U.S. Department of the Interior

U.S. Geological Survey

Water-Resources Investigations Report 03-4217

Prepared in cooperation with the 

U.S. Air Force, Arnold Air Force Base and the

University of the South

Tree-Regeneration and Mortality Patterns and Hydrologic 

Change in a Forested Karst Wetland—Sinking Pond, 

Arnold Air Force Base, Tennessee
Cover photographs: Left photograph is overcup oak in Sinking Pond under drained conditions, courtesy of K.C. Fitch, Arnold Engineering 

Development Center, 2001. Right photograph is overcup oak in Sinking Pond under flooded conditions.
Tree-Regeneration and Mortality Patterns 

and Hydrologic Change in a Forested Karst 

Wetland—Sinking Pond, Arnold Air Force 

Base, Tennessee

By William J. Wolfe, Jonathan P. Evans, Sarah McCarthy, W. Scott Gain, and 

Bradley A. Bryan

Prepared in cooperation with the 

U.S. Air Force, Arnold Air Force Base and the

University of the South

Water-Resources Investigations Report 03-4217

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Wolfe, W.J., Evans, J.P., McCarthy, Sarah, Gain, W.S., and Bryan, B.A., 2004, Tree-regeneration and mortality patterns 

and hydrologic change in a forested karst wetland—Sinking Pond, Arnold Air Force Base, Tennessee: U.S. Geological 

Survey Water-Resources Investigations Report 03-4217, 53 p.
iii

Contents

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

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

Purpose and Scope ............................................................................................................................. 3

Previous Studies .................................................................................................................................. 3

Description of Study Area ........................................................................................................................... 3

Karst Hydrology and Geomorphology .............................................................................................. 5

Hydrogeology ....................................................................................................................................... 5

Soils ................................................................................................................................................... 6

Climate................................................................................................................................................... 6

Vegetation ............................................................................................................................................ 6

Tree-Regeneration and Mortality Patterns .............................................................................................. 7

Overcup Oak Population Dynamics in Sinking Pond, 1997-2001 .................................................. 7

Seedling Establishment and Recruitment .............................................................................. 8

Adult Mortality ............................................................................................................................ 9

Spatial Patterns of Tree-Size Classes and Ponding Depth ........................................................ 10

Adults ........................................................................................................................................ 11

Small Adults and Saplings ....................................................................................................... 11

Seedlings .................................................................................................................................... 15

Age Distribution of Adult Overcup Oaks ........................................................................................ 15

Botanical Evidence for Hydrologic Change ................................................................................. 19

Hydrologic Change in Sinking Pond, 1854-2002 ..................................................................................... 20

Hydrologic Modeling ......................................................................................................................... 22

Model Input Data ...................................................................................................................... 22

Model Structure ........................................................................................................................ 23

Definition of Parameters .......................................................................................................... 24

Model Equations ....................................................................................................................... 25

Calibration and Verification .................................................................................................... 29

Simulated Hydrologic Record of Sinking Pond, 1854-2002 ......................................................... 30

Ecological Effects of Hydrologic Change in Sinking Pond .................................................................. 33

Hydrologic Constraints on Overcup Oak Recruitment and Survival ......................................... 33

Future Ecological Conditions in Sinking Pond .............................................................................. 36

Conclusion: Management and Scientific Implications of Hydrologic and 

Ecological Change in Sinking Pond ........................................................................................... 37

Acknowledgments ...................................................................................................................................... 38

References ................................................................................................................................................... 38

Appendix A. Flora of Sinking Pond. .......................................................................................................... 43

Appendix B. Evaluating Effects of Local Human Activity on the Hydrology of Sinking Pond ....... 47

Glossary ........................................................................................................................................................ 53
iv

Figures

1. Map showing major physiographic regions of Tennessee and locations of 

The Barrens, Arnold Air Force Base, and study area ............................................................ 2

2. Map showing location of Sinking Pond, adjacent drainage basins, monitored 

wells, and surface-water stations ............................................................................................ 4

3. Hydrograph showing water-surface elevations in Sinking Pond and adjacent 

well 355 for the period October 1, 1992, through February 12, 1995 .................................... 5

4. Graph showing normal annual precipitation at Tullahoma, Tennessee, 1960-2000 .......... 7

5. Photographs of areas in the interior and along the margins of Sinking Pond .................. 8

6. Graph showing aggregate mortality of adult overcup oaks by ponding depth and 

size class, assessed annually in 55 circular plots in Sinking Pond, 1998-2001 ............... 11

7. Map showing distribution of overcup oak by ponding-depth class and 

life-history state in a 2.3-hectare area of Sinking Pond ...................................................... 12

8. Graph showing size-class distribution of adult overcup oak in a 

2.3-hectare area of Sinking Pond by ponding-depth class. ................................................ 13

9. Map showing distribution of sweetgum by ponding-depth class and 

life-history state in a 2.3-hectare area of Sinking Pond ...................................................... 14

10. Graph showing size-class distribution of adult sweetgum in a 

2.3-hectare area of Sinking Pond by ponding-depth class. ................................................ 15

11. Map showing distribution of willow oak by ponding-depth class 

and life-history state in a 2.3-hectare area of Sinking Pond .............................................. 16

 12-16. Graphs showing:

 12. Size-class distribution of adult willow oak in a 2.3-hectare area of 

 Sinking Pond by ponding-depth class. ............................................................................ 17

 13. Size-class distribution of adult river birch in a 2.3-hectare area of 

 Sinking Pond by ponding-depth class. ............................................................................ 17

 14. Frequency distributions of tree diameters at 1.5 meters above 

 land surface and elevation for adult overcup oaks sampled 

 for tree-ring analysis and the adult overcup oak population 

 of the 2.3-hectare sampled area in Sinking Pond ......................................................... 19

 15. Elevations, ring-count ages, and age-frequency distribution of 

 overcup oaks sampled for tree-ring analysis in a 2.3-hectare 

 area of Sinking Pond. ......................................................................................................... 20

 16. Departures from 1900-1994 mean annual precipitation for national average 

 across the conterminous United States and for Tullahoma, Tennessee. .................. 21

17. Conceptual diagram of the water-balance model of Sinking Pond showing 

simulated hydrologic fluxes and storage compartments .................................................... 25

18. Schematic diagram of a parameterized water-balance model of Sinking Pond ............ 28

 19-21. Hydrographs showing:

 19. Observed and simulated stage, surface-water outflow, and volume 

 of Sinking Pond for calibration period February 11, 1993, 

 through February 12, 1995 ................................................................................................. 30

 20. Observed and simulated stage of Sinking Pond for verification period 

 October 1, 1999, through September 30, 2002. ............................................................... 31

 21. Simulated stage for Sinking Pond, January 1854 through September 2002 ............. 32
v

 22-25. Graphs showing:

 22. Days per water year from 1855-2002, in which simulated stage at 

 Sinking Pond exceeded the spillway elevation of 324.53 meters 

 above the National Geodetic Vertical Datum of 1929 ................................................... 32

 23. Temporal patterns in Sinking Pond hydroperiod, based on centered 5-year 

 average annual ponding duration above selected stages, 1857-2000 ....................... 33

 24. Timing and elevation of simulated annual 200-day inundation compared to 

 the ages and elevations of sampled overcup oaks in Sinking Pond, 1856-2002 ...... 34

 25. Frequency distributions for recruitment inundation for surviving overcup 

 oaks and all recruitment opportunities in a 2.3-hectare sampled area of 

 Sinking Pond during 148 years of simulated hydrologic record, pre- and 

 post-1970 recruitment opportunities in the sampled area, and shallow

 and deep/intermediate recruitment opportunities in the sampled

 area after 1970 .................................................................................................................... 35

Tables

1. Height and diameter criteria for tree-size classes ................................................................ 9

2. Aggregate density of overcup oak seedlings in Sinking Pond by 

cohort year and ponding depth, 1997-2001 .............................................................................. 9

3. Aggregate percent survival of overcup oak seedlings in Sinking Pond 

by cohort year and ponding depth, 1997-2001 ........................................................................ 9

4. Recruitment and survival of overcup oak saplings in sampled plots 

in Sinking Pond by ponding-depth class, 1997-2001 ............................................................ 10

5. Aggregate density of overcup oak adults by size class and ponding 

depth in sampled plots in Sinking Pond, 1997-2001 .............................................................. 10

6. Density of adult trees by species and ponding-depth class in a 

2.3-hectare area of Sinking Pond, 2001-2002 ........................................................................ 13

7. Density and percentage of 4-square-meter subplots containing overcup oak, 

sweetgum, and willow oak stems in a 2.3-hectare area of Sinking Pond 

by ponding-depth class and size class, 2001-2002 ............................................................... 18

8. Station data for climate stations used to develop input data set for the 

hydrologic model of Sinking Pond .......................................................................................... 23

9. Names, units, and calibrated values of constant terms in parameterized 

hydrologic model of Sinking Pond .......................................................................................... 26

10. Gage data for surface-water stations used in development, calibration, 

and verification of a water-balance model of Sinking Pond .............................................. 29
vi

Conversion Factors, Abbreviations, and Vertical Datum

Multiply By To obtain

millimeter (mm) 0.03937 inch

centimeter (cm) 0.3937 inch

basin centimeter (bcm) 0.3937 basin inch

meter (m) 3.281 foot

kilometer (km) 0.6214 mile

square kilometer (km2) 0.3861 square mile

Hectare (ha) 2.471 acre

cubic meter (m3) 35.31 cubic foot

cubic meter per second (m3/s) 35.31 cubic foot per second

Temperature in degrees Celsius (°C) can be converted to degrees Farenheit (°F) as follows:

°F = 1.8 x °C + 32

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29); horizontal coordinate information is referenced to the North American Datum of 

1927 (NAD 27).

Abbreviations and Acronyms

AAFB Arnold Air Force Base

AEDC Arnold Engineering Development Center

cdf cumulative distribution function

DBH diameter at 1.5 meter above land surface

DOQ digital orthophoto quadrangle

ET evapotranspiration

G1 globally extremely rare

GCM global circulation model

GIS geographic information system

GPS global positioning system

L3/t volume divided by time

NAVD 29 North American vertical datum of 1929

NCDC National Climatic Data Center

RTE rare, threatened, or endangered

USGS United States Geological Survey

USHCN United States Historical Climatology Network

TVA Tennessee Valley Authority
Abstract

Multiple lines of evidence point to climate change as 

the driving factor suppressing tree regeneration since 1970 in 

Sinking Pond, a 35-hectare seasonally flooded karst depression located on Arnold Air Force Base near Manchester, 

Tennessee. Annual censuses of 162-193 seedling plots from 

1997 through 2001 demonstrate that the critical stage for tree 

survival is the transition from seedling to sapling and that this 

transition is limited to shallow (less than 0.5 meters) ponding 

depths. Recruitment of saplings to the small adult class also 

was restricted to shallow areas. Analysis of the spatial and 

elevation distribution of tree-size classes in a representative 

2.3-hectare area of Sinking Pond showed a general absence of 

overcup oak saplings and young adults in deep (ponding depth 

greater than 1 meter) and intermediate (ponding depth 0.5-

1 meter) areas, even though overcup oak seedlings and mature 

trees are concentrated in these areas. 

Analysis of tree rings from 45 trees sampled in a 2.3-

hectare spatial-analysis plot showed an even distribution of 

tree ages across ponding-depth classes from the 1800s through 

1970, followed by complete suppression of recruitment in 

deep and intermediate areas after 1970. Trees younger than 

30 years were spatially and vertically concentrated in a small 

area with shallow ponding depth, about 0.5 meter below the 

spillway elevation. Results of hydrologic modeling, based on 

rainfall and temperature records covering the period January 1854 through September 2002, show ponding durations 

after 1970 considerably longer than historical norms, across 

ponding-depth classes. This increase in ponding duration 

corresponds closely with similar increases documented in published analyses of streamflow and precipitation in the eastern 

United States and with the suppression of tree regeneration at 

ponding depths greater than 0.5 meter indicated by tree-ring 

analysis. Comparison of the simulated stage record for Sinking 

Pond with the ages and elevations of sampled trees shows that 

prolonged (200 days or more per year) inundation in more 

than 2 of the first 5 years after germination is inversely related 

to successful tree recruitment and that such inundation was 

rare before 1970 and common afterwards.

Introduction

Wetlands in karst (bold terms can be found in Glossary) 

landforms are widely distributed in the upland valleys and 

plateaus of the southeastern and south-central United States. 

A recurrent theme in the literature on these wetlands is the 

presence of disjunct populations of northern or coastal plain 

plants and animals found far from their normal range (Barclay, 

1957; Greear, 1967; DeSelm, 1981; Ellis and Chester, 1989; 

Wolfe, 1996b). The disjunct biota of southern karst wetlands 

contribute to regional biodiversity and may provide insights 

into the resilience and stability of wetland ecosystems in the 

face of environmental change (Barclay, 1957; Greear, 1967; 

Tryon, 1990). 

Numerous studies describe biota in southern karst wetlands (Barclay, 1957; Greear, 1967; DeSelm, 1981; Ellis and 

Chester, 1989; Wolfe, 1996b; Tryon, 1990; McCarthy and 

Evans, 2000), but few of these studies combine ecological 

observations with detailed measurements of surface flooding, 

soil saturation, and ground-water levels (Hendricks and Goodwin, 1952; Greear, 1967; Wolfe, 1996a, b). Fewer still have 

undertaken long-term monitoring of hydrologic and ecological 

conditions or attempted to track their interaction through time. 

One karst wetland that has received interdisciplinary 

scientific attention for more than a decade is Sinking Pond 

(Patterson, 1989; Wolfe, 1996a, b; McCarthy and Evans, 

2000). Sinking Pond is a seasonally ponded karst depression 

whose interior is dominated by overcup oak (Quercus lyrata), 

a Coastal Plain tree endemic to bottomland hardwood forests 

(Wolfe, 1996a; McCarthy and Evans, 2000). Sinking Pond is 

located on Arnold Air Force Base (AAFB), a U.S. Air Force 

reservation occupying about 160 square kilometers (km2) on 

the divide between the Duck River and Elk River watersheds, 

near Manchester and Tullahoma, Tennessee (fig. 1). Sinking 

Pond and other natural areas provide a sight and sound buffer 

around Arnold Engineering Development Center (AEDC), a 

research and development facility operated by the U.S. Air 

Force at AAFB.

Tree-Regeneration and Mortality Patterns and Hydrologic 

Change in a Forested Karst Wetland—Sinking Pond, 

Arnold Air Force Base, Tennessee

By William J. Wolfe1, Jonathan P. Evans2, Sarah McCarthy3, W. Scott Gain1, and Bradley A. Bryan1

1

 U.S. Geological Survey

2

 University of the South

3

 University of Michigan
Study

Area

24

41 55

Woods

Reservoir

Arnold

Air Force Base

COFFEE CO

C

O

FFEE

C

O

GR

UND

Y

C

O

GRUNDY CO

FRANKLIN CO

Elk

Tullahoma

Estill

Springs

Normandy

Lake

Manchester

Duck

35°30'

86°15' 85°52' 30"

35°15'

0 25 50 KILOMETERS

0 25 50 MILES

Coastal Plain

Western

Highland

Rim Nashville

Basin

Eastern

Highland

Rim

Cumberland

Plateau

Valley and Ridge

Blue Ridge

Arnold Sequatchie Valley

Air Force

Base

The Barrens

TENNESSEE

River

River

41

ALT

Physiographic regions modified from

Miller, 1974. The Barrens based on soil

data from Soil Conservation Service, 1991

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Figure 1. Major physiographic regions of Tennessee and locations of The Barrens, Arnold Air Force Base, and study area.

2 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
Within Sinking Pond, overcup oaks are concentrated 

in the deepest areas subject to the longest annual inundation 

(Wolfe, 1996a). This concentration in deep areas reflects the 

high tolerance of overcup oak to inundation relative to other 

bottomland hardwood trees (Broadfoot and Williston, 1973; 

Solomon, 1990). Clearly, in the past, hydrologic conditions in 

the deep interior of Sinking Pond were suitable not only for 

the survival of mature overcup oaks but also for their reproduction and replacement by younger trees. The near absence 

of overcup oak saplings and young adults in the interior of 

Sinking Pond (McCarthy and Evans, 2000) suggests that 

hydrologic conditions have changed in a way that has made 

formerly suitable sites unsuitable for tree regeneration. 

Numerous studies show that regeneration and survival 

of overcup oak and other bottomland hardwoods are affected 

by environmental variables related to the hydrologic regime 

(Hosner and Boyce, 1962; Gill, 1970; Broadfoot and Williston, 1973; Harms and others, 1980; Reily and Johnson, 1982; 

Hook, 1984; Schupp, 1995; Wada and Ribbens, 1997). Such 

variables include basic climatic factors, such as precipitation 

and temperature, human influences ranging from direct hydrologic alteration of wetlands to indirect effects of land use in 

nearby uplands (Hewlett and Hibbert, 1961), and topographic 

effects on the timing, frequency, depth, and duration of inundation (Huenneke and Sharitz, 1986; Streng and others, 1989; 

Hall and Harcombe, 1998). 

Purpose and Scope

In 1999, the U.S. Geological Survey (USGS), in cooperation with the U.S. Air Force and the University of the 

South, began a 3-year study to examine the relation between 

hydrologic change and regeneration of overcup oak and other 

species in and around Sinking Pond. The main objectives of 

the study were to:

1. Determine the nature, extent, timing, and proximate cause 

of changes in tree regeneration and recruitment patterns 

in and around Sinking Pond; and

2. Reconstruct the historical ponding regime of Sinking 

Pond and evaluate climate and human activities as factors 

driving hydrologic change.

This report describes an analysis of the interaction 

between hydrology and tree regeneration in Sinking Pond. 

The analysis includes: (1) field observations of germination, recruitment, and mortality of overcup oak and other tree 

species; (2) analysis of the spatial relations among seedling, 

saplings, adult trees, and elevation; (3) tree-ring analysis, 

mapping, and topographic surveying to determine tree-age 

distribution among overcup oaks in relation to elevation; 

(4) development and analysis of a 154-year time series of 

daily temperature and precipitation; (5) development, calibration, verification, and application of an empirical model 

of water level in Sinking Pond as a function of historical 

rainfall and temperature; and (6) evaluation of the effects of 

climate change on tree recruitment and survival during the 

past 150 years. Additional activities, described in Appendix B, 

included field observations, air-photo analysis, and waterlevel monitoring to evaluate possible human influences on the 

hydrology of Sinking Pond.

Previous Studies

Patterson (1989) mapped soils and vegetation in a 

650-hectare (ha) area around Sinking Pond, noting high 

variability of vegetation on wetland soils and a strong correlation between moss lines and soil indicators of flooding. 

Wolfe (1996a, b) monitored surface-water and ground-water 

levels in and around Sinking Pond and other karst depressions 

on AAFB and related tree-distribution patterns to elevation 

and hydroperiods in the Sinking Pond drainage basin. Wolfe 

(1996a, b) documented the dominance of overcup oak (Quercus lyrata), a Coastal Plain disjunct, in the deepest parts of 

Sinking Pond, the absence of sub-canopy plants, including 

overcup oak saplings, and widespread germination and mortality of overcup oak seedlings in the interior of Sinking Pond. 

Wolfe speculated that oaks regenerate during infrequent periods of prolonged (several years) drought, producing a population composed of distinct age cohorts. 

McCarthy and Evans (2000) inferred the population age 

distribution of overcup oaks in Sinking Pond by coring 16 

adult overcup oaks, relating size to age, and measuring more 

than 300 additional trees. They concluded that prior to the 

1960s, age distribution was relatively uniform, with no distinct 

cohorts, but that afterwards, regeneration and recruitment of 

overcup oak had virtually ceased. McCarthy and Evans (2000) 

speculated that conditions in the pond had become sufficiently wetter to preclude regeneration where it had formerly 

occurred. However, the cause and mechanism of such a wetting trend remained unidentified, and the absence of hydrologic records for Sinking Pond prior to 1992 left the existence 

of such a trend open to question.

The results of previous studies raise several questions. 

What is the age distribution of the overcup oak population, 

and how is it related to elevation and ponding regime? What 

are current patterns of germination, recruitment, and mortality 

of overcup oak and other tree species in Sinking Pond? How 

have these patterns changed? Have human activities or climate 

change altered the ponding regime of the pond? 

Description of Study Area

The study was conducted in and around Sinking Pond, a 

seasonally ponded karst depression with a maximum flooded 

area of about 35 hectares (ha). Sinking Pond drains a basin 

of about 335 ha in the northwestern part of Arnold Air Force 

Base (figs. 1 and 2). The study area lies in the Highland Rim 

subsection of the Interior Low Plateau Physiographic Province 

(Fenneman, 1938) near the southern limit of The Barrens, a 

broad area of low-relief karst extending about 100 kilometers 

Description of Study Area 3
353

Sinking

Pond

Westall

Swamp

Huckleberry Creek

Crumpton Creek

To Crumpton Creek

Berm

Huckleberry

Creek

impoundment

Vertical

shaft

Berm

Berm

355

Sinking

Pond

outflow

358

359

356

Dixie Flats

Spillway

356

EXPLANATION

WELL LOCATION AND NUMBER

STREAMFLOW STATION

WATER-LEVEL RECORDER

ROAD OR TRAIL

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Subcatchment

boundary

Willow

Oak

Swamp Tupelo

Swamp

Sinkhole

Internal

drain

Catchment boundary

Intermittent

flow path

0 0.5 1 KILOMETER

0 0.5 1 MILE

86°05' 30" 86°02' 30"

35°26'

35°23' 30"

Grid of 20-m

by 20-m

vegetation

plots

0 100 200 300 400 METERS

N

Sinking Pond

boundary

20-m-wide

box transect

Internal

flow channel

Figure 2. Location of Sinking Pond, adjacent drainage basins, monitored wells, and surface-water stations. (Inset shows location of 

vegetation plot grid and box transects.)

4 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
(km) north from the Elk River (fig. 1). The Barrens is notable 

for supporting numerous plants and animals listed as rare, 

threatened, or endangered in Tennessee (Svenson, 1941; 

DeSelm and others, 1974; Kral, 1973; Bowen and Pyne, 

1995), including several that are Coastal Plain endemics for 

which The Barrens and analogous areas of Tennessee and Kentucky are the only significant inland occurrences (Pyne, 2000). 

The northern part of AAFB represents a major concentration 

of State-listed Barrens taxa as well as many Coastal Plain disjuncts, such as overcup oak in Sinking Pond (Pyne, 2000).

Karst Hydrology and Geomorphology

The name “Sinking Pond” dates from the 1830s (Jack 

Bennett, Archeological Assessments, Inc., written commun., 

1999) and reflects the pond’s abrupt seasonal rises and recessions (Wolfe, 1996a). The pond generally fills in the fall or 

winter and remains full for several months before receding 

in the summer. The seasonal rise can be rapid—commonly 

2 meters (m) or more from a fully drained condition in 1 or 

2 days (fig. 3).

The distinctive hydrologic behavior of Sinking Pond is 

largely a result of the pond’s karstic origin and its geomorphic 

expression. The ponded area of Sinking Pond comprises about 

10 percent of the area of the Sinking Pond drainage basin, but 

the pond’s 3.5 m of internal relief represents more than 30 percent of the overall relief in the basin (about 11 m). More than 

two thirds of Sinking Pond’s internal relief is contained in a 

complex of coalesced sinkholes and connecting channels that 

compose the pond’s internal drainage system. Several of these 

sinkholes receive concentrated recharge in the weeks preceding the seasonal rise and throughout the recession (Wolfe, 

1996a, b).

In addition to Sinking Pond, deep (greater than 2.5 m), 

complex sinkholes with efficient internal drains are found in 

Willow Oak Swamp, an 80-ha sub-basin northwest of Sinking Pond, in Westall Swamp, a seasonally flooded depression 

draining 150 ha located west of the Sinking Pond drainage 

basin, and along the intermittent channel that carries overflow 

from Sinking Pond to Crumpton Creek, a tributary of the Duck 

River (Wolfe, 1996a; fig. 2). Wolfe (1996a, b) noted fresh 

karst collapse features in the Sinking Pond area. 

Karst development in the Sinking Pond area is more 

active, and local relief higher, than is typical of The Barrens. 

More typical are numerous shallow (1.5 m or less) sinkholes 

without visible internal drains (Wolfe, 1996a). One shallow, 

perched sinkhole, Tupelo Swamp, is a sub-basin of Sinking 

Pond, draining an area of about 13 ha north of Sinking Pond 

(fig. 2). Other shallow depressions drain small areas on the 

broad, flat ridges. 

Hydrogeology

The bedrock geology of the Eastern Highland Rim is 

dominated by limestones and interbedded cherts and shales 

of Mississippian age that dip gently to the east, away from 

the Nashville Basin, a structural dome, and towards the base 

of the Cumberland Plateau. From top to bottom, the major 

Mississippian-age strata are the St. Louis Limestone, the 

Warsaw Limestone, and the Fort Payne Formation, which 

form the Highland Rim aquifer system. The lower boundary 

of the Highland Rim aquifer system is the Chattanooga Shale 

of Upper Devonian/Lower Mississippian age (Wilson, 1976; 

Burchett, 1977), a regional confining unit (Brahana and Bradley, 1986a, b; Haugh and Mahoney, 1994; Wolfe and others, 

1997). In general, the Mississipian strata are massive, thickly 

bedded, and contain a high percentage of calcium carbonate 

with less insoluble material near the upper units of the St. 

Louis Limestone and become progressively thinner-bedded 

Oct Jan Apr July Oct Jan Apr July Oct Jan 316

318

320

322

324

326

1992 1993 1994 1995

Ground-water

data collection

begins here

SINKING POND

WELL 355

Missing

ground-water

data

W

A

TER LEVEL, IN

METER

S

ABO

VE N

A

V

D 29

Figure 3. Water-surface elevations in Sinking Pond and adjacent well 355 for the period October 1, 1992, 

through February 12, 1995. (Modified from Wolfe, 1996a.)

Description of Study Area 5
and more heterogeneous, with a higher proportion of insoluble 

material, with increasing depth. 

The gentle dip of the strata and their compositional and 

textural trends down the stratigraphic column have produced 

distinctive topographic and hydrologic patterns across the 

surface of the Eastern Highland Rim. Near the base of the 

Cumberland Plateau, a sinkhole plain is developed on thick 

outcrops of the upper St. Louis Limestone. Karst development and sinkhole density decrease westward as the St. Louis 

Limestone thins out, eventually grading into the subtle, lowrelief karst of The Barrens, which has developed on the lower 

Warsaw Limestone and upper Fort Payne Formation. Farther 

west, along the margins of the Nashville Basin, the mechanical 

resistance of the lower Fort Payne Formation contributes to the 

formation of the steep, fluvially disected Highland Rim Ecarpment (Wolfe, 1996a, b; Wolfe and others, 1997).

Most of the AAFB area is mapped as Warsaw and St. 

Louis Limestones (Wilson, 1976). In the study area, both 

formations are weathered to clay-rich residuum. The uppermost unit of relatively unweathered bedrock is the Fort Payne 

Formation. 

The primary aquifers in the study area are, from top to 

bottom, the shallow aquifer, the Manchester aquifer, and the 

Fort Payne aquifer. The shallow aquifer consists of 1.5 to 23 m 

of clay-rich residuum and includes the soil cover and root 

zone. The Manchester aquifer is formed from the weathering 

of the lower Warsaw Limestone and the Fort Payne Formation (Burchett and Hollyday, 1974) and is the most productive 

and most complex aquifer in the study area. The upper part 

of the Manchester aquifer consists of chert gravel, weathered 

limestone, and rubble. The lower part includes fractures and 

solution openings in the Fort Payne Formation. The Fort Payne 

aquifer consists of that part of the Fort Payne Formation which 

is relatively dense, with few small fractures or solution openings. The characteristics that define the Fort Payne aquifer also 

limit its transmissivity and productivity (Haugh and Mahoney, 

1994).

Solution openings are most common near the top of bedrock in the Fort Payne Formation but have been found 25 m 

or more below the top of bedrock (Haugh and others, 1992). 

The lower part of the Manchester aquifer is more prevalent 

in areas, such as the northern part of AAFB, where weathering profiles are relatively shallow (less than about 15 m thick) 

and the concentration of solution openings in the Fort Payne 

Formation is relatively high (Haugh and others, 1992; Haugh 

and Mahoney, 1994); the distinctive karst topography of the 

Sinking Pond area is a surface expression of these hydrogeologic characteristics (Wolfe, 1996a). 

Soils

Soils in the Sinking Pond area belong to the DicksonMountview-Guthrie soil association and consist chiefly 

of Ultisols developed on a thin (about l.5 m), silty mantle 

overlying cherty limestone residuum (Love and others, 1959; 

Springer and Elder, 1980; Smalley, 1983; Patterson, 1989). 

The Dickson silt loam and Mountview silt loam are the dominant soils on well-drained slopes and ridges. Both of these 

soils are strongly to very strongly acid, moderately permeable 

in their surface horizons, and low in fertility; they differ primarily in that the Dickson soil has a discontinuous fragipan at 

the base of the silty upper mantle (Love and others, 1959).

The Guthrie and Purdy silt loams are characteristic soils 

of headwater wetlands in The Barrens and dominate the ponded area of Sinking Pond. These soils are developed on parent 

materials similar to those of the Dickson and Mountview soils. 

The main distinction between them is the higher clay content 

of the Purdy silt loam. These soils are strongly to very strongly 

acid and low in fertility. The Guthrie and Purdy silt loams 

differ from the Dickson and Mountview silt loams primarily in 

their poor drainage and landscape position (Love and others, 

1959; Springer and Elder, 1980; Patterson, 1989). Other soils 

within the Dickson-Mountview-Guthrie soil association are 

the moderately well-drained Sango silt loam and the somewhat 

poorly drained Taft (formerly Lawrence) silt loam (Love and 

others, 1959; Patterson, 1989).

Climate

Long-term weather records for Tullahoma, Tennessee, 

near the southwest boundary of AEDC, are representative of 

conditions in the study area. Normal (30-year average computed decennially) annual precipitation for the period 1971 

through 2000 is 152.5 centimeters (cm). Monthly normal precipitation ranges from 8.9 cm in August to 17.1 cm in March. 

Monthly normal mean temperatures range from 2.72  ̊C in 

January to 25  ̊C in July (National Oceanographic and Atmospheric Administration, 1900-2003). The normal precipitation 

at Tullahoma has not been stationary during the past several 

decades but has increased steadily since 1960 (fig. 4). This 

increase is consistent with increased precipitation and streamflow since 1970 documented in much of the eastern United 

States (Karl and others, 1996; McCabe and Wolock, 2002).

Vegetation 

The term “barrens” was used widely by English-speaking 

settlers east of the Mississippi River to describe areas where 

trees were sparsely distributed or stunted in growth. In Tennessee, the term was applied to areas of heterogeneous oaksavanna vegetation on the Highland Rim with physiographic 

characteristics that included: (1) low (less than 20 m) relief, 

(2) thick, infertile, residual soils, and (3) extensive areas of 

poor drainage interspersed with well or excessively drained 

slopes and ridges (Killebrew and Safford, 1874; Lollar, 1924; 

Fenneman, 1938; Wolfe, 1996b; Pyne, 2000). The Barrens 

(fig. 1) is the largest contiguous occurrence of these areas in 

Tennessee.

Information on the historical and present-day vegetation 

of The Barrens and analogous areas of the Highland Rim was 

6 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
reviewed and summarized by Pyne (2000). Fire suppression 

and other land-use change in the 19th and 20th centuries have 

altered the vegetative structure and composition of these areas, 

greatly increasing the forested area and the prevalence of 

formerly rare or absent tree species, such as fire-intolerant red 

maple (Acer rubrum L.) and commercially introduced loblolly 

pine (Pinus taeda L.) (Gattinger, 1901; Lollar, 1924; Chester 

and others, 1995, 1997; Pyne, 2000). Despite these changes, 

The Barrens retains considerable botanical importance because 

of its high concentration of plants and animals listed as rare, 

threatened, or endangered (RTE) by the State of Tennessee 

(Svenson, 1941; DeSelm and others, 1974; Kral, 1973; Bowen 

and Pyne, 1995). 

Pyne (2000) tabulated a list of 70 State-listed RTE plant 

taxa found in The Barrens. A striking characteristic of the 

list is the high proportion of taxa that are endemic to areas 

far removed in distance and physiography from the Highland 

Rim. Of the 70 taxa, 25 are endemic or nearly endemic to the 

Coastal Plain Physiographic Province—The Barrens and, in 

some cases, similar areas in Tennessee and Kentucky, appear 

as prominent and anomalous “inland stations” in their distribution. An additional 15 taxa are broadly disjunct from the 

Coastal Plain. Seven of the State-listed species of The Barrens are disjunct or peripheral from the northern United States 

or Canada, and six are disjunct from west of the Mississippi 

River. Fifteen taxa fit none of the above categories, including 

several that are widely distributed in the eastern United States 

and Canada; only two taxa are endemic or nearly so to the 

Interior Low Plateau (Pyne, 2000). In addition to the 40 Statelisted Coastal Plain disjuncts discussed previously, numerous 

non-listed Coastal Plain plants are found in The Barrens. Two 

of these, water tupelo (Nyssa aquatica L.) and overcup oak 

(Quercus lyrata Walt.), are prominent wetland tree species in 

the Sinking Pond drainage basin.

In the Sinking Pond area, vegetation is generally correlated with topography, drainage, and soil (Patterson, 1989). 

Well-drained ridges and slopes support deciduous trees such 

as scarlet oak (Quercus coccinea Muenchh.), southern red 

oak (Quercu falcata Michx.), and mockernut hickory (Carya 

tomentosa [Poir.] Nutt.). Moderately well-drained slopes are 

characterized by white oak (Quercu alba L.), hornbeam (Carpinus virginiana [Mill.] K. Koch), sourwood (Oxydendrum 

arboreum [L.] D.C.), and yellow poplar (Liriodendron tulipifera L.). The vegetation of poorly drained sites commonly 

includes sweetgum (Liquidambar styriciflua L.), blackgum 

(Nyssa sylvatica Marsh.), red maple (Acer rubrum L.), and 

willow oak (Quercus phellos L.). Some of the wettest sites 

support stands of coastal plain trees such as overcup oak and 

water tupelo (Patterson, 1989; Wolfe, 1996a; McCarthy and 

Evans, 2000). 

The deep areas of Sinking Pond are dominated by an 

assemblage of overcup oak, river birch (Betula nigra L.), and 

resurrection fern (Pleopeltis polypodioides L.). This assemblage has a preliminary ranking by the Nature Conservancy 

of G1—extremely rare globally (Tennessee Department of 

Environment and Conservation, 2002). Appendix A provides 

a comprehensive list of the flora in the ponded area of Sinking 

Pond. 

Tree-Regeneration and Mortality 

Patterns 

This study expands upon the results of McCarthy and 

Evans (2000) through the development of three lines of 

botanical evidence: (1) extension of the demographic census 

of overcup oak in Sinking Pond described in McCarthy and 

Evans (2000) to better characterize temporal patterns of tree 

regeneration, recruitment, and mortality with 5 years of continuous monitoring; (2) examination of the relation between 

elevation—an indicator for ponding depth and duration—and 

the spatial distribution of different size classes of overcup oak 

and other tree species in a representative area of Sinking Pond; 

and (3) analysis of the age-distribution of overcup oak and the 

relation of germination date to elevation in a representative 

area of Sinking Pond.

Overcup Oak Population Dynamics in Sinking 

Pond, 1997-2001

The establishment, survival, and recruitment of overcup 

oak seedlings and the survival and mortality of adult overcup 

oaks in Sinking Pond were studied in two 20-m-wide box 

135

140

145

150

155

FINAL YEAR OF 30-YEAR

AVERAGING PERIOD

PRECIPIT

A

TIO

N, IN

CEN

TIMETER

S

1960 1970 1980 1990 2000

Figure 4. Normal (30-year mean, computed 

decennially) annual precipitation at Tullahoma, 

Tennessee, 1960-2000. (National Oceanic and 

Atmospheric Administration, 1900-2003.)

Tree-Regeneration and Mortality Patterns 7
transects (fig. 2). The box transects were described by McCarthy and Evans (2000), and their center lines correspond to 

narrower vegetation transects established in a previous study 

(Wolfe, 1996a). The transects encompass the range of elevation and ponding depth and duration present in Sinking Pond 

(Wolfe, 1996a). 

Seedling Establishment and Recruitment 

Seedling establishment and recruitment within the overcup oak population in Sinking Pond was studied using 162 

circular seedling plots with a radius of 0.5 m established in 

1997 and described by McCarthy and Evans (2000), supplemented by an additional 31 plots established in 1998. The 

seedling plots were randomly located within two 20-m-wide 

box transects (fig. 2). The plots were assigned to pondingdepth classes, based on the height above land surface of the 

watermark (moss line) on the nearest tree. The watermark is a 

striking discoloration left on trees marking the typical highwater level within the pond (fig. 5), which approximates (plus 

or minus 15 cm) the spillway elevation, 324.5 m above the 

National Geodetic Vertical Datum of 1929 (NGVD 29). The 

distance between the watermark and the land surface was the 

designated ponding depth for each plot. 

Two ponding-depth classes were used in this analysis. 

“Shallow” plots had a ponding depth of 0.5 m or less; “deep” 

plots had a ponding depth greater than 0.5 m. Of the original 

162 plots, 76 had shallow ponding depths, and 86 had deep 

A

B

Figure 5. Photographs 

of areas (A) in the interior 

and (B) along the margins 

of Sinking Pond. Note the 

watermarks (moss lines at the 

bases of the trees and their 

pronounced visual effect on 

the pond’s interior. The understory in the background of 

photograph (B) includes overcup oak saplings and small 

adults, which are absent in 

most of the pond.

8 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
ponding depths. The 31 plots established in 1998 all had shallow ponding depths.

Seedlings were defined as individuals less than 0.5 m tall. 

First-year seedlings were identified by the persistent attachment of the acorn to the stem and recorded. Saplings were 

defined as individuals 0.5 m to 1.5 m tall at the time of census 

(table 1). Subsequent seedling cohorts were identified, measured, and tagged annually from 1997 through 2001. The 1997 

census identified seedlings from the 1996 age cohort. Each 

seedling cohort was monitored for survival and growth in all 

plots for the 5-year period from 1996 through 2001. 

Seedling density and survival differed between the two 

ponding depth classes. Seedling density was greater in deep 

plots than in shallow plots for each of the census years except 

1999 (table 2). The 1999 cohort represented germination 

from a very light seed rain in 1998 that was extremely patchy, 

particularly in the deep part of the two transects. However, no 

seedlings from the deep plots survived longer than 3 years past 

germination (table 3). In comparison, seedlings established 

in 1997 in shallow areas still had 8.8 percent survival 4 years 

after germination (table 3). 

Observed recruitment from seedling to sapling showed 

a strong relation to ponding-depth class. In ponding depths 

less than 0.5 m, a pool of saplings was maintained over the 

5-year sampling period through successful recruitment of 

saplings from the seedling pool (table 4). Attrition from this 

pool was mainly through transition of saplings back into the 

seedling category through stem dieback and subsequent height 

loss (table 4). In contrast, no seedlings that grew to saplings 

in deep plots survived more than 1 year during this study. 

All individuals that grew from seedling to sapling during the 

5-year study were individuals that were already seedlings prior 

to 1997. The transition from seedling to sapling appears to be 

the critical point at which the recruitment of new trees currently is failing in the deep areas of Sinking Pond.

Adult Mortality 

In 1997, 55 seedling plots were randomly selected as center points for 10-m-radius plots in which adult overcup oaks 

were measured for diameter at 1.5 meters above land surface 

(DBH), tagged for future reference, and recorded. Survival 

of established adult trees and recruitment of new adults were 

tracked in annual censuses of the 10-m-radius plots through 

2001. 

Overall density of adults was more than five times greater 

in the deep areas compared to the shallow areas (table 5). 

Throughout the 5-year period, there was a complete absence of 

small adults (trees taller than 1.5 m with DBH less than 5 cm) 

in the deep plots (table 5). This pattern was consistent with the 

seedling recruitment failure previously described and with the 

Table 1. Height and diameter criteria for tree-size classes.

[m, meters; cm, centimeters; DBH, diameter at 1.5 meters above land 

surface; --, no criteria; <, less than; ≥, equal to or greater than]

Size class Height range 

(m)

DBH range 

(cm)

Seedling 0-1.499 --

Sapling 0.5-0.99 --

Small adult >1.5 <5

Adult >1.5 ≥5

Table 2. Aggregate density of overcup oak seedlings in Sinking 

Pond by cohort year and ponding depth, 1997-2001.

[Seedling density in seedlings per hectare, determined from 0.5-meter-radius 

circular plots; Bold numbers represent density of first-year seedlings; n, number of plots; shallow, ponding depth 0.5 meter or less; deep, ponding depth 

greater than 0.5 meter]

Cohort year

Census year

1997 1998 1999 2000 2001

Shallow 

1996 (n = 76) 26,302 11,057 5,193 3,853 3,351

1997 (n = 76) 65,002 25,623 12,062 7,706 5,696

1998 (n = 107) 10,472 5,355 3,332 2,618

1999 (n = 107) 9,163 8,211 4,998

2000 (n = 107) 24,989 19,277

2001 (n = 107) 11,542

Deep (n = 86)

1996 17,766 2,517 148 0 0

1997 67,215 5,330 148 0 0

1998 15,545 148 0 0

1999 1,184 296 296

2000 59,813 35,088

2001 71,657

Table 3. Aggregate percent survival of overcup oak seedlings in 

Sinking Pond by cohort year and ponding depth, 1997-2001.

[Percent survival determined from 0.5-meter-radius circular plots; n, number 

of plots; shallow, ponding depth 0.5 meter or less; deep, ponding depth greater 

than 0.5 meter]

Cohort year

 Percent survival at annual census

1997 1998 1999 2000 2001

Shallow

1996 (n = 76) n/a 42.0 19.7 14.6 12.7

1997 (n = 76) 100 39.4 18.6 11.9 8.8

1998 (n = 107) 100 51.1 31.8 25.0

1999 (n = 107) 100 89.6 54.5

2000 (n = 107) 100 77.1

Deep (n = 86)

1996 n/a 14.2 0.83 0 0

1997 100 7.93 0.22 0 0

1998 100 0.95 0 0

1999 100 25.0 25.0

2000 100 58.7

Tree-Regeneration and Mortality Patterns 9
conclusion reached by McCarthy and Evans (2000) that seedlings currently are unable to become saplings and small adults 

at ponding depths greater than 0.5 m in Sinking Pond. In 

contrast, limited recruitment of small adults was documented 

in 2000 and 2001 in shallow areas of the pond (table 5) associated with light openings in the canopy (McCarthy and Evans, 

2000). For individuals greater than or equal to 5-cm diameter 

in both the deep and shallow areas, there were no other gaps 

in size-class distribution, suggesting continuous recruitment 

historically in these areas (McCarthy and Evans, 2000). 

Annual adult-tree mortality between 1997 and 2001 

ranged from negligible to nearly 30 percent and varied 

with size and ponding-depth classes (fig. 6). For all size 

and ponding-depth classes, the greatest adult-tree mortality 

occurred in the winter of 1998-99 (fig. 6), during and after ice 

storms. The ice storms had the greatest effect on trees with 

DBH of 5 to 40 cm in the shallow areas (fig. 6). Trees in the 

deep area, however, sustained a slightly higher rate of mortality following the ice storm (fig. 6). The rate of small adult 

recruitment in the shallow area appeared to compensate for 

the loss due to mortality (table 5). Such is not the case in the 

deep area where recruitment was absent. At the current rate of 

mortality absent replacement, the deep part of Sinking Pond 

will lose most of its forest canopy by the second half of this 

century. 

Spatial Patterns of Tree-Size Classes and 

Ponding Depth 

The critical stage at which tree-regeneration failure 

occurs is the transition from the seedling to sapling class. 

This transition appears to occur only at ponding depths of 

0.5 m or less. These results are consistent with earlier findings for overcup oak in Sinking Pond (McCarthy and Evans, 

2000). However, generalization of these results to the pond 

as a whole is limited by the linear sampling framework on 

which they are based, the imprecision of using watermarks as 

a surrogate for elevation, and the narrow focus on just one tree 

species. This section examines tree regeneration and mortality 

in the context of the overall forest community across a larger 

and more representative area of the pond.

In August 2001, a marked grid system of plots was established within a 2.3-ha area of forest (fig. 2) to examine spatial 

patterns of regeneration among the three most common tree 

species in Sinking Pond: overcup oak, willow oak, and sweetgum. The grid area encompasses the internal flow channel and 

spans the range of ponding depths from greater than 2 m to 

the edge of the pond. The grid consisted of 57 plots, 20 m by 

20 m, which were divided into 2-m by 2-m subplots. 

Within each 2-m by 2-m subplot, the density of overcup 

oak, willow oak, and sweetgum seedlings, saplings, and small 

adults were noted. Inside the larger 20-m by 20-m plots, adult 

overcup oak, willow oak, and sweetgum individuals were measured for DBH and mapped to the nearest meter in grid location. All trees with a DBH of 5 cm or greater were identified to 

species and mapped with grid location to the nearest meter.

The topography of the grid was surveyed to the nearest 

centimeter using a total station. Surveyed points included the 

corners and center of each 20-m by 20-m plot and topographic 

features such as sinkholes, channels, and major breaks in 

slope. Geographic coordinates were obtained for five points 

within the grid using a global positioning system (GPS), and 

the surveyed points were georeferenced, rectified, and converted into a surface of 2-m by 2-m cells, corresponding to the 

subplots of the same dimensions. 

All subplot and plot data were entered into a spatial database and mapped, using grid coordinates to link them to the 

rectified surface. For data analyses, the grid was divided into 

Table 4. Recruitment and survival of overcup oak saplings in 

sampled plots in Sinking Pond by ponding-depth class, 1997-2001. 

[Determined from 0.5-meter-radius circular plots; n, number of plots; --, no 

data; shallow, ponding depth 0.5 meter or less; deep, ponding depth greater 

than 0.5 meter]

Year

Overcup oak saplings

Total New Deaths Regress to 

seedling

Shallow plots

1997 (n = 76) 4 -- -- -- 

1998 (n = 107) 5 2 0 1

1999 (n = 107) 3 1 1 2

2000 (n = 107) 3 0 0 0

Deep plots (n = 86)

1997 14 -- -- -- 

1998 6 0 8 0

1999 0 0 6 0

2000 0 0 0 0

Table 5. Aggregate density of overcup oak adults by size class 

and ponding depth in sampled plots in Sinking Pond, 1997-2001.

[Determined from 55 circular plots (radius 10 meters); density in stems per 

hectare; shallow, ponding depth 0.5 meter or less; deep, ponding depth greater 

than 0.5 meter; DBH, diameter at 1.5 meters above land surface; cm, centimeters; <, less than; >, greater than]

DBH, in cm Number of adult overcup oaks, by year

1997 1998 1999 2000 2001

Shallow

<5 0 0 0 7 9

5-40 25 25 17 17 17

>40 30 30 25 25 25

Total 55 55 42 49 51

Deep

<5 0 0 0 0 0

5-40 205 204 188 184 181

>40 92 92 80 78 76

Total 296 296 268 262 257

10 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
three ponding-depth classes: shallow (less than 0.5-m ponding 

depth or at or above 324 m above NGVD 29); intermediate 

(0.5-1.0-m ponding depth or 323.5-323.99 m above NGVD 

29); and deep (greater than 1.0-m ponding depth or below 

323.5 m above NGVD 29). The distribution of subplots among 

depth classes was as follows: 26 percent (1,486) were shallow; 

35 percent (2,007) were intermediate; and 39 percent (2,207) 

were deep. 

Adults 

Overall density of adults in the deep area (626 trees per 

ha) was greater than the density of adults found in the intermediate (324 trees per ha) and shallow (355 trees per ha) 

areas (table 6). Nine tree species were present within the grid 

(table 6). All nine species were found in the shallow area, and 

tree-species richness decreased with increasing ponding depth 

(table 6). Only overcup oak, sweetgum, willow oak, and river 

birch composed the canopy of the deep area (table 6). Adult 

overcup oak density increased with ponding depth (table 6; 

fig. 7). Density of adult overcup oaks in the deep area was 

307 trees per ha, the highest of any species throughout the 

grid (table 6). The smooth distribution of overcup oak adult 

size classes for the deep areas 

(fig. 8) suggests that prior to the 

recent failure of sapling/small 

adult recruitment the size distribution of adult overcup oaks 

in deep areas of the pond was 

stable. Within each of the depth 

classes, there was some degree 

of spatial clumping of overcup 

oak individuals (fig. 7), as noted 

by McCarthy and Evans (2000).

Unlike overcup oak, 

sweetgum density decreased 

with ponding depth (fig. 9). The 

density of adult sweetgum in 

the shallow part of the surveyed 

area, 128 trees per ha, was the 

second highest density of adult 

trees found in this analysis 

(table 6). Sweetgum was the 

dominant canopy species in 

both the shallow and intermediate areas (table 6). The intermediate area was the only part 

of the sweetgum population 

that had representation in DBH 

size classes greater than 55 cm 

(fig. 10). This pattern, in combination with the high density 

of individuals having DBH less 

than 15 cm in the shallow area, 

suggests a recent population 

shift of sweetgum to shallower 

areas within the pond. 

Willow oak had the fourth highest density in each of the 

ponding-depth classes (table 6). The highest density of Willow 

oak was noted in the shallow area (table 6, fig. 11). However, 

the lack of trees in the 5-25 cm DBH class in the shallow 

area suggests that the population is on the decline in this area 

(fig. 12). Similar to the other species, recruitment of saplings 

is strongly concentrated in shallow areas (fig. 11). Recruitment 

of willow oak small adults appears to be generally failing, 

even in areas where sapling recruitment is successful (fig. 11). 

River birch was similar to overcup oak in being the only 

other species whose density was highest in the deep part of 

the surveyed area (table 6). The distribution of river birch, like 

that of overcup oak, is skewed toward smaller size classes in 

the intermediate and shallow areas, suggesting recent displacement of river birch from deeper to shallower areas of the pond 

(fig. 13). 

Small Adults and Saplings

Saplings and small adults of all three focal species 

(overcup oak, sweetgum, and willow oak) were found almost 

exclusively in the shallow part of the grid (table 7; figs. 7, 

1998 1999 2000 2001

0

10

20

30

CENSUS YEAR

�����������������������������������������������������������������������������������������������

����������������������������������������������������������������������������������������������

������������������������������ PERCENT MORTALITY SINCE PREVIOUS CENSUS

DIAMETER 5 - 40 CENTIMETERS,

 PONDING LESS THAN 0.5 METER

DIAMETER GREATER THAN 40 CENTIMETERS,

 PONDING LESS THAN 0.5 METER

DIAMETER 5 - 40 CENTIMETERS,

 PONDING GREATER THAN 0.5 METER

DIAMETER GREATER THAN 40 CENTIMETERS,

 PONDING GREATER THAN 0.5 METER

SHALLOW DEEP

EXPLANATION

Figure 6. Aggregate mortality of adult overcup oaks by ponding depth and size class, assessed 

annually in 55 circular plots (10-meter radius) in Sinking Pond, 1998-2001. Diameters measured 

1.5 meters above land surface.

Tree-Regeneration and Mortality Patterns 11
ADULTS

SAPLINGS

SMALL ADULTS

SEEDLINGS

PONDING-DEPTH CLASS

SHALLOW—Less than 0.5 meter 

INTERMEDIATE—0.5-1 meter 

DEEP—Greater than 1 meter

EXPLANATION

ADULT TREE

ADULT TREE SAMPLED FOR

 TREE RING ANALYSIS

LIFE-HISTORY STATE

NON-ADULT TREE—Indicates presence

 in 4-square-meter subplot

0 50 METERS

Figure 7. Distribution of overcup oak by ponding-depth class and life-history state in a 2.3-hectare area of Sinking Pond (see figure 2 

inset for location of distribution-grid site).

12 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
Table 6. Density of adult trees by species and ponding-depth 

class in a 2.3-hectare area of Sinking Pond, 2001-2002.

[Shallow, ponding depth 0.5 meter or less; intermediate, ponding depth 

0.5 - 1.0 meter; deep, ponding depth greater than 1.0 meter]

 Tree species

Tree density (stems per hectare)

Shallow Intermediate Deep

Sweetgum 128 112 22

Red Maple 74 17 0

Black Gum 96 26 0

Willow Oak 31 22 12

Overcup Oak 28 93 307

River Birch 9 7 14

Sourwood 3 0 0

Water Oak 2 0 0

Persimmon 1 0 0

Total 626 345 355

0

50

100

150

5-14.9 15-24.9 25-34.9 35-44.9 45-54.9 55-64.9 65-74.9 75-84.9 85-94.9 Greater

than

DIAMETER CLASS, IN CENTIMETERS 94.9

T

REE DENSIT

Y, IN S

TEM

S PER HEC

T

A

RE

PONDING-DEPTH CLASS

SHALLOW—Less than 0.5 meter

INTERMEDIATE—0.5-1 meter

DEEP—Greater than 1 meter

Figure 8. Size-class distribution of adult overcup oak in a 2.3-hectare area of Sinking Pond by pondingdepth class.

Tree-Regeneration and Mortality Patterns 13
PONDING-DEPTH CLASS

SHALLOW—Less than 0.5 meter

INTERMEDIATE—0.5-1 meter

DEEP—Greater than 1 meter

EXPLANATION

ADULT TREE

LIFE-HISTORY STATE

NON-ADULT TREE—Indicates presence

in 4-square-meter subplot

ADULTS

SAPLINGS

SMALL ADULTS

SEEDLINGS

0 50 METERS

Figure 9. Distribution of sweetgum by ponding-depth class and life-history state in a 2.3-hectare area of Sinking Pond (see figure 2 

inset for location of distribution-grid site).

14 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
9, 11). There were no saplings of any species found in the 

deep part of the grid. A few small adults were found in deep 

areas (figs. 7 and 9), but ring counts of recently dead small 

adult overcup oaks in these areas showed them to be stunted 

trees older than 30 years, rather than recently recruited. Overcup oak and sweetgum saplings and small adults were present 

in the intermediate part of the surveyed area in low densities 

(12 or fewer stems per ha), with less than 0.5 percent of subplots containing these size classes (table 7). The concentration 

of overcup oak saplings and small adults in shallow areas, the 

suppression of recruitment to these size classes elsewhere in 

the pond, and the continued mortality of overcup oak adults 

discussed in the previous section indicate a spatial shift of the 

overcup oak population from deep and intermediate areas to 

shallow areas in Sinking Pond. Few willow oak and sweetgum small adults or saplings were found in the intermediate 

area (table 7). No river birch saplings or small adults were 

observed anywhere within the grid. Within the shallow area, 

saplings and small adults were found in clumped patterns 

(figs. 7, 9, 11), most likely reflecting the distribution of light 

gaps in the canopy (McCarthy and Evans, 2000).

Seedlings

Seedlings of all three species were distributed throughout the grid, regardless of water depth (table 7; figs. 7, 9, 11). 

Overcup oak seedlings were by far the most widespread 

and numerous, being present in more than 65 percent of the 

subplots in each of the ponding-depth classes and representing 

the highest seedling densities of any species (table 7, fig. 7). 

Overcup oak seedling density increased with ponding depth; 

total density in the deep area exceeded 46,000 seedlings per 

ha, nearly triple the density of overcup oak seedlings in the 

intermediate area and more than six times the density in the 

shallow part of the sample grid (table 7). Sweetgum seedling 

density was highest in the intermediate area, and willow oak 

seedlings were most abundant in the shallow area. 

Age Distribution of Adult Overcup Oaks

Approximately 10 percent of live adult overcup oak in 

the 2.3-ha plot grid were sampled for tree-ring analysis to better characterize the age distribution of the overcup oak stand 

in Sinking Pond. At the time of sampling, information on site 

topography and tree sizes was insufficient to support a random 

sample stratified by elevation and size. However, the trees had 

been mapped, and their spatial distribution provided a framework for initial stratification and sampling. 

Adult overcup oaks in the plot grid were divided into five 

groups based on stem density in different areas of the grid. At 

least 10 percent of the trees in each group were selected for 

sampling. The spatial distribution of sampled trees within the 

population of adult overcup oaks in the plot grid is shown in 

0

20

40

60

80

100

120

140

160

180

5-14.9 15-24.9 25-34.9 35-44.9 45-54.9 55-64.9 65-74.9 75-84.9 85-94.9 Greater

than

DIAMETER CLASS, IN CENTIMETERS 94.9

T

REE DEN

SIT

Y, IN

S

TEM

S PER HEC

T

A

RE

PONDING-DEPTH CLASS

SHALLOW—Less than 0.5 meter

INTERMEDIATE—0.5-1 meter

DEEP—Greater than 1 meter

Figure 10. Size-class distribution of adult sweetgum in a 2.3-hectare area of Sinking Pond by 

ponding-depth class.

Tree-Regeneration and Mortality Patterns 15
PONDING-DEPTH CLASS

SHALLOW—Less than 0.5 meter

INTERMEDIATE—0.5-1 meter

DEEP—Greater than 1 meter

EXPLANATION

ADULT TREE

LIFE-HISTORY STATE

NON-ADULT TREE—Indicates presence

in 4-square-meter subplot

ADULTS

SAPLINGS

SMALL ADULTS

SEEDLINGS

0 50 METERS

Figure 11. Distribution of willow oak by ponding-depth class and life-history state in a 2.3-hectare area of Sinking Pond (see 

figure 2 inset for location of distribution-grid site).

16 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
0

2

4

6

8

10

12

14

16

18

20

5-14.9 15-24.9 25-34.9 35-44.9 45-54.9 55-64.9 65-74.9 75-84.9 85-94.9 Greater

than

DIAMETER CLASS, IN CENTIMETERS 94.9

T

REE DENSIT

Y, IN S

TEM

S PER HEC

T

A

RE

PONDING-DEPTH CLASS

SHALLOW—Less than 0.5 meter

INTERMEDIATE—0.5-1 meter

DEEP—Greater than 1 meter

Figure 12. Size-class distribution of adult willow oak in a 2.3-hectare area of Sinking Pond by 

ponding-depth class.

0

2

4

6

8

10

12

14

DIAMETER CLASS, IN CENTIMETERS

TREE DENSITY, IN STEMS PER HECTARE

5-14.9 15-24.9 25-34.9 35-44.9 45-54.9 55-64.9 65-74.9 75-84.9 85-94.9 Greater

than

94.9

PONDING-DEPTH CLASS

SHALLOW—Less than 0.5 meter

INTERMEDIATE—0.5-1 meter

DEEP—Greater than 1 meter

Figure 13. Size-class distribution of adult river birch in a 2.3-hectare area of Sinking Pond by 

ponding-depth class.

Tree-Regeneration and Mortality Patterns 17
figure 7. Selection of individual trees was made in the field. 

The frequency distributions of the elevations and diameters of 

the sampled trees are similar to those of the live adult overcup oak population in the plot grid (fig. 14). Trees with DBH 

greater than 75 cm were not sampled because such trees represent less than 2 percent of the plot-grid population (fig. 14), 

and commonly are hollow in the center. The final sample 

consisted of 45 of 442 live adult overcup oak in the plot grid.

Sample collection, processing, and analysis followed procedures described by Phipps (1985). Trees with diameters of 

less than 6 cm were sectioned with a crosscut saw, as was one 

9-cm tree that had sustained damage from a fallen neighboring 

tree. All other selected trees were sampled with an increment 

borer. Cores were mounted in wooden core clamps and shaved 

with a disposable scalpel. Rings on the mounted, shaved cores 

were counted using a dissecting microscope for magnification. 

For the purposes of this study, rings were counted from the 

outer edge to the center, but individual annual growth increments were not measured. 

The completeness and clarity of the rings on each core 

and section were evaluated, and the probable error associated with each ring count was estimated. Some of the samples 

failed to include the tree center. In others, the rings were small 

and indistinct. The maximum number of missing or false 

rings was estimated for each sample and expressed as an ageestimation error, with values ranging from -8 to 8 years. Positive values indicate less than complete samples with generally 

clear rings; negative values indicate complete samples with 

possible false rings. Plus-or-minus values indicate samples 

with very small, indistinct rings. 

For plotting and analysis, the ring-count for each sample 

was assumed to be the age of the tree, with the age-estimation 

error as an indication of the probable magnitude and direction 

of uncertainty associated with each age. Where two or more 

cores from the same tree yielded different ring counts, the 

most reliable ring count was used. Ring-count dates were calculated as 2001 minus the ring count. The topographic survey 

and surface-fitting described in the previous section were used 

to relate grid location and tree age to elevation.

The results of the tree-ring analysis show a sharp discontinuity in the relation between age and elevation at about 

30 years (fig. 15). All trees sampled in shallow areas (elevation above 324 m above NGVD 29) had ring-count ages of 

less than 30 years. The elevations of young adults revealed 

by tree-ring analysis thus correspond to the elevation range at 

which small adult and sapling overcup oaks were found in the 

Table 7. Density and percentage of 4-square-meter subplots containing overcup oak, sweetgum, and 

willow oak stems in a 2.3-hectare area of Sinking Pond by ponding-depth class and size class, 2001-

2002.

[Shallow, ponding depth 0.5 meter or less; intermediate, ponding depth 0.5 - 1.0 meter; deep, ponding depth greater than 

1.0 meter; stems/ha, stems per hectare; >, greater than; <, less than]

 Size class 

and species

Ponding-depth class

Shallow Intermediate Deep

Density 

(stems/ha)

Percentage 

of subplots

Density 

(stems/ha)

Percentage 

of subplots

Density 

(stems/ha)

Percentage 

of subplots

Adults

Overcup oak 47 1.9 116 4.6 307 11.7

Sweetgum 215 8.6 140 5.6 22 0.86

Willow oak 52 2.1 27 1.1 12 0.45

Small adults

Overcup oak 106 2.6 1 0.05 3 0.14

Sweetgum 230 6.4 12 0.45 1 0.05

Willow oak 10 0.34 0 0 0 0

Saplings

Overcup oak 907 15.2 30 0.80 0 0

Sweetgum 175 4.4 9 0.30 0 0

Willow oak 156 4.4 2 0.10 0 0

Seedlings

Overcup oak 7,206 73.5 >16,000 66.8 >46,000 77.4

Sweetgum 390 10.8 1,668 24.6 281 5.7

Willow oak 2,238 47.7 1,056 24.4 430 11.6

18 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
spatial analysis of tree-size classes and successful recruitment 

of saplings observed in the population dynamics study. Ringcount ages among trees sampled in the intermediate and deep 

areas ranged from 32 to 148 years. The older trees show greatest temporal concentration at about 50 to 65 years (fig. 15); 

but overall, the results support McCarthy and Evans’s (2000) 

conclusion that the age distribution of overcup oaks in Sinking 

Pond is relatively even for trees greater than about 30 years, 

without large gaps. 

Botanical Evidence for Hydrologic Change 

Several lines of botanical evidence suggest that hydrologic change is responsible for the absence of overcup oak 

saplings and young adults in the deep interior of Sinking Pond. 

Annual censuses of 162 seedling plots from 1997 through 

2001, and of 31 additional plots from 1998 through 2001, 

show that recruitment to large saplings and small adults is 

confined to shallow ponding depths, 0.5 m or less below the 

normal high-water surface. Spatial analysis of tree-size classes 

for overcup oak, willow oak, and sweetgum shows seedlings 

and large adults are widely distributed, but saplings and small 

adults are concentrated in areas with shallow ponding depths. 

Analysis of tree rings from 45 adult overcup oaks indicates 

a relatively even age distribution across the pond’s elevation gradient for trees dated from 1856 to 1968. Trees dated 

after 1970 were restricted to shallow ponding depths around 

324.2 m above NGVD 29. 

These mutually supporting lines of botanical evidence 

show that: (1) the distribution of overcup oaks relative 

to ponding depth has changed through time; (2) the most 

important change is a spatial shift in successful recruitment of 

0 10 20 30 40 50 60 70 0

10

20

30

40

50

60

70

80

90

100

TREE DIAMETER, IN CENTIMETERS

C

U

M

ULATIVE FREQ

UEN

C

Y, IN PER

CEN

T

SAMPLED TREES

POPULATION

321 322 323 324

ELEVATION, IN METERS ABOVE NATIONAL

GEODETIC VERTICAL DATUM OF 1929

SAMPLED TREES

POPULATION

A B

Figure 14. Frequency distributions of (A) tree diameters at 1.5 meters above land surface and (B) elevation 

for adult overcup oaks sampled for tree-ring analysis and the adult overcup oak population of the 2.3-hectare 

sampled area in Sinking Pond.

Botanical Evidence for Hydrologic Change 19
saplings and small adults from areas with deep and intermediate ponding depths to areas with shallow ponding depths; and 

(3) this change happened abruptly beginning around 1970. 

These observations make a strong circumstantial case that 

hydroperiods in Sinking Pond have increased during the 

past several decades and that increase has altered ecological 

conditions since about 1970. Questions left unaddressed by the 

botanical evidence include the nature and timing of hydrologic change in Sinking Pond, the cause of such change, and 

whether it is purely a local phenomenon or reflects a broader 

regional or global climatic pattern. 

Hydrologic Change in Sinking Pond, 

1854-2002

Two hypotheses were examined to identify the nature 

and probable cause of hydrologic change in Sinking Pond. 

The first hypothesis was that the pond’s hydrologic regime 

had been affected by human activity in and around the Sinking 

Pond basin. The second hypothesis was that climate change 

had altered Sinking Pond’s seasonal ponding pattern sufficiently to affect tree recruitment and survival. 

Direct modification of the pond’s spillway or internal 

drain, land-use and land-cover changes in the pond’s drainage 

basin, and impoundment of surrounding catchments all have 

the potential to affect hydrologic conditions in the pond. Based 

on field observations, collection and analysis of hydrologic 

records, and air-photo analysis, none of these factors explained 

tree-regeneration patterns and apparent hydrologic change 

in Sinking Pond (Appendix B). Absent evidence that human 

activity had altered the hydrology of Sinking Pond, analysis 

focused on climate change. 

Widely reported increases in historical streamflow and 

rainfall across the eastern United States during the second half 

of the 20th century (Groisman and Easterling, 1994; Lettenmaier and others, 1994; Karl and others, 1996; Karl and Knight, 

1998; Lins and Slack, 1999; Douglas and others, 2000; Easterling and others, 2000; Groisman and others, 2001; McCabe 

and Wolock, 2002) suggest climate change as an explanation 

for the apparent increase in the Sinking Pond hydroperiod. 

For example, McCabe and Wolock’s (2002) analysis of daily 

streamflow records from 400 stations in the conterminous 

United States for the period 1941-99 revealed distinct step 

increases around 1970 in annual minimum and median daily 

streamflow at nearly half (48 percent) of the stations, mostly 

those located in the eastern United States. Similarly, Karl and 

others (1996), describing precipitation records for the period 

1900-94 from 600 stations distributed across the conterminous 

United States, noted that “since 1970, precipitation has tended 

to remain above the 20th-century mean and has averaged about 

5 percent more than in the previous 70 years.” Precipitation 

increases of 10 to 30 percent have been documented in the 

southeastern United States during the 20th century (Burkett and 

others, 2001), with a regional increase of about 10 percent in 

Middle Tennessee (Karl and others, 1996). 

50 100 150

321.5

322

322.5

323

323.5

324

324.5

ELEVATION, IN METERS ABOVE NATIONAL GEODETIC

VERTICAL DATUM OF 1929

0 50

0

20

40

60

80

100

CUMULATIVE FREQUENCY, IN PERCENT

 SINKING POND SPILLWAY ELEVATION

RING-COUNT AGE, IN YEARS

DEEP PONDING DEPTH

INTERMEDIATE PONDING DEPTH

INTERMEDIATE PONDING DEPTH

SHALLOW PONDING DEPTH

CUMULATIVE

 FREQUENCY

INDIVIDUAL TREE

Figure 15. Elevations, ring-count ages, and age-frequency distribution of overcup oaks sampled for tree-ring 

analysis in a 2.3-hectare area of Sinking Pond.

20 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
Temporal patterns in the annual departures from longterm average precipitation at Tullahoma, Tennessee, show 

strong similarity with national averages (fig. 16) published 

by Karl and others (1996). The long-term (1900-94) average 

precipitation at Tullahoma (1,412 mm) is nearly twice the 

national long-term average of 737 mm, and annual departures 

from the mean at Tullahoma are proportionately larger than 

those from the national mean (fig. 16). Both time series show 

a marked increase in the frequency and magnitude of positive 

departures and corresponding decreases in negative departures 

beginning around 1970 (fig. 16). 

Local and national increases in precipitation and streamflow beginning around 1970 coincide closely with the suppression of overcup oak regeneration in deep and intermediate 

areas of Sinking Pond shown by tree-ring analysis in the previous section. This similarity in timing suggests a link between 

climate and changing recruitment patterns in Sinking Pond. 

Demonstrating such a link requires analysis of the relation 

between rainfall and the pond’s hydrologic conditions. That 

relation was examined through development of a hydrologic 

model of Sinking Pond, which used climatic records for input 

and local hydrologic records for calibration and verification.

-100

0

100

1900 1920 1940 1960 1980 2000 -500

-400

-300

-200

-100

0

100

200

300

400

500

YEAR

DEPA

R

T

U

RE FR

O

M

MEA

N,

IN

MILLIMETER

S

ANNUAL DEPARTURE

7-YEAR DISTANCE-WEIGHTED

AVERAGE

National Mean

Tullahoma

DEPA

R

T

U

RE FR

O

M

MEA

N, IN

MILLIMETER

S

Figure 16. Departures from 1900-1994 mean annual precipitation for (top) national average across the conterminous United States (modified from Karl and others, 1996) and for (bottom) Tullahoma, Tennessee.

Hydrologic Change in Sinking Pond, 1854-2002 21
Hydrologic Modeling

Evaluating climate-driven hydrologic change in Sinking 

Pond required a surrogate measure of the pond’s filling and 

draining behavior over historical time that could be explicitly linked to climate. Long-term monthly and daily climate 

records for several sites in south-central Tennessee and hydrologic records collected in and around Sinking Pond since 1992 

provided a basis for such a surrogate in the form of a simple 

hydrologic model. 

Developing a model required making numerous assumptions, which are described in detail below. The central assumption of the entire modeling exercise is that the hydrologic 

response of the Sinking Pond basin to climatic inputs has not 

changed through historical time. The intent of the modeling 

effort was not to calculate actual water levels in Sinking Pond 

but rather to evaluate the pond’s response to historical temperature and precipitation inputs, holding other independent 

factors—such as vegetation, human activity, and the hydraulic 

characteristics of the regional ground-water system—constant. 

The model had to be able to account for antecedent conditions 

in the basin, determine when conditions are right for routing 

water to the pond, and allocate water in the pond to runoff, the 

atmosphere, or the ground-water system. Additional constraints imposed by limitations of the input and calibration 

data were:

1. The model had to have the simplest possible structure 

capable of providing a reasonable representation of the 

Sinking Pond hydrologic system and an adequate calibration;

2. The number of unconstrained terms could not exceed the 

number of inflections from distinct storm peaks and seasonal rises and recessions in the calibration data set; and

3. Simulated surface-water discharge from the pond and 

simulated evapotranspiration had to approximate published local or regional values summed over the calibration period.

Model Input Data

Daily precipitation and mean temperature data are the 

inputs for the Sinking Pond hydrologic model. The best 

available data sets are historical precipitation and temperature records archived by the National Climatic Data Center 

(NCDC), located in Asheville, North Carolina. The long-term 

station most representative of the study area is Tullahoma, 

Tennessee (NCDC station 409155), located about 15 km west 

of Sinking Pond (fig. 1). The Tullahoma climate data was 

supplemented by data from several other stations in Tennessee 

(table 8).

The United States Historical Climatology Network 

(USHCN), maintained at NCDC, provides relatively complete 

monthly and daily time series for precipitation and temperature at Tullahoma beginning in 1893 and 1896, respectively. 

During model development, both time series extended through 

December 1997. The USHCN monthly data include adjustments intended to improve regional consistency among 

stations over time (Karl and others, 1996). However, monthly 

records without regional adjustments were favored for this 

effort to better represent local hydrologic conditions. Additional climate records for Tullahoma and other Tennessee 

stations, covering the period from 1998 to 2002, were obtained 

from NCDC’s national network of cooperative weather stations. 

Because monthly total precipitation is generally more 

robust than the set of daily observations, a complete set of 

monthly values was compiled and used to check and adjust 

the daily data for modeling. Of 1,314 monthly precipitation 

totals for Tullahoma between April 1893 and September 2002, 

17, or 1.2 percent, were estimated from monthly totals for 

other Tennessee stations (table 8) using simple linear regression. The station records used as independent variables for the 

regression estimation were ranked by proximity to the study 

area, elevation, and completeness of record; the highest ranked 

station with data for a given month was used to estimate precipitation for that month. 

The daily precipitation data set for the period April 1893 

through September 2002 consists of 39,994 records. Approximately 97.7 percent of the daily precipitation records were 

taken from the Tullahoma, Tennessee, daily precipitation data. 

The remaining 2.3 percent (912 records) were taken from 

other stations, in order of preference shown in table 8. For 

months having missing record, the assembled daily precipitation data were summed and uniformly scaled to equal monthly 

estimated totals. 

Monthly and daily time series for mean temperature were 

developed using the same sources described for precipitation. 

The procedure for assembling the monthly and daily temperature time series differed from that described above for precipitation in that monthly means, rather than totals, were used.

Synthetic daily precipitation and temperature time series, 

covering the period January 1854 through March 1893 were 

generated to extend the hydrologic simulation back to the earliest ring-count dates found in the tree-ring analysis presented 

above. The synthetic daily time series were based on disaggregated monthly climate records from McMinnville and Clarksville, Tennessee, beginning in 1872 and 1854, respectively. 

Because of its closer geographic proximity to the study area, 

the McMinnville monthly record was used whenever available.

Disaggregation of monthly totals was based on the application of a scaled distribution of daily precipitation values 

adapted from daily observations in a set of 12 typical reference months selected from the observed record (1893-2002). 

Selection of the reference months was based on two criteria: 

(1) a complete daily precipitation record for the month (no 

estimated values), and (2) a total number of rain days within 

2 days of the 1893-2002 average for that calendar month. 

The 12 selected reference months were: January through 

April 1905, May through September 1904, October 1905, and 

November and December 1904. Daily values in the collected 

reference months were used to represent the corresponding 

22 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
daily values for each day in the disaggregated calendar year. 

Disaggregated record for the period January 1854 through 

April 1893 was generated by multiplying daily reference 

precipitation within each month by the ratio of that month’s 

precipitation total to its corresponding reference total. 

A synthetic time series of daily mean temperatures for the 

period January 1854 through March 1893 was generated using 

methods similar to those described above for precipitation. 

Because temperature generally varies more smoothly from day 

to day than precipitation, the 1970-99 daily normal mean temperatures were used as the reference daily temperature values. 

The 1970-99 daily normal mean temperatures were multiplied 

by the ratios of the 1854-93 monthly mean temperatures to the 

corresponding 1970-99 monthly normal mean temperatures to 

produce the synthetic daily temperature time series. 

Model Structure

The starting point for modeling water fluxes into and out 

of Sinking Pond is a classical water budget in the form:

P GI RI + + = ∆S GO RO E + + + , (1)

where P is precipitation; GI is ground-water inflow; RI is 

surface-runoff inflow; ∆S is change in volume of stored water; 

GO is ground-water outflow; RO is surface-runoff outflow; 

and E is evapotranspiration (ET). The storage term ∆S has 

inherent dimensions of volume (L3). The remaining terms in 

equation 1 are hydrologic fluxes, which can be expressed as 

volumes by integrating them over a standard time step. For 

the hydrologic model presented in this report, the standard 

time step is daily, and water-storage and flux terms have units 

of basin centimeters (bcm). For the Sinking Pond basin (area 

335 ha), 1 bcm equals 33,500 cubic meters (m3).

The first task in developing a water-balance model is to 

specify the number and definitions of one or more storage 

compartments and to conceptualize the movement of water 

into, out of, and between them. For this study, the model must 

account for precipitation and runoff inputs to the pond and 

outflows from the pond to surface water, the ground-water system, and the atmosphere. The number of possible approaches 

for structuring such a model is very large. In addition to the 

surface water in Sinking Pond, the modeled system could be 

defined to include such additional compartments as surfacewater storage in sub-basins, vadose-zone storage, and shallow 

or deep ground-water storage beneath different parts of the 

Table 8. Station data for climate stations used to develop input data set for the hydrologic model of Sinking Pond.

[NCDC, National Climatic Data Center, Asheville, North Carolina; ID, identification; d-m, degrees-minutes; NGVD 29, National Geodetic Vertical Datum of 

1929; USHCN, United States Historical Climatology Network; *, station included in the U.S. Historical Climatology Network; D, daily; M, monthly; --, not 

applicable]

Station name NCDC ID 

number

Latitude

(d-m)

Longitude

(d-m)

Elevation 

(NGVD 29) USHCN Period of record used

Order of 

preference

Daily records Monthly records D M

Tullahoma, Tenn. 409155 35° 21′ N 86° 12′ W 319.4 * Apr. 1, 1893 

through 

Sept. 30, 

2002

Apr. 1893 through 

Sept. 2002

1 1

McMinnville, 

Tenn.

405882 35° 41′ N 85° 48′ W 286.5 * Jan. 1, 1927 

through 

Sept. 30, 

2002

Aug. 1872 

through Sept. 

2002

2 2

Lewisburg Experiment Station, 

Tenn.

405187 35° 27′ N 86° 48′ W 239.9 * Jan. 1, 1928 

through 

Sept. 30, 

2002

Apr. 1888 through 

Sept. 2002

3 3

Shelbyville Water 

Dept., Tenn.

408246 35° 30′ N 86° 29′ W 231.6 -- Jan. 1, 1928 

through 

Sept. 30, 

2002

Jan. 1928 through 

Sept. 2002

4 4

Nashville, Tenn. 406402 36° 8′ N 86° 41′ W 182.9 -- -- May 1876 -- 5

Clarksville Sewage Plant, Tenn.401790 36° 33′ N 87° 22′ W 116.4 * -- Jan. 1854 through 

July 1872

-- 6

Rogersville, Tenn. 407884 36° 25′ N 82° 59′ W 413 * Apr. 1, 1893 

through 

Dec. 31, 

1927

-- 5 --

Hydrologic Change in Sinking Pond, 1854-2002 23
basin and the pond. Water movement between these compartments could be conceptualized as unidirectional or bi-directional. Water in one compartment might be routed to another 

compartment or to a sink outside the modeled system. 

After consideration of several alternatives, a model form 

was selected based on two water-storage compartments: 

(1) water stored in the basin vadose zone, and (2) water stored 

in Sinking Pond. In terms of equation 1, each compartment 

receives rainfall, gains or loses stored water, returns water 

to the atmosphere through evapotranspiration, loses water to 

the ground-water system, and generates surface-water runoff. 

Neither compartment receives ground-water inflow. The two 

compartments are linked by the surface runoff outflow from 

the basin, which enters Sinking Pond as surface-water inflow. 

Modifying equation 1 to reflect the particular fluxes and linkage of the two compartments yields water-budget equations for 

the basin vadose zone:

BP = ∆SB BG BR BE + + + , (2)

and for Sinking Pond:

PP BR + = ∆SP PG PR PE + + + , (3)

where BP is precipitation falling on the basin outside the pond, 

SB is the water storage in the basin vadose zone outside the 

pond perimeter, BG is water flux from the basin vadose zone 

to ground water, BR is surface-water runoff from the basin to 

the pond, BE is ET from the basin outside the pond, PP is precipitation falling on the pond, SP is water storage in the pond, 

PG is water flux from the pond to ground water, PR is surface 

runoff from the pond, and PE is ET from the pond. A conceptual diagram of the two-compartment water-balance model for 

Sinking Pond is shown in figure 17.

Definition of Parameters

Equations 2 and 3 provide the basic framework for the 

model. Using that framework to produce a simulated historical 

hydrologic record required defining (1) initial and boundary 

conditions, (2) model parameters, and (3) algorithms to track 

water storage in the two compartments and the fluxes into, out 

of, and between them through time. As with the basic model 

structure, there were many possible approaches to parameterization that might produce acceptable results. Of the several 

alternatives that were evaluated, the approach described below 

was the simplest acceptable version, yielding an adequate 

calibration with the smallest number of terms.

The final model uses the 10 variables from equations 2 

and 3. Two of these, SB and SP are state variables (in bcm) 

that define water storage in the model compartments at any 

given time; the model’s initial conditions are specified by 

the starting values of SB and SP. Basin storage SB represents water stored in the basin vadose zone, which can either 

be saturated (water surplus) or unsaturated (water deficit). 

Conceptually, pond storage, SP, is more complex. Based on 

previous studies (Wolfe, 1996a, b), Sinking Pond is treated 

in the model as a karst window—the surface expression of a 

ground-water reservoir. For SP, water surplus is defined as the 

condition under which surface runoff is generated, and water 

deficit is defined as all other conditions. Deficit states for SP

encompass all pond stages below the spillway, including fully 

drained conditions. 

The remaining eight terms from equations 2 and 3, BP, 

BG, BR, BE, PP, PG, PR and PE, are flux variables (in bcm) 

that represent water movement into, out of, or between the 

compartments for a given daily time step.

The model utilizes two input terms, temp (degrees Celsius [°C]) and precip (in cm), which are climate data, based on 

records of daily mean temperature and precipitation, respectively. The input terms are used to determine two forcing functions, PT (in bcm), representing total precipitation, and PET

(in bcm), representing maximum potential ET. 

In addition to the variables, input terms, and forcing 

functions, the model includes 10 constant terms or parameters 

(table 9). Two of the constants are scaling factors used to 

determine spatial or temporal distribution of input terms or 

forcing functions. The pond-area scaling factor, PA (dimensionless), is set at 0.1, the ratio of the flooded area of Sinking 

Pond at spillway stage (34 ha) to the total area of the Sinking Pond drainage basin (335 ha) rounded to one significant 

figure. The potential ET scaling factor, FE (in bcm per 

degree Celsius squared [bcm/°C2]), is constrained to a value 

of 0.00115 bcm/°C2 by the requirement that cumulative PET

through the 2-year calibration period approximate the published regional average pan evaporation of 1,200 mm per year 

(U.S. Geological Survey, 1970, p. 96). 

The model specifies four threshold parameters—specific 

values of the state variables that represent discontinuities in 

the behavior of the system. The basin overflow threshold, BO

(in bcm), and the pond overflow threshold, PO (in bcm), are 

values of storage above which surface runoff flows from the 

basin and the pond, respectively. Both PO and BO were arbitrarily set at zero (table 9), but they differ in physical significance. The model assumes that the hydrologic response of the 

Sinking Pond drainage basin is dominated by the basin vadose 

zone, and that most of the basin storage is contained in the 

soil-root zone. Thus, the basin overflow threshold, BO, represents a soil-moisture condition, rather than a water-surface 

elevation. In contrast, the pond-overflow threshold, PO, represents spillway stage, with the corresponding volume of surface 

water in the pond equal to 2.39 × 105 m3 or 7.14 bcm, based 

on the stage-volume rating. Positive values of SP indicate that 

pond stage exceeds the spillway elevation, and negative values 

indicate stage is below the spillway, but do not necessarily 

mean that the pond is dry. 

ET is driven by temperature but limited by the availability of water. The model addresses that limit by two threshold 

terms, BAV and PAV, which represent the volume of water, in 

bcm, available for ET in the basin and the pond, respectively. 

Because they are defined as storage values below overflow 

conditions, both water-availability thresholds are represented 

24 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
as negative storage terms or deficits. BAV is assumed to be 

dominated by soil water held in the basin vadose zone. PAV is 

assumed to include soil water, but it is dominated by surface 

water in the pond.

BAV was determined from published available water 

capacities (bcm water per cm soil) of the Guthrie, Purdy, 

Mountview, and Dickson soil series (Conner, 2000; Jenkins, 

2000). Available water capacity is published as a range of 

values for three or four discrete depth increments to 152 cm 

below land surface for the Mountview, Dickson, and Guthrie 

soils (Jenkins, 2000) and to 165 cm below land surface for the 

Purdy soil (Conner, 2000). The maximum value for each depth 

increment was multiplied by that increment’s thickness (in 

cm) to yield a high estimate of how much available water the 

increment could hold. The incremental estimates for each soil 

were then summed to give a high estimate of maximum available water for areas mantled by the different soil series. The 

series estimates ranged from 19.8 bcm for 

the Dickson soil to 30.8 bcm for the Purdy 

soil. Each series estimate was weighted 

by the proportion of the total basin area 

represented by that soil series in the most 

recent available soil map of AEDC (Edwin 

Roworth, Arnold Engineering and Development Center, written commun., 2002). The 

spatially-weighted series estimates were 

then summed to give the overall value of 

23.4 bcm for BAV (table 8).

The pond water-availability threshold, 

PAV was determined by multiplying BAV

(23.4 bcm; table 9) by the pond-area scaling 

factor (0.1; table 9) and adding the total 

surface volume for the pond at spillway 

stage (7.14 bcm from stage-volume rating) 

to yield the value 9.48 bcm (table 9). 

The remaining four constant terms 

determine fluxes from or between the storage compartments. Three of the terms, KBR, 

KPR, and KBG, are rate coefficients, which 

specify the proportionality between storage 

in a given compartment and the flux leaving 

that compartment. The basin runoff (KBR) 

and pond runoff (KPR) coefficients determine 

runoff from the basin and pond as functions of SB and SP, respectively. The basin 

ground-water recharge coefficient, KBG, 

functions in an analogous way to determine 

ground-water losses from the basin vadose 

zone as a function of SB. Ground-water flux 

from the pond was assumed to be constant 

with a daily magnitude equal to the pond 

ground-water recharge constant-flux term, 

PG. Names and units of the constant terms 

used in the model and the magnitude of and 

basis for their assigned values are given in 

table 9.

Model Equations

The model combines 24 terms in 12 equations, of which 

6 have 2 or more conditional versions and 6 are unconditional. 

Using daily precipitation (precip) and temperature (temp) data 

from climate records as input, the model calculates the 12 

equations for each daily time step, i. 

The two forcing functions, daily total precipitation and 

maximum potential ET are calculated by equations 4 and 5, 

respectively. Daily total precipitation is taken directly from the 

precipitation input data set:

PT (4) i precipi =

where PTi is total precipitation for daily time step, and precip

is the corresponding precipitation value from the input data 

BP

BP

BE

BE

BR

BR

SB

SP

SP

SB

BG

BG

PG

PG

PE

PP

PP

PE

PR

PR

EXPLANATION

WATER STORED IN DRAINAGE-BASIN VADOSE ZONE

WATER STORED IN POND

PRECIPITATION ON DRAINAGE BASIN OUTSIDE POND

EVAPOTRANSPIRATION FROM DRAINAGE BASIN OUTSIDE POND

SURFACE-WATER RUNOFF FROM DRAINAGE BASIN TO POND

GROUND-WATER RECHARGE FROM DRAINAGE BASIN

EVAPOTRANSPIRATION FROM POND

PRECIPITATION ON POND

SURFACE-WATER RUNOFF FROM POND

GROUND-WATER RECHARGE FROM POND

Not to scale

Figure 17. Water-balance model of Sinking Pond showing simulated hydrologic 

fluxes and storage compartments.

Hydrologic Change in Sinking Pond, 1854-2002 25
set. Daily temperature determines calculation of maximum 

potential ET (eq. 5): 

For tempi > 0 °C, PETi =temp2

i * FE , (5a)

For tempi ≤ 0 °C, PETi = 0 , (5b)

where for each daily time step i, tempi (°C) is daily mean 

temperature from the input data set, PETi(bcm) is maximum 

potential ET, and FE (bcm/°C2) is an empirically determined 

scaling factor (table 9).

Equations 6 through 10 represent routing of water 

through the Sinking Pond drainage basin. Equation 6 is the 

basin water-balance equation, rearranged from equation 2:

SBi = SBi-1 + BPi – BEi – BRi – BGi, (6)

where SBi and SBi-1 represent the volume of water stored in 

the basin at time steps i and i-1, respectively; BPi is precipitation falling on the basin during time step i; and BEi

, BRi, and 

BGi represent water leaving the basin during time step i as ET, 

surface runoff, and ground-water recharge, respectively.

Precipitation inputs to the Sinking Pond basin are represented by a simple, unconditional scaling equation: 

BPi = PTi * (1 – PA), (7)

where BPi is precipitation falling on the basin exclusive of the 

pond during time step i, PTi is total precipitation for time step 

i (eq. 4), and PA is a dimensionless scaling factor representing the area of Sinking Pond relative to its drainage basin 

(table 9).

Water movement from the basin to the atmosphere (BE) 

is represented by one of three conditional expressions (eqs. 

8a, 8b, and 8c), depending on the magnitude of basin water 

storage (SB) relative to the basin overflow and water-availability thresholds (BO and BAV, respectively; table 9). When 

basin storage equals or exceeds the basin overflow threshold, 

soil saturation is sufficient to permit PET to be fully realized 

throughout the drainage basin: 

For SB > BO, BEi = PETi * (1 –PA), (8a)

where SB (bcm) is basin storage, BO (bcm) is the basin overflow threshold, BEi

 (bcm) is ET loss from the basin for time 

step i, PETi (bcm) is maximum potential ET for time step i, 

and PA is the dimensionless pond-area scaling factor. 

Negative basin storage (SB less than BO) indicates 

unsaturated soil-moisture conditions. The model assumes that 

ET continues for SB below BO in proportion to the difference 

between SB and the basin water-availability threshold BAV as 

long as SB exceeds BAV:

For BAV < SB ≤ BO, BEi = PETi * (1 –PA) * 

[(SBi

 -BAV)/(BO-BAV)]1/2,(8b)

where BAV (bcm) is the basin water-availability threshold and 

the other terms are as defined previously. Once SB reaches or 

Table 9. Names, units, and calibrated values of constant terms in parameterized hydrologic model of Sinking Pond.

[PET, potential evapotranspiration; bcm/°C2, basin centimeters per degree Celsius squared; cm, centimeter; bcm, basin centimeters]

Term Name Units Value Method for determining value

PA Pond-area scaling factor None 0.1 Map analysis and topographic survey

FE PET scaling factor bcm/°C2 0.00115 Set so the sum of daily PET over calibration 

period approximates regional average annual 

pan evaporation rate of 120 cm per year1

BO Basin overflow threshold bcm 0 Arbitrarily set at zero

PO Pond overflow threshold bcm 0 Arbitrarily set at zero

BAV Basin water-availability 

threshold

bcm -23.4 Spatially and depth-weighted mean published 

values of available moisture capacities of 

major soil series in Sinking Pond basin

PAV Pond water-availability 

threshold

bcm -9.48 Spillway volume of pond plus product of BXT 

multiplied by PA

KBR Basin runoff rate 

coefficient

None 0.2 Manual calibration

KPR Pond runoff rate 

coefficient

None 0.5 Manual calibration

KBG Basin ground-water rate 

coefficient

None 0.12 Manual calibration

PG Pond ground-water 

constant flux 

None 0.0762 Manual calibration

1

U.S. Geological Survey, 1970, p. 96.

26 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
falls below the basin water-availability threshold, BAV, ET 

loss from the basin is assumed to cease (eq. 8c, fig. 18).

For SB ≤ BAV, BEi = 0. (8c)

Surface runoff from the Sinking Pond basin (BR) is represented by either of two conditional equations (eqs. 9a and 9b). 

When SB exceeds BO, BR is proportional to the difference: 

For SB > BO, BRi = (SBi – BO) * KBR, (9a)

where KBR is a dimensionless coefficient (table 9). When SB

equals or falls below the basin runoff threshold BO, the model 

assumes that runoff losses from the basin are zero: 

For SB ≤ BO, BRi = 0. (9b)

The equations representing ground-water losses from 

the basin (eqs. 10a and 10b) are identical in form and governing conditions to those representing runoff (eqs. 9a and 9b) 

discussed above:

For SB > BO, BGi = (SBi – BO) * KBG, (10a)

For SB ≤ BO, BGi = 0, (10b)

where SBi (bcm) is basin storage at time step i, BO (bcm) is 

the basin overflow threshold, and KBG is a dimensionless rate 

coefficient. 

The daily water-balance equation for Sinking Pond 

(eq. 11) incorporates BR as a hydrologic input:

SPi = SPi-1 + PPi + BRi – PEi – PRi – PGi , (11)

where SPi is the volume of water stored in Sinking Pond at 

time step i, SPi-1 is pond storage from the previous time step, 

PPi is the daily precipitation input to the pond, BRi is daily 

runoff moving from the basin to the pond, PEi is daily ET 

losses from the pond, PRi is daily runoff losses from the pond, 

and PGi is daily ground-water losses from the pond. All terms 

in equation 11 have units of bcm.

Daily precipitation inputs to Sinking Pond are calculated 

by equation 12:

PPi = PTi * PA, (12)

where PPi (bcm) is precipitation entering the pond during time 

step i, PTi (bcm) is total precipitation for time step i, and PA is 

a dimensionless scaling factor (table 9).

Water movement from Sinking Pond to the atmosphere 

(PE) is represented by one of three conditional expressions 

(eqs. 13a, 13b, and 13c), depending on the magnitude of pond 

storage (SP) relative to the pond overflow and water-availability thresholds (PO and PAV, respectively; table 9). When pond 

storage equals or exceeds the pond overflow threshold, Sinking Pond is full, and maximum potential ET is fully realized 

for the entire pond area:

For SP > PO, PEi = PETi * PA, (13a)

where SP (bcm) is pond storage, PO (bcm) is the pond overflow threshold, PEi

 is the ET loss from the pond for time step 

i, PETi is the maximum potential ET for time step i, and PA is 

a dimensionless scaling factor.

Negative pond storage (SP less than PO) indicates lessthan-full conditions. The model assumes that ET continues 

for SP below PO in proportion to the difference between SP

and the pond water-availability threshold PAV as long as SP

exceeds PAV: 

where PAV (bcm) is the pond water-availability threshold, and 

the other terms are as defined previously. Once SP reaches 

or falls below the PAV, ET loss from the basin is assumed to 

cease:

For SP ≤ PAV, PEi = 0. (13c)

Surface runoff from Sinking Pond (PR) is represented by 

either of two conditional equations (eqs. 14a and 14b). When 

SP exceeds PO, calculated PR is proportional to the difference: 

For SP > PO, PRi = (SPi – PO) * KPR, (14a)

where KPR is a dimensionless coefficient (table 9). When pond 

storage SP equals or falls below the pond overflow threshold 

PO, the model assumes that runoff loss from the pond is zero: 

For SP ≤ PO, PRi = 0. (14b)

In contrast to surface runoff and ET, the model’s treatment of ground-water recharge from Sinking Pond differs 

from that used for the basin. Previous studies documented 

the importance of antecedent ground-water conditions to the 

filling behavior of Sinking Pond (Wolfe, 1996a, b). However, 

details of the interaction between ground-water conditions and 

the hydraulic behavior of Sinking Pond are poorly understood. 

The simplest approach that maintains the interdependence 

between the local ground-water system and the pond is to 

assume that ground-water recharge from the pond is constant: 

PGi = PG, (15)

where PGi is ground-water loss from the pond for time step 

i, and PG is the pond ground-water constant flux in bcm 

(table 9).

As noted above, Sinking Pond storage (SP) was conceptualized as a karst window comprising surface-water and 

ground-water components. The model sets no lower limit on 

For PAV < SP ≤ PO, PEi = PETi * PA *

 [(SPi

 - PAV)/(PO-PAV)]1/2 (13b)

Hydrologic Change in Sinking Pond, 1854-2002 27
EXPLANATION

PT PRECIPITATION

PET POTENTIAL EVAPOTRANSPIRATION

ET EVAPOTRANSPIRATION

BP PRECIPITATION INTO BASIN

 OUTSIDE POND

PP PRECIPITATION INTO POND

SB BASIN STORAGE

BO BASIN OVERFLOW THRESHOLD SET

 EQUAL TO 0

BAV BASIN WATER-AVAILABILITY THRESHOLD 

SP POND STORAGE

PO POND OVERFLOW THRESHOLD SET

 EQUAL TO 0

PAV POND WATER-AVAILABILITY THRESHOLD

BG BASIN GROUND-WATER RECHARGE

BR BASIN RUNOFF

BE BASIN EVAPOTRANSPIRATION

PG POND GROUND-WATER RECHARGE

PR POND RUNOFF

PE POND EVAPOTRANSPIRATION

BG

BE

PE

BR

PR

SB

SP

+

-

 PT

PET

+

-

+

-

BO

BAV

PAV

BP

PP

PO KPR

FORCING FUNCTION

STATE VARIABLE

FLUX VARIABLE

RATE COEFFICIENT—Subscripts

 following K indicate flux variable

THRESHOLD

WATER SURPLUS

WATER DEFICIT

CONSTANT FLUX 

KBG

KBR

KBR

PG

Figure 18. Schematic diagram of a parameterized water-balance model of Sinking Pond.

28 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
the magnitude of SP (fig. 18); deficits (negative values for SP) 

accumulate below the fully drained condition of the surface 

pond until reversed by the overall balance of hydrologic inputs 

and outputs.

Each value for SP has a corresponding value for pond 

volume and stage, which are calculated at every time step. At 

SP = 0 bcm, the modeled stage of Sinking Pond is defined to 

equal the spillway stage of 324.54 m, with a corresponding 

volume of water in the pond of 2.39 × 105 m3 or 7.14 bcm, 

based of the stage volume rating. For each time step i, the 

volume of water in Sinking Pond was calculated as:

Pvi [m3] = (Spi + 7.14) [bcm] × 33,500 [m3/bcm], (16)

where Pvi is the modeled volume of water in Sinking Pond at 

time step i, in cubic meters. The modeled stage for each time 

step was determined from the modeled volume, based on the 

stage-volume rating.

Calibration and Verification 

The calibration data set drew from USGS records of 

Sinking Pond stage and streamflow in the Sinking Pond overflow channel at a culvert about 0.9 km south and downgradient 

of the Sinking Pond spillway (fig. 2; table 10). Wolfe (1996a) 

reported details on construction and instrumentation of these 

stations. The calibration period, February 11, 1993, through 

February 12, 1995, covers the period for which continuous 

data are available for both stations. 

Calibration was based on graphical comparison of 

modeled and observed values for three closely related time 

series: (1) pond volume, (2) pond stage, and (3) pond runoff. 

Comparing the observed daily mean discharge at the Sinking Pond outflow gage with PR over the calibration period 

required converting the daily discharge values from cubic 

meters per second (m3/s) to daily basin centimeters of runoff. 

The gage drainage basin area is 399 ha (Wolfe, 1996a), about 

15 percent larger than the Sinking Pond drainage basin. Each 

basin centimeter at the gage represents 39,900 m3 of water. 

Daily mean discharge in basin centimeters was calculated by 

multiplying the daily mean discharge by 86,400 seconds per 

day to determine cubic meters per day and then dividing by 

39,900 m3/bcm. 

As noted in the previous section and in table 9, values 

for 6 of the model’s 10 constant terms, 2 scaling factors and 

4 threshold parameters, were determined independently or 

assigned arbitrarily, leaving 4 constant terms, KBR, KPR, KBG, 

and PG, available for use in calibrating the model. The pond 

runoff coefficient, KPR, was partly constrained by the requirement that cumulative modeled runoff closely approximate the 

cumulative water yield 51 bcm observed at the Sinking Pond 

outflow gage during the period (Wolfe, 1996a). Within that 

constraint, values for the four rate coefficients were determined through an iterative process in which the computed 

values of daily pond stage, surface-storage volume, and runoff 

were compared to those of the calibration data set using different values of the various coefficients. 

Given the close relation between stage and volume, comparison of the two time series may seem redundant, but the 

different shapes of the two hydrographs (fig. 19) made them 

complementary for calibrating the model. The pond-volume 

hydrograph rises and falls more gradually than the stage 

hydrograph because volume is determined by the balance of 

hydrologic inputs and outputs and thus is insensitive to the 

geometry of the pond. Because the model is based on volumetric water balance, comparison of positive values for observed 

and modeled volume hydrographs was the main tool used to 

adjust the general magnitude of the most sensitive parameters, PG, BR, and BG. Observed volume, based on the stage 

record, has zero as its lower limit, whereas, modeled volume, 

like the state variable SP on which it is based, can accumulate 

deficits lower than the fully drained condition of the surface 

pond (fig. 19). Only positive values for modeled and observed 

volume were used in calibration.

Comparison of the modeled and observed stage and 

outflow hydrographs were used to adjust BR to match storm 

peaks. The model produces reasonably good approximations 

of recessions and wet-season peaks in Sinking Pond for the 

calibration period, but these approximations tend to lag behind 

the observed values during early filling (fig. 19). This lag 

probably reflects the limited information available on groundwater storage directly beneath Sinking Pond and its interaction 

with the regional ground-water flow system. Adding terms or 

Table 10. Gage data for surface-water stations used in development, calibration, and verification of a water-balance model of 

Sinking Pond.

[d-m-s, degrees, minutes, seconds; NGVD 29, National Geodetic Vertical Datum of 1929; km2, square kilometer]

Station name

USGS site 

identification 

number

Latitude and 

longitude 

(d-m-s)

Station type Gage datum 

(NGVD 29)

Drainage 

area (km2) Period of record used

Sinking Pond 03596075 35° 24′ 36′′

86° 04′ 11′′

Continuous 

stage

321.03 3.35 Feb. 11, 1993 – Feb. 12, 1995;

Oct. 1, 1999 – Sept. 30, 2002

Sinking Pond 

outflow

035960755 35° 24′ 00′′

86° 03′ 41′′

Continuous 

streamflow

308.575 3.996 Feb. 11, 1993 – Feb. 12, 1995

Hydrologic Change in Sinking Pond, 1854-2002 29
relaxing some of the model constraints might have reduced or 

eliminated this early-rise lag, but also would have overspecified the model. 

A continuous stage recorder was re-established in Sinking Pond in October 1999 and operated through September 

2002 (table 10), which provided data for three complete filling 

and draining cycles to verify the hydrologic model of Sinking Pond. As noted during calibration, the greatest departures 

of calculated from observed stage values occurred during the 

early filling stages, with modeled stage consistently lagging 

behind the observed rises (fig. 20). For the purpose of assessing broad temporal trends driven by climate, the verification 

indicates that the model produces an adequate simulation of 

the basin and pond’s hydrologic response. 

Simulated Hydrologic Record of Sinking Pond, 

1854-2002

The model equations were applied to daily precipitation 

and temperature data covering the period beginning January 1, 

1854, and ending September 30, 2002. As the initial condition, 

storage in Sinking Pond (SP) and the basin vadose zone (SB) 

were set at their respective overflow thresholds, PO and BO

(table 9). Figure 21 shows the 148-year simulated hydrologic 

record for Sinking Pond. In general, the record shows increasing frequency and duration of ponding, beginning around 

1945, which after 1970 reached levels unprecedented in the 

simulated record.

321

322

323

324

325

S

T

A

GE, IN

METER

S

OBSERVED

SIMULATED

0

0.5

1

1.5

2

2.5

R

U

N

OFF, IN

CEN

TIMETER

S

OBSERVED

SIMULATED

APR JULY OCT JAN APR JULY OCT JAN -4x105

-2x105

0

2x105

4x105

V

OLU

ME, IN

C

UBIC

METER

S

OBSERVED

SIMULATED

A

B

C

1993 1994 1995

Figure 19. (A) Observed and simulated stage, (B) surface-water outflow, and (C) volume of Sinking Pond 

for calibration period February 11, 1993, through February 12, 1995.

30 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
One measure of ponding frequency is how often the 

water-surface elevation exceeds that of the spillway (324.53 m 

above NGVD 29). From 1855 through 1943, the simulated 

record shows flooding above the spillway occurring in 46 of 

89 years, indicating that, on average, the pond had a 52 percent probability of overflowing in any given year. From 

1944 through 2002, that probability increased to 86 percent, 

with flooding above the spillway occurring in 51 of 59 years 

(fig. 22). 

Simulated duration of ponding at or above the spillway 

elevation also increased after 1943. From 1855 through 1943, 

inundation of the spillway equaled or exceeded 50 days per 

water year in 9 years; the longest annual spillway inundation 

during this period was 64 days in 1882 and in 1884 (fig. 22). 

From 1944 through 2002, the spillway was flooded for 50 or 

more days per water year 15 times. Annual inundation of the 

spillway equaled or exceeded 70 days in 7 years, 5 of which 

had durations greater than 80 days (fig. 22). 

The simulated hydrologic record shows increasing ponding duration for all three ponding-depth classes discussed in 

previous sections. Complete inundation of shallow areas (324 

to 324.5 m above NGVD 29) corresponds to stages of 324.5 m 

or higher. Prior to 1944, the centered 5-year average annual 

inundation of shallow areas reached or exceeded 50 days per 

water year during only one 5-year period, 1921-25 (fig. 23). 

After 1944, the 5-year average annual inundation of shallow 

areas reached or exceeded 50 days during three distinct periods: water years 1947 through 1960, 1971 through 1978, and 

1989 through 2000. After 1944, the simulated record shows a 

steady increase in the peak average annual inundation of shallow areas: 72 days per water year for the 5 years centered on 

1950; 84 days for the period centered on 1974, and 95 days for 

the period centered on 1998 (fig. 23). 

The simulation shows an increase in ponding duration of 

intermediate (323.5 to 323.99 m above NGVD 29) and deep 

(below 323.5 m above NGVD 29) areas beginning around 

1970. Before 1970, the simulated hydrologic record shows 

average annual inundation of intermediate areas exceeding 

150 days per water year for only one 5-year period, centered 

on 1918. For 1970 through 2000, the simulated 5-year average annual ponding duration of intermediate areas exceeds 

150 days for 18 of 31 5-year periods (fig. 23). The average 

annual inundation of deep areas approaches 200 days only 

once before 1970, reaching 194 days for the 5-year period 

centered on 1918. During the periods 1971 through 1978 and 

1991 through 2000, the simulated 5-year average annual inundation of deep areas remains above 185 days per water year, 

exceeding 200 days for 11 of 18 years (fig. 23). 

Deep areas (below 323.5 m above NGVD 29) compose 

only about 20 percent of Sinking Pond’s maximum flooded 

area. However, these areas encompass roughly two thirds of 

the pond’s overall relief and contain the core of the overcup 

oak habitat (Wolfe, 1996a, b). More than half of the overcup 

oaks surveyed previously by Wolfe (1996a, b) and more than 

60 percent of those surveyed in this study were found below 

323.5 m above NGVD 29 (fig. 15). In both studies, more than 

85 percent of surveyed overcup oaks were found at intermediate or deep ponding depths. The results of hydrologic 

modeling indicate that ponding durations in these areas have 

increased substantially since 1970, with important implications for the ecology of Sinking Pond. 

OCT JAN APR JULY OCT JAN APR JULY OCT JAN APR JULY

321

322

323

324

325

S

T

A

GE, IN

METER

S

OBSERVED

SIMULATED

1999 2000 2001 2002

Figure 20. Observed and simulated stage of Sinking Pond for verification period October 1, 1999, through 

September 30, 2002.

Hydrologic Change in Sinking Pond, 1854-2002 31
1855 1860 1865 1870 1875 1880 1885 1890 1895 1900 321

322

323

324

325

1905 1910 1915 1920 1925 1930 1935 1940 1945 1950 321

322

323

324

325

STAGE, IN METERS

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 321

322

323

324

325

YEAR

Figure 21. Simulated stage for Sinking Pond, January 1854 through September 2002.

D

AY

S PER

WATER

YEA

R

150

100

50

0

YEAR

1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Figure 22. Days per water year from 1855-2002, in which simulated stage at Sinking Pond exceeded the spillway 

elevation of 324.53 meters above the National Geodetic Vertical Datum of 1929.

32 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
Ecological Effects of Hydrologic 

Change in Sinking Pond 

Comparison of the density of overcup oak seedlings with 

that of surviving adults indicates that under pre-1970 hydrologic conditions, seedlings in Sinking Pond were subject to 

rigorous attrition. In 2001, overcup oak seedlings were present 

in 70 percent of the 4-m2 subplots in the surveyed area (fig. 7), 

with an average density of 10 seedlings per subplot. If, on 

average, 5 seedlings germinate on each subplot during each of 

the past 148 years, the period of hydrologic simulation would 

have seen more than 4 million seedlings germinated in the 

surveyed area, of which only about 400 seedlings survived 

through 2001, giving an attrition rate of 99.99 percent. Competition for light, herbivores, pathogens, fire, wind, lightning, 

ice, and all the other mortality factors operating in an oak 

forest contributed to this attrition, but the harsh hydrologic 

environment (Solomon, 1990) also constrained recruitment 

and survival. Despite such constraints, recuitment and survival 

were sufficient to maintain an apparently stable, closedcanopy stand of overcup oak in Sinking Pond for at least 150 

years.

Hydrologic change in Sinking Pond since 1970 appears 

to have pushed the overcup oaks in deep and intermediate 

areas across a critical ecological threshold. The most striking changes in Sinking Pond’s hydrologic regime is the 

pronounced increase in ponding duration, particularly the 

increased frequency of annual inundation exceeding 200 days 

per year in deep and even intermediate areas (fig. 23). The 

ecological threshold that has been crossed is the change from 

nearly total to total attrition of seedlings 

before they can recruit into the sapling 

stage.

Hydrologic Constraints on 

Overcup Oak Recruitment and 

Survival

Comparing the ring-count dates and 

elevations of sampled overcup oaks with 

the historical changes in the elevations 

subjected to prolonged ponding shows 

a strong relation between the overcup 

oak recruitment and ponding duration 

(fig. 24). Before 1970, model results 

indicate that inundation of any part of 

the pond for 200 days or more was relatively uncommon. Eighty-five percent of 

the sampled overcup oaks and 100 percent of the sampled trees in deep and 

intermediate areas (below 324 m) germinated during this period. After 1970, 

the frequency of simulated 200-day 

filling events increases sharply, with 

corresponding decreases in the temporal 

density and range of elevation of sampled trees. Hydrologic 

simulation indicates inundation of 200 days or more in 18 of 

33 years from 1970 through 2002, compared with only 20 out 

of the previous 115 years (fig. 24). 

Before and after 1970, tree regeneration and survival 

appear to be most successful at times and elevations free from 

recurrent, prolonged ponding over several successive years. 

Survival of trees dated to the dry period from 1885 through 

1900 (six trees from 15 years or 0.28 tree per year) is about 

twice that from a more recent wet period from 1910 through 

1933 (three trees from 24 years or 0.125 tree per year). The 

densest concentration of sampled trees, including those at the 

lowest and third-lowest elevations in the sample, was dated 

to the dry period 1934-45 (fig. 24). The seven post-1970 trees 

in the sample have elevations tightly clustered near 324 m. 

Five of these seven trees have ring-count dates occurring in 

the drought years 1985-88, in which the simulated hydrologic record shows no 200-day ponding durations in the pond 

(fig. 24).

The strong relation between the 200-day ponding elevation and the dates and elevations of sampled trees supports the 

previous conclusion that prolonged inundation during the first 

few years after germination is a critical constraint on seedling 

survival and recruitment. The inset in figure 24 shows the 

distribution of 200-day inundation exceeding the elevations 

of sampled overcup oak during the first 5 years after their 

ring-count dates. Forty of 45 trees (89 percent) have ringcount dates and elevations that show 200 days of inundation 

in one or none of their first 5 years. Just one tree (2 percent) 

has an elevation and ring-count date that indicates 3 of the first 

1850 1900 1950 2000

0

50

100

150

200

250

YEAR

A

VER

A

GE PO

N

DIN

G

D

U

R

A

TIO

N, IN

D

A

Y

S PER

W

A

TER

YEA

R

STAGE, IN METERS

324.5—All sites below spillway inundated

324—Deep and intermediate sites inundated

323.5—Only deep sites inundated

Figure 23. Temporal patterns in Sinking Pond hydroperiod, based on centered 5-year 

average annual ponding duration above selected stages, 1857-2000.

Ecological Effects of Hydrologic Change in Sinking Pond 33
5 years with 200 days or longer of inundation, and no trees 

had more than 3 such years (fig. 24). 

The simulated inundation histories of the sampled trees 

provide a more detailed picture of the relation between postgermination ponding and the recruitment and survival of overcup oaks. One measure of a tree’s post-germination inundation 

history is the recruitment inundation, defined for this report 

as the median annual inundation, in days inundated per water 

year, for the 5-year period following germination. Recruitment inundation can be calculated for any tree for which 

elevation and germination date are known or estimated and 

for any site or group of sites in any set of 5-year recruitment 

periods. Recruitment inundations were determined for the 

sampled trees, based on each tree’s elevation and ring-count 

date, ranked in ascending order, and plotted as a cumulative 

frequency distribution (fig. 25). A smooth curve through the 

points on the cumulative frequency plot (fig. 25A) provides 

an estimate of the cumulative distribution function (cdf) of 

recruitment inundation for the population from which the 

sample was drawn (Iman and Conover, 1983), in this case the 

surviving adult overcup oaks in the 2.3-ha sampled area.

Similarly, recruitment inundation was calculated for 

the potential germination sites in the sampled area across 

the 148-year period of hydrologic simulation. The simulated 

hydrologic record contains 144 periods of 5 consecutive years, 

each of which represents a possible recruitment window, and 

each 4-m2 subplot in the sampled area is a potential germination site with known elevation. Multiplying 5,700 germination 

sites by all 5-year periods yields 820,800 possible recruitment 

opportunities in the sampled area during the period of hydrologic simulation. 

The 820,800 calculated values of recruitment inundation 

were ranked in ascending order and plotted as a frequency 

distribution, which was then compared to the estimated cdf 

for surviving overcup oaks in the sampled area (fig. 25A). For 

the first 78 percent of the two distributions, the estimated cdf 

for surviving overcup oaks lies to the right (wetter) side of 

the distribution of all possible recruitment opportunities. For 

example, the 50th percentile for all recruitment opportunities 

is 130 days inundated per year, compared with 148 days for 

surviving overcup oaks (fig. 25A). The two distributions cross 

at the 78th percentile (about 170 days), and the upper limit for 

all recruitment opportunities exceeds that of the estimated cdf 

for overcup oaks by 55 days. Seven percent of all recruitment 

opportunities had recruitment inundations exceeding 200 days, 

approaching the upper limit (100th percentile) of the estimated 

overcup oak cdf (fig. 25A).

The effect of hydrologic change after 1970 on the frequency distribution of recruitment inundation in the 2.3-ha 

sampled area is shown in figure 25B. The distribution of 

recruitment inundation for pre-1970 recruitment opportunities 

lies to the left (drier) side of the estimated cdf for surviving 

overcup oaks, suggesting that in the past, prolonged inundation in the first few years was not a critical constraint on seedling survival and recruitment. The rightward (wetter) shift of 

the distribution of recruitment inundation after 1970 represents 

the increase in the ponding duration experienced by seedlings 

in their first few years. The median recruitment inundation for 

1860 1870 1880 1890 1900 1910

YEAR

1920 1930 1940 1950 1960 1970 1980 1990 2000

322

323

324

325

326

ELEVATION, IN METERS ABOVE NATIONAL GEODETIC

VERTICAL DATUM OF 1929

0 1 2 3 4 5 0

5

10

15

20

NUMBER OF TREES

SAMPLED OVERCUP

 OAKS, n = 45

NUMBER OF YEARS INUNDATED

200 DAYS OR LONGER DURING FIRST

5 YEARS AFTER RING-COUNT DATE

ELEVATION INUNDATED 200 DAYS

SAMPLED OVERCUP OAK RING-COUNT

 DATE AND ELEVATION WITH AGE-

 ESTIMATION ERROR BARS

Figure 24. Timing and elevation of simulated annual 200-day inundation compared to the ages and elevations of sampled overcup oaks 

in Sinking Pond, 1856-2002.

34 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
10

20

30

40

50

60

70

80

90

100

CUMULATIVE FREQUENC

Y, IN PER

CENT

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300

10

20

30

40

50

60

70

80

90

100

5-YEAR MEDIAN ANNUAL INUNDATION, IN DAYS INUNDATED PER WATER YEAR

SAMPLED OVERCUP OAK

ESTIMATED CUMULATIVE DISTRIBUTION

FUNCTION FOR SURVIVING OVERCUP OAKS

ALL AREAS, 1855-2002

SURVIVING OVERCUP OAKS

ALL AREAS, 1855-1969

ALL AREAS, 1970-2002

SURVIVING OVERCUP OAKS

SHALLOW AREAS, 1970-2002

DEEP AND INTERMEDIATE AREAS, 1970-2002

C

B

A

Figure 25. Frequency distributions for recruitment inundation (5-year median annual inundation) for (A) surviving overcup oaks and all recruitment opportunities in a 2.3-hectare sampled area of Sinking Pond during 

148 years of simulated hydrologic record, (B) pre- and post-1970 recruitment opportunities in the sampled 

area, and (C) shallow and deep/intermediate recruitment opportunities in the sampled area after 1970.

Ecological Effects of Hydrologic Change in Sinking Pond 35
pre-1970 recruitment opportunities is 120 days, compared with 

181 days for post-1970 recruitment opportunities and 148 days 

for the estimated cdf for surviving overcup oaks (fig. 25B). 

Nearly 32 percent of post-1970 recruitment opportunities have 

recruitment inundation exceeding 200 days (fig. 25B).

Under current (post-1970) hydrologic conditions, the concentration of successful overcup oak recruitment has shifted 

from deep and intermediate to shallow sites in Sinking Pond. 

The relation of recruitment inundation to this spatial shift is 

shown in figure 25C. In general, the recruitment-inundation 

distribution for shallow post-1970 recruitment opportunities 

lies close to the estimated cdf for surviving overcup oaks, indicating that post-1970 recruitment inundation of shallow sites 

approximates the recruitment-inundation regime that in the 

past permitted overcup oaks to recruit and survive. In contrast, 

the post-1970 recruitment inundation for deep and intermediate sites is much farther to the right (wetter) side of the cdf for 

surviving overcup oaks. The median post-1970 recruitment 

inundation for deep and intermediate sites is 192 days, which 

equals the 98th percentile of the estimated cdf for surviving 

overcup oaks; 42 percent of post-1970 deep and intermediate 

recruitment opportunities had recruitment inundation exceeding 200 days (fig. 25C).

As noted in this report and by McCarthy and Evans 

(2000), successful recruitment of overcup oak saplings and 

small adults currently is confined to areas with ponding depths 

less than 0.5 m, corresponding to elevations between 324 m 

and 324.5 m. The analysis of recruitment inundation confirms 

that current ponding regimes in these shallow areas are consistent with those that formerly permitted overcup oak recruitment in deeper parts of Sinking Pond—typically corresponding to recruitment inundation of 100 to 180 days (fig. 25). 

The apparent preference of surviving overcup oaks for this 

range of recruitment inundation does not reflect a physiological requirement for poorly drained soil or periodic inundation 

(Solomon, 1990). Rather, the overcup oaks in Sinking Pond 

are exploiting sites that may be barely tolerable for them but 

are even less tolerable for their competitors. 

The spatial shift of suitable recruitment inundation from 

deep and intermediate to shallow sites represents a fundamental change in the ecological structure of Sinking Pond. In 

the past, overcup oak and associated tree species maintained 

a closed-canopy forest across most of the inundated area of 

Sinking Pond. Current patterns of recruitment and mortality indicate a transition towards a progressively more open 

canopy. Tree-ring analysis suggests that the former structure, 

a closed-canopy forest with the deepest areas dominated by 

overcup oak, has been relatively stable for at least 150 years. 

Neither the structural characteristics of the emerging ecosystem nor the climatic conditions under which it is developing 

seem likely to produce similar stability in the future.

Future Ecological Conditions in Sinking Pond

Increased precipitation and streamflow in the eastern 

United States since 1970 is consistent with the results of 

global circulation models (GCMs) that simulate global climate response to greenhouse gas emissions (Karl and others, 

1996; Burkett and others, 2001; McCabe and Wolock, 2002). 

Applying the GCM developed by the Hadley Climate Centre 

(United Kingdom) to the southeastern United States, Burkett 

and others (2001) projected regional mean annual temperature 

increasing by about 2.3 °C and precipitation increasing by 

about 20 percent between 2000 and 2100. The implications of 

this projection for Sinking Pond would be persistent prolonged 

(greater than 200 days per year) annual ponding into the foreseeable future. 

 Persistence of the wetting trend of the past several 

decades would sustain the spatial patterns of tree mortality 

and survival documented in this report. The former core of the 

overcup oak stand in Sinking Pond—deep and intermediate 

areas below 324 m above NGVD 29—would become progressively more open as adult trees die without being replaced. 

Shallow areas below the spillway elevation might sustain 

overcup oak as recruitment of competing species suffered 

under the new ponding regime. However, the resulting stand 

would have spatial and ecological characteristics markedly 

different from those of the historic stand. 

The historic overcup oak stand, centered in the deep areas 

of Sinking Pond (about 20 percent of the maximum ponded 

area), was spatially compact and surrounded by buffer zones 

of intermediate to shallow ponding depth. When a canopy gap 

developed in the core area, overcup oak seedlings, saplings, or 

small adults were available to fill the gap. Gaps in the intermediate and shallow buffer zones provided additional opportunities for overcup oak release, especially during relatively wet 

years, when recruitment by competing species was suppressed. 

The spatial arrangement of a compact, monospecific core, surrounded by ecological buffer zones, favored structural stability and longevity for the overcup oak stand in Sinking Pond 

across the range of climatic fluctuations and other environmental stresses that occurred before 1970.

In contrast, the spatial characteristics of the emerging 

overcup oak stand are likely to exacerbate the stand’s vulnerability to competition and environmental stress. Under current 

conditions, overcup oak regeneration in Sinking Pond is confined to areas with elevations between 324 and 324.5 m above 

NGVD 29, which compose about 30 percent of the maximum 

ponded area of Sinking Pond. These shallow areas currently 

have ponding regimes similar to those that characterized the 

deep core of the overcup oak stand before 1970. However, 

the shallow areas below the spillway elevation do not form 

a single compact core; rather these areas extend in a more 

or less narrow band around the pond’s perimeter. Continued 

shifting of the overcup oak stand from the pond’s interior to its 

perimeter would result in a progressively more dispersed, less 

concentrated seed source. 

36 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
The emerging overcup oak stand would likely form a discontinuous linear swath between areas too wet for overcup oak 

recruitment and drier sites where overcup oak is at a competitive disadvantage. Even if shallow ponded areas were too wet 

for establishment of sweetgum and other competing species, 

upland trees near the pond margin could still compete with 

nearby overcup oaks for light. Because the spillway limits the 

elevation of prolonged inundation, further increases in ponding duration could make even shallow ponded areas untenable 

for overcup oak recruitment without creating new opportunities at higher elevations. 

The broad structural change in the Sinking Pond ecosystem discussed above would likely be accompanied by many 

changes in ecological function, including changes in the interactions of fauna with the pond. Several hundred pairs of great 

blue herons currently nest in the deep and intermediate areas 

of Sinking Pond. Continued attrition without replacement of 

adult trees in these areas would steadily reduce the number 

of available nesting sites. Concurrently, loss of acorn production would reduce the value of Sinking Pond as a foraging site 

for deer, wood ducks, and other wildlife. On the other hand, 

upstream movement of fish into Sinking Pond from Crumpton Creek would likely increase with increased duration and 

magnitude of surface outflow. In response to increased fish 

numbers and residence time, snapping turtles and other aquatic 

predators might visit Sinking Pond in greater numbers and 

spend more time there.

Changes in hydrology and biota, such as those described 

above, would affect carbon and nutrient cycles in Sinking 

Pond. Breakdown of organic matter might slow down or 

proceed through different chemical pathways in response to 

longer inundation. 

Conclusion: Management and 

Scientific Implications of Hydrologic 

and Ecological Change in Sinking Pond 

This report presents several lines of mutually supporting 

botanical and hydrologic evidence leading to the following 

conclusions:

(1) Areas of Sinking Pond with normal high-water ponding 

depths greater than 0.5 m have become untenable for 

overcup oak regeneration, even though these areas are 

dominated by adult overcup oaks;

(2) Failure of tree regeneration in Sinking Pond occurs at the 

critical stage of recruitment from seedling to sapling size 

class;

(3) Failure of tree regeneration in Sinking Pond began 

abruptly around 1970 and has persisted since then;

(4) Prolonged (greater than 200 days per year) inundation 

during the first 5 years after germination is the proximate 

cause of tree-regeneration failure in Sinking Pond; 

(5) Increased ponding duration within Sinking Pond since 

1970 has been driven by a step increase in precipitation, 

which occurred around that time; and

(6) Increased rainfall since 1970 is not a local phenomenon 

but has been documented across much of the eastern 

United States.

These conclusions have several implications for wetland 

management in and around the study area. One implication 

is that local management options for enhancing regeneration 

and survival of overcup oak in Sinking Pond are limited. Little 

or nothing can be done to reduce ponding duration in deep 

or intermediate areas. Even massive excavation of the spillway and outflow channel would leave deep areas of the pond 

unaffected (Appendix B). Selective thinning of red maple, 

sweetgum, and other trees may enhance light availability for 

overcup oak saplings and small adults in shallow areas of the 

pond. Such thinning would help maintain a remnant population of overcup oak around the pond margins but would not 

reverse broad changes in ecological structure.

A second implication of climate driven hydrologic 

change in Sinking Pond is that nearby wetlands have experienced similar increases in precipitation. The likely ecological 

response of nearby wetlands to increased precipitation would 

depend on both hydrologic and ecological factors. At present, 

little is known about ecological change in nearby wetlands, but 

previous studies (Wolfe, 1996a, b) and the results of this study 

suggest some hypotheses. 

Wetlands whose hydrologic regimes are most responsive to seasonal ground-water conditions and whose biota 

are most sensitive to increased ponding duration would most 

likely experience the most pronounced ecological response to 

increased precipitation. For example, Willow Oak Swamp and 

Westall Swamp (fig. 2) are geomorphically and hydrologically 

similar to Sinking Pond in having maximum depths greater 

than 2 m, relatively efficient internal drains, and bottom elevation well within the normal range of seasonal water-table fluctuation (Wolfe, 1996a, b). Both of these depressions support 

forests dominated by species, such as willow oak and sweetgum, which are less tolerant of inundation than the overcup 

oaks of Sinking Pond. The combination of hydrologic similarity to Sinking Pond and biota with comparatively low tolerance for increased ponding duration would tend to make these 

wetland ecosystems vulnerable to the kind of climate-driven 

ecological change that has been documented in Sinking Pond.

In contrast, Tupelo Swamp is dominated by water tupelo, 

a species more tolerant of inundation than overcup oak, and 

has geomorphic and hydrologic characteristics that differ from 

those of Sinking Pond. Shallow (less than 1.5 m) maximum 

depth, lack of an efficient internal drain, and a bottom elevation higher than all but the highest recorded local water-table 

elevations make the hydrologic regime of Tupelo Swamp less 

dependent on ground-water conditions than that of Sinking Pond (Wolfe, 1996a). This hydrologic difference and the 

relatively high tolerance of its biota to inundation may tend to 

limit the ecological effect of increased precipitation in Tupelo 

Swamp and similar depression wetlands.

Conclusion: Management and Scientific Implications of Hydrologic and Ecological Change in Sinking Pond 37
Hydrologic monitoring of Sinking Pond and other depression wetlands would be useful in testing the hypotheses summarized above. Such monitoring also could support refining 

the water-budget model described in this report and developing similar models for other depression wetlands, which 

could be used to project hydrologic conditions under different 

climatic scenarios. Ecological monitoring of Sinking Pond, for 

example permanently marking the 2.3-ha area used for spatial 

analysis and resurveying its vegetation at regular intervals, 

would improve understanding of the mechanics and trajectory 

of ecological change and may detect management opportunities that might otherwise be missed. Similarly, ecological 

monitoring of nearby depression wetlands would improve 

understanding of the extent, rates, and mechanism of wetland 

ecological response to changing climate at AAFB.

Beyond local management concerns, this study shows 

that current hydrologic conditions cannot be assumed to represent the ecological or historical norm without risk of substantial error. A 30-year hydrologic record is rare for most wetland 

settings, but even such a relatively long record would give a 

distorted view of the hydrologic conditions under which the 

overcup oak stand in Sinking Pond developed. 

Carter and others (1979) listed five urgent research needs 

in wetland hydrology: (1) better hydrologic measurement 

techniques; (2) measurement of hydrologic inputs and outputs 

of representative wetlands with appropriate error analysis; 

(3) improved understanding of wetland soil-water-vegetation relations; (4) detailed long-term studies of representative 

wetlands in different environmental settings; and (5) predictive 

and analytical models of wetlands based on hydrologic data. 

Arguing that these research needs have been fully addressed 

during the 25 years since Carter and others pointed them out 

would be difficult. Nevertheless, the results of this study indicate the need for a sixth item on the list: comparison of current 

to past wetland hydrologic regimes and analysis of wetland 

response to climate change.

Acknowledgments

Many individuals representing numerous public agencies and private organizations contributed to this investigation 

and report. We owe particular thanks to Geoffrey Call, Mark 

Moran, Kevin Fitch, and Edward Roworth of Arnold Engineering Development Center (AEDC) for logistical support, background data, and intellectual exchange through the course of 

the investigation. Members of the AEDC Integrated Planning 

Core Team assisted with numerous questions, discussions, and 

criticisms at different stages of the study. We also thank Mary 

Priestley and Yolande Gottfried of the Sewanee Herbarium 

for compiling the plant species list for Sinking Pond (Appendix A) and for their assistance with data collection in the field. 

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42 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
Appendix A. Flora of Sinking Pond.

The following table lists plants observed within the seasonally inundated perimeter of Sinking Pond. Scientific and common names are from Kartesz and 

Meacham (1999). 

Family Genus Species Common Name

Ferns

Aspleniaceae Asplenium Platyneuron (L.) B.S.P. Ebony spleenwort

Dryopteridaceae Athyrium filix-femina (L.) Roth Subarctic lady fern

Dryopteridaceae Polystichum acrostichoides (Michx.) Schott Christmas fern

Ophioglossaceae Botrychium sp. Sw. Grape fern, Moonwort

Osmundaceae Osmunda regalis var. spectabilis (Willd.) Gray Royal fern

Polypodiaceae Pleopeltis polypodioides ssp. polypodioides (L.) Andrews & Windham Resurrection fern

Thelypteridaceae Phegopteris hexagonoptera (Michx.) Fée Broad beech fern

Thelypteridaceae Thelypteris noveboracensis (L.) Nieuwl. New York fern

Monocots

Cyperaceae Carex debilis var. debilis Michx. White-edge sedge

Cyperaceae Carex folliculata L. Northern-long sedge

Cyperaceae Carex gracilescens Steud. Slender loose-flower sedge

Cyperaceae Carex intumescens Rudge Greater bladder sedge

Cyperaceae Carex joorii Bailey Cypress-swamp sedge

Cyperaceae Carex lupulina Muhl. ex Willd. Hop sedge

Cyperaceae Carex tribuloides Wahlenb. Blunt broom sedge

Cyperaceae Carex typhina Michx. Cat-tail sedge

Cyperaceae Cyperus pseudovegetus Steud. Marsh flat sedge

Cyperaceae Rhynchospora corniculata (Lam.) Gray

Short-bristle horned beak 

sedge

Cyperaceae Scirpus cyperinus (L.) Kunth Cottongrass bulrush

Juncaceae Juncus brachyphyllus Wieg. Short-leaf rush

Juncaceae Juncus bufonius L. Toad rush

Juncaceae Juncus coriaceus Mackenzie Leathery rush

Juncaceae Juncus effusus L. Lamp rush

Liliaceae Maianthemum racemosum (L.) Link

Feathery false Solomon’sseal

Poaceae Andropogon virginicus L. Broom-sedge

Poaceae Brachyelytrum erectum (Schreb. ex Spreng.) Beauv. Bearded shorthusk

Poaceae Chasmanthium latifolium (Michx.) Yates Indian wood-oats

Poaceae Chasmanthium laxum (L.) Yates Slender wood-oats

Poaceae Danthonia spicata

(L.) Beauv. ex Roemer & J. A. 

Schultes Poverty wild oat grass

Poaceae Echinochloa crus-galli (L.) Beauv. Large barnyard grass

Poaceae Microstegium vimineum (Trin.) A. Camus Nepalese browntop

Poaceae Panicum rigidulum Bosc ex Nees Red-top panic grass

Poaceae Panicum verrucosum Muhl. Warty panic grass

Poaceae Saccharum baldwinii Spreng. Narrow plume grass

Poaceae Sphenopholis obtusata (Michx.) Scribn. Prairie wedgescale

Smilacaceae Smilax ecirrata (Engelm. ex Kunth) S. Wats Upright carrion flower

Smilacaceae Smilax rotundifolia L. Horsebrier

Dicots

Aceraceae Acer rubrum L. Red Maple

Anacardiaceae Rhus copallinum L. Winged sumac

Anacardiaceae Rhus glabra L. Smooth sumac

Anacardiaceae Toxicodendron radicans (L.) Kuntze Eastern poison-ivy

Appendix A 43
Family Genus Species Common Name

Dicots—Continued

Annonaceae Asimina triloba (L.) Dunal Common Pawpaw

Apiaceae Cryptotaenia canadensis (L.) DC. Canadian honeworth

Apiaceae Ptilimnium nuttallii (DC.) Britt Laceflower

Apocynaceae Trachelospermum difforme (Walt.) Gray Climbing-dogbane

Aquifoliaceae Ilex opaca Ait. American holly

Asteraceae Bidens frondosa L. Devil’s-pitchfork

Asteraceae Erechtites hieraciifolia (L.) Raf. ex DC. American burnweed

Asteraceae Erigeron strigosus Muhl. ex Willd. Prairie fleabane

Asteraceae Eupatorium capillifolium (Lam.) Small Dog-fennel

Asteraceae Eupatorium serotinum Michx.

Late-flowering thoroughwort

Asteraceae Helenium autumnale L. Fall sneezeweed

Asteraceae Pluchea camphorata (L.) DC. Plowman’s-wort

Asteraceae Solidago ulmifolia Muhl. ex Willd. Elm-leaf goldenrod

Asteraceae Symphyotrichum lateriflorum var. lateriflorum (L.)A.&D.Love Farewell-summer

Asteraceae Symphyotrichum pilosum var. pilosum (Willd.) Nesom

White oldfield Americanaster

Asteraceae Xanthium strumarium L. Rough cockleburr

Berberidaceae Podophyllum peltatum L. May-apple

Betulaceae Betula nigra L. River birch

Betulaceae Carpinus caroliniana Walt. American hornbeam

Betulaceae Corylus cornuta Marsh. Beaked hazelnut

Bignoniaceae Campsis radicans (L.) Seem. ex Bureau Trumpet-creeper

Calycanthaceae Calycanthus floridus L. Eastern sweetshrub

Campanulaceae Lobelia cardinalis L. Cardinal-flower

Caprifoliaceae Lonicera japonica Thunb. Japanese honeysuckle

Caprifoliaceae Symphoricarpos orbiculatus Moench Coral-berry

Celastraceae Euonymus americana L. American strawberry-bush

Clusiaceae Hypericum crux-andreae (L.) Crantz St. Peter’s-wort

Clusiaceae Hypericum hypericoides (L.) Crantz St. Andrew’s-cross

Clusiaceae Triadenum walteri (J. G. Gmel.) Gleason

Greater marsh-St. John’swort

Cornaceae Cornus florida L. Flowering dogwood

Cornaceae Cornus foemina P. Mill. Stiff dogwood

Cornaceae Nyssa aquatica L. Water tupelo

Cornaceae Nyssa sylvatica Marsh. Black tupelo

Ericaceae Rhododendron canescens (Michx.) Sweet Mountain azalea

Ericaceae Rhododendron viscosum (L.) Torr. Clammy azalea

Ericaceae Vaccinium corymbosum L. Highbush blueberry

Ericaceae Vaccinium fuscatum Ait. Black blueberry

Euphorbiaceae Chamaesyce maculata (L.) Small Spotted sandmat

Fabaceae Desmodium nudiflorum (L.) DC. Naked-flower tick-trefoil

Fabaceae Lespedeza frutescens (L.) Hornem. Shrubbery bush-clover

Fagaceae Quercus lyrata Walt. Overcup oak

Fagaceae Quercus nigra L. Water oak

Fagaceae Quercus palustris Muenchh. Pin oak

Fagaceae Quercus phellos L. Willow oak

Appendix A. Flora of Sinking Pond.—Continued

The following table lists plants observed within the seasonally inundated perimeter of Sinking Pond. Scientific and common names are from Kartesz and 

Meacham (1999). 

44 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
Family Genus Species Common Name

Dicots—Continued

Fagaceae Quercus rubra L. Northern red oak

Fagaceae Quercus velutina Lam. Black oak

Gentianaceae Gentiana sp. L. Gentian

Grossulariaceae Itea virginica L. Virginia sweetspire

Haloragaceae Proserpinaca palustris L. Marsh mermaidweed

Hamamelidaceae Liquidambar styraciflua L. Sweet-gum

Juglandaceae Carya alba (L.) Nutt. ex Ell. Mockernut hickory

Lamiaceae Lycopus rubellus Moench Taper-leaf water horehound

Lamiaceae Pycnanthemum tenuifolium Schrad. Narrow-leaf mountain-mint

Lamiaceae Scutellaria lateriflora L. Mad dog skullcap

Lauraceae Sassafras albidum (Nutt.) Nees Sassafras

Lemnaceae Lemna valdiviana Phil. Pale duckweed

Magnoliaceae Liriodendron tulipifera L. Tuliptree

Menispermaceae Cocculus carolinus (L.) DC. Carolina coralbead

Moraceae Morus rubra L. Red mulberry

Oleaceae Fraxinus americana L. White ash

Onagraceae Ludwigia alternifolia L. Seedbox

Onagraceae Ludwigia palustris (L.) Ell. Marsh primrose-willow

Phytolaccaceae Phytolacca americana L. American pokeweed

Platanaceae Platanus occidentalis L. American sycamore

Polygonaceae Polygonum virginianum L. Jumpseed

Pyrolaceae Chimaphila maculata (L.) Pursh Striped Prince’s-pine

Rhamnaceae Frangula caroliniana (Walt.) Gray Carolina false buckthorn

Rosaceae Amelanchier arborea (Michx. f.) Fern. Downy service-berry

Rosaceae Photinia pyrifolia (Lam.) Robertson & Phipps Red chokeberry

Rosaceae Prunus serotina Ehrh. Black cherry

Rosaceae Rubus argutus Link Saw-tooth blackberry

Rubiaceae Cephalanthus occidentalis L. Common buttonbush

Rubiaceae Diodia virginica L. Virginia buttonweed

Rubiaceae Galium circaezans Michx. Licorice bedstraw

Rubiaceae Galium obtusum Bigelow Blunt-leaf bedstraw

Rubiaceae Mitchella repens L. Partridge-berry

Scrophulariaceae Gratiola neglecta Torr. Clammy hedge-hyssop

Scrophulariaceae Gratiola virginiana L. Round-fruit hedge-hyssop

Solanaceae Solanum americanum P. Mill. American black nightshade

Styracaceae Styrax americanus Lam. American snowbell

Ulmaceae Ulmus rubra Muhl. Slippery elm

Urticaceae Boehmeria cylindrica (L.) Sw. Small-spike false nettle

Vitaceae Parthenocissus quinquefolia (L.) Planch. Virginia-creeper

Vitaceae Vitis sp. L. Grape

Appendix A. Flora of Sinking Pond.—Continued

The following table lists plants observed within the seasonally inundated perimeter of Sinking Pond. Scientific and common names are from Kartesz and 

Meacham (1999). 

Appendix A 45

Appendix B. Evaluating Effects 

of Local Human Activity on the 

Hydrology of Sinking Pond 

Three hypothetical anthropogenic alterations 

of hydrologic conditions in Sinking Pond were 

considered: (1) direct alteration of the spillway 

or main internal drain, (2) land-use change in the 

Sinking Pond drainage basin, and (3) waterfowl 

impoundments in three adjacent small watersheds 

north, west, and east of the Sinking Pond drainage 

basin. 

Condition of the Sinking Pond Spillway 

and Main Internal Drain

Previous geomorphic and hydrologic investigations of the Sinking Pond area found no 

evidence for human modification of the Sinking 

Pond spillway or internal drain (Wolfe, 1996a, b). 

The spillway was traversed several times without 

noting evidence for disturbance. The broad, flat 

topography and mature hardwood vegetation of 

the Sinking Pond spillway contrast sharply with 

constructed berms in nearby basins (fig. B1). 

Level notes and discharge measurements made 

between 1992 and 1995 (Wolfe, 1996a) indicate 

a channel elevation at the Sinking Pond outflow 

gage of 323.6 m above NGVD 29 and typical wetseason water-surface elevations of 323.9 m—less 

than 1 m below the current spillway and more than 

2 m above the internal drain; dredging the entire 

outflow channel to the level of the culvert would 

affect only sites in the highest one third of Sinking 

Pond’s internal relief. Water levels in Sinking 

Pond and an adjacent well (fig. 3) indicate that the 

current internal drain is hydraulically efficient. 

Artificial blockage of the internal drain appears an 

improbable mechanism for increasing hydroperiods in Sinking Pond.

Additional field observations from 2000 

through 2002 confirm that the spillway and 

internal drain of Sinking Pond remain in a basically natural condition. Soil borings in both areas 

reveal gray silty clay or silt loam with distinct soil 

horizons, hydromorphic features, and a fragipan at 

0.5-1 m below land surface. Similar soil profiles 

are found throughout the pond and are consistent 

with the mapped Guthrie and Purdy soil series 

(Love and others, 1959; Edwin Roworth, Arnold 

Engineering Development Center, written commun., 2001). 

A

B

C

Figure B1. Photographs showing vegetation and topography of (A) the 

Sinking Pond spillway and the constructed berms at (B) Westall Swamp 

and (C) Huckleberry Creek.

Appendix B 47
Land-Use Change in the Sinking Pond Area, 

1938-1997

Land-use patterns in the Sinking Pond area were mapped 

using aerial photographs that spanned a 60-year period from 

1938 to 1997. For the purposes of this investigation, the Sinking Pond area includes the 335 ha drained by Sinking Pond 

and the 65 ha downgradient of the Sinking Pond spillway that 

drains to the Sinking Pond outflow gage (fig. 2). The objective of this work was to evaluate the possible relation between 

major changes in land use in the Sinking Pond area and 

hydrologic change associated with tree-regeneration failure 

documented in this report. Of particular interest were land-use 

activities, such as ground clearing, soil disturbance, and major 

changes in ground cover that could affect the basin’s hydrologic response. 

During the 60-year study period, management of the 

Sinking Pond catchment was transferred from private ownership (pre-1941) to State ownership (1941-1949), then to 

Federal ownership (1949-present). During the period of State 

ownership, the site was heavily utilized for military training 

exercises associated with Camp Forrest and extensive timber harvesting was conducted by the State caretakers (Jack 

Bennett, Archeology Associates, written commun., 2002). In 

1954, much of the Sinking Pond catchment was designated a 

Wildlife Management Area, and in 1976 the site was designated a National Natural Landmark by the U.S. Park Service. 

In the 1960s, the AEDC forestry program began establishing 

pine plantations on former agricultural fields in the catchment 

(David Bynum, Arnold Air Force Base, written commun., 

1997).

Changes in land use in the Sinking Pond area were 

determined by creating and analyzing a land-use/land-cover 

GIS database. This process involved the following steps: 

(1) acquiring imagery, (2) scanning and rectifying imagery, 

(3) digitizing land-use/land-cover polygons, and (4) assigning land-use/land-cover categories to each polygon across 

the 60-year temporal sequence. Table B1 shows the dates and 

sources of aerial photographs used in this analysis. The 1938 

and 1943 imagery was received in a usable, georeferenced 

digital format. The 1950-1992 imagery was obtained in hard 

copy form from the Tennessee Valley Authority. Each image 

was scanned using a UMAX Powerlock 2100xl (at 400 dpi) 

and rectified to the USGS 1997 Manchester digital orthophoto 

quadrangle (DOQ) using ArcView Image Analysis software. 

The perimeter of the Sinking Pond outflow basin was 

scanned from Wolfe (1996a) and rectified against the 1997 

Manchester DOQ. This area represents the 335-ha catchment of Sinking Pond plus an additional 65 ha drained by the 

Sinking Pond outflow channel below the pond’s spillway. The 

perimeter boundary then was digitized and used to define the 

area for land-use determinations in each image. A total of 100 

land-use polygons, which maintained relative homogeneity 

throughout the period of analysis, were identified and digitized. Each polygon was examined in each of the 10 sequential 

images and was assigned to one of seven land-use/land-cover 

classes (table B2) for each of the 10 years. Prior to classification of the polygons, the seven land-use/land-cover classes 

were field checked using the 1997 photos.

The GIS analysis of the land-use patterns over the 

60-year study period indicates that human disturbance within 

the Sinking Pond area reached a maximum level around 1943, 

with less than 25 percent of the land area having a hardwood forest cover of 40 percent or greater (fig. B2). During 

this same time, 57 percent of the landscape was in a heavily disturbed and erosion-prone condition (cleared bare and 

disturbed 10-40 percent) compared to 23 percent in 1938. 

Visual inspection of the imagery indicates that this increase 

can be attributed to heavy clearcutting in and around Willow 

Oak Swamp and around the perimeter of Sinking Pond and to 

off-road vehicle traffic and explosion impacts on the west side 

of the drainage.

From 1950 to 1997, there was a steady increase in the 

percent of landscape with continuous forest cover and a 

decrease in overall soil disturbance. In 1960, there was a brief 

increase in cleared-bare area as former agricultural fields were 

converted to pine plantation. By 1971, however, 85 percent of 

the land area had hardwood forest cover greater than 40 percent or contained established pine plantations, and areas 

of bare ground represented less than 1 percent of the basin 

(fig. B2). None of the photos indicate direct disturbance of the 

ponded area of Sinking Pond during the 60-year period.

The general relation of vegetation structure to the hydrologic cycle in humid, mid-latitude settings is well established 

from many studies (Wilde, 1958; Pereira, 1973; Pritchett, 

1979). Other factors being equal, forests have higher infiltration and soil-water retention (Heyward and Barnette, 1934; 

Pereira, 1973; Pritchett, 1979), less runoff (Hibbert, 1967), 

greater interception and evapotranspiration losses (Johnson, 1964; Rutter, 1972; Stanhill, 1970), lower water yields 

(Hewlett and Hibbert, 1961; Hibbert, 1967; Hornbeck and 

others, 1970), and lower ground-water levels (Trousdell and 

Hoover, 1955) than areas under row crops, pasture, savanna, 

or bare ground. Stands of evergreen conifers exhibit similar 

hydrological differences in comparison to deciduous hardwoods (Johnson, 1964; Stanhill, 1970; Helvey, 1974; Swank 

and others, 1972; Swank and Douglas, 1974). 

Table B1. Sources and dates of aerial photographs used in landuse analysis of the Sinking Pond drainage basin.

Source Date

National Archives1 April 1938

National Archives1 June 18, 1943

Tennessee Valley Authority February 16, 1950

Tennessee Valley Authority April 1, 1960

Tennessee Valley Authority February 26, 1969

Tennessee Valley Authority November 3, 1971

Tennessee Valley Authority March 22, 1976

Tennessee Valley Authority March 22, 1984

Tennessee Valley Authority December 13, 1992

Tennessee Valley Authority March 29, 1997

1

National Archives materials obtained through Arnold Air Force Base.

48 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
1938 1943 1950 1960 1969

1971 1976 1984 1992 1997

EXPLANATION

PINE PLANTATIONEVERGREEN SUCCESSIONHARDWOOD, 70-100 PERCENT FORESTHARDWOOD, 40-70 PERCENT FOREST

DISTURBED GROUND, 10-40 PERCENT FORESTGRASSLANDBARE GROUND

0 0.5 1 KILOMETER

0 0.5 1 MILE

Figure B2. Changes in land cover in the area drained by the Sinking Pond outflow gage, 1938-1997.

Appendix B 49
Table B2. Land-use/land-cover classes used in land-use analysis of the Sinking Pond area.

Land-use/land-cover class Definition

Hardwood, 70-100 percent forest cover Hardwood forest with 70 to 100 percent continuous canopy cover; forest habitat with minimal evidence of recent human disturbance.

Hardwood, 40-70 percent forest cover Hardwood forest with 40 to 70 percent continuous canopy cover; low-density canopy such 

as found in savanna-like habitat or recent selective logging activity; some disturbance of 

soil surface may be present (less than 10 percent mineral soil exposed).

Disturbed ground, 10-40 percent forest cover Forest with 10 to 40 percent continuous canopy cover; early successional habitat (mixture 

of tree, shrubs, and herbaceous cover); strong evidence of vehicular or other disturbance 

of soil surface (10 to 40 percent mineral soil exposed).

Evergreen succession Early- to mid-successional habitat; dominant canopy species is red cedar; some disturbance of soil surface may be present (less than 10 percent mineral soil exposed).

Pine plantation Various stages of loblolly pine plantation development with varying degrees of pine 

canopy cover; varying degrees of soil disturbance, depending on stand age.

Grassland Predominantly herbaceous vegetation with less than 10 percent tree cover; strong evidence 

of vehicular or other disturbance of soil surface (10 to 40 percent mineral soil exposed).

Bare ground Cleared land with 40 to 100 percent mineral soil exposed; lands in crop rotation, in 

preparation for pine plantation, or receiving high off-road vehicular traffic such as traffic 

associated with military training exercises.

Land-use changes in the Sinking Pond basin from 1943 

through 1997—a steady increase in forested area and establishment of pine plantations (fig. B2)—could be expected to 

lower the water table, reduce runoff and throughflow, and 

increase evapotranspiration. These hydrologic effects would 

reduce, not increase, the hydroperiod in Sinking Pond resulting from a given annual rainfall. Neither the altered patterns in 

tree regeneration indicated by the botanical evidence discussed 

above nor the hydrologic changes that apparently have driven 

those patterns can be attributed to land-use change.

Impoundment of Adjacent Drainage Basins

In the 1960s, berms were built across Huckleberry Creek, 

Dixie Flats, and Westall Swamp, directly to the north, east, 

and west of the Sinking Pond basin, respectively (fig. 2). The 

berms were intended to provide habitat and resting places for 

waterfowl and are equipped with control structures to permit 

seasonal draining (Mark Moran, Arnold Engineering Development Center, written commun., 2001). The hypothesis was 

considered that the berms might potentially increase groundwater recharge or compete with Sinking Pond for drainage. An 

extreme example of such an effect would be the case of two 

basins each having highly conductive internal drains leading 

to the same karst conduit. In such a case, artificially ponding 

water in one basin would pressurize the other basin’s internal 

drain, creating an effect similar to that of a backed up drain in 

a bathtub.

The hydraulic connection between the Sinking Pond 

basin and one of the neighboring impounded basins was examined through a simple impoundment-draining experiment. 

Continuous water-level recorders were installed behind the 

Huckleberry Creek berm, at a seasonally filled vertical shaft 

near the divide between the Huckleberry Creek and Sinking 

Pond basins, and in three wells north of Sinking Pond (fig. 2). 

The control structure at Huckleberry Creek dam was closed, 

and the area behind the dam was allowed to fill. The control 

structure was opened on February 13, 2002, and the waterlevel response in Sinking Pond, the wells, and the vertical 

shaft was monitored as the pond drained.

Water levels at Huckleberry Creek dam, Sinking Pond, 

and the monitored wells and vertical shaft are shown in 

figure B3. There is no indication that draining Huckleberry 

Creek dam had any effect on the other stations. Short-term 

recessions from minor rainfall on February 7, 2002, began 

before Huckleberry Creek dam was drained and continued 

with no obvious change in slope through the drainage event. 

Lack of a strong signal does not mean that the berms have 

had no effect on the Sinking Pond hydroperiod. Even after 

drainage, some water remained ponded behind Huckleberry 

Creek dam. Similarly, the berms at Dixie Flats and Westall 

Swamp retain water even with the control structures open. 

The artificially ponded water imposes hydrostatic pressure on 

the local ground-water system and may marginally increase 

seasonal recharge (C.J. Haugh, U.S. Geological Survey, 

Nashville, Tenn., written commun., 2002). The effects of such 

changes across basin divides, however, cannot be demonstrated and appear to fall short of the abrupt, high-magnitude 

hydrologic change implied by the ecological and tree-ring data 

for Sinking Pond.

50 Tree-Regeneration and Mortality Patterns and Hydrologic Change...Sinking Pond, Arnold Air Force Base, Tennessee
FEB 11 FEB 13 FEB 15 FEB 17 FEB 19

324

325

326

327

328

DATE

329

330

331

332

333

ELEVATIO

N

OF WATER

S

U

RFA

CE, IN

METER

S

ABO

VE

N

ATIO

N

AL GEO

DETIC

VER

TIC

AL D

AT

U

M

OF 1929

WATER LEVEL, IN

METER

S

ABO

VE N

ATIO

N

AL GEO

DETIC

VER

TIC

AL D

AT

U

M

OF 1929

SINKING POND

VERTICAL SHAFT

HUCKLEBERRY CREEK IMPOUNDMENT

WELL 353

WELL 356

WELL 358

WELL 359

Figure B3. Water levels in monitored wells and surface-water stations during drainage of Huckleberry Creek impoundment, 

February 2002.

Appendix B 51

Basin centimeter The volume of 1 centimeter of precipitation distributed over a drainage 

basin.

Biodiversity The number, variety, and variability of species in an ecosystem. 

Canopy The cover formed by the branches 

and leaves of forest trees.

Cohort A group of organisms of the same 

species and age, for example, a group of overcup oak trees that germinated in a single year.

Disjunct An isolated population of plants 

or animals far from their typical geographic 

range. 

Endemic Native to or typical of a particular 

region or ecosystem.

Fragipan A subsurface soil layer of relatively high density and low permeability to 

water.

Hydroperiod The amount of time a wetland 

is covered by or saturated with water. 

Karst A landscape in which chemical 

dissolution has enlarged joints, fractures, 

bedding planes, or other openings in soluble, 

underlying bedrock; may be characterized by 

sinkholes, caves, and disappearing streams.

Karst conduit A discrete, solution-enlarged 

opening in karst bedrock large enough to 

permit turbulent flow, typically greater than 

5 millimeters across.

Recruitment The transition of an individual 

organism into one of several successive developmental stages, such as acorn to seedling, 

seedling to sapling, or sapling to mature tree.

Regeneration The maintenance of a biological population through the reproduction and 

survival of new individuals.

Vadose zone The zone between the land 

surface and the dry-season water table.

Water budget The balance between water 

inputs, outputs, and changes in storage during 

a fixed time interval.

Glossary

Glossary 53
Wolfe, Evans, McCarthy, Gain, and Bryan—Tree-Regeneration and Mortality Patterns and Hydrologic Change in a Forested Karst Wetland—Sinking Pond, 

Arnold Air Force Base, Tennessee—Water-Resources Investigations Report 03-4217

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