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

Results and Discussion

Vegetation

The composition of the tree layer of the plot was generally similar to that of the forest as a whole (Tables 4, 11) except for the absence of Quercus, Juniperus, and Pinus species. The tree density in the plot is somewhat less than the mean for the forest; however, the basal area (33.9 m²/ha) is greater, particularly that contributed by Ilex opaca. Individuals in the plot have greater heights, large diameters, and are more widely spaced than in the Sunken Forest as a whole.

In contrast, the shrub layer in the plot is less well developed, having about one-fifth the basal area and one-fifth the density of the average shrub development in the forest (Tables 5, 11). The herbaceous cover of the plot was similar to the entire Sunken Forest, but the species composition differed (Tables 3, 12). In the plot there was greater cover of Aralia nudicaulis and Smilacina stellata and less of Rhus radicans than in the herb layer of the forest as a whole.

The plot, with an ocular estimated midsummer canopy cover of 85-90%, was similar to the relatively undisturbed areas of the Sunken Forest, which are dominated by mature Ilex opaca, Sassafras albidum, and Amelanchier canadensis individuals. In general, the plot is representative of the areas of maximum biomass and stability within the forest. Therefore, the biomass, net primary productivity, and biogeochemical relationships of the ecosystem analysis plot cannot be neatly extrapolated to the whole of the maritime forest ecosystem on Fire Island.

Biomass

The living biomass of the Sunken Forest plot (17,083 g/m²) is dominated by the tree layer, which contains over 95% of the total (Table 13). About 3% of the measured biomass is in above-ground Smilax rotundifolia, about 0.6% in shrubs, and 0.2% in the herb layer. The tree layer biomass (16,408 g/m²) is 75% Ilex opaca, 16% Amelanchier canadensis, 6% Sassafras albidum, and 3% other species (Table 14). Although Sassafras albidum had a greater basal area (Table 11), its contribution to the total biomass of the tree layer was less than half that of Amelanchier canadensis. This appears to be due to the earlier deterioration of the large Sassafras individuals, most having poorly formed crowns and many having rotted heartwood. Sassafras individuals of a given stem diameter generally have lower tree heights than the other dominant species in the plot (Fig. 38).

The tree layer biomass in the plot is fairly evenly distributed between roots (30%), stems (32%), and branches plus leaves (37%), although the distributions within individual species vary. Leaves comprise 2-3% of the total biomass for all species. Branches range from a low of 4% for Sassafras albidum to a high of 40% for Ilex opaca, with Amelanchier canadensis intermediate at 29%. Distributions of stem biomass for the three major tree species are the opposite of those for branch biomass; Sassafras has 67% of its biomass in stem wood plus bark, while Amelanchier and Ilex have 42% and 27%, respectively.

The root systems of the three dominant tree species comprise 25-32% of the total species biomass. Excavation showed the root systems to be generally concentrated in the upper 30 cm of soil, but Sassafras and Amelanchier had shallower root systems than Ilex (Fig. 39). However, the lateral extension of the roots was as much as 10 m from the trunk. The relatively high water table and common vegetative reproduction may be factors favoring the development of the shallow and extensive root systems. The mean root:woody shoot ratio in the plot was 0.445, ranging from 0.324 for the dominant Amelanchier canadensis trees to 0.477 for Ilex opaca (Table 14).

Fig. 39. Excavated root system of a Shadbush with Sassafras root in background (meter-stick gives scale).

In contrast to the tree layer, the shrub layer exhibited greater proportions of biomass in stems and roots (36% each) than in branches (18%) (Table 15). Current twigs plus leaves were a relatively greater component of the shrub biomass (9%) than in the tree layer (2%). Amelanchier canadensis accounted for over 50% of the total shrub biomass. However, the total contribution of the shrub layer to the plot biomass was nearly insignificant. This was also true for the herbaceous layer. Lianas, of which only the aerial portions of Smilax rotundifolia were measured, have a biomass more than three times that of the entire shrub and herb layers combined.

The total biomass of 17,083 g/m² of the Sunken Forest ecosystem analysis plot is less than the mean for temperate forests (30,000g/m²) given by Whittaker (1970), but is greater than the biomasses of many temperate and boreal forests reported in the literature (Art and Marks 1971). The above ground biomass in the Sunken Forest plot amounts to 11,545 g/m², approximately equal to the mean biomass of Ilex aquifolium stands in England reported by Peterken and Newbould (1966). The magnitude of the total biomass in the Sunken Forest plot was surprising under the prevailing environmental conditions in which the height growth of the trees is limited to a maximum of 8 m by wind and the impaction of salt-spray aerosols.

In comparison with other forested ecosystems (Table 16) (Rodin and Bazilevich 1967; Art and Marks 1971), the Sunken Forest has a greater proportion of its biomass in branches and less in stems. The 5673 g/m² of branch bark plus wood represents 35% of the total biomass and 49% of the above ground biomass. In contrast, the contribution of branches to the total biomass in other forested ecosystems ranges from 7% in some boreal stands to 15% in the mixed oak-pine forest (Table 16). The biomass in leaf tissues in the Sunken Forest is about the same as other deciduous forest ecosystems. There are no data from inland stands of Ilex opaca, but inland stands of Ilex aquifolium in England have relatively about half as much of the above ground biomass in branch tissues as in the Sunken Forest (Peterken and Newbould 1966). If the distribution of biomass in Ilex opaca on inland sites is similar to that of Ilex aquifolium in England, it would appear that in the limitation of vertical growth of the bole by salt-spray impaction, there has been an increase in branch biomass relative to stem biomass. Ilex opaca growing on inland sites tends to have a strongly excurrent, pyramidal crown, while on Fire Island where it is subjected to salt spray, the crown is broadest at the top of the canopy and decreases groundward (Fig. 26).

The 30% of the total biomass in the root systems in the Sunken Forest plot is a higher proportion than in other forested ecosystems (Table 16) with the exception of the fire-adapted pine-oak Brookhaven Forest (Whittaker and Woodwell 1968). The revegetation of these pine-oak stands is primarily by root sprouting, leading to large root systems that have been accumulating biomass over a longer period of time than their associated shoots. In the Sunken Forest there is also the potential for root systems to be older than shoots since reproduction appears to be by vegetative means.

Net primary production

The tree layer accounted for 94% of the 1075 g/m²/year total net primary production in the Sunken Forest. The leaves of Smilax rotundifolia accounted for 4%, the current twigs and leaves of the shrub layer, 1%, and the herb layer shoots, 2% of the measured production. The distribution of production between strata, as would be expected, was similar to the biomass distributions (Table 13).

The production of shrub and herb tissues not measured probably represents a total of 50 g/m²/year, making the probable total net primary production of the plot slightly greater than 1100 g/m²/year. Production estimates of shrubs and herbs and lianas were based solely on above-ground clipping. If the root production:shoot production ratios for the Sunken Forest herbs are similar to those species reported by Bray (1963) (a mean ratio of 0.66), the production of herb roots in the Sunken Forest plot would be approximately 11 g/m²/year. Smilax rotundifolia is a monocotyledonous plant and therefore does not have growth rings from which stem production can be estimated. As a result, only the current leaf biomass was used in the estimation of production for this liana. The branch, stem, and root production of shrubs are also missing from the estimated total production. Whittaker and Woodwell (1969) report that the current twig and leaf production was approximately 30% of the total production in the shrub stratum of the Brookhaven Forest. Applying this ratio to the Sunken Forest plot, the production of the layer would be approximately 30-35 g/m²/year.

The net primary production of about 1100 g/m²/year in the Sunken Forest ecosystem analysis plot is well above the production of most woodlands, shrub lands, and boreal forests (Art and Marks 1971) and is close to the suggested temperate forest mean of 1300 g/m²/year (Whittaker 1970). The above-ground production in the Sunken Forest plot (882 g/m²/year) is slightly lower than the above-ground production per unit of biomass; the Sunken Forest system, with a ratio of 0.063, is relatively more productive than the average temperate forest whose ratio is 0.043 (Whittaker 1970), but is not accumulating biomass at as high a relative rate (0.1 to 0.5) as early successional Prunus pensylvanica stands (Marks 1971). The ratio of above-ground productivity to above-ground biomass is typical of forests and woodlands approaching climax but subjected to disturbance (Whittaker et al. 1974).

Ilex opaca contributes 66% of the tree primary production; Amelanchier canadensis, 27%; Sassafras albidum, 4%; and other species, 2% (Table 14). The differences in the production distributions for various species largely reflect the differences in branch and stem biomass distributions. The relative branch production of Sassafras was only one-third that of Ilex or Amelanchier. However, Sassafras had a greater relative leaf production (44%) than either of the other two dominant tree species (30% and 33%).

Thirty-six percent of the total tree layer net primary production is in current twigs and leaves; 27%, in branches; 18%, in stems, and 19% in roots.

The distribution of production between perennial and annual tissues in the Sunken Forest resembles that of a wide variety of other forested ecosystems. The leaves of deciduous forests account for about 35% of the total production, while leaves in coniferous stands account for approximately 45% of the total production (Rodin and Bazilevich 1967).

Surface area relations

The surface area relations, as in the case of biomass, were dominated by the tree layer in general and Ilex opaca in particular (Table 13). The total stem surface area:land surface area ratio of 0.495 m²/m² fell between the ranges of 0.2-0.4 for small open forests and 0.5-0.7 for mature closed forests (Whittaker and Woodwell 1967)

Major Sunken Forest tree species have unusually high branch surface:stem ratios (Whittaker and Woodwell 1967), 22.3 for Amelanchier canadensis and 25.8 for Ilex opaca. Branch surface areas were estimated using formulae of Whittaker and Woodwell (1967) and are probably overestimates since the formulae use the distance from the branch base to the terminal twig end, which is the maximum branch length, not the mean distance to all twig ends. However, the branches of both I. opaca and A. canadensis are profuse and finely dissected (Fig. 40). The total branch surface area:land surface area ratio of 9.5, which must be regarded as an approximation, is far greater than the 1.5-1.6 m²/m² given by Whittaker and Woodwell (1967) for the branch surface ratios of mature, closed forests.

Fig. 40. Branches of a mature American holly in the Sunken Forest.

Branch surfaces in the Sunken Forest may play a unique role in nutrient cycling and photosynthesis. The concentration of the large branch surface area in the top of the canopy undoubtedly provides an effective surface for the impaction of salt-spray aerosols and thereby serves as a site for nutrient inputs for the ecosystem. Indeed, periodic, intense impaction of salt-spray aerosols on twig surface and the resulting toxic effects on apical meristems are important factors in producing a large branch surface (Boyce 1954).

The branch and stem bark of Ilex opaca, which have a greater total surface area than leaves, appear to be rich in chlorophyll and may represent a significant photosynthetic component as was indicated for Populus tremuloides (quaking aspen) bark by Pearson and Lawrence (1958).

The leaf surface:ground surface ratio (leaf area index) for the ecosystem analysis plot as a whole is 5.9 m²/m². The tree layer comprises 77% of the total leaf area; 11 % is in Smilax, 8% is in herbs, and 3% is in shrubs. The leaf area index for the plot is at the upper end of the range of 4.0-6.0 given for closed-canopy deciduous forests by Whittaker and Woodwell (1967), and in the middle of the range 4.9-6.6 for Ilex aquifolium forests in Great Britain given by Peterken and Newbould (1966).

The stems of Smilax rotundifolia (478 g/m²) remain green after autumnal leaf fall, and therefore may be photosynthetically active through the winter since maximum air temperatures were below freezing less than 30 days during the year in which meteorologic records were kept.

Organic debris

The conversion of living vegetation to organic debris is an important transfer taking place within the organic compartment of forested ecosystems (Bormann and Likens 1967). The production of organic debris plays an obvious role in the transfer of nutrients from the living biomass to other components in the intrasystem nutrient cycle. In the Sunken Forest ecosystem, the organic debris has an extremely important influence on both the physical and chemical characteristics of the soil system.

Soil organic matter greatly increases both the water-holding capacity and the stability of porous, sandy soils (Lutz and Chandler 1946). Furthermore, in sandy soils with low clay contents, the organic matter may provide the bulk of the colloidal surface in the soil system. Therefore, the status of the available nutrient compartment in the Sunken Forest may be determined largely by the amount and production rate of organic debris. To quantify the dimensions of the organic debris component the weights of above-ground organic matter were determined.

Above-ground organic debris

The Sunken Forest ecosystem plot has not developed to the stage in which large dead individuals of the dominant species are found.

The total weight of the above-ground organic debris (dead trees, fallen branches, and dead wood on living branches, excluding entire dead branches still on the trees) was 606 g/m² or only 5% of the total above-ground live biomass (Table 17). Sassafras albidum accounted for the greatest amount (70%) of the 485 g/m² of dead trees and fallen branches. The Sassafras organic debris is about equally divided between five dead medium-sized trees and large branches on the ground. All Amelanchier organic debris on the ground (37 g/m²) is in fallen branches, while all the Ilex material (38 g/m²) is from a single holly tree cut prior to the initiation of this study.

Another component of the above-ground organic debris is dead branch material. The dead organic matter on living branches (121 g/m²) amounted to about one-quarter of that in dead trees and fallen branches. Dead branch wood on Ilex opaca branches was concentrated near the top of the canopy, the apparent cause of death of much of this material being osmotic desiccation by impacted salt-spray aerosols.

Litter fall

The total litter fall of 557 g/m²/year deposited by the living plant biomass (Table 18) represents a substrate for secondary production by heterotrophs and detritivores. Ultimately, nutrients in this material are released by mineralization or leaching and are transferred out of the organic compartment. The total litter fall in the Sunken Forest is close to the mean for temperate forests (550 g/m²/year) given by Bray and Gorham (1964).

Eighty-three percent of the total litter fall is annual tissues (77% leaves and 6% flowers plus fruit) while the deposition of wood and bark material amounts to only 17% of the total. The estimated leaf litter fall of Ilex, Amelanchier, and Sassafras (323 g/m²/year) is in excellent agreement with the estimated total leaf production of these three species (310 g/m²/year). The amounts of annual tissues deposited by various species reflect their net production of leaves, but the deposition of wood and bark does not reflect the species differences in bark and wood production. Sassafras albidum accounts for 34% of the above-ground perennial tissue fallout, but only 4% of plot total above-ground wood plus bark production.

The total litter fall pattern, which is dominated by leaf fall, shows two distinct seasonal peaks (Fig. 28). The mid-October to mid-November peak is due to the leaf fall of deciduous species, while the mid-May to mid-June peak is caused by the fall of Ilex opaca leaves. The distribution of flower and fruit fall has a peak in May and June caused by the falling of tree flowers. Ilex fruit fall peaks in November and December, falling most heavily during periods of intense winds. The deposition of perennial tissues seems more closely related to these periods of intense winds than to any distinct seasonal pattern.

Soil organic matter

Soil organic matter comprises 94% of the total organic debris (10,125 g/m²) in the Sunken Forest plot (Table 17). Properly, the material contained in the soil organic fraction is both nonliving substrate and a living microfauna and flora involved in the processes of organic decomposition.

The >2-mm fraction of the surface soil layer consists entirely of organic matter and therefore gives an indication of the soil litter layer. The 879 g/m² is a slight overestimation of the average weight of the litter layer since samples were taken in July about a month after peak Ilex opaca leaf fall, and in addition some living root material was included in the >2-mm fraction. Observations of the Sunken Forest throughout the year disclosed only a thin litter (L) layer present, except in periods immediately following the vernal and autumnal litter fall maxima. These observations suggest a very rapid and nearly complete turnover of the L layer on an annual basis. The annual decomposition rate of all organic matter above the mineral soil in the tropics averages between 39 and 63%, but only 6-12% for black oak forests and 1-3% for pine forests in California (Jenny et al. 1949).

The structure of the Sunken Forest floor is that of a mull humus held together by fibrous root systems which are concentrated in the surface layer. Whole leaves are rapidly broken down physically by the abundant ground-feeding birds in their search for food, and by soil isopods (largely Porcellia scaber), which are numerous in the humus layer. The latter consume both leaf fragments and entire leaves. Leaf debris is further reduced by earthworms which appear to have a major role in the mixing of organic matter with sand grains. Most of this mixing is localized in the surface 15 cm of soil where organic matter comprises 7.1% of the <2 mm soil. In contrast, the 15-30-cm layer is only 0.38% organic matter and has the appearance of unaltered beach sand.

The organic debris represents a component of the total organic matter that is frequently bypassed in considerations of ecosystems biomass. The 10,125 g/m² organic matter (to a depth of 30 cm) represents 37% of the total organic matter in the Sunken Forest plot. A similar distribution of organic matter is found in Pseudotsuga menziesii (Douglas fir) forests in which 10,621 g/m² of organic debris, at the same depth, represents 34% of the total organic matter (Cole et al. 1967).

Soil organic matter plays an especially important role in the biogeochemical cycling in ecosystems such as the Sunken Forest that have sandy substrates. The organic matter not only increases the water-holding capacity and physical stability of the soil surface but also is a major factor in the circulation and retention of available nutrients within the ecosystem.

Soil organic matter is responsible for virtually the entire cation exchange capacity (c.e.c.) of the soil in the Sunken Forest (Fig. 41). The concentration of organic matter in the surface layer of the soil therefore leads to an enhancement of ability of the ecosystem to hold cations at the soil surface and zone of maximum root distribution. The c.e.c. of the surface 15 cm soil layer is 10.2 milliequivalents (me)/100 g soil. The c.e.c. of both surface layer and 15-30 cm layer samples which had organic matter removed by ignition is 0.15 me/100 g soil. The c.e.c. of these samples is identical to that of beach sand which has an organic content of only 0.04%.

Fig. 41. Soil organic matter, exchangeable cations, and cation exchange capacity relations.