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The resource economics of chemical and structural defenses across nitrogen supply gradients.
Craine, Joseph1, 2, Lee, William2, Bond, William3, Reich, Peter1, Ollinger, Scott4, 1 University of Minnesota, Saint Paul, MN, USA2 Landcare Research, Dunedin, 00, New Zealand3 University of Cape Town, Cape Town, 00, South Africa4 University of New Hampshire, Durham, NH, USA
ABSTRACT- Plant defenses are a fundamental regulator of the transfer of energy and materials to higher trophic levels and can require a substantial fraction of the resources acquired by plants. In order to better understand the role of nutrient supplies in determining the prevalence of plant defense types, we investigated the theoretical relationships between ecosystem N supply and the net C gain of shoots that were undefended or defended in one of three ways: by N-free chemical compounds, N-containing chemical compounds, or structural defenses. We extended economic models of shoot resource balance to include the relative value of C and N (derived from a model of ecosystem C and N uptake) as well as depreciation, and amortization. In general, the relative net C gain of plants along N supply gradients were dependent on the type of defenses present and similar to changes in their generally understood abundance along an N supply gradient. For example, at low N supply, the additional C acquired when investing N in photosynthesis and C in defense is much higher than investing N in defenses. Only at high N supply is it better to invest large quantities of N in defense rather than additional photosynthesis. In a sensitivity analysis, net C gain of shoots was most sensitive to factors that affect the relative value of C and N and the rate of herbivory. This work expands our understanding of the determinants of the distribution of chemical defenses and extends into structural defenses, whose resource economics have largely been ignored.
Key words: herbivory, plant defenses, nitrogen, resource availability