Inspired by the importance of globally well-constrained carbon:nitrogen: phosphorus (C:N:P) ratios in planktonic biomass to the understanding of nutrient cycles and biotic feedbacks in marine ecosystems, we looked for analogous patterns in forest ecosystems worldwide. We used data from the literature to examine the stoichiometry of C, N, and P in forest foliage and litter on both global and biome levels. Additionally, we examined the scaling of nutrient investments with biomass and production both globally and within biomes to determine if and when these ratios respond to macroscale ecosystem properties (such as nutrient availability). We found that, while global forest C:N:P ratios in both foliage and litter were more variable than those of marine particulate matter, biome level (temperate broadleaf, temperate coniferous, and tropical) ratios were as constrained as marine ratios and statistically distinct from one another. While we were more interested in the relative constancy of the C:N:P ratios than their numerical value we did note, as have others, that the atomic ratios calculated for foliage (1212:28:1) and litter (3007:45:1) reflect the increased proportion of C-rich structural material characteristic of terrestrial vegetation. Carbon : nutrient ratios in litter were consistently higher than in comparable foliar data sets, suggesting that resorption of nutrients is a globally important mechanism, particularly for P. Litter C:N ratios were globally constant despite biome-level differences in foliar C:N; we speculate that this strong coupling may be caused by the significant contribution of immobile cell wall bound proteins to the total foliar N pool. Most ratios scaled isometrically across the range of biomass stocks and production in all biomes suggesting that ratios arise directly from physiological constraints and are insensitive to factors leading to shifts in biomass and production. There were, however, important exceptions to this pattern: nutrient investment in broadleaf forest litter and coniferous forest foliage increased disproportionately relative to C with increasing biomass and production suggesting a systematic influence of macroscopic factors on ratios.

Redfield noted the similarity between the average nitrogen-to-phosphorus ratio in plankton (N:P = 16 by atoms) and in deep oceanic waters (N:P = 15; refs 1, 2). He argued that this was neither a coincidence, nor the result of the plankton adapting to the oceanic stoichiometry, but rather that phytoplankton adjust the N:P stoichiometry of the ocean to meet their requirements through nitrogen fixation, an idea supported by recent modelling studies. But what determines the N:P requirements of phytoplankton? Here we use a stoichiometrically explicit model of phytoplankton physiology and resource competition to derive from first principles the optimal phytoplankton stoichiometry under diverse ecological scenarios. Competitive equilibrium favours greater allocation to P-poor resource-acquisition machinery and therefore a higher N:P ratio; exponential growth favours greater allocation to P-rich assembly machinery and therefore a lower N:P ratio. P-limited environments favour slightly less allocation to assembly than N-limited or light-limited environments. The model predicts that optimal N:P ratios will vary from 8.2 to 45.0, depending on the ecological conditions. Our results show that the canonical Redfield N:P ratio of 16 is not a universal biochemical optimum, but instead represents an average of species-specific N:P ratios.

T. 2005. Recent advances in ecological stoichiometry: insights for population and community ecology. Á/ Oikos 109: 29 Á/39.Conventional theories of population and community dynamics are based on a single currency such as number of individuals, biomass, carbon or energy. However, organisms are constructed of multiple elements and often require them (in particular carbon, phosphorus and nitrogen) in different ratios than provided by their resources; this mismatch may constrain the net transfer of energy and elements through trophic levels. Ecological stoichiometry, the study of the balance of elements in ecological processes, offers a framework for exploring ecological effects of such constraints. We review recent theoretical and empirical studies that have considered how stoichiometry may affect population and community dynamics. These studies show that stoichiometric constraints can affect several properties of populations (e.g. stability, oscillations, consumer extinction) and communities (e.g. coexistence of competitors, competitive interactions between different guilds). We highlight gaps in general knowledge and focus on areas of population and community ecology where incorporation of stoichiometric constraints may be particularly fruitful, such as studies of demographic bottlenecks, spatial processes, and multi-species interactions. Finally, we suggest promising directions for new research by recommending potential study systems (terrestrial insects, detritivory-based webs, soil communities) to improve our understanding of populations and communities. Our conclusion is that a better integration of stoichiometric principles and other theoretical approaches in ecology may allow for a richer understanding of both population and community structure and dynamics.S. J. Moe, Norwegian Inst. for Water Research, NO-0411 Oslo, Norway (jannicke. moe@niva.no) and

Liebig's law of the minimum, which states that only one element limits the growth of organisms at any given time, is widely used in ecology. This principle is routinely applied to organisms, populations and communities, but can it really be applied indistinguishably across these different scales? Here we show, by prediction of a resource ratio conceptual model and with an experimental test carried out in microcosms with bacteria that, unlike single species, communities are likely to adjust their stoichiometry to that of their resources. This adjustment results from competitive exclusion and coexistence mechanisms, and is sensitive to the overall diversity of species in the community. It guaranties co-limitation, i.e. simultaneous limitation by multiple resources, at the community scale and optimal use of resources and maximization of community biomass for wide ranges of resource ratios. These results question the applicability of the Liebig's law of the minimum at the community level, and the relevance of ecosystem models relying on this principle.

Because phytoplankton live at the interface between the abiotic and the biotic compartments of ecosystems, they play an important role in coupling multiple nutrient cycles. The quantitative details of how these multiple nutrient cycles intersect is determined by phytoplankton stoichiometry. Here we review some classic work and recent advances on the determinants of phytoplankton stoichiometry and their role in determining ecosystem stoichiometry. First, we use a model of growth with flexible stoichiometry to reexamine Rhee and Goldman's classic chemostat data. We also discuss a recent data compilation by Hall and colleagues that illustrates some limits to phytoplankton flexibility, and a model of physiological adaptation that can account for these results. Second, we use a model of resource allocation to determine the how the optimal nitrogen-tophosphorus stoichiometry depends on the ecological conditions under which species grow and compete. Third, we discuss Redfield's mechanism for the homeostasis of the oceans' nitrogen-to-phosphorus stoichiometry and show its robustness to additional factors such as iron-limitation and temporal fluctuations. Finally, we suggest areas for future research.