This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
Limits on Regeneration Processes
in Southeastern Riverine Wetlands1
R. R. Sharitz and L. C. Lee 2
Abstract.--Principal factors that affect seedling recruitment in mature cypress-tupelo forests include seed production,
microsite availability and hydrologic regime. Studies on the
Savannah River floodplain in South Carolina show that although
seed production seems adequate, microsite characteristics and
water level changes limit regeneration success. Management
of water levels on regulated streams must account for species
regeneration requirements to maintain floodplain wetland
community structure.
INTRODUCTION
Development and maintenance of forested
wetlands is dependent, in part, upon the ability
of species to replace themselves or of other
species to replace them. Both vegetative and
sexual means of reproduction may be significant
in determining the species composition and structure of wetland stands.
In highly disturbed
wetlands, sprouts from the stumps or root systems
of damaged individuals may be the chief means
of forest recovery (Fonda 1974; Hawk and Zoebel
1974; Gardner 1980; Lee 1983). In natural
forested wetland communities, reproduction by
seed, especially in canopy gaps, often plays
a major role in species regeneration and the
long term maintenance of stand composition
(Whittaker and Levin 1977). Species establishment processes in wetland ecosystems are important for several reasons.
It has been shown
that patterns and rates of elemental cycling
through wetlands change with successional development (Sloey et al. 1978; Klopatek 1978). The
net primary productivity of wetland sites and
associated stream ecosystems can vary significantly, depending largely upon the species composition and condition of wetland plant communities (Palmisano 1978; Minshall et al. 1983).
The value of wetlands to fish and wildlife
species can often be directly linked to the structure imparted to wetland habitats by the existing
vegetation (Williams and Dodd 1978; Thomas et
al. 1979).
Dominant plant species in floodplain forests
typically are distributed along environmental
gradients that can be associated with topographic
and hydrologic factors (Robertson et al. 1978;
Wharton et al. 1982; Van Cleve et al. 1980).
Along many of the major river systems in the
southeastern coastal plain, bald cypress (Taxodium
distichum 3 ) and water tupelo (Nyssa aquatica),
occur as canopy codominants in areas that are
inundated on a nearly permanent basis (Larsen
et al. 1981; Wharton et al. 1982). Transition
areas from these extensively flooded sites to
adjacent uplands are characterized by a shift
in canopy dominance from cypress and tupelo to
species such as red maple (Acer rubrum), oaks
(e.g. Quercus laurifolia, g. nigra, Q. lyrata),
sweetgum (Liquidambar styraciflua) and other
bottomland hardwoods that are tolerant of poorly
drained soil conditions (Fowells 1965; Teskey
and Hinckley 1977; Hook 1984).
In southern floodplain forests minor shifts in edaphic and hydrologic features usually result in mosaic patterns
of species distributions and wetland community
types (Robertson et al. 1978).
lpaper presented at the first North American
riparian conference, "Riparian Ecosystems and
Their Management: Reconciling Conflicting Uses."
[Tucson, AZ, April 16-18, 1985].
Many wetland forest species have specific
environmental requirements for seed germination
and seedling establishment (Schopmeyer 1974;
Teskey and Hinckley 1977). For example, seeds
of both bald cypress and water tupelo require
a moist, yet non-flooded substrate to germinate
2Rebecca R. Sharitz and Lyndon C. Lee are
Division Head and Research Manager, respectively,
of the Division of Wetland Ecology, University
of Georgia Savannah River Ecology Laboratory,
Aiken, SC. Research conducted under contract
DE-AC0976SR008l9 between the U.S. Dept. of Energy
and the University of Georgia.
3All taxonomic nomenclature follows
Radford et al. (1968).
139
(Schopmeyer 1974). Furthermore, field and laboratory studies have demonstrated 'that seedlings
of neither species can tolerate prolonged submergence of their foliage (Mattoon 1916; Teskey
and Hinckley 1977; Hook 1984). Therefore, for
successful regeneration of cypress and tupelo
stands to occur, moist but exposed substrates
must exist during portions of the growing season.
On both regulated and natural streams, it is
unlikely that such conditions will occur every
year, or even with a predictable frequency or
pattern. As a result, cypress and tupelo stands
commonly consist of cohorts of even-aged individuals that become established during those occasional growing seasons when specific environmental conditions favorable for successful seedling establishment occur (Schlesinger 1976).
Major factors potentially responsible for
limited regeneration of bald cypress and water
tupelo include:
(1) low seed production, viability and dispersal, (2) lack of suitable microsites
for germination and seedling 'establishment, (3)
physiological and growth restrictions of seedlings
exposed to combinations of non-lethal environmental perturbations, and (4) catastrophic environmental events such as major fluctuations in water
levels. An evaluation of the relative importance
of these factors in limiting the regeneration
success of bald cypress and water tupelo in a
3800 ha floodplain forest on the u.S. Department
of Energy's Savannah River Plant (SRP) in South
Carolina is underway (fig. 1). Here, cypress
and tupelo are canopy codominants over 47% of
the area (table 1). Examination of the composition of these stands, especially of the representation of individuals in the seedling and sapling
size classes, suggests that the dominant canopy
species are not being replaced effectively (Sharitz
et al.
in press).
Table l.--Major plant community types on the
SRP Savannah River floodplain (from Jensen
et al. 1984).
Community
Type
Areal Coverage
%
ha
Persistent
Emergent
Marsh
54.6
1.4
latifolia
cYEerinus
Leersia spp.
Nonpersistent
Emergent
152.2
Marsh
4.0
Hydrolea guadrivalvis
Polygonum spp.
Ludwigia spp.
Scrub-Shrub
154.6
4.1
Mixed
Deciduous
1793.6
Swamp Forest
47.2
Taxodium distichum
Nyssa aguatica
1528.1
40.2
Quercus spp.
Acer rubrum
Liguidambar
styraciflua
83.4
2.2
Dominant
Species
~
Mixed
Deciduous
Bottomland
Forest
Needle-leaved
Evergreen
Forest
Sciq~us
Salix spp.
CeEhalanthus
occidentalis
Pinus taeda
FACTORS AFFECTING REGENERATION
Seed Production, Viability and Dispersal
N
Annual seed production of bald cypress and
water tupelo was examined during two consecutive
years by trapping dispersing seeds. Floating
seed traps were positioned within randomly located
0.20 ha quadrats to quantify seed rain.
In addition, water dispersal of seeds was measured to
quantify the potential seed input to a given
area of the swamp. Tetrazolium and germination
tests (Shuel 1948) were used to determine viability and potential for germination success of
the second year's seed crop.
During the winter of 1983-1984, seed fall
of both species began in September and continued
until December for water tupelo and February
for bald cypress (Schneider and Sharitz in prep.;
Sharitz et al.
in press).
Seed rain peaked
in November at an input of 4 seeds m- 2 day-1
for cypress and 1 seed m- 2 day-l for tupelo.
Although seed production was lower in 1984-1985,
seed release again occured in late fall and early
winter. During the winter and early spring months
of both years, flooding substantially increased
potential seed input to sites in the swamp by
D-Swomp
?
Ip
y
km
SAVANNAH
Figure l.--The Savannah River Plant in South
Carolina indicating location of Savannah
River floodplain and major tributary
streams.
140
and Hinckley 1977; Hook 1984), growth and certain
physiological processes may be altered under
combinations of flooding and other environmental
stresses. On the SRP, discharges of heated water
from the cooling systems of nuclear reactors
increase the temperature and depth of floodplain
water and sediments. McLeod and Sherrod (1981)
found that moderate increases in water temperature
enhanced cypress seedling growth. Similarly,
Donovan and McLeod (submitted) observed increases
in aboveground and belowground biomass of cypress
seedlings at moderately increased water temperatures (5°C above ambient), compared with biomass
under ambient conditions. However, after four
months' growth under combinations of high temperature (10°C above ambient) and flooding (6 cm
above the substrate surface), root carbohydrate
concentrations were lower than in seedlings grown
under ambient temperatures or non-flooded conditions. These results suggest that reduced carbohydrate storage may be a major factor contributing to the eventual decline of bald cypress
seedlings and mature trees in thermally impacted
areas of the SRP floodplain forest.
several orders of magnitude. Data from the winter of 1983-1984 indicate that as many as 100-200
cypress and tupelo seeds moved across any given
m2 area during periods of high water in January
and February (Sharitz et al.
in press). However,
many of these seeds may not have been deposited
on sites suitable for germination.
Although total seed production and availability seem adequate for community regeneration,
low seed viability, especially of bald cypress,
may be an important limiting factor. Estimates
of insect parasitism and seed abortion were made
on seeds collected over the two dispersal periods,
and viability was tested (Mitchell et al. 1985).
Preliminary results in 1983-1984 indicated that
21% of the water tupelo seeds were either aborted
or lost to frugivory while in the tree.
Insect
parasitism was observed in 26% of the cypress
seeds.
In addition, tetrazolium staining techniques indicated approximately 65% viability
of the remaining tupelo seeds and only 12% viability of cypress. Most of the non-viable cypress
seeds lacked embryos.
Availability of Suitable Microsites
Catastrophic Environmental Events
Given an adequate viable seed source, a
major factor limiting the success of cypresstupelo forest regeneration on the Savannah River
floodplain is the availability of substrates
suitable for germination and seedling establishment. Because floodplain substrates may be inundated during much of the year, microsites that
meet the necessary requirements of moist but
non-flooded conditions may be unavailable.
In
cypress-tupelo forests, several microsite types
can be distinguished (Huenneke and Sharitz 1985).
These include the bases of trees and cypress
knees (live wood substrates), stumps, woody
debris and fallen logs (dead wood substrates),
and several types of organic muck substrates.
Microsites differentially trap water dispersed
seeds and provide varied conditions for germination and growth, thus affecting seedling recruitment.
Differential seedling survivorship and growth
in various microenvironments will, under natural
conditions, limit the number of individuals that
survive to become dominants in the wetland forest
canopy. However, catastrophic environmental
perturbations may control regeneration in many
floodplain forests associated with regulated
streams. For example, discharges from reservoirs
on the Savannah River have modified the annual
hydrograph during the past three decades (fig.
2). Maintenance of high river levels during
SA VANNAH RIVER FLOWS
50
1926-1936 1966-1976 ----
'0
40
o
o
The recovery of marked seeds released into
the floodplain environment at random locations
indicates that potentially more than 50% of the
seeds are retained within 500 m of the parent
tree. Trapping of these water dispersed seeds
occurs differentially among knee, log, stump,
tree base and leaf litter microsite types
(Schneider and Sharitz 1985). In addition,
Huenneke and Sharitz (1985) showed that woody
seedlings were distributed not in proportion
to the abundance of microsite types. The relative stability of a microsite during winter floods
appears to be one determinant of its value as
a substrate for successful seedling regeneration.
!!.
30
2
Q)
ro
c::
20
~
o
u::
10
26
66
28
68
30
70
32
72
Year
Figure 2. Discharges from reservoirs on the
Savannah River have modified the river's
annual hydrograph during the past three
decades. Annual water level fluctuations
at the SRP for a representative interval
prior to upstream dam construction (19261936) and following dam construction (19661976) are compared.
Physiological Restrictions
Although bald cypress and water tupelo have
high tolerances to prolonged inundation (Teskey
141
major regeneration events on floodplains of natural rivers of the Southeast are pulsed and relatively infrequent. However, the cumulative impact
of annual regeneration failures in floodplain
forests is potentially significant.
the growing season and continuous inundation
of the floodplain may limit tree regeneration
success by reducing the availability of exposed
substrates suitable for seed germination and
seedling establishment. Furthermore, flood events
that inundate seedlings during the growing season
cause high mortality. Sharitz et ala
(in press)
reported 89% survival of a cohort of 1200 bald
cypress and water tupelo seedlings during the
typical winter floods of 1984. However, following two desynchronized floods resulting from
abrupt high volume discharges from an upstream
reservoir (one during May and the second during
early August) seedling mortality was greater
than 99% (fig. 3).
180
160
100
.-----------.
\
80
140
60
w
ro
ro
~
l
~
w
~
80
\
.,
~
~
~
\
40
20
~
LITERATURE CITED
20
Connor, W. H., J. G. Gosselink and R. T. Parrondo.
1981. Comparison of the vegetation of three
Louisiana swamp sites with different flooding
regimes. Am. J. Bot. 68:320-331.
0
Jun
1963
With increases in water resources development on Southeastern rivers, the potential longterm effects of management practices on floodplain communities should be considered. This
study suggests that river discharge management
that does not provide for forest regeneration
may lead to changes in the structure, productivity and habitat values of floodplain wetlands.
Currently, our ability to predict all the consequences of degradation of wetland portions
of watersheds is limited. Whereas several
studies have addressed impacts to particular
sites resulting from water level management
(Connor et al. 1981), the relationships between
the condition of floodplain forests
and major watershed processes have not been
addressed. Additional research is necessary
to establish watershed management techniques
that will satisfy maintenance requirements of
floodplain forests.
Aug
Oct
1964
Figure 3. Relationships between percent survivorship of bald cypress and water tupelo seedlings and flood events in the Savannah River
floodplain.
Donovan, L. A. And K. W. McLeod. Morphological
and root carbohydrate responses of bald
cypress seedlings to water temperatures
and water level regimes. Therm. BioI.
(submitted).
DISCUSSION
The natural establishment, development and
maintenance of floodplain forests in the Southeast is largely dependent on the coincident availability of viable seeds with low water levels
during periods in the growing season when germination and seedling establishment can occur. Our
understanding of the major factors that limit
regeneration in cypress-tupelo forests of the
Southeast indicates that alteration of stream
discharges without adequate attention to the
autoecological requirements of floodplain species
is an oversight in current management practices.
Fonda, R. W.
1974. Forest succession in relation
to river terrace development in Olympic
National Park, Washington. Ecology 55:927942.
Fowells, H. A. (ed).
1965. Silvics of forest
trees of the United States.
762 p. USDA
For. Sera Ag. Hdbk. No. 271. Washington,
D.C.
Gardner, A. C.
1980. Regeneration problems
and options for white spruce on river flood
plains in the Yukon Territory.
p. 19-24.
In Murray, M. and V. Van Velhuizen (eds).
Forest regeneration at high latitudes.
USDA PNW Gen. Tech. Rpt. 107.
Watershed management seldom focuses on maintenance of floodplain wetlands. Most river discharge events in the Southeast are timed for
recreational boating and fishing purposes or
for electrical power generation. Accurately
timed river drawdowns that allow natural wetland
ecosystem processes to occur are uncommon. Results
of the SRP studies summarized here suggest that
major desynchronized floods of the Savannah River
have led to degradation of floodplain forest
regeneration processes. Although the potential
for natural regeneration exists, managed water
level changes generally preclude establishment
and maintenance of major cohorts of the dominant
species on the SRP floodplain.
In any given
year, failure of seedling establishment is probably not significant.
In fact, it appears that
Hawk, G. and D. Zobel.
1974. Forest succession
on alluvial landforms of the McKenzie River
Valley, OR. Northwest Sci. 48: 245-265.
Hook, D. D.
1984. Waterlogging tolerance of
lowland tree species of the south. So.
J. Applied Forestry 8:136-149.
Huenneke, L. F. and R. R. Sharitz.
1985. Microsite abundance and the distribution of woody
seedlings in a South Carolina cypress-tupelo
swamp. Am. MidI. Nat.
(in press).
142
Jensen, J. R., E. J. Christiansen and R. R.
Sharitz. 1984. SRP swamp vegetation map.
29 p. DPST-84-372. E. I. DuPont de
Nemours & Co. Aiken, SC.
Schneider, R. L. and R. R. Sharitz. Factors
affecting seed availability in a southeastern
riverine swamp.
(in prep.).
Schneider, R. L. and R. R. Sharitz.
1985. Fate
of water-dispersed seeds in a southeastern
cypress-tupelo riverine swamp. Bull. Ecol.
Soc. Am.
66:
Klopatek, J. M.
1978. Nutrient dynamics of
freshwater riverine marshes and the role
of emergent macrophytes. p. 195-216. ~
Good, R. E., D. F. Whigham and R. L. Simpson (eds.). Freshwater wetlands: ecological processes and management potential.
Academic Press, NY.
Schopmeyer, C. C. (ed.).
1974. Seeds of woody
plants in the United States. 883 p. USDA
For. Sere Agric. Hdbk. No. 450. Washington,
DC.
Larsen, J. S., M. S. Bedinger, C. F. Bryan, S.
Brown, R. T. Huffman, E. L. Miller, D. G.
Rhodes and B. A. Touchet. 1981. Transition from wetlands to uplands in southeastern bottomland hardwood forests. p. 225-273.
In J. R. Clark and J. Benforado (eds.).
Wetlands of bottomland hardwood forests.
Developments in agricultural and managed
forest ecology. Elsevier Sci. Publ. Co.
Amsterdam.
Sharitz, R. R., R. L. Schneider and L. C. Lee.
Composition and regeneration of a disturbed
river floodplain swamp in South Carolina.
Proc. EPA Bottomland Hardwoods Symp. St.
Francisville, LA. (in press).
Shuel, R. W.
1948. Seed germinability tests
with 2, 3, 5-triphenyltetrazolium chloride.
Sci. Agric. 28:34-38.
Lee, L. C. 1983. The floodplain and wetland
vegetation of two Pacific Northwest river
ecosystems. 268 p. Ph.D. Dissertation.
Univ. Washington, Seattle.
Sloey, W. E., F. L. Spangler and C. W. Fetter.
1978. Management of freshwater wetlands
for nutrient assimilation. p. 321-340.
In Good, R.E., D.F. Whigham and R. L. Simpson (eds). Freshwater wetlands: ecological
processes and management potential.
Academic Press, NY.
Mattoon, W. R. 1916. Water requirements and
growth of young cypress. Soc. Am. For.
Proc. 11:192-197.
Teskey, R. O. and T. M. Hinckley. 1977. Impact
of water level changes on woody riparian
and wetland communities. 46 p. Vol. II
(The southern forest region). FWS/OBS-77/58,
U.S. Fish & Wildl. Sere Washington, DC
McLeod, K. W. and K. C. Sherrod Jr.
1981.
Baldcypress seedling growth in thermally
altered habitats. Am.J. Bot. 68:918-923.
Minshall, G.W., R. C. Peterson, K. W. Cummins,
T. L. Bott, J. R. Sedell, C. E. Cushing
and R. L. Vannote.
1983. Interbiome
comparison of stream ecosystem dynamics.
Ecol. Monog. 53:1-25.
Thomas, J. W., C. Maser and J. E. Rodick.
1979.
"Riparian zones." p. 40-47. In Wildlife
habitats in managed forests. USDA For.
Sere Agri. Hdbk. No. 553. Chap. 3.
Mitchell, C. E., R. L. Schneider and R. R.
Sharitz. 1985. Seed demography of
two floodplain swamp tree species.
Bull. Ecol. Soc. Am. 66:
Van Cleve, K., T. Dyrness and L. Viereck.
1980.
Nutrient cycling in interior Alaska floodplains and its relationship to regeneration
and subsequent forest development. p. Il-lR.
In Murray, M. and R.M. Van Veldhuizen (eds).
Forest regeneration at high latitudes, Proc.
of an Intern. Wkshp., Nov. 15-17, 1979.
(Fairbanks, AK). USDA For. Sere Gen. Tech.
Rpt. PNW-l07.
Palmisano, A. W.
1978. Harvest from wetlands.
p. 589-597.
In Greeson, P. E. , J. R.
Clark and J. ~. Clark (eds).
Wetland
functions and values: the state of our understanding. Am. Water Res. Assoc., Minneapolis,
MN.
Wharton,C. H., W. M. Kitchens, E. C. Pendleton
and T. W. Sipe. 1982. The ecology of bottomland hardwood swamps of the Southeast:
a community profile. 133 p. U.S. Fish
& Wildl. Ser., FWS/OBS-8l/37. Washington,
DC.
Radford, A. E. , H. E. Ahles and C. R. Bell.
1968. Manual of the vascular flora of the
Carolinas.
1183 p. Univ. North Carolina
Press. Chapel Hill.
Whittaker, R. H. and S. A. Levin. 1977. The
role of mosaic phenomena in natural communities. Theoret. Pop. BioI. 12: 117-139.
Robertson, P. A., G. T. Weaver, J. A. Cavanaugh.
1978. Vegetation and tree species patterns
near the northern terminus of the southern
floodplain forest. Ecol. Monog. 48:249-267.
Williams, J. D. and C. K. Dodd. 1978. Importance
of wetlands to endangered and threatened
species. p. 565-575.
In Greeson, P. E.,
J. R. Clark and J. E. Clark (eds). Wetland
functions and values: the state of our understanding. Am. Water. Res. Assoc., Minneapolis, MN.
Schlesinger, W. H.
1976. Biogeochemical limits
on two levels of plant community organization
in the cypress forest of Okefenokee Swamp.
142 p. Ph.D. Dissertation. Cornell Univ.,
Ithaca.
143