Evidence of Floods on the Potomac River 

From Anatomical Abnormalities in the 

Wood of Flood-Plain Trees

U;.S-GEOLOGICAL SURVEY PROFESSIONAL PAPER 1296
Evidence of Floods on the Potomac River 

From Anatomical Abnormalities in the 

Wood of Flood-Plain Trees

By THOMAS M. YANOSKY

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1296

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1983
DEPARTMENT OF THE INTERIOR 

William P. Clark, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Yanosky, Thomas M.

Evidence of floods on the Potomac River from anatomical abnormalities in the wood of flood-plain trees.

(U.S. Geological Survey professional paper ; 1296)

Bibliography: p.

Supt. of Docs, no.: I 19.16:1296

1. Floods Potomac River. 2. Ash (Tree) Climatic factors. 3. Ash (Tree) Growth. 4. Tree-rings.

I. Title, II. Series: Geological Survey professional paper ; 1296.

GBL1399.4. P67Y36 1983 551.48'9'0287 83-^500163

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304
CONTENTS

Page

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

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

Methods .................................................................................... 2

Botanical methods ......................................................................... 2

Hydrologic methods ........................................................................ 4

Tree growth and wood anatomy ............................................................... 4

Flood rings within the latewood zone ........................................................... 5

Flood rings formed in 1972 .................................................................. 5

Flood rings formed after June floods, 1930 to 1979. ............................................. 8

Flood rings formed after July and August floods, 1930 to 1979 .................................... 9

Flood rings formed after May floods, 1930 to 1979 .............................................. 11

Flood rings within the earlywood zone .......................................................... 16

Jumbled and extended earlywood ........................................................... 17

Combination flood rings .................................................................... 21

Depauperate earlywood .................................................................... 22

Discolored earlywood ...................................................................... 23

Mechanisms and timing of flood-related growth.................................................. 25

Field experiments with defoliated growth ..................................................... 31

Botanical and hydrologic factors associated with flood-related growth .............................. 33

Flood rings formed prior to 1930 ............................................................... 37

Applications ................................................................................ 38

Summary ................................................................................... 40

References cited ............................................................................. 42

ILLUSTRATIONS

Page

FIGURE 1. Map showing Potomac River near Washington, D.C., and study areas near

Difficult Run and Chain Bridge .............................................. 3

2-13. Photomicrographs of the wood of ash showing:

2. Typical growth rings of an undamaged ash tree ............................ 6

3. Flood rings formed after the great flood of June 1972 ....................... 7

4. Flood rings formed in 1972 at increasing stem heights of one tree. ............ 10

5. Flood ring formed in 1972 that has the appearance of an extra ring ........... 13

6. Typical flood rings formed near stem bases after large floods in 1949

and 1951 .......................................................... 14

7. Typical flood rings formed after minor floods in 1959 and 1974. ............... 15

8. Flood rings formed after late-season floods in 1945 and 1956 ................. 16

9. Flood rings of jumbled vessels within the earlywood ........................ 18

10. Flood rings in which the earlywood zone appears extended.................. 20

11. Growth rings with enigmatic vessel patterns or with jumbled and extended

vessels along the same radius ....................................... 23

12. Combination flood rings formed after floods in May 1942 .................... 26

13. Depauperate earlywood formed after an ice jam in 1948 .................... 28

14. Photograph of the wood of ash showing discolored earlywood ...................... 29

15-17. Graphs showing:

15. Ages of 18 trees at times of flood-ring formation ........................... 34

16. Trunk diameters of 18 trees at times of flood-ring formation ................. 35

17. Ages at last minor-flood ring of 35 trees in zones of different flood

frequency and exposure ............................................ 36

18. Photomicrographs of flood ring and narrow growth rings formed after the great

flood of June 1889 ........................................................ 39

in
TABLES

Page

TABLES 1-4. Frequency of latewood flood rings formed:

1. After the great flood of June 1972 ........................................... 7

2. In June-flood years, 1930 to 1979 ............................................ 14

3. In July-and August-flood years, 1930 to 1979 ................................. 15

4. In early May (May 1 -15)- and late May (May 16-May 31)- flood years, 1930 to 1979 17 

5-7. Frequency of:

5. Earlywood flood rings, 1930 to 1979.......................................... 25

6. Combination flood rings, 1930 to 1979. ....................................... 27

7. Depauperate earlywood, 1930 to 1979 ....................................... 28

8. Results of manual defoliation of ash trees, 1980 to 1982 .............................. 31

9. Summary of flood rings formed in 49 trees, 1889 to 1979. ............................. 42

IV
Evidence of Floods on the Potomac River 

From Anatomical Abnormalities 

in the Wood of Flood-Plain Trees

By Thomas M. Yanosky

ABSTRACT

Ash trees along the Potomac River flood plain near 

Washington, D.C., were studied to determine changes 

in wood anatomy related to flood damage. Samples 

were collected as cross sections, and anomalous growth 

was compared with flood records for April 15 to August 31, 1930 to 1979. Collectively, anatomical evidence 

was detected for 33 of the 34 growing-season floods 

during the study period. Evidence of 12 floods prior to 

1930 was also noted, including catastrophic floods in 

1889 and 1924.

Trees damaged after the transition from early wood 

to latewood growth typically formed "flood rings" of 

enlarged vessels within the latewood zone. Trees damaged during the earlywood-growth interval developed 

flood rings within, or contiguous with, the early wood 

zone. Both patterns are assumed to have developed 

after refoliation of flood-damaged crowns. Flood rings 

formed when trees produced a second crop of leaves. 

Trees damaged by high-magnitude floods developed 

well-formed flood rings along the entire height of the 

stem in which the growth ring was present and around 

the entire circumference of the stem. Smaller floods 

were generally associated with diffuse or discontinuous 

anomalies restricted to stem apices. Peak stages were 

positively related to the frequency of abnormal growth 

within samples, and the intraring position of abnormal 

growth relative to total ring width corresponded to the 

approximate time of the flood.

Some trees provided evidence of numerous floods. 

Those with the greatest number of flood rings grew on 

frequently flooded surfaces subject to flood-flow velocities of at Jeast 1 meter per second, and more typically 

greater than 2 meters per second. Tree size, more than 

age, was related to flood-ring formation. Trees kept 

small by frequent flood damage had more flood rings 

than taller trees of comparable age. Flood rings formed 

only if tree crowns were inundated, presumably because 

leaves were stripped or damaged.

Anatomical evidence of floods, in addition to methods 

involving age determinations of flood scars and sprouts, 

can be used to document or extend streamflow records. 

Reconstructing tree heights in a year of flood-ring formation can provide estimates of peak stages along 

local stream reaches. Time of flood generation during 

the tree-growth season can be estimated from the radial position of anomalous growth relative to annual 

ring width. Further studies might define the minimum

anatomical "signal" representing flood-affected growth 

in ash and other woody species, thus permitting more 

direct estimates of magnitude.

INTRODUCTION

Deciduous trees growing on the Potomac River 

flood plain near Washington, D.C., are occasionally flooded during the growing season. If crowns 

are damaged, the subsequent radial growth of 

stems produces wood that is often anatomically 

different from that of undamaged trees. A study 

was undertaken to relate anomalous wood to 

documented floods on the Potomac River and to 

determine if different patterns of anomalous 

growth were related to flood frequency and magnitude. These methods in turn may be used to 

document floods where hydrologic data are incomplete or lacking.

Typical spring growth in certain deciduous species is characterized by large-diameter cells 

(springwood or earlywood) and by smaller, thicker 

walled fibrous cells (summerwood or latewood) 

for the remainder of the growth year. Generally, 

earlywood growth in the Potomac basin commences in mid-April and continues into May. The 

transition to latewood growth is fairly abrupt, 

and subsequent radial growth may continue for 

several months. Under unusual circumstances, 

however, an extra, or false, ring of cells may develop within a growth increment. In deciduous 

species, false rings have been reported as a result 

of frost injury (frost rings) and from defoliation by 

insects (Fritts, 1976) but not from flooding.

Flood damage can produce a false ring, however. A large flood that crested on the Potomac 

River on June 24, 1972, killed great numbers of 

trees and severely damaged many others. Growth 

of many survivors was interrupted because of 

leaf and bud damage, but growth resumed after 

trees produced a second crop of leaves. The initial 

flush of radial growth after the flood typically resembled that of earlywood; that is, rings of large1
diameter cells abrupty formed within the latewood. 

Abnormal growth may likewise develop within 

the earlywood zone if flood damage occurs in the 

early part of the growing season. Thus, "flood 

rings" 1 can be used to document the occurrence 

of floods, and the relative time of the flood can 

be estimated from the intraring position of the 

abnormal growth.

Previous uses of botanical evidence to document 

the occurrence of floods have primarily involved 

scars or sprouts growing from flood-tipped trunks. 

For example, Sigafoos (1964) found that trees 

along parts of the Potomac River flood plain were 

scarred by numerous floods and that the year of 

damage could be determined by counting the 

number of annual growth rings formed after the 

flood scar. Harrison and Reid (1967) similarly constructed a flood-frequency graph from scarred 

trees for the Turtle River, North Dakota, and 

C. R. Hupp of the U.S. Geological Survey (oral 

commun., 1982) used stem-deformation dates to 

extend the flood record of Passage Creek, Virginia. It was found that the ages of trees along 

parts of the Potomac River flood plain were related to the degree of sheltering during high 

flood-flow velocities (Yanosky, 1982a). Although 

most trees exposed to the full forces of a catastrophic flood in 1972 were destroyed, none had 

been growing prior to the last great flood in 

1942. Numerous trees predating the 1942 flood, 

however, grew along nearby reaches that were 

sheltered from the maximum velocity of the 1972 

flood. It was concluded that the ages of trees 

from reaches with a high probability of flood 

damage could be used to estimate the date of the 

last catastrophic flood. Another study comparing 

the widths of the 1972 and 1973 rings showed 

that many severely damaged trees formed a typically narrow 1973 rings (Yanosky, 1982b). Minimally damaged trees, however, often formed 

a typically wide 1973 rings if large numbers of surrounding trees had been destroyed. It was concluded that unusual departures from mean annual 

ring width can be used as indicators of catastrophic flooding and can also aid in determining 

variations in the local severity of flood flows.

Although these studies may be valuable, they 

are not without several practical limitations. 

Sprouts sometimes grow from trunks that have 

been pushed over by wind throw or slope failure 

rather than by flooding. The exact year of a flood 

cannot be determined from sprouts because growth

'The term "flood rings" to describe false rings due to flooding 

was originated by R. L. Phipps and was first used formally by 

Yanosky (1982b).

can begin in the same year as the flood or in the 

following year. Old sprouts may have rotten center rings that preclude an exact determination of 

age, and floods often prune limbs and sprouts in 

which evidence of earlier floods was preserved. 

A tree may have scars that cannot be observed 

unless entire cross-sectional discs are collected at 

numerous stem heights.

Flood rings, however, can generally be observed 

in wood samples collected with an increment 

borer. Flood rings have been observed at all stem 

heights at which a growth ring is present and 

along most or all of the ring circumference. Hence, 

evidence is not subsequently lost unless the tree 

is destroyed. In addition, the time of a flood can 

be estimated to within 2 weeks based on the infraring position of abnormal growth. Most importantly, abnormal growth has not been found in 

annual increments formed when trees were not 

flooded or in upland trees; that is, it seems that 

abnormal growth observed in this study formed 

only in response to flood damage. If anomalous 

growth were due to a factor other than flooding, 

such as defoliation by insects, upland trees would 

also be expected to develop atypical radial growth.

METHODS

Botanical Methods

Ash trees (Frax/nus amer/canum L. and F. pennsy/van/ca Marsh.) were sampled along reaches of 

the Potomac River flood plain near Difficult Run, 

Virginia, and Chain Bridge, Washington, D.C. 

(fig. 1). Ash at both locations grows at most levels 

between the low-water channel and adjoining uplands. The flood plain near Chain Bridge is broad 

and gently sloping, with patchy tracts of alluvium 

and outcropping bedrock. Trees on low flood-plain 

levels are small due to frequent flooding (the return interval of flooding is about 0.33 year) and 

rarely exceed 40 years of age. Trees at higher 

levels are larger and generally older. At the site 

downstream from Difficult Run, the flood plain is 

narrow and more topographically varied than that 

near Chain Bridge. Trees close together may differ 

strikingly in age, size, and form. A more detailed 

account of the study reaches is given by Yanosky 

(1982a, b), and descriptions of individual trees 

and local site features are in appropriate parts of 

this text.

Forty-six trees were harvested with a bowsaw, 

and cross-sectional discs were taken near the 

base of the trunk and at succeeding stem heights
77»12'30" 77"10' 77»7'30"

38«57'30" -

Figure 1. Potomac River near Washington, D.C., and study areas (shaded) near Difficult Run and Chain Bridge.

of about 1 to 2 m. Most trees were about 3 to 5 m 

in height, although a few were as tall as 12 m. 

Many had been uprooted or otherwise severely 

damaged by floods, and five were stumps from 

which only one section could be taken. Sections 

from four other trees cut in 1960 were also obtained. Additionally, the wood of 53 trees was 

sampled with an increment borer rather than as 

cut sections. The size range of these trees was 

comparable to that of harvested samples, but a 

larger percentage was tall. Two cores per tree 

were taken on opposite sides of the trunk and 

perpendicular to the channel. Although the term 

"sample" applies to cut sections or increment corings, it also refers to individual trees when the 

frequency of growth anomalies in specific years 

was tallied for the entire collection; for example, 

a tree with a 1972 flood ring was tallied only 

once even if abnormal 1972 growth was detected 

in more than one cut section or coring.

Samples were sanded on a drill press with a 

100-grit garnet disc, followed successively by grits 

of 220, 320, and 400. Samples were then buffed 

with a lamb's wool pad. Sections were studied 

anatomically with a Bausch and Lomb dissecting 

microscope ("Microzoom"), generally at X20 to

X80. A Pentax Spotmatic with Kodak Plus-X Pan 

film (ASA 125) was affixed to the trinocular eyepiece, and a Dyonics fiber-optic lamp was used 

for photographic illumination. 2

Flood rings were classified according to intraring 

position. Those within the earlywood zone were 

designated "earlywood flood rings," and those 

within the latewood, as "latewood flood rings." 

Variations in the two types of flood rings were 

described, as was anomalous growth that appeared 

intermediate between the two. All anatomical descriptions, measurements, and photomicrographs 

are of tissues in transverse section.

Flood rings within the 1972 growth increment 

were present in the greatest number of sampled 

trees and were studied first. Flood rings formed 

in other years were tallied in each sample and 

compared with flood records for the Potomac 

River gaging station near Washington, D.C. (fig. 1). 

The study period (1930-79) included all floods occurring from April 15 to August 31 of each year.

2Any use of trade names in this publication is for descriptive 

purposes only and does not constitute endorsement by the 

U.S. Geological Survey.
Anatomical variation of flood rings was noted 

among individual trees, different years within the 

same tree, and at various stem heights within the 

same tree. The presence of flood rings was compared to tree size and age, flood magnitude, and 

the degree of flood-flow exposure.

Hydrologic Methods

For each tree, the stage of the Potomac River 

was determined by the height of the water as it 

reached the trunk base. The discharge of this 

stage was determined by taking the simultaneous 

stage reading for the gage near Washington, D.C., 

and computing the corresponding discharge from 

a rating table for the station. The discharge that 

covers the trunk base is generally insufficient to 

damage the tree but can be used to determine 

the number of trees flooded by a given discharge. 

In this report, flooding refers only to the submergence of the trunk, whereas inundation implies 

the complete submergence of the tree. For each 

flood, the number of inundated trees was determined by comparing local crest stages with tree 

heights. Because most sampled trees had been 

kept small by frequent flood damage, the probability of crown inundation during specific events 

could be determined with reasonable certainty. 

The approximate height of taller trees in specific 

flood years, however, was estimated from typical 

age/height relations of trees growing along sampled reaches.

The average discharge of the Potomac River 

near Washington, D.C., from 1930 to 1980 was 

325 mVs. The mean annual flood is about 3,800 

m3 /s, and the partial duration flood series base is 

1,270 mVs. Accordingly, the term "flood" in this 

report is used for peak discharges equal to or 

greater than the base discharge. Although the 

term "flood plain" typically refers to surfaces 

flooded on the average once every 2 years, the 

definition is broadened here to include lower levels. Surfaces near a low-water channel are reworked during flows smaller than the mean annual flood and may be regarded as part of the 

active channel (Osterkamp and Hedman, 1977). A 

flood was considered catastrophic if peak flow 

was more than twice that of the mean annual 

flood. Catastrophic floods occurred during the 

tree-growth season in June 1889, May 1924, April 

1937, and June 1972 and during periods of dormancy in March 1936 and October 1942. All streamflow records cited in this report were compiled by 

the U.S. Geological Survey.

Most trees sampled for anatomical study grew 

along the active channel, including some on surfaces flooded by the average discharge. The degree of flood-flow exposure was determined near 

each tree from field observations during the base 

discharge (1,270 mVs) or, if trees grew on higher 

levels, at the discharge flooding the trunk to a 

depth of approximately 1 m. Trees were classified 

as "sheltered" if estimated velocities were less 

than 1 m/s, as "moderately sheltered" if velocities 

were betwen 1 and 2 m/s, and as "unsheltered" 

if velocities exceeded 2 m/s. As stage increases, 

velocities within these zones seem to increase in 

a fairly linear fashion, although velocities within 

some highly sheltered zones remain unchanged 

except during catastrophic floods. Trees sampled 

near Chain Bridge grew only in unsheltered flood 

zones, whereas those near Difficult Run were 

found in unsheltered, moderately sheltered, and 

sheltered zones.

TREE GROWTH AND WOOD ANATOMY

Trees grow from the continued divisions of a 

thin sheath of cells composing the vascular cambium. The cambium, which lies just beneath the 

bark, tapers from the trunk to the apices of 

stems and roots and may be described geometrically as a double paraboloid joined at ground 

level (Phipps, 1967). Wood (xylem) is produced by 

cambial divisions oriented toward the center of 

the tree. This radial growth is in a series of annual 

increments, or tree rings, that can be counted to 

determine the age of the tree. The cambium remains between the outermost growth increment 

and the bark. Although radial growth occurs at 

all stem heights, longitudinal growth occurs only 

at stem tips. Initials carved on a tree trunk, for 

example, remain at their original height even 

though the tree grows taller. At increasing heights 

in the stem, however, diameter decreases because 

there are fewer growth rings; that is, age decreases at succeedingly higher parts of the stem, 

and all branch and root apices are 1 year of age.

Cells produced by the cambium enlarge and differentiate into specific xylary tissues. Cells elongate along their longitudinal axis, and a complex 

mixture of celluloses and lignins is deposited just 

inside the thin wall that surrounds each cell. This 

"secondary" wall confers rigidity to the wood 

after cells die.

Cells with the largest diameters in porous 

woods (including Frax/nus) are vessel elements,
which typically are connected end-to-end. At maturity, the vessel elements die and the end walls 

partly or completely disappear, thus forming 

open tubes termed vessels. Fibers and tracheids 

are longer and thinner than vessel elements and 

are difficult to distinguish from each other in 

transverse section. Vessels and tracheids transport 

water, minerals, and certain organic molecules, 

whereas the thicker walled fibers provide mechanical support. A fourth type of xylem tissue 

remains alive at maturity and is oriented radially 

or longitudinally. Radial parenchymatous cells 

comprise "wood rays" that function for transport 

and storage of organic materials. Other parenchyma 

cells may be scattered or in tangential bands 

throughout the xylem of hardwoods but are lacking in softwoods.

The woods of tree species differ in the proportions and dimensions of these four xylary tissues. 

Wood is either nonporous or porous, depending 

on whether vessels (pores) are present. Conifers, 

or softwoods, have tracheids but not vessels, 

and, consequently, the wood is considered nonporous. The porous woods, or hardwoods, have 

vessels as well as tracheids and are subdivided 

according to the degree of contrast in vessel diameters across growth rings. In ring-porous species, vessels formed at the beginning of the growth 

season (earlywood vessels) are conspicuously 

larger than those formed during the remainder of 

the growth year (latewood vessels). Oak, ash, 

and hickory are ring porous. Woods in which vessel size is fairly constant within a growth ring are 

"diffuse porous." Ring boundaries are easier to 

distinguish in ring-porous than in diffuse-porous 

woods.

No anatomical differences pertinent to the objectives of this study were observed between 

woods of the two ash species. Early wood vessels 

are large and thin walled and are surrounded by 

fibers, parenchyma, and some tracheids. Earlywood vessels are typically in two to three radial 

rows (ranks), followed abruptly by thick-walled 

fibers comprising the latewood zone. Vessels 

scattered within the latewood are smaller than 

early wood vessels and are typically surrounded 

by longitudinal parenchyma distributed tangentially. The size difference between cells in the 

latewood of one year and the early wood of the 

next permits an exact determination of ring boundaries. The transition from early wood to latewood 

growth in a single ring can also be determined, 

although with less exactness. A typical sequence 

of growth rings from an ash tree sampled near 

Difficult Run is shown in figure 2.

The initiation and timing of ash growth in the 

lower Potomac basin is variable from year to 

year and from tree to tree. Bud break and the 

concurrent enlargement of early wood vessels develop in mid-April, although individuals have 

been observed that lagged behind nearby trees 

by as much as 2 to 3 weeks. Generally, however, 

leaves are fully expanded by early May, and the 

early wood zone is complete or nearly complete 

by mid- to late May. Latewood growth occurs 

thereafter and continues at least into August.

FLOOD RINGS WITHIN THE LATEWOOD ZONE

A flood ring consists of abnormally large vessels 

that develop from cells that otherwise would 

have remained much smaller. These vessels are 

highly conspicuous when they develop among the 

smaller cells composing the latewood zone. However, abnormal growth within or near the earlywood zone is difficult to distinguish from the typically large vessels in the early wood. For this reason, latewood flood rings are discussed first. 

Flood rings within the latewood correlate with 

floods in June, July, and August; both latewood 

and earlywood flood rings are associated with 

May floods.

Flood Rings Formed in 1972

A flood of 10,200 mVs crested on the Potomac 

River near Washington, D.C., on June 24, 1972, 

and was exceeded in this century only in 1936 

and 1942. The 1972 flood, however, was the largest that has occurred during a growing season. 

Accounts of the impact of this flood upon vegetation along specific reaches near Washington, D.C., 

are found in Yanosky (1982a, b).

Of 46 harvested trees from which cross sections 

were obtained, 38 were growing at the time of 

the June 1972 flood, and 32 of these developed a 

flood ring within the 1972 growth increment. All 

trees with flood rings were inundated during the 

June flood. Four trees without flood rings had 

crowns that remained above the crest of the 

flood. Only two trees with flood-covered crowns 

did not develop flood rings in 1972 and both had 

extremely narrow rings prior to and including 

1972; rings consist mostly of earlywood zone and 

minimal latewood. Thus, radial growth of the two 

trees had greatly slowed or ceased prior to the 

flood, and postflooding radial growth apparently 

did not develop. In addition to the harvested 

trees, 53 trees were cored with an increment
borer to examine the 1972 annual rings. Of 27 

trees inundated by the June flood, 25 formed 

flood rings; 26 additional trees were flooded but 

not inundated, and 1972 flood rings were not observed. The frequency of flood rings in the 1972 

growth increments of cored and harvested trees 

is summarized in table 1.

Flood rings were first described anatomically 

from the basal sections of harvested trees at 

stem heights from about 0.2 to 1.0 m. Nearly all 

flood rings in these sections were continuous; 

that is, they extended along the entire circumference of the 1972 ring. However, the samples varied 

strikingly in the size of the largest flood vessels. 

Tangential diameters of vessels were generally 

less than one-half those of typical earlywood vessels. Considerable variation was also observed in 

the number of flood vessels per unit area; flood 

rings varied from jumbled groups of diffuse vesFigure 2. Typical growth rings of an undamaged ash 

tree. A, Four growth rings. Earlywood vessels are 

the rings of large cells that mark the beginning of 

each growth year. B, Ring formed during one growth 

year. Largest earlywood vessels are 125 to 225 /im in 

tangential diameter, whereas latewood vessels are 

typically 30 to 60 /im. Smallest cells are primarily 

fibers and parenchyma. A, X20; B, X70.
Table 1. Frequency of flood-ring formation in 91 trees flooded on June 24, 1972 

(Peak discharge 10,200 mVs)

Crowns flooded Crowns unflooded

Method of sampling

Number of trees Number of trees 

Number of trees with flood ring Percentage Number of trees with flood ring Percentage

Total

Average 

34 32 94.1 4 00

27 25 92.6 26 00

61 57 30 0 

93.4 0

sels to closely grouped cells in a definite ring pattern that resembled vessel distribution in the 

earlywood (fig. 3). Flood vessels generally developed tyloses, which are living cells that protrude 

into the cavities of nonconducting vessels. Following the flood ring, a narrow zone of thin-walled 

cells typically formed the remainder of the 1972 

latewood. The narrow width of this zone resulted, 

to some extent, because the flood occurred fairly 

late in the growing year. In addition, photosynthates produced by the second crop of leaves 

were probably minimally utilized for postflooding

radial growth. Instead, it seems reasonable to assume that the bulk of new resources was used to 

set buds for the 1973 growing season and to store 

food reserves in the roots. Severely damaged 

trees, particularly those with broken crowns, had 

the least postflooding growth. In a few samples, 

however, postflooding growth accounted for nearly 

50 percent of the width of the latewood, possibly 

because damage was not severe.

Anatomical descriptions of flood rings to this 

point have been from cross sections at stem height 

ranging from about 0.2 to 1.0 m. Sections higher

Figure 3. Flood rings formed after the flood of June 24, 1972 (10,200 mVs), in the lower trunks of three trees.

Figure 3A. Well-formed flood ring. Largest flood vessels are between 140 and 165 /mi, although most are between 

60 and 120 jim. Spacing of vessels is comparable to that of typical earlywood vessels. (X70.)
 ft

^^ 1070:^3-

Figure 3B. Moderately formed flood ring. Most large flood vessels are between 90 and 110 jum and are more 

diffuse than earlywood vessels. (X70.)

in the stems also had flood rings in the 1972 

growth increments. In none of the trees did a 

1972 flood ring form at only one stem height. In 

other words, unlike a scar, the 1972 flood ring 

was a growth response within the entire 1972 

growth paraboloid. In most trees, flood rings 

were more distinct in sections near the stem 

apex where flood vessels had larger diameters 

and were generally less diffuse than vessels in 

basal sections. Figure 4 shows the 1972 flood ring 

in one tree at four successive stem heights. Figure 5 shows wood near the apex of another tree 

in which the 1972 flood ring has the local appearance of a true ring.

Flood Rings Formed After June Floods, 

1930 to 1979

In June, nine floods exceeded 1,270 mVs during 

the study period (1930-79). Table 2 lists the number of samples forming flood rings in these nine 

flood years. The largest flood peaks were on 

June 15, 1951 (3,120 mVs), June 20, 1949 (3,820 

mVs), and June 24, 1972 (10,200 mVs). As mentioned previously, flood rings were observed 

within the 1972 annual rings of 32 harvested

trees. Flood rings formed in 1951 were detected 

in 9 of 35 samples, and 1949 flood rings developed 

in 13 of 32 trees. Flood rings within the 1949 and 

1951 growth increments (fig. 6), as well as those 

in 1972, are generally well developed throughout 

the height of the tree, although one sampled tree 

formed a flood ring (1949) only near the stem 

apex. The other six June floods ranged from 

1,670 to 2,340 mVs. Flood rings formed in each of 

these flood years but less frequently than in the 

3 years of larger floods. No sample had more 

than one flood ring in these 6 years, and flood 

rings in only 2 flood years (1946 and 1959) were 

tallied in more than one tree.

Variations in the anatomy of flood rings also 

seem related to flood magnitude, even though all 

samples with flood rings were inundated. Flood 

rings corresponding to the three largest June 

floods are generally well formed, but flood vessels 

formed in the six minor-flood years are typically 

small and diffuse (fig. 7). In addition to a scattered, poorly defined appearance, these flood 

rings generally are locally missing (discontinuous) 

along parts of the ring circumference.

Attempts were also made to correlate flood 

rings with June flows of less than 1,270 mVs.
Figure 3C. Poorly formed flood ring. Largest vessels are as much as 100 /mi, although most are less than 70 /* 

Note paucity of flood vessels. (X70.)

This was done to describe the appearance of 

flood rings induced by small flows. Although 

trees growing on surfaces flooded by as little as 

280 mVs were studied, no flood rings were detected. Instead, periods of root flooding produced 

fibers with atypically thin walls and large lumens. 

Unlike the formation of flood vessels, these responses are apparently associated with a growth 

spurt rather than with the damage that induces 

flood rings. These bands of anomalous fibers may 

permit the documentation of small summer flows.

Flood Rings Formed After July and 

August Floods, 1930 to 1979

Floods in July and August were less frequent 

than during the peak tree-growth months of May 

and June (table 3). July floods exceeding 1,270 

mVs were recorded in 1949, 1956, 1970, and 

1972, whereas August floods occurred in 1933, 

1937, 1945, and 1955. The largest July-August 

flood peaked on August 20, 1955 (6,120 mVs). 

This discharge was exceeded in this century only 

in May 1924, March 1936, April 1937, October 

1942, and June 1972. No other late-season floods 

exceeded 2,630 mVs.

The flood of July 21, 1956 (2,050 mVs), is one 

of only four late-season flow events for which 

corresponding flood rings have been detected. 

Two trees that developed a flood ring in the terminal part of the 1956 latewood zone are shown 

in figure 8. The ring of flood vessels in both trees 

is continuous along most of the outer circumference of the 1956 growth ring, although the vessels 

were much smaller and more diffuse than vessels 

typically formed after June floods. One of these 

trees grew near Chain Bridge on a surface flooded 

by approximately 850 mVs and had been damaged 

repeatedly to such an extent that in 1980 most of 

the upstream-facing bark and wood had been destroyed. Although the tree was in its first year of 

growth in 1956, the flood ring did not develop in 

a basal section; however, flood vessels formed in 

another section closer to the apex. The other 

tree, which was in its second year of growth at 

the time of the 1956 flood, grew about 600 m 

downstream from Difficult Run on a surface flooded 

by flows of 420 mVs.

Minor floods on August 1, 1945 (1,390 mVs), and 

July 11, 1970 (1,510 mVs), are associated with 

one and two trees, respectively, that developed 

flood rings. Each flood ring is composed of small,
Figure 4. Flood rings formed in 1972 at increasing stem heights of one tree. Tree height in 1980 was about 5 m. 

Photographs are along radii extending in the same compass direction. Flood vessels near the apex are larger and 

more closely grouped than those in the lower trunk.

Figure 4A. At 0.2 m, most large vessels are 60 to 80 ^m and are highly diffuse. (X70.)

diffuse flood vessels similar to those formed in 

1956. The tree with the 1945 flood ring was 5 years 

of age at the time of the flood and grew near 

Chain Bridge on the upstream part of a rocky island. The roots of the tree were flooded by 310 

mVs. The 1945 flood ring was observed in a section taken at a stem height of 1 m but was absent 

from a basal section. One tree with a 1970 flood 

ring also grew near Chain Bridge but on a surface 

flooded by approximately 850 mVs. The basal 

section started to grow in 1945. Sections 0.5, 1.0, 

and 1.5 m above the basal section, however, had 

center rings that formed in 1968, suggesting that 

much of the tree had been destroyed before the 

1968 growing season and that a sprout had subsequently grown from the damaged trunk. An ice 

jam in January 1968, which severely damaged 

many trees on low flood-plain levels near Chain 

Bridge, may have damaged this stem (Yanosky, 

1982b). The 1970 flood, which crested at approximately 1 to 1.5 m above the roots of the tree,

most likely tipped, and thus inundated, the sprout. 

A 1970 flood ring developed in two sections near 

the apex but not in lower sections. The other tree 

with a 1970 flood ring grew on a low level near 

Chain Bridge and was inundated by the 1970 flood.

A flood ring was also observed in the outer 

latewood of the 1938 growth ring of a tree near 

Difficut Run. Growth of the tree began in 1937. 

Vessels comprising the 1938 flood ring are small 

but closely spaced and extend along approximately one-third of the ring circumference. Although flows exceeding 1,270 mVs did not occur 

during the 1938 growing season, records indicate 

an average flow of 850 mVs on May 25-26 and 

an average flow of 600 mVs on July 26. The flood 

ring in the outer latewood may have resulted 

from damage in July because flood vessels are 

separated from the earlywood by a broad band of 

latewood growth. The base of the tree is flooded 

by a discharge of 370 mVs, and a flow of 600 

mVs (the minimum peak discharge) increases the

10
 1973 

1972 flood ring

Figure 4B. At 1.0 m, most large vessels are 70 to 100 jum. (X70.)

depth of flooding to approximately 1 m. Velocities 

at this discharge would be expected to tip, and 

possibly damage, a small tree. The flow of late 

July 1983 is the smallest discharge for which a 

corresponding flood ring has been detected.

The flood of July 19, 1949 (2,590 mVs), was of 

sufficient magnitude that flood rings were highly 

likely to form. However, an earlier flood (3,820 

mVs on June 20) is assumed to have induced the 

flood rings observed in the growth increments, 

primarily because flood vessels were generally 

larger and more numerous than those formed 

after the late-summer floods in 1945, 1956, and 

1970. In other words, the pattern of anomalous 

growth in 1949 was similar to that formed in 1951 

and 1972. Similarly, none of the 1972 flood rings 

probably formed as a result of the minor flood of 

July 1 (1,460 mVs).

Flood Rings Formed After May Floods, 

1930 to 1979

Although flood rings were detected for all 

June-flood years, latewood flood rings were discerned in only 6 of 16 May-flood years during the 

study period. This lower frequency to some extent 

may be because six May floods were less than 

1,410 mVs, whereas the smallest June flood was 

1,670 mVs. However, four larger May floods, 

ranging from 2,330 to 4,760 mVs, were not associated with flood-ring formation in sampled trees. 

Consequently, a relation between latewood floodring frequency and flood magnitude is not immediately apparent for May floods. However, a relation 

results when flood-ring observations are tallied 

by subdividing May-flood years (table 4) into an 

"early May group" (May 1-May 15) and a "late 

May group" (May 16-May 31). In the early May 

group, 9 of 10 flood years are without corresponding latewood flood rings, whereas, in the 

late May group, 5 of 6 flood years are associated 

with flood rings in the wood samples. In 1 year 

(1960), a large flood occurred in early May and a 

smaller flood was generated in late May; flood 

rings in 1960 were placed in the early May group.

A probable explanation for differences in the 

frequency of flood rings in May-flood years is that 

radial growth during early May was different 

from that of late May. In the lower Potomac River

11
Figure 4C. At 2.5 m, most large vessels are 90 to 110 fim. (X70.

basin, the date of transition from earlywood to 

latewood growth may be highly variable but is 

generally not beyond late May. Floods in the 

early part of the growth year, therefore, are during the earlywood-growth interval. Flood vessels, 

if formed, would be expected to be in the earlywood zone and might be difficult to distinguish 

from typical earlywood vessels. Abnormal growth 

of this type is discussed in the following section. 

Flood rings forming after the transition to latewood 

growth, however, are easily discerned because of 

the sharp contrast between large-diameter flood 

vessels and smaller latewood cells.

The largest late May flood (3,940 mVs) was on 

May 24, 1942, and 9 of 20 samples developed a 

well-formed flood ring along the entire circumference of the 1942 ring. Flood vessels generally 

were closer to the earlywood zone than were 

those formed after the late June floods of 1949, 

1951, and 1972. In contrast, none of 30 samples 

developed flood rings after the flood of May 17, 

1978 (3,310 mVs). Similarly, the flood of May 14, 

1932 (4,760 mVs), was the largest May flood during the study interval, and yet no latewood flood 

rings were observed in the five samples growing

at the time of this flood. Of the remaining 4 years 

in which late May floods occurred, 1 year (1968) 

is associated with flood rings in the wood of 2 of 

26 samples. The peak discharge of the 1968 flood 

was 2,310 mVs. Anatomical evidence for another 

small flood during late May (1,910 mVs on May 22, 

1943) was observed in only one sample and consisted of a poorly formed group of scattered vessels just outside the earlywood zone. The smallest 

late May flood peaked on May 20, 1950 (1,270 

mVs), and was followed by the development of 

diffuse, discontinuous flood rings in 2 of 15 trees.

In addition to abnormal growth associated with 

peaks above the base floodflow, a flood ring 

formed in one sample after the flow of 850 mVs 

on May 25-26, 1938. A conspicuous, though discontinuous, band of small flood vessels formed 

just outside the earlywood. This tree also developed 

a discontinuous flood ring in the outer latewood 

after inundation in late July and was described 

more fully in the previous section. This is the only 

sample with two latewood flood rings within the 

same annual increment.

The greater frequency of flood rings formed in 

response to late May rather than early May floods

12
Figure 4D. At 4.0 m, most large vessels are 100 to 130 /^m and are closely grouped. (X70.)

suggests that the timing of the growth transition 

between earlywood and latewood is closely related to the type of subsequent abnormal growth. 

Flood magnitude is less important than the timing 

of this growth activity. In other words, trees damaged by floods prior to the initiation of latewood 

growth develop abnormal earlywood growth (see 

next section) but not latewood flood rings. Despite 

the small size of floods in 1943, 1950, and 1968, 

flood-damaged trees developed abnormal latewood zones because latewood growth had commenced prior to these floods. The largest late 

May flood (1942) was associated with the greatest 

number of trees having flood rings.

In 1 of the 16 May flood years (1960), floods occurred on May 10 (3,510 mVs) and May 30 (1,450 

mVs). Two trees had latewood flood rings. From 

the previous discussion, it would be expected

Figure 5. Flood ring formed in 1972 that has the local

appearance of an extra ring. Section is near the apex of a 

tree that was about 4.0 m tall in 1972. Vessels are somewhat smaller and more diffuse along much of the remainder of the ring circumference at this height. Note that 

flood vessels are in more than one rank and generally lack 

.tyloses. (X30.)

13
Table 2. Frequency of latewood flood rings formed in June-flood years, 1930 to 1979. Floods are ranked in order of increasing 

magnitude. Collectively, flood rings formed in 11.8 percent of trees inundated by the six smallest floods, compared to 53.5 

percent of trees inundated by large floods in 1949, 1951, and 1972

Peak discharge

Number of trees

Date of flood (cubic meters per second) With crown inundated1 With flood rings

June 4 1974

June 10, 1955

June 4, 1959

June A 1946

June 2, 1940

June 1 1971

June 15 1951

June 20 1949

June 24, 1972

1,670 

1,710 

1,740 

1,950 

2,250 

2,340 

3,120 

3,820 

10,200

19 

15 

17 

20 

16 

23 

35 

32 

34

1 

1 

2 

7 

1 

1 

9 

13 

32

'Determined by comparing local crest altitudes with estimated tree heights.

1950 

1949 ftood ring

Figure 6. Typical flood rings formed near stem bases after 

large floods. A, June 20, 1949 (3,820 mVs). B, June 15, 

1951 (3,120 mVs) and in June 20, 1949 (3,820 mVs). 

A, X70; B, X20.

that both flood rings developed after the flood of 

May 30, even though this flood was smaller than 

that of May 10. Additional evidence to support 

this view is that the 1960 growth increments of 

eight other trees had earlywood flood rings (see 

next section), all of which presumably formed 

after the larger flood of May 10. However, in one 

of the two trees with latewood flood rings, the 

appearance of the wood suggests that abnormal

growth formed after the flood of May 10. Although 

its trunk base is covered by floods greater than 

550 mVs, this tree grows along a reach where 

minor floods are unlikely to damage trees. Only 

a few trees sampled along this reach showed 

flood-related abnormal growth, and damage 

there was generally in response to floods exceeding 2,500 mVs. Thus, the flood of May 10, 1960, 

would have been more likely to damage this tree

14
_ _ _ _ _ _ _ _ _ _ ^ _ ___ 1QCQ flstosl vliem/1^ "*** 

Figure 7. Typical flood rings formed after minor floods. 

A, June 4, 1959 (1,740 mVs). B, June 4, 1974 (1,670 mVs). 

Note that flood vessels are smaller and more diffuse than 

those formed after larger June floods (see fig. 6). 

A, X30; B, X20.

B

1974 

1973 latewood

Table 3. Frequency of latewood flood rings formed in July- and August-flood years, 1930 to 1979. Floods are ranked in order of 

increasing magnitude

Peak discharge

Number of trees

Date of flood (cubic meters per second) With crown inundated1 With flood rings

July floods:

July 26, 1938 

July 1, 1972 

July 11, 1970 

July 21, 1956 

July 19, 1949 

2 600

1,460

1,510

2,050

2,590

'Determined by comparing local crest altitudes with estimated tree heights.

2Mean daily discharge.

3Not applicable; larger flood occurred earlier in growth year.

4

15

14

19

25

1 

3NA

2

2

3NA

August floods: 

August 1,1945

August 27, 1937

August 25, 1933

August 20, 1955

1 990

9 SOO

2

£.on

6

1 or\

11

13 

5 

36

1 

0 

0 

0

than would the May 30 flood because velocities 

were higher at this larger discharge. In addition, 

this tree had a 1964 flood ring in which latewood 

flood vessels gradually merge into the earlywood

zone along parts of the ring circumference. Thus, 

by May 1, when the 1964 flood crested (2,570 

mVs), the tree had already made the transition 

to latewood growth along part of its circumfer15
Figure 8. Flood rings formed after late-season floods. 

A, Flood ring formed after the flood of July 21, 1956 (2,050 

mVs). A narrow band of thin-walled fibers precedes the 

flood vessels. Largest flood vessels are 50 to 60 /^m, compared to typical vessels (25-35 /^m) within the latewood. B,

-1946 

1945 flood ring

1945 late wood

Flood ring formed after the flood of August 1, 1945 (1,390 

mVs). Flood vessels are 45 to 70 /^m. The large solitary 

vessel (110 /^m) in the center of the ring is not part of the 

1945 flood ring. A, X70; B, X70.

ence. In other words, because of genetic or unusually favorable site factors, the tree was in an 

advanced state of growth relative to most surrounding trees. The flood of May 10, 1960, was 

even later in the year than the 1964 flood and 

was preceded by the warmest April during the 

50-year study period. Thus, latewood growth in 

the tree probably had already begun by early 

May 1960.

FLOOD RINGS WITHIN THE EARLYWOOD

ZONE

In addition to flood rings within the latewood 

zone, abnormal earlywood growth is commonly 

observed within the annual rings of flood-plain 

trees. As mentioned previously, earlywood growth 

in the lower Potomac basin typically begins in 

mid-April and continues into May. In this study, 

April 15 to May 31 is considered the interval of 

earlywood formation. It seems reasonable to expect that flood damage during earlywood growth 

would result in growth anomalies within the earlywood zone. Floods in mid- or late May, however,

would be followed by latewood flood rings in 

some trees and earlywood flood rings in others, 

depending on the timing of the transition to latewood growth. Furthermore, abnormal growth 

might form within both the earlywood and latewood of a ring if flood damage occurred before 

the growth transition was complete along the entire ring circumference.

Anomalous earlywood growth is more variable 

than anomalous latewood growth and also more 

difficult to distinguish from typical growth. Diffuse, 

discontinuous latewood flood rings are generally 

easier to identify than is the minimum anatomical 

"signal" that represents anomalous earlywood 

growth. Accordingly, the designation of a particular ring as "anomalous" was made by comparing 

that ring not only to corresponding rings of other 

trees in the collection but also to several adjacent 

rings in the sample. This was particularly practical 

if patterns of earlywood growth varied little from 

year to year, even during most years of flooding. 

Unvaried patterns were common in trees growing 

along sheltered reaches or at high flood-plain altitudes. In contrast, frequently flooded trees along 

moderately sheltered and unsheltered reaches

16
Table 4. Frequency of latewood flood rings formed in early May (May 1-15) and late May (May 16-31) flood years, 1930 to 1979. 

Floods are ranked in order of ascending magnitude

Date of flood

Peak discharge 

(cubic motors por second)

Number of trees

With crown inundated1 With flood rings

Early May

AAn\/ *?

May 9,

May 1 1 ,

Mnv T3

May 9,

May 7,

May 1,

May 10,

AAn\/ *?n

AAnv "I A

group: 

1Q75

1Q/.4.

TOf.~7

1QT3

1959

1 944 '

ioc:a '

1 QZ. A '

IQftO '

1960

1939 A

V\f\

OOA

39fi

Ann

,790

? 9OO

2 T3f)

2,570

3,510

4*»n

i 7*n

16

12

12

2

9T

14

29

25

oo . 33)

14 *

5

0

0

0

0

0

0

0

0

2

0

Late May group: 

May 25 26, 1938

May 20, 1950

May 22, 1943

May 30, 1968

May 17, 1978

May 18, 1942

May 24, 1942

2 50

1,270

1,910

2

01A

3 31 0

1,720

3

OAf\

5 

15 

13 

26 

30 

13,

20 5

1 

2 

1 

2 

0

9

'Determined by comparing local crest altitudes with estimated tree heights. 

2Mean daily discharge.

occasionally developed abnormal earlywood growth 

in numerous annual rings. In rings formed near 

the center during flood years, it was sometimes 

difficult to determine if a minor anatomical variation was caused by flooding because growth patterns in central rings commonly vary more than in 

outer rings. In all cases, therefore, the designation 

of anomalous earlywood growth was made 

conservatively.

Jumbled and Extended Earlywood

A common pattern of abnormal earlywood 

growth is the clustering of distorted vessels among, 

or replacing, typical vessels. These abnormal vessels generally have unusually small diameters, 

may be discolored or tylosic, and generally do 

not greatly increase the width of the earlywood 

zone. Local variations in the size and spacing of 

these vessels impart a "jumbled" appearance relative to typical earlywood growth. These jumbled 

patterns are similar to the clusters of irregular

vessels composing latewood flood rings. On occasion, the entire width of the earlywood zone consists of jumbled vessels, but, generally, abnormal 

growth is confined to part of the width (fig. 9). 

The most striking examples of jumbled earlywood 

growth are generally related to floods that occurred prior to mid-May.

In another pattern of flood-related growth, an 

earlywood zone of typical width is formed, followed by a radially "extended" zone of abnormal 

vessels that is generally continuous around the 

circumference of the ring. Extreme variations 

were observed in the number, size, and spacing 

of these vessels and, hence, in the width and pattern of the extended zone. In the most striking 

cases, the extended zone consists of numerous 

vessel ranks that double or triple the typical 

width of the earlywood. Vessels are mostly uniform in size and arranged in diffuse, often tapered 

patterns. Zones of extended vessels are wider 

than latewood flood rings and are not separated 

from the earlywood by a conspicuous band of 

latewood. However, it may be difficult to distin17
Figure 9. Flood rings of jumbled vessels within the early wood.

Some rumbled vessels

Figure 9A. Growth ring formed in 1960. The first rank of large vessels is followed by a jumbled group of smaller 

vessels, generally with tyloses. Floods occurred near the end of the earlywood-growth interval (May 10 3,510 

mVs and May 30 1,450 mVs). (X70.)

guish the first rank of extended vessels from the 

terminal rank of typical earlywood. This type of 

growth is observed most frequently following 

mid- or late May floods. Examples of extended 

earlywood growth are shown in figure 10.

The designation of anatomical patterns as jumbled or extended was generally difficult. In many 

cases, both growth patterns were simultaneously 

observed in the same part of a ring. For example, 

part of the typical width of the earlywood zone 

and all of the extended zone may be composed 

of jumbled vessels; in other rings, the degree of 

jumbling or vessel extensions may be difficult to 

determine, or patterns may be highly variable 

and discontinuous around the ring circumference 

(fig. 11). In years in which a tree was flooded 

twice during earlywood growth, it is not always 

possible to determine which flood affected growth 

or if anomalous growth developed as a result of 

damage from both floods. Even a single flood 

sometimes produced a great range of growth responses, presumably due to differences in the 

stage of growth around the stem at the time of

the flood or to differences in the amount of flood 

damage. As will be discussed later, it seems 

likely that anomalous growth patterns which develop from local injury to the cambium or expanding vessels differ from patterns that develop from 

damage to the crown. Because the kind of anomalous earlywood growth formed seems to be a 

result of many factors, all the anomalous patterns 

are in general referred to simply as earlywood 

flood rings.

Earlywood flood rings (jumbled and extended 

growth collectively) were observed within treegrowth rings formed in 26 of the years from 1930 

to 1979. In 24 of these years, floods equaled or 

exceeded 1,270 mVs during the approximate 

time of earlywood formation. In the other 2 years 

(1938 and 1953), smaller peak discharges occurred, 

but each was large enough to inundate trees in 

which 1938 and 1953 earlywood abnormalities 

were observed. None of the earlywood flood 

rings developed during years in which floods 

were not generated during the earlywood growth 

interval. Minor floods in 1964, 1966, and 1970 are

18
1952 

1951 latnwood

Figure 9B. Growth ring formed in 1952. Jumbled vessels compose much of the earlywood, although some vessels 

in the first rank appear typical. Flood damage in 1952 occurred at an earlier stage of earlywood growth than in 

1960 (April 29 4,190 mVs and May 13 1,790 mVs). (X70.)

Figure 9C. Earlywood growth in 

1973 (April 29 2,360 mVs), in 

which jumbled vessels compose 

almost the entire earlywood 

zone. Note the size range of 

vessels in the first rank. (X70.)

19
Figure 10. Flood rings in which the earlywood zone appears extended. Vessels may be jumbled and tylosic but 

generally to a lesser extent than those in figure 9.

Figure 10A. Flood ring formed after the flood of April 28, 1937 (9,830 mVs). Extended vessels are similar to those 

in figure 10C, but the width of the flood ring is more variable. (X40.)

the only floods for which anomalous earlywood 

growth was not detected. Table 5 lists the 26 

years of flooding and the corresponding earlywood 

flood rings that developed in 27 of the samples. 

In 9 flood years, two floods occurred during the 

earlywood growth interval. Collectively, 81 earlywood flood rings were observed compared to 58 

latewood flood rings (excluding 1972) within the 

same period.

For single-flood years, the flood of April 29, 

1973 (2,360 mVs), corresponds to anomalous ring 

development in the largest number (seven) of 

samples. Floods in 1937, 1939, 1940, 1944, 1950, 

1951, 1953, and 1978 are each associated with 

anomalous growth in at least three trees. In 1932, 

1938, 1961, and 1967, only one tree had atypical 

growth. Jumbled flood rings were more common 

than extended-vessel zones after floods in April 

and early May (particularly in 1932, 1948, 1949, 

1951, 1967, and 1973). In years of floods in midto late May, however, the earlywood zone is 

generally not jumbled but is followed by a ring of 

extended vessels that may or may not appear 

jumbled.

In 9 of the 26 years in which earlywood flood 

rings were formed, two floods occurred during 

the earlywood growth interval. In 4 of these years 

(1943, 1952, 1958, and 1960), at least four trees 

developed abnormal growth. In 1952, for example, 

floods peaked on April 29 (4,190 mVs) and May 13 

(1,790 mVs), and 11 samples developed anomalous growth. The earlywood zones of 9 of these 

11 trees were mostly jumbled, and those of the 

other 2 trees were both jumbled and extended. 

It seems reasonable that the April-flood-damaged 

trees before the earlywood zone was complete, 

and, thus, much of the earlywood growth appears 

jumbled. The other two trees may have been 

damaged by the flood in mid-May, or earlywood 

growth may have been more nearly complete at 

the time of flood damage in April. The greater 

proportion of trees with jumbled earlywood is 

consistent with the hypothesis that the larger 

April flood damaged more trees than did the May 

flood.

In 1943, small floods occurred near the beginning and end of the earlywood growth interval 

(2,370 mVs on April 22 and 1,910 mVs on May 22).

20
Figure 10B. Flood ring formed after the flood of April 1937 in which the width of the extended zone is only about 

three ranks of vessels. (X70.)

Unlike floods in 1952 and 1960, the floods in 1943 

were of approximately the same magnitude. In 

the five trees that developed anomalous growth, 

several growth patterns were observed. In one 

tree, the earlywood zone was extended by a diffuse row of vessels documenting the May flood. 

In another tree, the first rank of vessels was jumbled, as was much of the remainder of the earlywood, and a widely extended zone of diffuse 

vessels also developed. Three other trees had 

jumbled growth within the first-formed part of the 

earlywood along small parts of the ring, whereas 

the remainder of the circumference was composed 

of predominately typical earlywood vessels followed by diffuse, extended zones. Generally, the 

annual rings were narrow near the jumbled vessels and wider where earlywood growth was 

more typical.

Combination Flood Rings

Eleven growth rings from nine harvested trees 

formed latewood flood rings along parts of the 

annual-ring circumference and earlywood flood 

rings along other parts (fig. 12). These 11 growth

rings formed during 9 flood years between 1942 

and 1974 (table 6). Generally, the position of 

latewood flood vessels gradually changes along 

the circumference until the flood vessels locally 

replace part of the typical earlywood and form an 

earlywood flood ring. One sample, however, developed an earlywood flood ring along all of the 

circumference except for a local flaring of vessels 

preceded by a band of typical latewood growth. 

The total ring width was greatest in the region of 

the latewood flood ring.

The single combination flood ring in 1964 was 

the only example of flood-related growth for that 

year in the entire collection. For the other 8 years 

in which combination flood rings formed, other 

trees in the collection had earlywood or latewood 

flood rings singly (see tables 2, 4, 5). For example, in 1942, 1943, 1946, 1960, 1968, and 1974, a 

total of 22 latewood flood rings were also observed, whereas a total of 34 earlywood flood 

rings formed in 1942, 1943, 1952, 1958, 1960, 

1968, and 1974. 3

3The 1974 flood ring formed after an early June flood and 

thus is not listed in table 5.

21
Figure 10C. Flood ring formed in 1943 (April 22 2,370 mVs and May 22 1,910 mVs) in which the extended zone 

composes approximately one-half the total ring width. Note the diffuse, symmetrical arrangement of vessels and 

that the extended zone is several times wider than the typical earlywood. Presumably, the extended zone is 

evidence of the May flood. (X35.)

Depauperate Earlywood

Another general category of abnormal growth 

is a narrow earlywood zone composed of vessels 

that were smaller and more diffuse than those 

typically formed. This abnormal pattern extends 

along part or all of the circumference and generally imparts a "depauperate" appearance to the 

earlywood zone (fig. 13). Generally, vessels are 

30 to 50 percent of typical size and may be discolored or in jumbled clusters. Occasionally, vessels 

are so small and diffuse that earlywood appears 

discontinuous.

Depauperate earlywood was observed in 23 

growth rings corresponding to seven of the years 

from 1930 to 1979 (table 7). The growth year most 

frequently associated with depauperate earlywood was 1961 (eight trees). It is followed by 

1979 (four trees), 1958 and 1978 (each with three 

trees), and 1936 and 1948 (each with two trees), 

and 1968 (one tree). Each of these 7 years was 

marked by large floods and (or) heavy ice flows

during times of tree-growth dormancy. For example, the flood of March 19, 1936 (13,700 mVs), is 

the largest known in the lower Potomac River 

basin. In 1979, another large flood (5,830 mVs) 

was generated on February 26, and smaller 

floods developed on January 26 (3,480 mVs) and 

March 7 (2,650 mVs). Ice jams or ice flows in the 

remaining 5 years associated with depauperate 

earlywood include a catastrophic jam in midFebruary 1948, which destroyed many trees along 

the Chain Bridge reach (Sigafoos, 1964), and a 

smaller, less destructive jam in mid-January 1968, 

along with three small winter floods (Yanosky, 

1982b). The flood of January 28, 1978 (4,110 

mVs), carried large ice flows, and, although jamming did not occur, vegetation was severely 

damaged along much of the reach between Difficult Run and Chain Bridge. Other floods in 1978 

occurred on March 16 (4,330 mVs) and March 28 

(3,480 mVs). The frequency of flooding from 

January 1978 to February 1979 is striking in that 5 

of the 29 largest floods during the study period 

were generated during this time.

22
Figure 11. Growth rings with enigmatic vessel patterns or with jumbled and extended vessels along the same 

radius.

Figure 11 A. Anomalous growth formed in 1951. The first 

rank is composed of jumbled groups of atypically 

small, tylosic vessels. Ranks of larger vessels are 

typical of earlywood cells but are extremely diffuse 

(compare to fig. 10). Floods occurred during the 1951

growing season on April 14 (1,710 mVs) and on June 

15 (3,120 mVs). In addition, seven floods (1,420 mVs or 

greater) occurred between the end of the 1950 growth 

year and mid-April 1951. It is unknown which floods 

are associated with the growth zones. (X70.)

Depauperate and jumbled earlywood have 

been detected within separate annual rings formed 

in 1958. Jumbled earlywood (see table 5) probably 

developed after damage from the flood of May 7 

(2,330 mVs). Depauperate earlywood in 1958, on 

the other hand, is believed to have resulted from 

ice damage before the onset of spring growth. 

According to records, stage-discharge relations of 

the gage near Washington, D.C., were affected 

by ice from January 6 to 14. On January 15, the 

daily average discharge increased from the previous 9-day average of approximately 130 mVs to 

at least 870 mVs, and, according to Sigafoos 

(1964), blocks of ice were deposited on parts of 

the flood plain near Chain Bridge. Another minor 

flood occurred on March 1 (1,390 mVs) but, according to discharge records, was preceded by 17 

days of low flow, during which daily gage readings were affected by ice. Thus, the sudden rise 

in stage probably also resulted in ice-laden flows

that damaged some trees. In addition, a small 

flood occurred on March 29 (1,900 mVs).

Floods were generated in 1961 on February 21 

(3,290 mVs) and on February 27 (2,860 mVs). According to surface-water records, the first flood 

was preceded by 66 days during which stagedischarge relations were affected by ice. This 

flood carried large amounts of ice and severely 

damaged many trees (R. S. Sigafoos, U.S. Geological Survey, oral commun., 1965). In addition, 

average daily discharges exceeded 850 mVs for 

16 days during March, and a minor flood (2,620 

mVs) occurred on April 14 (just prior to the initiation of earlywood growth.

Discolored Earlywood

A final category of flood-related growth is the 

discoloration of tissues that otherwise are typically 

shades of amber yellow. Discolorations range

23
Figure 11B. Growth ring formed in 1940. Jumbled vessels in the first TWO ranks of the earlywood are followed 

abruptly by a zone of diffuse vessels. It appears that growth responded to two episodes of flooding (April 22- 

3,030 mVs and June 2 2,250 mVs). (X40.)

from shades of reddish brown to black. Discolored 

tissues are in discrete groups that may be irregular and diffuse or in nearly contiguous patterns 

that impart a "bruised" appearance even when 

viewed with the unaided eye. Groups of discolored cells were observed along the entire circumference of some growth rings. Despite differences 

in the spacing between groups, the area of each 

group is fairly uniform 0.5 to 1.0 mm2 . Ranks of 

discolored cells rarely extend radially for more 

than 1.0 mm but may traverse the entire width of 

extremely narrow rings. More commonly, however, the discolored tissues are primarily the 

thin-walled cells of the earlywood and first part 

of the latewood and part of the latewood of the 

previous year (fig. 14).

This pattern differs from the uniform darkening 

of the heartwood region that occurs with advancing tree age and from color changes associated 

with scar formation. In scars, wood cells are replaced by a mass of darkened, undifferentiated 

wound tissue ("callus"). The discolored tissues described in this report are composed of ordinary 

wood cells that are atypical only in color, even

though they were frequently observed in growth 

increments in which flood rings formed.

Zones of heavily discolored wood along the entire ring circumferences were most frequent in 

1972 annual rings. Latewood flood rings were 

present in all of these. Generally, discolorations 

were darkest in the 1972 earlywood, even in 

wide rings, and discolored vessels commonly developed tyloses. In some 1972 rings, discolorations 

also were observed within the latewood flood 

rings. Heavy discolorations along the entire circumference were also observed in growth rings 

formed in 1960 and 1967. Similar heavy discoloration along smaller parts of the ring circumference 

was observed in many rings corresponding to 

flood years from 1930 to 1979. Regardless of the 

time of flooding, heaviest discoloration was generally within earlywood growth regions.

For a particular growth ring, discolored tissues 

were observed at random stem heights. Flood 

rings, on the other hand, are most highly developed near the stem apex and, in many samples, 

were observed at all stem heights. This strongly 

suggests that discoloration develops not as a re24
Table 5. Frequency of earlywood flood rings, 1930 to 1979. Interval of earlywood growth is April 15 to May 31

Date of flood

Peak discharge 

(cubic meters per second)

Number of trees

With crown inundated1 With flood rings

May 14, 1932

Anril 99 1Q99

May 11, 1933

April 28, 1937

May 25 26, 1938

Anril 19 1999

April 22, 1940

May 18, 1942

May 24, 1942

Apr! 122, 1943

May 22, 1943

May 9, 1944

April 16, 1948

Apr! 11 6, 1949

May 20, 1950

April 15, 1951

Apr! 129, 1952

May 13, 1952

May 18, 1953

Anril^O 1QSR

May 7, 1958

May 10, 1960

May 30, 1960

Apr! 11 8, 1961

April 22, 1964

Mayl, 1964

May3, 1966

May 9, 1967

May 30, 1968

Apr! 11 6, 1970

Apr! 126, 1970

Anril 9Q 1Q79

Apr! 127, 1975

May 6, 1975

May 17, 1978

4,760

3 Ann

1,400

Q RTO

2p^n

2,200

9 mo

1,720

9 940

2

07O

1,910

2

O/V\

2,760

1,340

1,270

3 1 STO

4,190

1 ~TOf\

21 /yi/"|

1,410

9 T3O

3,510

1 450

2,070

1 9SO

2,570

1 9RO

i ion

9 Tin

1,770

1,710

9 TftO

1,440

1 T3O

S oifl

5 

5 i

2 ' 

11 

5 

15 

18 

13,

20 i 

13,

13 ' 

14 

17 

12 

15 

18 

35 ,

23 f 

10 

16 ,

29 1 

33 >

14 1 

20

11 \

25 J 

12 

12 

26

15 1

15 I 

27 

16 >

16 1 

30

1

1

4

1 

3 

3

i

5

5 

2 

2 

3 

3

11

6

4

1 

0

0 

1 

2

0

7 

2

3

'Determined by comparing local crest altitudes with estimated tree heights.

2Mean daily discharge.

3Mean daily discharge; preceded by flood peak on April 14 (1,710 mVs).

suit of flood-induced crown damage but rather 

from bark damage that permits the entry of microorganismal fungi. It is possible that tissues are 

partly decomposed, and hence discolored, from 

the metabolic activities of these microorganisms 

and that the flaring of the discoloration to the 

preceding year's increment results from the radial 

spread of fungi through ray parenchyma. The discoloration may, however, result from the deposition of materials (for example, tannins) produced 

by flood-injured tissues. Regardless of the mechanisms of formation, it seems most probable that 

discolored tissues are a consequence of injury.

MECHANISMS AND TIMING 

OF FOOD-RELATED GROWTH

Earlywood and latewood flood rings did not develop in trees in which crowns were not inundated. Apparently, these types of abnormal growth 

formed only if leaves and buds were damaged,

25
Figure 12. Combination flood ring formed after the floods of May 18 (1,720 mVs) and May 24 (3,940 mVs), 1942.

Figure 12A. Typical latewood part of the 1942 flood ring. The transition from earlywood to latewood growth had 

occurred along the part of the ring prior to the floods. Note that considerable radial growth developed after the 

formation of the flood ring. (X70.)

and a new leaf crop was subsequently produced. 

Entire branches may be stripped by flood waters, 

or, alternatively, leaves and buds may be locally 

removed. Many remaining leaves may be torn, 

covered with mud and debris, or shed after flood 

waters recede. Adventitious leaves and new 

sprouts typically form along the trunk or broken 

crown.

The formation of flood-induced growth may be 

explained by the mechanism believed to regulate 

the development of vessels. The enlargement of 

earlywood vessels (see fig. 2) is, to a large extent, 

induced by hormones produced in the buds and 

expanding leaves and transported basipetally to 

the newly formed cambial derivatives when dormancy is broken in early spring (Wareing and 

others, 1964). It is here suggested that refoliation 

is physiologically similar to this early spring 

growth. A flush of growth regulators, manufactured by the second crop of breaking buds and 

expanding leaves, initiates the development of 

large vessels within the latewood or earlywood.

The size and number of flood vessels formed 

would be expected to be a function of the distance 

from the stem apex and the amount of hormones 

produced. It seems reasonable to assume that 

trees which completely refoliate produce a greater 

flush of hormones than partly retaliated trees 

and, therefore, are more likely to develop continuous rings of large-diameter flood vessels along 

the entire stem. Partly refoliated trees probably 

form distinct flood rings only near the apex because the supply of hormones is depleted before 

it reaches the base of the tree.

The hormonal control of flood-ring formation 

may also explain the positive relation between 

peak stages and the corresponding frequencies of 

flood rings in inundated trees. In addition to damaging more trees than small floods, large floods 

also presumably damage a greater proportion of 

leaves and buds of many trees. This may be due 

to increased velocities during large floods relative 

to smaller floods and to greater durations of 

crown submergence. A larger crop of new leaves,

26
Figure 126. Earlywood part of the 1942 flood ring along a different part of the growth increment. Flood vessels 

appear as part of the earlywood zone because growth was locally less advanced at the times of the floods. Note 

the difference in total ring width from that in figure 12A. (X70.)

Table 6. Frequency of combination flood rings, 1930 to 1979. Anomalous latewood and earlywood growth developed along 

different parts of an annual ring after a flood

Date of flood

Peak discharge 

(cubic meters per second)

Number of trees 

with combination flood rings

May 18, 1942

Mav 94 1 Q49

Anril 99 ]QAt

Mav 99 1 Q4*}

June 4 1946

April 29 1952

May 1 3 1 952

* Anril *r* * v%30 *» 1958 *****

May 7 1 958

Mnv ID 1 QAO

May 30 1 960

Anril 99 IQ/vd

Mayl, 1964

May 30 1 968

lime* A 1Q74

1,720 1

O QAf) \

9 ^70 i

1,910 1"

1 QSf)

4,190 j

1,790 1

1,410 i

S oon |

3,510 )

1,450 1

1,350 i

2,570 J

9 Tin

1 ft7f)

2

1

1

1

i1

1

]

2

l

27
Figure 13. Depauperate earlywood formed after an ice jam in mid-February 1948. Earlywood vessels are smaller 

(25-85 (im) than those of the 1947 ring. Note tyloses in vessels of the depauperate zone. (X70.)

Table 7. Frequency of depauperate earlywood, 1930 to 1979, in 23 growth rings of 17 trees

Date of flood

Peak discharge 

(cubic meters per second)

Number of trees 

with depauperate earlywood

February 27, 1936

March 13, 1936

March 19, 1936

February 16, 1948

January 15, 1958

February 21, 1961

February 27, 1961

January 15, 1968

Inniinrv9ft l°7fi

March 16, 1978

March 28, 1978

January 26 1979

February 26, 1979

March 7, 1979

3,740 .

3 000 >

n TOO )

'2 120

2Q"7/"\

21 900 i

2 860 J

770

24,110 -v

4 iin \

1 4RO J

1 4RO

«; Rin i.

2,650 /

2

Q

1

4

'Bank-to-bank jamming of ice near Chain Bridge. 

2 lce-block deposition on flood plain near Chain Bridge.

28
/ '+ ,V*

Figure 14. Discolored earlywood. Note that darkened tissues are in small clusters along ring circumference. The 

number of clusters varies considerably, occasionally forming dense, nearly continuous patterns. (X2.)

and consequently a greater flush of hormones, 

would be expected in severely damaged trees 

than in trees in which only a small part of the 

original crown was damaged. In severely damaged 

trees, flood rings are likely to form at considerable distances from the stem apex. When damage 

is minor, flood rings are unlikely to form at or 

near stem bases unless trees are extremely small. 

Flood rings rarely form after late summer floods, 

probably because radial growth of trees in the 

lower Potomac basin is completed or greatly reduced from that in May or June. The timing of the 

reduction and the eventual cessation of annual 

growth probably depends to a large extent on environmental and genetic factors. However, the 

study of sample cross sections indicates that 

young trees often grow later into the summer 

than older trees on comparable sites. Radial files 

of fibers with strikingly thin secondary walls and 

large lumens have been observed within the latewood zones of young but not of older trees. These 

atypical fibers appear to form during growth 

spurts and are related to periods of intense precipitation and concomitant increases in Potomac 

River discharge. Thus, older trees, even if defoliated or damaged by a large flood such as that of 

August 20, 1955 (6,120 mVs), have already ceased 

radial growth or are growing so slowly that postflooding growth is negligible. Defoliation does 

not result in the production of a new leaf crop, 

and latewood flood rings do not form. Even young, 

vigorous trees may not produce new leaves in 

August.

Defoliation in July, however, is followed by refoliation and significant postflooding growth in 

some trees. Thus, flood rings are hypothetically 

more likely to form after July floods than after 

those of August, although less likely than after 

May or June floods. The trees forming flood rings 

after late-season floods were young, with relatively wide annual rings. If precipitation is generally abundant during most of the growing season 

preceding a late-year flood, it seems reasonable 

to assume that some small trees will still be 

growing vigorously enough to replace damaged 

leaves. If so, evidence of late-season floods would 

more likely be present in the inner than in the 

outer rings of old trees.

At the onset of the study, it seemed reasonable 

to assume that jumbled earlywood developed

29
only as a result of direct damage to expanding 

vessel members. Direct damage probably occurred 

where vessels are jumbled only along a small 

part of the ring circumference. However, in growth 

increments with fairly continuous zones of jumbled 

earlywood, anomalous growth was generally less 

pronounced in basal than in apical sections. In 

several trees, jumbled zones were absent from 

wood samples near the tree base but well developed in sections near the apex. This strongly suggests that jumbled vessels, like flood vessels 

within the latewood zone, develop in response to 

a second flush of growth regulators following 

crown damage. If this were not the case, jumbled 

earlywood would have formed at random with respect to stem height. Furthermore, if jumbled 

growth resulted only from mechanical stresses, 

jumbling would be expected to be more frequent 

along upstream-facing sides of trunks than along 

downstream-facing sides. This has not been observed. A zone of jumbled earlywood, therefore, 

is a flood ring in which flood vessels develop primarily within, rather than outside, the terminal 

rank of typical earlywood growth.

Similarly, the extended earlywood of most 

trees is more fully developed near stem apices 

than bases. Vessel patterns of the narrowest extended zones are sometimes similar to those of 

latewood flood rings. In the widest zones, however, vessel arrangement differs from that of 

latewood flood rings; vessels are extremely diffuse and may form a radial zone several times 

the typical width of the earlywood (see fig. 10C), 

and, thus, the entire annual ring is unusually 

wide. If the rate of formation of earlywood were 

the same as that of a widely extended zone, 

there would be insufficient time for both an extended zone and latewood to develop. Therefore, 

it seems more likely that the growth rate of a 

widely extended zone exceeds that of the earlywood. Accordingly, the extended zone probably 

does not develop as a result of defoliation and 

retaliation because flood damage generally has a 

negative impact upon ring width. Nevertheless, 

the most striking examples of extended earlywood 

were detected in trees flood in April- and Mayflood years. It may be, therefore, that extended 

growth in some trees is a response to flooding 

but not to flood damage; that is, prolonged root 

flooding may be favorable to growth if crowns 

are not damaged. Radial growth is accelerated, 

and large vessels continue to form, although size 

and spacing patterns differ from those of typical 

earlywood growth.

The extended-growth pattern may be somewhat 

analogous to radial growth in conifers, in which

the transition from earlywood to latewood seems 

to be controlled in large part by water availability. 

Earlywood growth in conifers continues until 

water becomes limiting, and, thus, there may be 

great yearly differences in the proportions of the 

two zones. In ring-porous species, however, the 

transition from earlywood to latewood growth 

seems largely controlled by endogenous factors, 

and the width of the earlywood is often remarkably constant (R. L. Phipps, oral commun., 1982). 

This does not always appear to be the case in 

flooded ash trees, in which an environmental factor (extreme water availability) seems to prolong 

the period of earlywood formation. A widely extended zone, therefore, probably forms during a 

period of increased growth rate, producing a 

gradual rather than abrupt transition from earlywood to latewood growth. In 1953, for example, 

most growth rings are unusually wide relative to 

adjacent rings, and the extended zone of flooded 

trees is diffuse and flared. May 1953 was the 

wettest May during the study interval, although 

only a minor flood was generated.

Extended growth, like latewood and jumbled 

flood rings, is presumably regulated by hormones. 

As previously noted, it can be argued that refoliation produces large amounts of diffusable hormones that mediate the formation of abnormal 

growth. Thus, latewood and jumbled flood rings 

are generally more highly developed near stem 

apices than in lower trunk regions. This growth 

trend with respect to tree height is also observed 

in annual increments with extended earlywood 

zones, probably because it depends on a hormonal gradient from the crown down the trunk. If 

a hypothesis is correct that widely extended zones 

develop as a result of a growth-rate change, the 

controlling hormones must be produced from new 

leaves at stem tips, rather than from refoliation 

along entire branches. In other words, extendedearlywood growth is probably related to rapid 

shoot elongation. This seems reasonable because 

favorable conditions are generally associated 

with relative increases in both radial and apical 

growth. It should be emphasized, however, that 

some narrowly extended earlywood probably 

does form after refoliation, particularly when vessel patterns resemble those of latewood flood 

rings. It can be argued that narrowly extended 

zones are latewood flood rings that develop adjacent to the earlywood and, hence, are not true 

extensions of earlywood growth. In light of the 

great variety among abnormal growth patterns of 

all types, however, the simple classification of 

"latewood" and "earlywood" flood rings seems 

justified. Of greater importance than precise clas30
sification is the correlation of abnormal earlywood 

growth with early-season floods.

Depauperate earlywood is related to flooding 

before the growing season and to ice jams, but 

growth abnormalities do not appear to have resulted solely from damage to the cambium or developing vessels. Rather, it would seem that the 

small size of earlywood vessels resulted from a 

low concentration of diffusible hormones at the 

time of spring growth. Because of earlier damage 

to stems and leaf buds, bud break was unusually 

small, and expansion of vessels was retarded. 

This hypothesis is strengthened by studying cross 

sections from different stem heights. Although 

this could be done in only 5 of the 16 trees that 

developed depauperate earlywood, it was clear 

that the degree of abnormal growth was greatest 

in basal sections and became progressively less 

pronounced in apical sections. This suggests that 

abnormal growth was caused by a diminishing 

gradient of hormones from the apex to the lower 

trunk. Thus, it seems that trees with depauperate 

earlywood were severely damaged between the 

onset of growth and the cessation of growth in 

the previous year. Because the formation of depauperate earlywood and flood rings seems to be 

controlled by growth-regulating substances, it is 

reasonable to consider depauperate earlywood a 

special category of flood ring. However, unlike 

rows of flood vessels within the latewood zone 

and additional vessels jumbled among or just outside the earlywood, depauperate vessels develop 

only within the earlywood. This type of flood ring 

is evidence of floods in the fall, winter, or early 

spring, although at present there seems to be no 

practical means to estimate the time of occurrence 

of individual floods.

Field Experiments With Defoliated Trees

To test the hypothesis that flood rings develop 

after damage to leaves and buds, groups of small 

trees were manually defoliated (table 8) at monthly 

intervals from mid-May 1980 to mid-August 1980. 

Other trees were defoliated in mid-June 1981 and 

in spring 1982. Typically, study trees were 1 to 3 m 

in height and 2 to 5 cm in basal diameter. Ages 

ranged from 1 to 11 years. Trees defoliated in 

1980 were harvested in October, and those defoliated in 1981 were cut down in mid-August. Samples from the trunk base, middle stem, and apex 

were collected, and, in some trees, a few sections 

from lateral branches were also taken.

Of 14 trees defoliated in June 1980 and 1981, 8 

grew complete or moderately complete crops of 

new leaves. Refoliation typically began within 1 

week, and first-formed leaves grew in small clusters at stem tips. No adventitious sprouts from 

trunks or roots were observed. Leaf sizes near 

the end of the growing season ranged from 50 to 

100 percent of those of nearby trees. Abnormalities were not observed within the growth increments of June-defoliated trees that did not produce 

a second leaf crop. Refoliated trees, however, 

developed abnormal radial growth similar to latewood flood rings. In some trees, particularly 

when the second leaf crop approximated that of 

the original leaf cover, abnormalities were observed at all stem heights sampled. Partly refoliated trees developed abnormal growth only near 

stem apices. Anomalous growth in all refoliated 

trees formed the terminal ranks of the latewood 

so that no subsequent radial growth developed.

Of five trees defoliated in mid-July 1980, three 

refoliated, although only a small percentage of

Table 8. Summary of manual defoliation of ash trees, 1980 to 1982. Anomalous growth similar to flood rings developed in 

numerous defoliated trees but only when trees produced a second leaf crop

Date Number of trees

Defoliated Harvested Defoliated Refoliated

'Refoliated only near apex.

J ln one tree, abnormal growth at apex only.

Number of trees 

with abnormal radial growth

Refoliated Not refoliated

June 17, 1980

June 23, 1981

July 21, 1980

August 22, 1980

May 15-16, 1980

May 1, 1982

October 22, 1980

August 17, 1981

October 17, 1980

October 17, 1980

October 17, 1980

May 29, 1982

4

10

5

6

8

9

3

5

'3

0

8

9

3

5

23

0

4

2

0

0

0

0

0

0

31
the original leaf crop was replaced. The timing of 

bud break was comparable to that of trees defoliated in mid-June. One of the three trees fully refoliated only along the apical part of the stem. In 

this tree, rings of abnormal vessels developed in 

the outermost latewood zone from the apex to a 

point approximately half way to the trunk base. 

Latewood fibers surrounding these abnormal vessels appeared to have thinner walls and larger 

lumens than those formed earlier in the latewood 

and imparted a whitish appearance to the wood. 

Sections near the stem base did not develop abnormal rings. A second tree defoliated in July 

formed three fully expanded leaves along the terminal leader but otherwise did not refoliate. The 

pattern and distribution of abnormal vessels were 

the same as in the first tree. The third defoliated 

tree produced six leaves at the apex of a lateral 

branch, although leaves expanded only to about 

10 percent of typical size. Abnormal vessels were 

observed in a twig section 2 cm from the branch 

apex but in no other stem sections.

None of the six trees defoliated in mid-August 

1980 produced new leaves, even though buds on 

several trees had been left intact. Swelling of 

buds was not observed.

The growth responses of trees defoliated during 

the earlywood-growth interval were more variable 

than those of trees defoliated in June, July, and 

August. All nine of the trees defoliated on May 1, 

1982, grew second sets of leaves and were harvested on May 29, 1982. Twigs were collected 

from most trees at the time of defoliation to determine the extent of earlywood growth. At harvest, control samples were taken from trees of 

comparable size which had not been defoliated.

Only one tree defoliated in early May developed 

a completely formed abnormal ring in the latewood. This tree was 1 m in height and grew along 

a tract of exposed bedrock near Difficult Run. The 

abnormal ring was well developed in a section 

0.5 m from the apex but had not formed at the 

base. At the apex, abnormal vessels appeared to 

be forming but were not as large or numerous as 

those in the middle section. In the control tree, 

stem elongation by May 29 was nearly 0.6 m, and 

basal ring width was approximately twice that of 

the defoliated tree.

In another defoliated tree, clusters of abnormal 

vessels appeared to be developing at the time of 

harvest. Vessels were mostly contiguous with the 

first-formed ranks of the earlywood but seemed 

to be in jumbled patterns. No latewood growth 

was detected. In the control, a sprout from the 

parent trunk of the defoliated tree, the earlywood

zone had completely formed, and considerable 

latewood growth had developed.

The other seven trees defoliated in early May 

appeared to be still forming earlywood upon harvest, and unambiguous patterns of earlywood 

vessels were not observed. Rather, defoliation 

was associated with a retardation of radial growth 

compared to the controls.

Of eight small trees defoliated in mid-May 

1980, seven refoliated completely, and one formed 

only a few new leaves. At the time of defoliation, 

this last tree had a heavy fruit crop that was left 

intact. In 1980, unlike in May 1982, trees were 

not harvested until the completion of the growing 

season. Four trees did not develop abnormal 

growth and had extremely narrow 1980 rings. In 

one of the four trees that did develop growth 

similar to a flood ring, the 1980 ring was extremely 

narrow, and virtually no latewood was formed. 

The terminal rank of vessels appears jumbled relative to preceding vessels. Another tree, about 

3 m in height, developed an abnormal ring at all 

stem heights. In the basal section, the abnormal 

growth was mostly within the latewood but appeared to merge with the earlywood along parts 

of the ring. In the narrowest parts of the ring, 

abnormal growth was not observed. Tyloses were 

locally abundant where the abnormal growth was 

most conspicuous. In sections closer to the apex, 

abnormal growth was continuous and formed in a 

greater proportion of the latewood. In a section 

near the apex, abnormal vessels were totally 

within the latewood. At all stem heights, there 

was little or no growth after the abnormal vessels, 

and ring width in 1980 was much narrower than 

that in 1979.

The two remaining trees defoliated in mid-May 

1980 were approximately 1 m in height and appeared to grow from a single parent stump. Near 

the apex, each tree developed a multiranked 

zone of extremely jumbled vessels followed by a 

typical zone of latewood. Along most of the circumference of each ring, it was not possible to 

mark a clear transition between typical and atypical earlywood vessels. In basal sections, however, 

abnormal growth was less conspicuous than near 

apices. At the base of one of these trees, the 

width of the 1980 growth ring was highly asymmetrical. The earlywood appeared typical where 

the ring was narrowest and locally atypical along 

wider parts. Vessels were less jumbled and crowded 

than in the apical section. In the other tree, the 

1980 ring was narrow near the trunk base, and 

jumbling was minimal and highly discontinuous.

32
The results of these field experiments explain 

to some extent why flood rings are most commonly associated with May and June floods. Many 

trees flooded in May and June typically replace 

most or all damaged leaves. Refoliation, however, 

apparently occurs only to a limited extent later in 

the growing season, when even severely damaged 

trees may not fully replace stripped or damaged 

leaves. Partial refoliation is associated with anomalous growth that forms only near stem apices. 

Variations in the type of growth responses after 

May defoliations are fairly consistent with observations in spring-flood years. The formation of 

latewood or jumbled flood rings appears to depend not only on the timing of tree-growth activities but probably also with rates of growth before 

and after crown damage. For example, growth 

conditions were probably unfavorable in 1980 and 

early 1982. Precipitation was low during both periods, and daily maximum temperatures during 

parts of summer 1980 were unusually high. Total 

growth (and presumably the rate of growth) of 

most trees, particularly defoliated trees, was thus 

probably somewhat reduced. Trees defoliated in 

May 1980 that produced new leaves, but without 

concomitant abnormal growth, had extremely 

narrow growth rings in 1980. In 1982, refoliated 

trees were harvested after only 4 weeks, but 

most had narrower rings than nearby trees, and 

most did not have abnormal vessels. It seems, 

therefore, that anomalies do not develop if growth 

rates are significantly limited. This may account 

for the low frequency of abnormal growth in 

manually defoliated trees. If water availability is 

the most important environmental factor correlated with the rate of tree growth, then trees 

damaged by floods are probably not as likely as 

manually defoliated trees to be growth limited by 

water stresses.

BOTANICAL AND HYDROLOGIC FACTORS 

ASSOCIATED WITH FLOOD-RELATED 

GROWTH

Selected trees were studied to determine the 

hydrologic and botanical factors most frequently 

associated with the formation of flood rings. 

These data support the hypothetical mechanism 

of flood-ring formation and may also facilitate the 

selection of samples for applied field studies. Because of considerable differences between trees, 

sites, and hydrologic histories, a factor is difficult 

to assess independently; nevertheless, several 

general conclusions are possible.

Only harvested trees were studied because discontinuous growth anomalies may not be detected 

in increment-core samples. To relate tree sizes 

(estimated from trunk diameters near breast 

height) and ages to years of flood-ring formation 

(figs. 15, 16), a subsample of 18 trees was 

chosen. These trees germinated between 1931 

and 1956 and formed at least two flood rings during years of minor floods, as well as a flood ring 

after the catastrophic 1972 flood. All grew along 

unsheltered or moderately sheltered reaches and 

on surfaces flooded by less than 1,700 mVs, although these were not preconditions for sample 

selection.

The average age of the 18 trees when the first 

flood rings formed was 4.6±6.9 years, coefficient 

of variation (c.v.) equals 150 percent. Fourteen 

trees formed flood rings within the first 3 years, 

including three in the first growth year; one tree, 

however, was 31 years of age. The average stem 

diameter when the first flood ring formed was 

1.3±1.1 cm (c.v. = 84.6 percent). For last-formed 

flood rings associated with minor floods (that is, 

excluding 1972), tree ages ranged from 4 to 41 

years (x=20.0±11.3 years; c.v. = 56.5 percent), 

and diameters averaged 4.9±2.1 cm (c.v. = 42.8 

percent). In six of these trees, the last minor-flood 

ring was formed after 1972. The diameter of 

these six trees (x = 4.7±1.7 cm) did not differ significantly at the 0.05 level from that of the 12 

trees forming a final minor-flood ring before 1972 

(x = 4.9±2.3 cm). However, flood rings in this latter group developed, on the average, 18.5±7.1 

years before 1972. Furthermore, one of the six 

trees forming a final minor-flood ring after 1972 

developed its initial flood ring in 1968, 31 years 

after germination. Trunk diameter in 1968, however, was only 3.2 cm, and tree height in 1980, 

was only 2 m.

These data indicate that formation of flood 

rings varies more with age than with size. The 

high frequency of minor-flood rings in young 

trees is because small trees are inundated even 

by minor floods. In addition, anomalous growth is 

generally easy to distinguish in the wide rings 

characteristic of young trees. When trees are 

older, and thus generally taller, crowns are inundated only by large, infrequent floods. Rings 

are also typically narrower than those near the 

center, and anomalous growth, therefore, may be 

more difficult to detect. If trees remain small because of frequent flood damage, however, they 

may develop flood rings despite increasing age.

In 1972, average age and diameter of the 18 

samples were 31.0±6.3 years and 7.5±3.2 cm,

33
C/)

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LJJ

C/) 

LL 

O

DC 

LJJ 

CD 

^ 

ID 

Z

15

14

13

12

DC

Q H 

O

g 10

LL

8

7

1

0

EXPLANATION

First-formed flood ring 

Last-formed flood ring 

1972 flood ring

6 9 12 15 18 21 24 27 30 33 36 39 42

AGE-CLASS DISTRIBUTION, IN YEARS

Figure 15. Ages of 18 trees at times of flood-ring formation. Each tree formed at least two flood rings in response to 

minor floods and also a flood ring in 1972. Average age when flood rings first formed was 4.6±6.9 years. Flood rings 

last formed after minor floods when trees were 20.0±11.3 years of age, and trees with flood rings in 1972 were 

31.0±6.3 years of age.

respectively; two other trees not included with 

the 18 samples formed 1972 flood rings at ages 

51 and 53 years, respectively. It is concluded that 

older trees are physiologically capable of recovering from flood damage and that the low frequency of minor-flood rings in older trees is because minor floods do not inundate the crowns of 

large trees.

Hydrologic factors associated with flood-ring 

formation were studied by comparing tree ages 

at the last minor-flood ring with flood frequency 

and exposure (fig. 17). The entire collection of 

harvested trees was considered, although trees

were excluded if germination was after 1956 or if 

roots were above the stage of the 2-year flood.

Seven trees grew in moderately sheltered zones 

downstream from Difficult Run. Five were clustered in the lower part of the study area (see 

fig. 1), and minor-flood rings last formed when 

the trees were from 10 to 16 years of age; two 

other trees were in the upstream part of the 

study area and had lower age values (fig. 17). Of 

the five clustered trees, one was approximately 

12 m in height in 1980, but all flood rings were 

formed within the first 16 years of growth. For 

another tree, stem height in 1980 was 6 m, and

34
EXPLANATION

I I First-formed flood ring 

Last-formed flood ring 

1972 flood ring

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

STEM-BASE DIAMETER, IN CENTIMETERS

Figure 16. Trunk diameters of 18 trees at times of flood-ring formation. Samples are the same as those in figure 15. 

Average diameter when flood rings first formed was 1.3±1.1 cm. Flood rings last formed after minor floods when trees 

were 4.9±2.1 cm, and average diameter of trees with flood rings in 1972 was 7.5±3.2 cm.

the interval between germination and formation 

of the final minor-flood ring was 14 years. This 

interval for three other trees was 10, 10, and 16 

years, although, in 1980, each tree was small because aerial parts above breast height had been 

destroyed (one during the flood of January 1978; 

another by an undetermined, earlier flood; and 

the third was felled by beavers after the 1979 

growth year). Nearby, undamaged trees of comparable ages and trunk diameters were 6 to 12 m 

tall, and it seems reasonable to assume that the 

three damaged trees had previously attained similar heights. These data indicate that flood-flow

velocities, even during minor floods, are sufficient 

to damage small trees in moderately sheltered 

areas. Damage, however, is apparently less severe than along unsheltered reaches because 

trees attain greater height.

Trees from unsheltered reaches formed flood 

rings until an older age than did trees from moderately sheltered or sheltered areas. Of 35 trees, 

21 grew along unsheltered areas and, at harvest, 

ranged from 1 to 4 m in height. In some of these 

trees, vertical height was much less than stem 

length because the trees had been partly uprooted. 

Unsheltered trees on surfaces flooded by less

35
LU

Q

O 

O

Q 

LU

^ 

DC 

O

LU 

O 

<

LU 

LU 

DC

50

40

30

20

10

0

I I

EXPLANATION

 Unsheltered

o Moderately sheltered

A Sheltered

00

o

OA

A

A

I T

0 300 600 900 1200 1500 >1800

DISCHARGE FLOODING ROOTS, IN CUBIC METERS PER SECOND

Figure 17. Ages at last minor-flood ring of 35 trees in 

zones of different flood frequency and exposure. All 

trees began growth before 1957. Because 4 of these 

35 trees did not form flood rings, they are not

graphed. These four trees, however, grew in sheltered zones. Two trees were flooded by 850 mVs and 

1,100 mVs, respectively; the two others were 

flooded by about 2,800 mVs.

than 1,800 mVs were kept small by frequent 

damage, and the last flood rings of nine trees 

formed at ages greater than 28 years. Age values 

of the other 10 trees, however, ranged from 2 to 

25 years. No relation was generally evident between age and decreasing flood frequency up to 

1,800 mVs, although most age values of the unsheltered collection were greater than those of 

moderately sheltered and sheltered trees. Two 

small unsheltered trees on a surface flooded by 

about 2,500 mVs developed a final minor-flood 

ring in their second and sixth growth year, respectively. Both trees were about the same size 

and age as more frequently flooded trees that

developed numerous minor-flood rings. It seems 

reasonable that minor-flood rings did not form 

late in the lives of both trees because flood damage is infrequent on high flood-plain levels.

Sheltered ash trees show few external signs of 

flood damage other than occasional corrosion 

scars, and ring-width patterns are less variable 

than those of trees from other flood-exposure 

zones (Yanosky, 1982b). Seven trees from sheltered locations were harvested along the Difficult 

Run study site, and numerous others were sampled with an increment borer. Four harvested 

trees did not develop minor-flood rings, and two 

others formed final minor-flood rings at 2 and 3

36
years of age, respectively. The seventh sheltered 

tree began growth in 1946 and formed a flood 

ring in 1952. Although the base of this tree was 

flooded by 1,400 mVs, it grew directly downstream 

from a large protective rock wall. This tree, and 

possibly much of the lowest part of the wall, 

were inundated by the April 1952 flood, the largest growing-season flood (4,190 mVs) between 

germination of the tree and 1972. In other words, 

some of the sheltering effect was lost, and the 

tree was damaged. Minor-flood rings were not observed in other sheltered trees cored with an increment borer, although, as mentioned previously, 

growth anomalies are sometimes not detected by 

this sampling method.

The data in figure 17 strengthen the hypothesis 

that, along uniform altitudes of the Potomac River 

flood plain, flood-channel geometry is the major 

factor controlling the growth and development of 

flood-plain forests. Trees in high-damage zones 

continue to suffer periodic damage and thus remain small, whereas trees in more sheltered 

zones commonly attain crowns above all but the 

greatest floods. Tree size (height) and form are, 

to a great extent, functions of flood-flow dynamics 

(Yanosky, 1982a, b).

The presence of numerous flood rings in certain 

trees indicates that these trees are "recorders" of 

many floods. Fifteen trees each developed at 

least 5 minor-flood rings, three trees had 10 or 

more, and one formed 18 flood rings. These three 

trees grew along an extremely unsheltered reach 

near the main channel at Chain Bridge. The sparse 

vegetation along much of this flood plain does 

not reduce flood velocities along downstream 

parts of the reach, as is often typical where vegetation density is great. Crowns are kept narrow 

by constant flood pruning, and sprouts from surviving branches are smaller than the older stems 

they replace. This crown form, in turn, may increase the probability of flood-ring formation because damage sustained by floating debris may 

be greater than if the crown were larger and 

more robust. A small crown is a small "target," 

but this would decrease the probability of damage 

only if paths of debris are random with regard to 

the flood wave. Observations from shore and 

from a canoe suggest that paths are not random, 

even within a particular zone of flood exposure. 

Rather, debris is routed along flow "corridors." 

Velocities may differ between nearby debris corridors or may change with flow volumes, presumably 

because of changes in flood-channel roughness. 

Plow differences within these corridors may be 

somewhat analogous to the simultaneous temperature differences in adjacent areas commonly recorded during microclimatic studies. Trees with 

numerous flood rings may be those in zones 

where debris flows are heaviest. It is also possible 

that the preponderance of flood rings in some 

trees is due in large part to genetic or edaphic 

factors. Nevertheless, it is striking to find anatomical evidence in one tree for 18 of 26 minor 

floods.

FLOOD RINGS FORMED PRIOR TO 1930

Although flow data for the Potomac River near 

Washington, D.C., were not recorded prior to 

1930, a gaging station at Point of Rocks, Md. 

(about 50 km upstream), has been in operation 

since 1895. The drainage area at this location is 

25,000 km2 , whereas it is 29,950 km2 near Washington, D.C. Peak-flow data from the Point of 

Rocks station were used as minimum estimates of 

pre-1930 peak flows within the study area. Treering abnormalities, therefore, can be related to 

known floods even though local peak-discharge 

data are unavailable.

Only two of the harvested trees were growing 

prior to 1930. Of these, the older tree had an indistinct series of narrow center rings but appears 

to have begun growth in approximately 1920. The 

younger tree started to grow in 1922. A closely 

spaced row of flood vessels just outside the 1924 

earlywood zone of the younger tree indicates that 

the tree was flooded in the early part of the 

growing season. The Point of Rocks gaging station 

recorded a flood peak of 7,850 mVs on May 13, 

1924. The flood peak near Washington, D.C., was 

probably slightly greater than at Point of Rocks, 

making it the fifth largest flood of the century. No 

other flood rings were formed in either tree during 

the 1920's.

Wood samples from several large trees were 

extracted with an increment borer rather than as 

cut sections. These trees grow along a reach of 

Virginia shore just downstream from a convex 

channel bend near Difficult Run, and trunks are 

flooded every 2 to 5 years. None formed flood 

rings in 1972 because tree crowns remained 

above the crest elevation of the flood. The innermost rings, however, which formed when trees 

were smaller, were expected to contain anatomical evidence of large floods.

A latewood flood ring in the 1924 growth ring 

was observed in a tree that in 1981 measured 

18 cm in diameter and approximately 10 m in 

height. The trunk is flooded by approximately

37
3,500 mVs but grows along a highly sheltered 

reach where current velocities at this discharge 

are less than 0.1 m/s. At the start of the 1924 

growing season, the diameter of the tree was approximately 2.5 cm at a stem height of 1.5 m. The 

height of the tree was probably about 3 m, which 

is a conservative estimate because the tree grows 

in a wooded tract where competition for light is 

presumed to limit growth. The crown would have 

been damaged in 1924 if debris accumulated in 

this backwash zone had tipped the tree or if the 

flood stage were high enough to increase current 

velocities within the zone. Neither occurrence is 

likely during small floods, but deposition of debris 

and damage to small trees in this vicinity have 

been observed after large floods in June 1972 

and February 1979 (5,830 mVs). Thus, the anomalous growth observed in this tree is consistent 

with flood damage in 1924.

Another large tree near the previous sample 

grows in a less protected reach and is flooded 

about three times each year. The tree began 

growing in about 1890 and shows evidence of at 

least three, and possibly five, floods prior to 

1930. An early wood flood ring developed in the 

1893 growth increment and would be expected to 

have formed after a flood in early to mid-May. A 

diffuse flood ring in the 1894 latewood suggests 

flood damage in late May or June. Both of these 

flood years were before gaging records were 

kept at Point of Rocks. Diffuse flood rings also 

appear to be present within the inner latewood 

zones of the 1907 and 1908 growth rings. Abnormal vessels in 1908 are larger and in a more ringlike pattern than those in 1907. Peak flows at 

Point of Rocks were 2,660 mVs (June 3) in 1907, 

and 2,750 mVs (May 8) and 3,480 mVs (May 23) 

in 1908.

At the time of the flood of May 1924, this tree 

was at least 10 cm in diameter near the base and 

probably no less than 5 m in height. As in the 

tree growing nearby at a higher altitude, a flood 

ring developed within the 1924 latewood zone. 

The diameters of some of the flood vessels are 

nearly as large as those of typical early wood vessels. No flood rings were observed in growth increments forming after 1924.

The oldest tree sampled with an increment 

borer started to grow in approximately 1865 just 

downstream from a rock cliff that protects the 

tree from high current velocities during minor 

flooding. The base of the tree is flooded approximately twice every 3 years. The tree has a single, 

well-defined flood ring in the outer latewood in 

1889, followed by a series of 30 narrow, indistinct

rings (fig. 18). In 1889, the tree was approximately 9 cm in diameter and possibly as much as 

5 m in height, and only a high-magnitude flood 

could have inundated the crown. The large diameter of vessels near the base of the tree and the 

narrow rings beginning in 1890 are evidence that 

the crown was severely damaged in the 1889 

growing season. A peak discharge of 13,000 mVs 

on June 2, 1889, was estimated from flood marks 

at Point of Rocks and is approximately that of the 

maximum known flood near Washington, D.C. 

(13,700 mVs on March 19, 1936).

Growth rings from two trees collected in 1960 

by R. S. Sigafoos were also studied for anatomical evidence of flooding prior to 1930. Because 

Sigafoos cut down trees rather than sampling 

them with an increment borer, a more precise 

anatomical study was possible. These trees 

started to grow in 1905 and 1920, respectively, 

along a rocky, frequently flooded reach near 

Chain Bridge where flood-flow velocities are 

great. The younger tree formed an earlywood 

flood ring in 1924 (7,850 mVs at Point of Rocks) 

and a latewood flood ring in 1921 (2,510 mVs on 

May 6). In the older tree, evidence was found for 

10 floods from 1907 to 1924. Earlywood flood 

rings developed in 1908 (2,750 mVs on May 8 and 

3,480 mVs on May 23), in 1919 (1,830 mVs on 

May 12), in 1921, and in 1924. Well-defined, generally continuous latewood flood rings formed in 

1907 (2,660 mVs on June 3), in 1910 (4,390 mVs 

on June 18), in 1913 (1,880 mVs on May 29 and 

1,440 mVs on June 1), and in 1915 (3,600 mVs on 

June 4). A diffuse flood ring corresponded to a 

minor flood on June 17-18, 1916 (1,390 mVs). The 

intraring position of abnormal growth in both 

trees corresponds to the approximate date of 

flooding, with flood rings forming within the 

earlywood after floods in early to mid-May and 

within the middle or outer latewood after late 

May or June floods.

APPLICATIONS

Sampling strategies consistent with the findings 

of this study are recommended in using anatomical data to reconstruct flood histories. Strategies 

will further depend on whether the researcher 

desires to document as many floods as possible 

or to estimate the relative magnitude of floods. 

These may be most useful along streams where 

hydrologic records are lacking. Short-term records 

can be extended by flood-ring evidence, catastrophic floods can be documented, and the mini38
Figure 18. Wood of a tree damaged by the catastrophic 

flood of June 2, 1889 (estimated discharge 13,000 mVs 

near Point of Rocks, Md.). A, Flood ring formed in the 

lower trunk. Note that postflood growth in 1889 was 

minimal even though the flood occurred in the middle part 

of the growth year. B, Part of increment core showing ringwidth patterns before and after 1889. The 20 growth rings 

prior to 1889 had an average width of about 2.0 mm. The 

30 rings forming after 1889 averaged only about 0.4 mm, 

and ring widths comparable to those before 1889 did not 

develop again until the early 1920's. Narrow rings probably 

resulted from extremely severe damage in 1889. A, X70; 

B, X20.

mum crest stage can be estimated along local 

stream reaches.

Although only one genus (Frax/nus) was 

studied, flood rings have also been noted in 

swamp white oak (Quercus b/co/or Willd.) collected near Chain Bridge and possibly in a post 

oak (Quercus stellate Wang.) specimen collected 

near Great Falls by J. F. Hill. Flood-related growth 

anomalies might be expected in elm (U/mus) and 

hickory (Car/a), both of which have easily distinguishable ring boundaries. Other flood-plain 

species, such as maple (Acer), willow (Sa//x), and 

sycamore (Platanus), are abundant on most flood 

plains in eastern North America, but their ring 

boundaries are difficult to determine. Willow, in 

particular, might be a useful indicator of recent 

floods because it commonly grows near the lowwater channel. It is unknown whether floodrelated growth develops in coniferous species.

Despite these possibilities, the author highly 

recommends ash. This genus grows under a wide 

range of edaphic and hydrologic conditions, has 

conspicuous growth rings, and occasionally attains 

advanced age. Although primarily a north temperate species, the genus is distributed southward into parts of Cuba and Mexico and is also 

found in northern Africa, southern Asia, and Java 

(U.S. Dept. of Agriculture, 1965). Ash may be 

damaged by floods smaller than those on a large 

river such as the Potomac. For example, flood 

rings have been observed in trees sampled along 

Difficult Run, Va., a small tributary of the Potomac 

River, and in sections harvested along Passage 

Creek, Va., by C. R. Hupp.

If the goal is to document as many floods as 

possible, trees along reaches of the low-water 

channel should be sampled. Trees with floodtrained crowns and numerous abrasion scars 

probably have the most flood rings. Trees can be 

cored first with an increment borer to determine 

whether anomalous growth is present. Most large 

floods are associated with well-defined, continuous flood rings that can be easily detected in 

increment cores. It may be advisable, however, 

to harvest several trees to become familiar with 

growth variations around the circumference and 

to detect diffuse flood rings that may otherwise 

not be observed. An upland tree might also be

39
sampled for comparison. Stem and branch sections taken at intervals of 1 m or less are recommended. Skill in detecting latewood flood rings in 

ash is quickly gained even without an extensive 

anatomical background. Although anomalous 

earlywood growth may be difficult to recognize, 

the most striking examples can generally be 

identified. A 20X dissecting microscope is sufficient for studying samples that have been properly sanded and polished (see "Methods").

As the researcher becomes adept at recognizing patterns of abnormal growth, future samplings 

can be taken exclusively with an increment borer. 

Four core samples from each tree, preferably 

along the points of the compass, should be sufficient to detect most diffuse or discontinuous flood 

rings. It is also desirable to reach the center ring 

in at least two corings. Trees should then be 

marked in the event that additional samples are 

needed. If possible, it is also advisable to sample 

wood near the apex.

Trees of various sizes, ages, and growth forms 

should be sampled. Generally, flood rings are 

easiest to observe in the wide rings typically 

formed early in life, when trees were more likely 

to be damaged. Thus, the samples should include 

trees that began growth in successive decades, 

rather than just the oldest trees. This is desirable 

even if trees of different ages are about the 

same size. Trees sampled in this overlapping 

manner may provide a fairly complete flood-information series.

If the goal is to estimate relative flood magnitude (stage), several considerations are necessary. Flood rings common to a large percentage 

of samples may indicate a large flood. Smaller 

floods are associated with fewer flood rings and 

typically are restricted to trees on low flood-plain 

levels. The percentage of samples with flood 

rings during a flood year, therefore, is a rough 

measure of flood magnitude. In addition, large 

floods are typically associated with well-formed, 

continuous flood rings in basal stem sections, 

whereas anomalous growth restricted to apices is 

generally associated with minor-flood damage.

The crest stage of a flood can be estimated by 

sampling trees at progressively higher flood-plain 

levels until no further flood rings are found for 

that year. The height of the tallest tree damaged 

at the time of the flood can then be used as a 

minimum estimate of the flood crest. Height can 

be inferred by comparing the diameter in the 

flood year with that of trees presently of similar 

diameter. Alternatively, the tree can be cored at 

successive stem heights to determine the floodyear apex. An approximate crest stage can then 

be calculated by adding this height to the altitude 

of the trunk above the low-water channel.

Trees kept small by frequent flood damage are 

of little value for determining the crest stages of 

catastrophic floods. Evidence for catastrophic 

floods should instead be sought along reaches 

where damage is unlikely during smaller floods. 

Trees in low-damage zones commonly attain 

greater sizes and ages than those likely to be 

severely damaged or uprooted. Thus, trees downstream from convex channel bends may preserve 

anomalous growth that significantly extends the 

record of high-magnitude floods.

SUMMARY

The wood anatomy of ash commonly changes if 

flood-damaged crowns produce a second crop of 

leaves. Flood vessels may form in any part of the 

ring but are most conspicuous in the latewood 

zone. Latewood flood rings develop after floods 

from mid-May to early August. Anomalous earlywood growth develops after floods in the early 

part of the growing season and is generally more 

variable and difficult to interpret than anomalous 

latewood growth. Occasionally, a single flood is 

associated with both growth patterns along different parts of the circumference of an annual 

ring. Thus, anomalous growth not only documents 

a flood, but, in most cases, the time of the flood 

can be estimated from the position of the pattern.

The number of flood rings corresponding to 

flood years on the Potomac River during the 

growing season (approximately April 15 to August 

31) is summarized in table 9. From 1930 to 1979, 

anatomical evidence was found for each flood 

year except 1966. Evidence was also found for 

catastrophic floods in 1889 and 1924 and for 

smaller floods from 1893 to 1921.

Within unsheltered and moderately sheltered 

flood zones, the frequency of flood rings seems in 

large part related to tree size and flood magnitude. For example, floods in 1942, 1949, 1951, 

1952, and 1960 were among the largest (excluding 1972) during the study period, and numerous 

corresponding flood rings were detected. Fewer 

flood rings formed after smaller floods. Most tall 

trees developed flood rings only before they 

became large, whereas trees remaining small 

from frequent flood damage commonly formed 

flood rings throughout life; that is, increased age 

did not preclude the development of flood rings.

40
A catastrophic flood in 1972 was associated with 

anomalous growth in large trees only if crowns 

were inundated.

Anomalous growth patterns associated with 

large floods are generally more continuous 

around the ring circumference than are those 

from small floods. Minor-flood vessels are typically small, highly diffuse, and discontinuous and, 

consequently, may be difficult to observe unless 

entire cross sections are available. More importantly, minor-flood rings typically develop only 

near the stem apex. Flood rings associated with 

larger floods also develop near the base, although 

vessels are larger and more closely spaced near 

the apex. Apparently, this is because anomalous 

growth develops basipetally in response to a 

diminishing gradient of hormones produced during refoliation. It is assumed that the amount of 

diffusable hormones is proportional to the degree 

of refoliation, which, in turn, seems to depend on 

the amount of crown damage. Large floods are

more likely than small floods to damage the 

crowns of most trees and, therefore, are more 

likely to initiate flood rings near the base of the 

trunk. Small trees near the channel, however, 

may be damaged even by minor floods and thus 

develop anomalous growth at all stem heights.

Trees with the most flood rings grew along unsheltered or moderately sheltered reaches. Within these zones, trees near the low-water channel 

formed more flood rings than did those at less 

frequently flooded altitudes. Trees that are presently tall developed abnormal growth only before 

crown height exceeded the crest altitude of most 

floods or if floods were extremely large. Trees in 

sheltered areas are seldom damaged during small 

floods and are taller and older than trees along 

exposed reaches; nevertheless, they may be 

damaged during catastrophic floods. Thus, evidence for maximum-record floods may be preserved in sheltered trees longer than in trees at 

more exposed sites.

Table 9. Summary of flood rings formed in 49 trees, 1889 to 1979. Three trees were sampled with an increment borer, and 

46 were cut down. Depauperate and discolored tissues are not included 

[ , no abnormal growth detected]

Year Late wood

Number of flood rings per year

Early wood Combination Total

1889

1893

1894

1907

1908

1910

1913

1915

1916

1919

1921

1924

1932

1933

1937

1938

1939

1940

1942

1943

1944

1945

1946

1948

1949

1950

'1

'1

22

1

 

1

'2

2

1

9

1

1

7

13

2

'1

1

1

1

2

1

1

4

1

3

3

3

5

5

2

2

3

 1

 1

 1

_ 2

_ 2

 2

_ 4

 1

 1

 4

 3

 3

 4

2 14

1 7

 5

 1

1 8

 2

 15

 5

41
Table 9. Summary of flood rings formed in 49 trees, 1889 to 1979. Three trees were sampled with an increment borer, and 

46 were cut down. Depauperate and discolored tissues are not included Continued

Number of flood rings per year

Year

1951

1952

1953

1955

1956

1958

1959

1960

1961

1964

1967

1968

1970

1971

1972

1973

1974

1975

1978

Total

Latewood

9

1

2

2

2

2

2

1

33

1

103

Earlywood

3

11

6

4

8

1

1

2

3 1

7

3 1

2

3

89

Combination

_

1

1

1

1

2

1

11

Total

12

12

6

1

2

5

2

11

1

1

1

6

2

2

33

7

3

2

3

203

'Sampled with increment borer.

2One tree sampled with increment borer.

3Formed after early June flood and thus not listed in table 5.

REFERENCES CITED

Fritts, H. C., 1976, Tree rings and climate: London, 

Academic Press, 567 p.

Harrison, S. S., and Reid, J. R., 1967, A flood frequency 

graph based on treescar data: North Dakota Academy 

of Science, Proceedings, v. 21, p. 23-33.

Osterkamp, W. R., and Hedman, E. R., 1977, Variation 

of width and discharge for natural high-gradient 

stream channels: Water Resources Research, v. 13, p. 

256-258.

Phipps, R. L., 1967, Annual growth of suppressed chestnut oak and red maple, a basis for hydrologic inference: U.S Geological Survey Professional Paper 

485-C, 27 p.

Sigafoos, R. S., 1964, Botanical evidence of floods and

flood-plain deposition: U.S. Geological Survey Professional Paper 485-A, 35 p.

U.S. Department of Agriculture, 1965, Silvics of forest 

trees of the United States: 762 p.

Wareing, P. F., Haney, C. E. A., and Digby, J., 1964, 

The role of endogenous hormones in cambial activity 

and xylem differentiation, in Zimmerman, AA. H., ed., 

The formation of wood in forest trees: London, 

Academic Press, 562 p.

Yanosky, T. AA., I982a, Effects of flooding upon woody 

vegetation along parts of the Potomac River flood 

plain: U.S. Geological Survey Professional Paper 

1206, 21 p.

____ 1982b, Hydrologic inferences from ring widths of 

flood-damaged trees, Potomac River, Maryland: 

Environmental Geology, v. 4, p. 43-52.

42