Within‐Stand Nutrient Cycling in Wetland Ecosystems 1

Within-stand nutrient-cycling studies have occurred primarily in temperate and tropical terrestrial ecosystems where plants generally have well-developed conservation mechanisms. We sought to determine the degree to which the findings and hypotheses developed from studies of terrestrial ecosystems were applicable to wetland ecosystems and, where appropriate, to develop new ideas and hypotheses relevant to wetlands. Less information is available for wetland ecosystems, particularly oligotrophic wetlands that experience changes in nutrient status through anthropogenic activities. The authors of this Special Feature were asked to address issues related to nutrient conservation, retention, and cycling among wetlands of different trophic status from a global perspective. Some hypotheses and predictions that the authors were asked to consider are: 1) Mechanisms that help conserve nutrients for plants in low-nutrient wetlands become less efficient as availability of the limiting nutrient increases. This hypothesis predicts that the efficiency of wetland plants to resist leaching and withdraw nitrogen (N) and phosphorus (P) prior to leaf abscission decreases as availability of a limiting nutrient increases. A corollary of this hypothesis is that sclerophylly and extended leaf longevity, typically characteristic of plants in low-nutrient habitats, decrease as nutrient availability increases. 2) High-nutrient litter decomposes faster and immobilizes smaller amounts of N and P than low-nutrient litter. This hypothesis predicts that decomposition rates of plants from high-nutrient wetlands will be greater than for plants from low-nutrient wetlands. 3) Plants growing in low-nutrient conditions have higher tissue C:N ratios, resulting in higher concentrations of carbon-based polyphenolics (tannins and lignin). This hypothesis predicts that, because of lower levels of polyphenolics, higher leaf-litter decomposition rates occur in highnutrient wetlands. It also predicts that a greater percentage of the nutrient standing stock will be cycled via grazing pathways when nutrient availability is high. The five papers consider these issues and others for arctic, boreal, temperate, subtropical, and tropical wetlands. Sven Jonasson and Gaius Shaver review internal nutrient-cycling processes in arctic and boreal wetlands where nutrient availability is usually low due to low temperatures and anoxic soils. Arctic vegetation typically has a higher percentage of biomass in perennial structures, resulting in lower rates of nutrient loss and reduced annual nutrient demand. Jonasson and Shaver find that plant and nutrient turnover increases with increasing soil fertility across gradients of nutrients and moisture. Vegetation composition apparently has little influence on nutrient turnover at the ecosystem level, and there is strong competition between microbes and plants for nutrients. Barbara Bedford, Mark Walbridge, and Allison Aldous review the literature and provide examples from their own research for patterns of nutrient availability and plant diversity in temperate wetlands in North America. They find that species diversity decreases across broad gradients from low to high nutrient availability but that the pattern does not hold within many vegetation types. They further note that interpretations of vegetation-nutrient patterns would benefit greatly by consideration of bryophytes. Bryophytes are major elements of some wetlands, and it can be expected that they would present a different pattern than higher plants. The authors' analysis suggests that most North American temperate wetlands are P-limited with some co-limited by N and F? Rien Aerts, Jos Verhoeven, and Dennis Whigham also consider temperate wetlands but focus on plant-mediated processes that influence nutrient cycling at the levels of individual species and ecosystems. Their review considers fens and bogs dominated by monocots (i.e., sedges and grasses) as well as scrub-shrub and forested wetlands. They conclude that rates of nutrient cycling are predicted best by N and P concentrations in mature leaves and that plant growth form predicts nutrient-cycling rates better than does wetland vegetation type.