N:P stoichiometry of land plants: integration and coordination of the composition of leaves, stems, roots and reproductive structures.
Kerkhoff, Andrew*,1, Fagan, William2, Elser, James3, Enquist, Brian 1, 1 University of Arizona, Tucson2 University of Maryland, College Park3 Arizona State University, Tempe
ABSTRACT- Patterns of plant allocation have long been a central concern in ecology. The allocation of nitrogen (N) and phosphorus (P) to different plant organs are of particular interest because N and P play pivotal roles in many aspects of plant biology, and their availability frequently limits plant growth and primary productivity. Globally, the N and P content of plant tissues vary approximately 50-fold, and (in leaves) they are positively correlated across species and environments. Here we examine correlations in nutrient content among plant organs, and assess the strength of phylogenetic signals in stoichiometric variation. We take a comparative approach using a global data compilation detailing the N and P concentrations of leaves, stems, roots, and reproductive structures of 1,726 species in 224 land plant families. P and N content are highly correlated within all organ types, as has been previously documented in leaves. With one exception, nutrient contents and N:P ratios are also significantly correlated across organs, indicating a high degree of functional integration, though differences exist between woody and herbaceous taxa. Furthermore, nested ANOVAs demonstrate that most of the variance (0.42 – 1.0) in these relationships is accounted for at the level of genera and species, which is consistent with a high degree of lability and ecotypic variation in plant stoichiometry. Still, for many relationships, a significant fraction of the variation is accounted for at the family level and above. Independent contrasts on family means are used to explore the phylogenetic structuring of this remaining variation. Our results demonstrate that the stoichiometric composition of land plant organs exhibits a high degree of functional integration. The combination of high variability at low taxonomic levels and significant phylogenetic signals suggests that the coordination of organ stoichiometries is the result of both the constraints of evolutionary history and more direct environmental influences. By providing more complete knowledge of the partitioning of nutrients among plant organs, our findings can contribute to evolutionary explanations of plant functional diversity, the development of more accurate nutrient budgets from sparse and costly data, and the parameterization of models of ecosystem function.—–
Key words: ecological stoichiometry, functional diversity, macroecology
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