FIRE ISLAND Ecological Studies of the Sunken Forest, Fire Island National Seashore, New York NPS Scientific Monograph No. 7

Quantitative data on the Atlantic maritime forest are sparse although general descriptive accounts of the vegetation are numerous. The maritime live oak forest in North Carolina reported by Bourdeau and Oosting (1959) has an average basal area of 40.64 m²/ha. The major species in the forest is Quercus virginiana with 62% of the basal area. Other important species present with respective basal area percentages in descending order are: Persea borbonia (8.5%); Carpinus caroliniana (American hornbeam) (5.5%); Juniperus virginiana (4.4%); and Pinus taeda (3.7%).

The canopy cover in the red cedar woodland on Island Beach, New Jersey, is Juniperus virginiana 87%, Ilex opaca 20%, Prunus serotina 15%, and Amelanchier canadensis 10% (Martin 1959). Without giving any basal area or density data by species, Ogden (1961) reported that Quercus alba (white oak), Fagus grandifolia, and Sassafras albidum constituted more than 60% of the density on better sites in the modern woodlands of Martha's Vineyard, Massachusetts.

The Sunken Forest, by contrast, has a mean basal area of 22.59 m²/ha, or about half that of the live oak maritime forest in North Carolina reported by Bourdeau and Oosting (1959). However, the individual sampling plots within the Sunken Forest had basal areas ranging from 4.07 m²/ha to 43.52 m²/ha. The biomass and productivity plot had a basal area (33.94 m²/ha) much above the mean for the forest as a whole. Therefore the more stable and well-developed portions of the Sunken Forest have about the same basal area as the well-developed maritime forests in North Carolina.

The species composition of the Sunken Forest is one factor making it unique compared to the other maritime forest. Ilex opaca with 38% of the basal area appears to achieve its greatest dominance on the entire Atlantic barrier island chain in the Sunken Forest. There are also higher basal area percentages of Sassafras albidum (23%) and Amelanchier canadensis (19%) in the Sunken Forest than in any other maritime forests reported in the literature. The Sunken Forest appears to be the northernmost Ilex opaca-dominated maritime forest on the Atlantic barrier island system. The development of the Sunken Forest system with a greater canopy closure and lower percentages of Pinus rigida, Juniperus virginiana, and Quercus spp. compared to Island Beach, N.J., Sandy Hook, N.J., and Montauk, N.Y., is probably a function of the greater protection afforded to the Sunken Forest by an unbroken system of primary and secondary dunes.

Although the species composition of the Sunken Forest differs from other maritime forests, the general vegetational organization of Fire Island is similar to other barrier islands on the Atlantic Coast. In the Sunken Forest area the vegetation exhibits increasing cover and structural complexity along a complex-gradient composed of salt spray, wind disturbance, and sand movement. The greatest vegetational cover and structural complexity are found in the Sunken Forest which appears to have exhibited spatial stability over the last 150-200 years. Spatial changes in the dune-swale and salt-marsh communities appear to be localized at their interfaces with the Atlantic Ocean and Great South Bay. Otherwise, the spatial relationship between the various vegetative communities are relatively constant with no indication of succession between communities at present.

Within the community zones perturbations in the environmental complex-gradient lead to vegetational readjustments. Perturbations in the form of human disturbance and wind erosion occur most frequently in the dune and swale community. The terminal stage of vegetational development of any location on the Fire Island area is determined by the nature of the complex-gradient at that point. Disturbances are such frequent events in the barrier island environment that a variety of successional stages is perpetuated, "climax" never being achieved synchronously throughout any of the communities.

Even though the Sunken Forest is situated in what might be classified as a harsh environment, the net primary productivity of the ecosystem appears to be about the same as many inland forests of the same latitude. The toxic effects of salt spray appear to be manifest more in a restriction of the accumulation of biomass than in the inhibition of production. The normal productivity by an ecosystem of subnormal biomass may be a direct result of the enormous surface area of the dominant vegetation acting as both a photosynthetic surface and a nutrient trap (Art 1971).

The Sunken Forest is a relatively young ecosystem in an environment of continual, periodic disturbance. Several lines of evidence suggest that it has taken approximately 200-300 years for a mature maritime forest to develop on Fire Island. This rate of ecosystem development is as rapid as in many temperate forests undergoing secondary succession and more rapid than in some (Braun 1950).

The rapidness of ecosystem development in the Sunken Forest is even more apparent when it is compared to the rate of succession on fresh water dunes. The forests on the Indiana Dunes of Lake Michigan have taken 850-12,000 years to develop basal areas (ca. 20 m²/ha) similar to the Sunken Forest (Olson 1958c). The accumulation of soil organic matter in the Sunken Forest, as well as in other maritime forests (Salisbury 1925), also appears to be a much more rapid process than in fresh-water dune forests.

The major factor appearing to govern differentially the rates of ecosystem development and function of these two forested ecosystems on nearly identical sand substrates (Cowles 1899) is the availability of nutrients. The geographic position of the Sunken Forest is such that nutrient deficiencies of the highly weathered soil can be compensated for by large inputs of nutrients from the ocean via salt spray (Art 1971); in the inland fresh-water dune ecosystems this compensation is not possible.

The composition, structure, and function of the maritime forest on Fire Island appear to be an expression of the interaction between marine and terrestrial environments. Meteorological, geological, hydrological, and biological process are so closely interwoven that they become nearly inseparable.