Few wetland studies from temperate North America have related either species richness or plant community composition to any direct measure of nutrient availability, or examined changes in species composition following experimental nutrient additions. Studies of wetlands in western Europe and of other terrestrial ecosystems in North America frequently show that nutrient enrichment leads to changes in species composition, declines in overall plant species diversity, and loss of rare and uncommon species. We therefore used an extensive literature survey and analysis of data on plant species composition, species richness, productivity or standing crop, and C:N:P stoichiometry in plant tissues and surface soils to draw conclusions about the nature of nutrient limitation in temperate North American bogs, fens, marshes, and swamps, and to infer their potential response to nutrient enrichment. We searched all major bibliographic data bases for studies containing such data (through March 1998) and added relevant data from our own ongoing research. We analyzed plant and soil data sets by wetland type and by wetland soil type, and the plant data set also by growth form. Existing studies appear to confirm common generalizations: (1) plant community type changes across broad nutrient gradients; (2) species richness declines as various indicators of nutrient availability increase beyond some threshold; and (3) rare and uncommon species are almost always associated with species‐rich communities. However, (1) these generalizations do not always hold within community types; (2) for many community types, the threshold beyond which richness declines has not been established, and high or low diversity may occur below that threshold; and (3) the failure of many studies to include bryophytes precludes drawing strong conclusions about nutrient availability and diversity in peatlands. Marshes had significantly lower mean nitrogen:phosphorus (N:P) ratios in live tissue than other wetland types (bogs, fens, and swamps), which did not differ significantly from each other. Mean N:P ratios in live tissues were significantly higher in peatlands than in mineral‐soil wetlands. Nitrogen:phosporus ratios in litter did not differ significantly between peatlands and mineral‐soil wetlands but were higher than in live tissues. Among growth forms, the highest mean N:P ratios in live tissues occurred in bryophytes, and the lowest in vascular herbaceous species. Only bryophyte live tissues differed significantly from other growth forms; litter N:P ratios were not significantly different among growth forms. Average N:P ratios in surface soils were lower in marshes and swamps than in bogs and fens. Wetlands on mineral soils had lower average N:P ratios than wetlands on peat soils. Average surface soil N:P ratios rose sharply at high soil organic‐matter contents (≥90%) and were generally greater than 16 at organic‐matter concentrations above 20%. In combination, plant tissue and surface soil N:P ratios suggest that a large proportion of North American wetla...

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A natural landscape gradient of P—deficient pocosin (evergreen shrub bog) and bay forest ecosystems was used to test three hypotheses concerning P availability and P reabsorption efficiency: (1) that variations in vegetation height and aboveground biomass among even—aged pocosin stands are related to variations in soil P availability; (2) that microbial processes control P availability in these soils; and (3) that the proportion of P reabsorbed from senescing foliage in native plants is inversely related to soil P availability. The natural gradient from short pocosin to bay forest was found to represent a gradient of increasing P availability, as indicated by increases in: (1) total P in surface (0°15 cm) soils, from 19.6 to 118.3 kg/ha; (2) seasonal and annual PO43— supply to in situ anion—exchange resins; and (3) P concentrations in native plant foliage. Increases in P availability were accompanied by a significant decline in the N:P ratios of both plant leaves and surface soils. Microbial processes strongly influenced the supply of available P. Soil microorganisms immobilized up to 90% of added 32PO43— during laboratory incubations, but when microbial uptake was inhibited, as much as 90% of added 32PO43— remained in the available pool. A decline in the ratio of microbial: total P in surface soils, from 56.3 to 37.4%, and an increase in the ratio of net: gross P mineralization (estimated by isotope dilution), suggested that as P availability increased, microbial control decreased. Native plants reabsorbed both P and N efficiently from senescing foliage, reaching maxima of 86.6 and 80.5%, respectively; natural variations in soil P availability did not affect the proportion of either nutrient reabsorbed. In general, however, plants tended to reabsorb greater proportions of foliar P than N. Natural variations in P availability, the factors controlling P availability in these soils, and the potential for differential nutrient reabsorption by native plants have important implications for the cycling and accumulation of P and N in pocosin and bay forest soils. Landscape—Level processes may have contributed to the observed variations in vegetation and P availability along this natural gradient.

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