Stoichiometry
Megan McGroddy, Tanguy Daufresne, Lars Hedin

Megan McGroddy, Tanguy Daufresne and Lars Hedin are collaborating on a project using an empirical approach to examine large-scale patterns stoichiometry in terrestrial ecosystems. Marine biogeochemists have used well characterized stoichiometric ratios in both the biota and the seawater matrix to better understand the biological- geochemical interactions that drive the cycling of essential nutrients in those ecosystems. We have borrowed some of these ideas from our marine colleagues to examine terrestrial vegetation. Using data from over a hundred published studies we found well constrained ratios in foliage and litter on both global and biome scales. Further analyses showed that large scale shifts in nutrient availability (moving from generally P-limited tropics to generally N-limited temperate ecosystems) are paralleled in shifts in C:N:P in both foliage and litter with apparently much greater sensitivity shown for P availability. We also found that most of the nutrient ratios, particularly P:N scaled linearly across gradients in litter productivity and foliar stocks suggesting that physiological requirements determine the relationship. There were, however, some interesting exceptions to the linear scaling pattern where we see disproportionate increases in nutrient richness with increased foliar biomass or litter productivity. These cases suggest that the ecosystem scale factors (competitive strategies etc.) impose patterns on top of the fundamental physiological requirements