Processing rates of autumn-shed leaves in aquatic habitats are highly variable. It has been hypothesized that these processing rates may, in part, be regulated by the concentrations of residual tannins in the leaves. Tests of this hypothesis have been inconclusive, and experimental designs may have been compromised by the use of both processing rates and tannin concentrations taken from a variety of sources using highly variable methods, sites, and experimental conditions. Here, processing rates of 48 species of deciduous leaves are measured using uniform conditions, and related to concentrations of leaf tannins, N, P, C:N, lignin, and toughness. The results indicate that condensed tannin, N, C:N, and lignin are significantly correlated with processing rates, although the predictive power of these simple relationships is weak. A multiple regression model using tannins, measured as total phenolics, N, and lignin explained almost 50% of the variation in processing rates, suggesting that the inhibition of processing by tannins is modified by other measures of leaf quality.

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Summary1. In the face of human-induced declines in the abundance of common species, ecologists have become interested in quantifying how changes in density affect rates of biophysical processes, hence ecosystem function. We manipulated the density of a dominant detritivore (the cased caddisfly, Limnephilus externus) in subalpine ponds to measure effects on the release of detritus-bound nutrients and energy. 2. Detritus decay rates (k, mass loss) increased threefold, and the loss of nitrogen (N) and phosphorus (P) from detrital substrates doubled across a range of historically observed caddisfly densities. Ammonium and total soluble phosphorus concentrations in the water column also increased with caddisfly density on some dates. Decay rates, nutrient release and the change in total detritivore biomass all exhibited threshold or declining responses at the highest densities. 3. We attributed these threshold responses in biophysical processes to intraspecific competition for limiting resources manifested at the population level, as density-dependent per-capita consumption, growth, development and case : body size in caddisflies was observed. Moreover, caddisflies increasingly grazed on algae at high densities, presumably in response to limiting detrital resources. 4. These results provide evidence that changes in population size of a common species will have nonlinear, threshold effects on the rates of biophysical processes at the ecosystem level. Given the ubiquity of negative density dependence in nature, nonlinear consumer density-ecosystem function relationships should be common across species and ecosystems.

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Tannin concentrations were determined in the autumn-shed leaves of 48 species of deciduous trees. Chemical measures of tannins (total phenolics and condensed tannins) were significantly correlated with protein-precipitating capacity. None of these measures of tannin concentration, however, were significantly related to published leaf processing rates, measured as weight loss versus time, or to microbial colonization, measured as rates of lipid synthesis on conditioned leaves. These data suggest that the large variation seen in leaf processing rates is due to factors other than tannin concentration and that much of this variation is due to abiotic factors such as leaching and physical fragmentation. Microbial activity on leaves showed much less variation than did processing rates.