Restoration of Longleaf Pine Ecosystems
Seed bank viability in disturbed longleaf pine sites
Cohen, Susan1, Richard Braham2,
and Felipe Sanchez 1
1USDA Forest Service, Southern Research Station, 3041 Cornwallis Rd.
Research Triangle Park, NC 27709 and 2North Carolina State University,
Department of Forestry, Biltmore Hall, Raleigh, NC 27695
Abstract
Some of the most species-rich areas and highest concentrations of threatened
and endangered species in the Southeast are found in wet savanna and
flatwood longleaf (Pinus palustris Mill.) communities. Where intensive
forestry practices have eliminated much of the natural understory vegetation
of longleaf ecosystem, the potential for re-establishment through a
seed bank presents a valuable restoration opportunity. Longleaf pine
sites converted to loblolly pine plantations and non-disturbed longleaf
sites on the Coastal Plain of North Carolina were examined for seedbank
viability and diversity. Conducting vegetation surveys and examining
the seed bank using the seedling emergence technique, allowed for verification
of the seed bank presence, as well as evaluation of the quality of the
seed bank in disturbed longleaf pine sites. Over 35 species and 1000
individuals germinated, and the seed banks of both stand types contained
species not noted during the vegetation survey. While many of these
species were weedy and typical of disturbance, numerous taxa were indicative
of stable longleaf communities. This study confirms both the presence
and quality of seed banks in highly disturbed longleaf pine sites.
Introduction
Due to human activities, the area occupied by longleaf pine ecosystems
has been reduced from an estimated 80 million acres in presettlement
times to less than 3 million acres (Frost 1990). Generally categorized
according to moisture conditions, longleaf pine ecosystems range from
xeric sand hills to wet flatwoods and savannas, and associated with
many different soils are unique understory conditions. Land conversion
to agriculture and loblolly pine (Pinus taeda L.) plantations account
for much of reduction on the more moderate to poorly drained sites.
Roughly 95 percent of today’s remnant longleaf stands are found
on xeric to dry-mesic sites that are too infertile to support agriculture
or intensive plantation culture (Frost 1990). The wetter savanna and
flatwood communities are more interesting, however, because they contain
higher understory diversity (Christensen 1981). Some of the most species-rich
areas and highest concentrations of endangered and threatened species
in the Southeast are found on these wet sites (Walker and Peet 1983,
Rome 1988, Frost 1990).
Because the wetter longleaf communities are rare, restoration efforts
on wet sites will promote conservation of an endangered ecosystem. Where
the natural understory vegetation has been eliminated by severe disturbance,
the potential for re-establishment through a seed bank presents a valuable
opportunity (Simpson et al 1989). Sowing and transplanting of native
species is costly, time consuming, and may not be possible if seed is
not commercially available. In addition, the increasing isolation of
intact, rare community types makes natural seed dispersal of desired
understory species unreliable (van der Valk and Pederson 1989).
Determining the presence of a seed bank and its composition is an essential
first step before a management or restoration plan that relies on the
seed bank is attempted (van der Valk and Pederson 1989). If the seed
bank of disturbed sites proves to be viable with species typical of
intact longleaf ecosystems, more sites will be available for restoration.
This is particularly important for the rare, wet longleaf ecosystems.
The objectives of this study were to verify the existence of a persistent
seed bank in disturbed longleaf pine sites, and if present, evaluate
the quality of the seed bank.
Study Sites
Study sites are on the Croatan National Forest, located in Jones, Craven
and Carteret counties. Two treatments--disturbed and non-disturbed--were
established with 4 replicate study sites each. All sites are predominantly
on Leon soils--a poorly drained soil of coarse sediment, with a weakly
cemented spodic horizon (Bh) (Goodwin 1987, Goodwin 1989, and LeBlond
1999).
The four sites of the disturbed treatment have all undergone severe
understory disturbance associated with forestry activities, and are
currently in loblolly pine plantations established between 1971 and
1975. These sites have undergone mechanical site preparation on a minimum
of two occasions and are sporadically burned for fuel reduction and
wildlife purposes (Cherry, pers com 1999). The four sites of the non-disturbed
treatment are fire maintained natural longleaf stands that served as
vegetative standards in the study. These sites primarily contain second
growth longleaf established between 1897 and 1900, with occasional scattered
old growth (Croatan National Forest 1997). The nondisturbed sites have
never experienced understory disturbance, and are considered to be the
best approximation of pristine longleaf pine flatwood or savannas sites
on Leon soils within the Croatan National Forest (LeBlond 1999, Braham
1999, and Schafale 1994).
Methods
The methodology was designed to allow for sampling and evaluation of
the seed bank contained in the soil from each study site and to allow
for comparison of the species that germinate from each treatment with
the standing vegetation currently present on each site.
A modified version of the North Carolina Vegetation Survey (NCVS) was
employed to examine the standing vegetation of each site (Peet et al.
(1998). Percent foliar cover of shrubs and ground line species was determined,
as was density and basal area of tree species. Evaluation of the seed
bank was completed using the seedling emergence technique. Soil was
collected and seeds germinated in a greenhouse using the most favorable
germination conditions with regards to light, temperature, and moisture.
Soil sample collections were made in August 1999 and May 2000, and each
collection was monitored for 9 months. Plants germinated from seed were
collected and identified, with the date of germination and study site
recorded.
Summary of Results
Vegetation survey
The nondisturbed sites were dominated by longleaf pine and the only
other tree species was Pinus serotina (pond pine). Many species typically
occurring in these types of sites have not reached tree size probably
due to the regular prescribed burn pattern used to manage these sites.
As expected, loblolly pine accounted for over 75 percent of the basal
area in the disturbed sites. Hardwood species typically found on wet
mineral soils of the Coastal Plain, such as Acer rubrum, Nyssa sylvatica
, Magnolia virginiana and Persea borbonia were also present. The basal
are a in the disturbed sites is approximately twice that found in the
nondisturbed site and there are almost 9 times as many stems/hectare.
With regards to the shrub and groundline vegetation there is a shift
from shrub to groundline vegetation dominance going from disturbed to
nondisturbed sites. There is probably a higher occurrence of shrubs
on the nondisturbed sites due to the dormant season growing regime.
Although the sites share many of the same species -particularly with
reference to shrubs, the abundance of these species varies widely among
site type. The primary shift being that from herbaceous to woody species
when going from the nondisturbed to disturbed sites, in addition to
the typical pocosin shrubs gaining dominance in the disturbed sites.
The disturbed sites do exhibit high resiliency to disturbance and in
addition to many of the ruderal type species, many species typical of
intact longleaf communities can be found. Complete species list, as
well as percent cover by species, will be present in the manuscript
currently in preparation.
Seed bank evaluation
The disturbed sites had greater numbers of individuals and greater numbers
of species than the nondisturbed sites. This is to be expected as dominant
plants in natural ecosystems tend not to accumulate large seed banks
and equal disturbance on a predictable schedule--like low intensity,
high frequency fire--tends not to select for the accumulation of a large
seedbank.
The table presented below provides a brief summary of the numbers of
species detected in the seed banks from both the August 1999 and May
2000 collections. Species were considered indicative if cited as consistently
occurring in intact longleaf pine sites by sources such as plant community
inventories and vegetation studies. Weedy plants were those typical
of highly disturbed areas. These species are not likely to perpetuate
themselves in fire maintained ecosystems.
Almost half of the indicative species, germinated in the greenhouse
from the disturbed sites in the 1999 collection, were not found in the
standing vegetation. That number increases to almost 75% from the disturbed
sites from the 2000 collection. Lack of correspondence between standing
vegetation and seed bank is a very common finding in seed bank research
from other community types. In addition, weedy species are not well
represented, again following the literature.
There were a greater number of species in the May 2000 collection and
greater numbers of individuals from the August 1999 collection. These
differences may be due to seasonal variation in dormancy breaking requirements
based on collection time, uneven distribution of seeds in the soil,
or some other factor. Detailed results will be present in the manuscript
currently in preparation.
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