 1-12.

Dean, G. W., 1969, Forests and forestry in the Dismal Swamp: Virginia 

Jour. Sci., v. 2O, no. 4, p. 166-173.

Fritts, H. C., 1976, Tree rings and climate: New York City, Academic 

Press, 567 P«

Fritts, H. C., Mosimann, J. E., and Bottorff, C. P., 1969, A revised 

computer program for standardizing tree-ring series: Tree-Ring 

Bull., v. 29, p. 15-20.

Kearney, T. H., 19O1, Report on a botanical survey of the Dismal Swamp 

region: U. S. Department of Agriculture, Contributions from the 

U. S. National Herbarium, v. 5, no. 6, p. 321-550.

Lichtler, W. F., and Walker, P. N., 1974, Hydrology of the Dismal Swamp, 

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20O West Grace Street, Richmond, Virginia 2322O.

Mason, C. R., 1952, Mapping in Dismal Swamp: The Military Engineer, 

v. 44, no. 298, p. 120-125.

Oaks, R. Q., Jr., and Coch, N. K., 1973, Post-Miocene stratigraphy and 

morphology, southeastern Virginia: Virginia Division of Mineral 

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of the Dismal Swamp, southeastern Virginia and northeastern North 

Carolina. Proc. First Dismal Swamp Symposium, Norfolk, Va., 197^, 

(in press).

Reid, J. H., 1952, Reports to Camp Manufacturing Company on timber holdings 

of Camp, Richmond Cedar Works, Roper Lumber Company, and other landowners in the Great Dismal Swamp: Typewritten reports on file in 

offices of Union Camp Corporation, Franklin, Virginia.

25
Shaler, N. S., 1890, General account of the fresh-water morasses of

the United States, with a description of the Dismal Swamp District 

of Virginia and North Carolina: 19th Annual Report to the Director, 

U. S. Geological Survey, p. 255-339-

Stockton, C. W., and Fritts, H. C., 1971? Conditional probability of

occurrence for variations in climate based on width of annual tree 

rings in Arizona: Tree-Ring Bull. v. 31? P- 3-24.

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Charles T. Main, Inc., Engineers: Duplicated report on file at 

Great Dismal Swamp National Wildlife Refuge, Suffolk, Va.

U. S. Weather Bureau, Climatological Data, 1917-1977-

Whitehead, D. R., 1972, Developmental and environmental history of the 

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 £ U.S. Government Printing Office: 1979 281-359/24

26