Human impacts on leaf economics in heterogeneous landscapes: the effect of harvesting non-timber forest products from African mahogany across habitats and climates.
Abstract
Non-timber forest products (NTFP) are harvested by millions of people for their livelihood. To define sustainable harvest limits it is critical to understand the biological impacts of harvest. In the last decade we have improved our understanding of the demographic mechanisms driving population level responses to harvest. Our understanding of the ecophysiological underpinnings for these processes is still limited. We tested the effect of foliage harvest by indigenous Fulani people on leaf stoichiometry and economics in Khaya senegalensis (Meliaceae) in two vegetation types (fallows vs. forest) and in two ecological regions (dry vs. moist) in Benin, West Africa. Leaf mass per area (LMA) increased with aridity. Across sites and treatments, LMA correlated negatively with nitrogen and phosphorus per mass (Nmass and Pmass respectively), more strongly than with concentrations per area (Narea and Parea respectively) consistent with world-wide trends in leaf economics. The effect of foliage harvest on foliar nutrient concentrations was dependent on plant size and habitat. Harvesting increased Nmass and Pmass in larger trees, and altered the LMA-Nmass relationship between vegetation types and the LMA-Parea relationship between ecological regions, but it did not affect stoichiometry (N:P or C:N ratios). Synthesis and applications. The context-dependent effect of harvest on leaf economics emphasizes the importance of considering plant size and climate in predicting the biological consequences of NTFP harvest, and explains some of the ecophysiological mechanisms underlying demographic responses to harvest. The plant size- and climate-dependent effects of harvest on leaf composition suggest a different approach for assessing the impact of NTFP harvest on population dynamics. NTFP harvest should be modelled explicitly as a size dependent factor and integral projection models provide the framework for this. Our findings suggest that to reduce harvesting pressure on wild populations, it is important to encourage Fulani owned K. senegalensis plantations, especially in the dry region where harvest had greater effects.