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

The nutrient cycling model suggested that in some ecosystems the sole source of nutrients might be in the form of meteorologic inputs (Fig. 42). A basic question raised by this possibility is: How do the structure and function of ecosystems in which meteorologic inputs are the main nutrient source differ from those in ecosystems which have weathering as the primary nutrient source? To answer this question, the Sunken Forest was examined in depth to determine community patterns, biomass, primary production, surface area, and nutrient relationships in an ecosystem in which weathering is negligible and meteorologic inputs are the predominant source of nutrients.

The Sunken Forest is the terminal development of 200-300 years of succession, apparently beginning on barren sand. At present, the biomass of the Sunken Forest ecosystem is slightly greater than 17,000 g/m², being fairly evenly distributed between roots, stems, and branches plus leaves. The production of the Sunken Forest is approximately 1100 g/m², the majority of which is in leaves and twigs.

The biomass and productivity of the Sunken Forest generally fall between those of woodlands and temperate forests (Table 16; Art and Marks 1971). The distribution of biomass in the Sunken Forest is atypical in that branches and roots account for a far greater proportion of the biomass than in other forested ecosystems. This unusual distribution of biomass probably is the result of the restriction of vertical stem growth by the toxic effects of salt-spray aerosols.

The development of a 17,000 g/m² maritime forest on Fire Island in a 200-300 year time period has not resulted from supplies of nutrients derived from soil minerals. The capital of nutrients held in the Sunken Forest soil mineral compartment, and in sea sands in general, is extremely low compared to other soils (Table 27). The levels of potassium, sodium, and calcium in the ocean-worked sand minerals of the Sunken Forest are even lower than the weathered residues of podzolic soils or the surface layers of some red and yellow soils (Table 27). The magnesium levels, although similar to the weathered residue layer of podzols, are below those of many soils.

The retention of nutrients in the soil system in the Sunken Forest is a function of the organic matter rather than inorganic colloids. The pattern of high organic matter and nutrient concentrations in the surface layer of soil is common in a variety of maritime forests and coastal ecosystems (Au 1969; Wright 1955, 1956; Ovington 1950). This pattern is due to the gradual accumulation of organic colloids in relatively infertile sands. This accumulation with its adsorbed cations largely deter mines the magnitude of the available nutrient compartment.

Even though the soil mineral compartment of the Sunken Forest is relatively depauperate and the available nutrient compartment is concentrated in the surface 15 cm of soil, the potassium, calcium, and magnesium concentrations in herb shoots and tree leaves plus twigs are not greatly different from those in other ecosystems (Table 28). As would be expected, the sodium concentrations in the Sunken Forest plant tissues far exceed those of inland ecosystems.

The amounts of cations per unit biomass in the Sunken Forest ecosystem as a whole are distinguished from those in other temperate forest ecosystems mainly by the greater amounts of sodium and magnesium, the calcium and potassium concentrations being similar (Table 29). The primary production of the Sunken Forest contains slightly less calcium, about the same amounts of potassium and magnesium, and vastly greater amounts of sodium than in other forested ecosystems reviewed by Ovington (1965).

The movement of cations to the forest floor in litter fall and gross leaching (gross throughfall and gross stemflow, which includes aerosols plus rainout) in the Sunken Forest is generally greater than in other tmperate forest ecosystems (Table 30). On coasts with predominantly onshore winds, the amounts of cations in the gross leaching in temperate, forested ecosystems appear to generally decrease with increasing distance from the ocean. This trend is most apparent for sodium and magnesium, the two cations most abundant in salt-spray aerosols.

The "tightness" of nutrient cycling within ecosystems theoretically increases through succession (Odum 1969). In the Sunken Forest the entrapment of airborne nutrients is dependent upon the profuse development of finely divided vegetative surfaces. The retention of nutrients within the ecosystem is dependent upon the accumulation of living biomass and soil organic matter. Undoubtedly, nutrient inputs, accumulation and retention within the Sunken Forest all increase as the ecosystem develops toward a steady-state condition of maximal biomass.

The amounts of cations circulating in the gross throughfall and litter all in the tropical rain forest are greater than in any of the temperate forests in Table 30. The intrasystem nutrient cycling patterns of moist tropical forests and the Sunken Forest may be similar since both have low soil mineral nutrient levels and large proportions of cations in the living biomass. Both ecosystems are dependent upon the rapid circulaation of nutrients within the organic and available nutrient compartments although the soil organic matter is greater in the maritime forest system (7%) than in the tropical rain forest (2-3%) (Langdale-Brown 1968; Richards 1964).

There are two major sources of nutrients for terrestrial ecosystems, inputs from outside the system and weathering of soil minerals within the system (Bormann and Likens 1967). In the Sunken Forest the weathering source is extremely small and is completely overshadowed by the meteorologic input. In contrast, the major source of nutrients in inland, forested ecosystems is usually the weathering of soil minerals (Table 31). While the weathering for the Sunken Forest and Hubbard Brook Forest systems were calculated from cation concentrations of minerals at different levels in soil profiles, the weathering for the Brookhaven Forest and the Cedar River Forest in Table 31 were estimated from the relationship equating inputs plus weathering to outputs plus immobilization in the living biomass. The major error associated with the latter means of estimation is the difficulty in assessing the net annual immobilization of nutrients by the living biomass.

Even though they are entirely of meteorologic origin, the total annual sources of cations for the Sunken Forest are in amounts exceeding or equal to those in the Hubbard Brook Forest, a mature maple-beech-birch stand growing on a podzol soil in central New Hampshire. Allowing for errors in the estimation of weathering in the Brookhaven and Cedar River forests, the total annual sources of cations for the Sunken Forest appear to be surprisingly close to those of a variety of other ecosystems.

The magnitude of the meteorologic input of cations is largely a function of the weather patterns combined with the distance of the ecosystem from the ocean or terrestrial sources of pedogenic particles (Tamm 1958). The weathering transfers are largely dependent upon parent material, vegetation, climate, history, and other factors within the ecosystem. The meteorologic inputs into the four ecosystems in Table 31 decrease with increasing distance from the ocean. However, the Brookhaven, Hubbard Brook, and Cedar River forests, in contrast to the Sunken Forest, are not entirely dependent upon the meteorologic inputs as the sole nutrient source due to the nature of their soil mineral compartments.

The Sunken Forest is at the positive end of a gradient of meteorologic inputs. The deficiencies in the soil mineral compartment tend to be compensated for by the cation-rich aerial environment. In the Sunken Forest, as in other coastal areas, the losses of cations from the soil by leaching appear to be balanced by meteorologic inputs (Etherington 1967). The effectiveness of the meteorologic inputs is determined by the buildup of organic matter capable of retaining and differentially increasing available amounts of nutritionally important elements.

Ecosystems with heavily weathered or nutrient-deficient soils as well as ecosystems with no soil mineral compartment (raised bogs and epiphytic systems) are dependent upon meteorologic inputs of one or more nutrient elements (Tamm 1958; Gorham and Cragg 1960; Whittaker 1970). The development inland of these ecosystems, with low meteorologic inputs coupled with nutrient-deficient soil mineral compartments, may be an extremely slow process. For example, the development of fresh-water dune ecosystems with sand similar to that of Fire Island may take thousands of years to reach the forest stage (Cowles 1899; Olson 1958c). In contrast, the accumulation of organic matter in the soil and the development of maritime forest ecosystems in protected coastal areas appears to be a relatively rapid process (Salisbury 1925, 1952; Wilson 1960).

The importance of meteorologic inputs in biogeochemical cycling is not limited to either coastal areas or sites with nutrient-deficient soils. The global circulation of the chloride ion is largely dependent upon meteorologic pathways and the impaction of aerosols on vegetative surfaces (Conway 1942; Eriksson 1955; Juang and Johnson 1967). Similarly, the incorporation of both radioactive fallout and airborne industrial pollutants in terrestrial ecosystems is by processes identical to the meteorologic input of nutrients (Romney et al. 1963; Gorham 1958). Clearly, an understanding of the relationships between meteorologic inputs and the functioning of ecosystems is vital for man's survival in an increasingly polluted biosphere.