Springer ****

Plant Ecology (****) ***:*** ***

DOI 10.1007/s11258-005-9045-8

Extant and potential vegetation of an old-growth maritime Ilex opaca forest

Jodi A. Forrester and Donald J. Leopold*

Faculty of Environmental and Forest Biology, State University of New York College of Environmental Science

and Forestry, Syracuse, NY 13210, USA; *Author for correspondence (e-mail: abphtx@r.postjobfree.com; fax: +1-

315-***-****)

Received 9 December 2004; accepted in revised form 10 August 2005

Key words: Gap regeneration, Ilex opaca, Maritime forest, Seed bank, Treefall gap, Vegetation structure

Abstract

The extant and potential (seed bank) vegetation of a rare maritime holly forest on Fire Island, New York

was described to assess whether treefall gaps act as a mechanism for the persistence of the species com-

position of this plant community over time. The Sunken Forest overstory is dominated by Ilex opaca,

Amelanchier canadensis and Sassafras albidum. A survey of canopy gaps indicated canopy openings

compose 11.3% of the land within the Sunken Forest (16 ha). The composition and density of the seed bank

were described using the emergence method. Germination from soil samples placed in the greenhouse was

monitored over 2 years. Sixteen species germinated with an average propagule density of 215 41 germi-

nants per square metre. An early successional species (Rhus copallinum) dominated the seed bank, but the

late-successional, shade-tolerant I. opaca was also present. Though only one species in the seed bank did

not appear in the current vegetation, species abundance differed between vegetation strata. The mean cover

and density of the ground-layer flora were higher beneath treefall gaps than closed canopy. Sapling density

did not differ between the two canopy conditions, but the dominant species differed with A. canadensis

occupying several closed canopy plots and P. serotina saplings appearing more often in gap plots. Most of

the dominant canopy species are present in the seed bank and ground layer but are not present in the shrub

and sapling layer, with the exception of A. canadensis. Current (2002) sapling density is lower than three

decades ago for all species except P. serotina, which is now the dominant woody species in the Sunken

Forest understory. The results of this study indicate that if the cause of the sapling reduction is lessened or

removed, the characteristic species of the overstory of this unusual plant community may rebound and

redevelop a sapling and shrub layer akin to that present before the increase in Odocoileus virginianus on the

island.

disturbances inherent to maritime locations result

Introduction

in a higher likelihood of coastal plant communities

Maritime forests are long recognized for their being diverted along alternate successional se-

unique plant communities (Wells 1939; Boyce quences (Hayden et al. 1991; Day et al. 2001).

1954; Oosting 1954; Stalter and Odum 1993; Bellis Maritime forest communities have become con-

and Keough 1995). The physical environment, servation priorities due to intense urban develop-

especially storm events, plays a dominant role in ment and recreational pressure on barrier islands

shaping these dynamic coastal communities (USFWS 1997). Understanding the ecological

(Ehrenfeld 1990; Day et al. 2001). The frequent processes and dynamics underlying unusual

350

associations of plant species is necessary for their Applying this relationship between disturbance

conservation (Motzkin et al. 1993). frequency and seed bank composition to the vege-

Small-scale disturbances important for the tation zones typical of barrier islands, seed bank

maintenance of species diversity in inland forests diversity would be expected to increase from dune

may not serve the same role in maritime systems. plant communities, having small to non-existent

We were interested in examining the regeneration soil seed banks, to inland communities having the

dynamics in a late-successional, rare vegetation most developed (e.g., high species richness and

type occurring on a barrier island on the Atlantic density) seed banks (Fahrig et al. 1993; Looney and

Ocean. The maritime holly forest plant commu- Gibson 1995; Crawford and Young 1998). Poten-

nity type occurs only on barrier islands in New tially, forested communities that tend to occur in

York and New Jersey and has a global heritage the most sheltered locations of an island would

status rank of G1 (NatureServe 2004) and is have a rich seed bank, dissimilar in composition to

especially vulnerable in NY State (S1; NYNHP the aboveground vegetation of the forest.

2002). An understanding of the regeneration po- Chronic levels of herbivory by an elevated

tential within this unusual association of species density of Odocoileus virginianus on Fire Island

is critical for its conservation as chronic herbiv- has changed the cover, density and species richness

ory by Odocoileus virginianus (white-tailed deer) of the understory flora of the Sunken Forest

may alter the long-term dynamics of this plant (Forrester 2004). The aim of this study was to

community. compare the forest canopy constituents with the

In many forested systems, canopy gaps are an vegetation in the understory, including individuals

important microsite for regeneration, in which growing beneath closed canopy, within canopy

future canopy individuals are recruited (Runkle gaps, and in the seed bank to assess if the future

1981). Treefall gaps are important sites for both forest canopy is likely to resemble the current

germination of seeds and increased growth of state. We address several questions: (1) Do dif-

established plants (Grubb 1977). Canopy gaps ferences in the cover, density and species richness

typically provide increased sunlight, a release of of the understory exist beneath canopy gaps or

nutrients, and exposed inorganic soil (Canham closed canopy? (2) Does the size of the canopy

et al. 1990; Bazzaz and Wayne 1994; Denslow opening or the light available in the opening

1995). Species whose regeneration strategies are influence the cover, density or richness of the

well-adapted to the conditions created by the dis- vegetation? (3) What is the composition of the seed

turbance are most likely to be recruited to the bank? (4) Are the relative importance of species in

canopy (Oliver and Larson 1990). However, indi- the seed bank, understory and overstory vegeta-

viduals established prior to gap formation will tion similar?

often come to dominate a gap indicating that the

sapling layer in gap and closed canopy conditions

is important for understanding gap regeneration Methods

(Runkle 1990).

Soil seed banks can re ect the historical vege- Study area

tation of a location and contribute to the future

The study was conducted in the Sunken Forest, a

plant community (Fenner 1985; Rossell and Wells

maritime Ilex opaca forest on Fire Island, a barrier

1999). Many viable, but ungerminated seeds in

island located south of Long Island, New York

forest soils emerge following environmental chan-

(40 39 N, 73 07 W). Portions of the island,

ges such as treefalls and rainfall events, serving as

including the Sunken Forest comprise Fire Island

a major pool for the above-ground plant com-

National Seashore. The island is bordered by the

munity (Hyatt and Casper 2000). Frequently dis-

Great South Bay to the north and the Atlantic

turbed locations typically have similar species

Ocean to the south. The 16 ha forest lies between

compositions in the seed bank and the current

the towns of Oakleyville and Cherry Grove. The

vegetation, but as disturbance frequency lessens

forest canopy is dominated by Ilex opaca,

compositions become more dissimilar (Fenner

Sassafras albidum, Amelanchier canadensis, and

1985). For example, forest seed bank composition

Nyssa sylvatica.

is often dominated by early-successional species.

351

Mean annual temperature is 10 C and the an- directly beneath the opening within the canopy

nual precipation is about 114 cm. The average created by the falling of a tree and the expanded

length of the frost-free period is 180 days. The soil gap (the area extending to the bases of the canopy

of the Sunken Forest is a mull humus bound to- trees bordering the gap; Runkle 1992).

gether by brous root systems concentrated near All woody stems 1 cm tall

were recorded within a 25 m2 plot located 3 m

the surface (Art 1976). Almost all of the particles

>2 mm in size and 7% of the particles 1.6). Amelanchier

conducted using PC-ORD (version 4, MjM Soft-

canadensis (55%), Vaccinium corymbosum (24%)

ware Design, Gleneden Beach, OR). The number

and Rhododendron viscosum (18%) were most

of dimensions was decided based on a Monte

important beneath closed canopy, while Prunus

Carlo test and the amount of stress (unexplained

serotina (36%), Gaylussacia baccata (26%) and

variation). The vectors visually represent the rel-

again V. corymbosum (31%) were most important

ative strength and directionality of gradients de-

in canopy gaps (Table 1).

rived from a second matrix of species importance

The mean cover and density of herbaceous and

values. The direction of a vector indicates how well

woody vegetation 0.05) Table 3.

354

All spp. P. serotina

100

80

Plant cover 60

40

20

0

*-**-**-**-**-** 60 70

Gap area (m2)

Figure 1. Percent cover of all herbaceous and woody seedlings (d) and Prunus serotina (D) vs. the area of the actual gap opening.

Linear regressions are shown for all species (solid line, r2=0.31, p=0.025) and P. serotina (dashed line, r2=0.56, p=0.001).

Baccharis halimifolia and Polygonum hydropipero-

Propagule bank

ides (r=)0.459 and )0.470, respectively) and pos-

Rhus copallinum, S. rotundifolia, and Digitaria itively correlated with S. rotundifolia (0.423).

sanguinalis were the most abundant germinants

from the propagule bank (57.2 15.6, 37.5 13.3,

and 31.9 10.4 stems m)2). Ilex opaca germina- Composition of the overstory, understory

tion occurred only after the second cold stratifi- and seed bank

cation period (from Feb to July 2003) eventually

reaching 17.8 6.7 stems m)2. Several species The ordination of the three vegetation layers also

common in the forest had very low germination revealed strong separation into groupings based on

including S. albidum and A. canadensis. Nyssa plot location (Figure 3). The final stress for a three-

dimensional ordination was 17% (Monte Carlo

sylvatica, V. corymbosum and several herbaceous

p=0.02). The ordination explained 76% of the

species were absent from the seed bank.

The ordination performed on the plant compo- variation; 31, 24, and 21 based on the first, second

sition within canopy gaps, closed canopy, and the and third axes. The first axis was positively corre-

seed bank revealed a strong separation of canopy lated with R. copallinum (r=0.542), and negatively

correlated with S. albidum (r=)0.496). Distribu-

and understory species (Figure 2). Stress (unex-

plained variation) was 21% for a two-dimensional tion along the second axis was primarily due to the

ordination, therefore no further axes were used to importance of I. opaca or P. serotina and

(r=)0.665 and 0.561, respectively). The third axis

describe these data (Monte Carlo, p=0.02). Axis 1

explained 53% of the variation in plots while axis 2 is not displayed but was negatively correlated with

A. canadensis (r=)0.603). Though the overstory,

explained an additional 18%. The first axis is highly

negatively correlated with P. serotina (r=)0.605) ground-layer vegetation, and seed bank appear

and positively correlated with D. sanguinalis and grouped in ordination space, the sapling layer is

R. copallinum (r=0.644 and 0.619, respectively). much more dispersed indicating a more variable

The second axis was negatively correlated with composition. Not appearing in the ordination are

355

DIGI

PRSE

RHCO

Axis 2

GROUP

UG

UCC

PB

Axis 1

Figure 2. Ordination of extant and potential species composition of the maritime Ilex forest. Filled triangles represent plots within

canopy gaps, open triangles represent plots beneath closed canopy and filled circles indicate propagule bank samples. Vectors indicate

strong relationship with species importance: PRSE=Prunus serotina, DIGI=Digitaria sanguinalis, and RHCO=Rhus copallinum.

the many sapling plots where no individuals >1 m same size categories was estimated as 10,570

2400 stems ha)1 in 1967 (Art 1976), while in 2002,

tall and

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