Scaling of C:n:p Stoichiometry in Forests Worldwide: Implications of Terrestrial Redfield-Type Ratios

Abstract

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.

Global-scale latitudinal patterns of plant fine-root nitrogen and phosphorus

“…For all fine-root ( < 2 mm in diameter) ) green-leaf n:P ratio (data from references 6,8 ) is best described by a linear function (y = 28.2-0.334×latitude, r 2 = 0.174, n = 903, green circles with green trend line), and senesced-leaf n:P ratio (data from references 8,9 ) is best described by a power function (y = 122.3×latitude − 0.547 , r 2 = 0.284, n = 636, yellow circles with yellow trend line), all relationships are significant at P < 0.0001. (a) Fine-root n concentration, (b) Fine-root P concentration, (c) Fine-root n:P ratio.…”

“…Therefore, plant tissues such as leaves and fine roots of fast-growing species at high latitudes, based on the growth rate hypothesis 25 , are expected to have high P demand for P used in P-rich RNA in ribosomes that are needed for synthesis of proteins, thus leading to low N:P ratio at high latitudes. However, at present, we do not know whether global-scale plant fine-root latitudinal patterns are similar to the linear decline in N:P in green leaves 6 or to the nonlinear decline in N:P in senesced-leaf litter 8 .…”

“…The global N pool in fine roots is estimated to be 4.8×10 8 Mg, approximately 1/7 of all terrestrial vegetation 1 . Nutrient release from decomposing roots is a key pathway of significant nutrient flux in terrestrial ecosystems 2 .…”

“…So far, terrestrial ecologists and physiologists have largely focused on ratios of N:P in plant green leaves 6,7 , but also somewhat on senesced-leaf litter 8,9 . By contrast, our understanding of N and P stoichiometry in root systems, especially at a global scale, is much less developed.…”

“…Plant green-leaf N and P concentrations have been shown to vary among climatic and soil environments, presumably reflecting both plant adjustment (acclimation and adaptation) 13 and biogeographical differences in relative N and P supply 14 . Studies on both plant leaves [6][7][8]15 and on litterfall 8,9 indicate that broad latitudinal and biome-level differences in soil N and P supplies can result in differences in plant nutrition and ecosystem nutrient cycles. As metabolically active tissues, fine roots, similar to green leaves, might be expected to have similar N-to-P stoichiometry 7 at local and global scales.…”