The ecological role of silicon in tropical forests: effects on plant nutrient stochiometry, drought resistance and herbivory

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Silicon (Si) is the second most abundant element in the Earth´s crust after oxygen. It is widely recognised to have a variety of functions in plants, including the alleviation of biotic and abiotic stress, changing nutrient relations and promoting biomass production. Plants can accumulate Si or avoid accumulation, and plant species thus exhibit a wide range of Si concentrations in their leaves. The ecology and physiology of Si has been best studied in grasses, which include many Si accumulators. Nevertheless, significant foliar Si accumulation has also been documented in a number of non-monocot angiosperm families, and even for non-accumulators variation of foliar Si can have substantial ecological impacts both at the plant and ecosystem level. Tropical forests are among the most species rich systems on earth and provide important ecosystem services. Because they have the highest Si uptake rates of any biome, they have been suggested as a model system to study the ecological role of Si. At the same time, Si is known to influence several key processes that are important for plant performance, species distribution and community composition in tropical forests, including nutrient and water relations, as well as herbivory. Nevertheless, our knowledge about variation of Si across plant species and its ecological role remains rudimentary for tropical forests. We recently showed that Si concentrations vary tremendously within and across tropical tree species, which should have pervasive, as yet unrecognized, ecological consequences, relevant for understanding mechanisms of maintenance of biodiversity and for managing tropical forests under global change.Building upon this initial study, we will take the next step and evaluate, whether the potential effects of Si on nutrient and water relations, and on herbivore defenses actually occur in tropical forests. Towards this aim, we will integrate observational data with experimental approaches in the screenhouse, the laboratory and the field. We will (1) conduct an extensive survey of leaf Si concentrations within a diverse local tree community in Central Panama, and analyse relations to phylogeny and ecology. For a subset of 12 species we will (2) assess the consequences of different soil Si availability for leaf Si contents and C:N:P stochiometry in a Si fertilization experiment. Based on experimental manipulations of leaf Si concentrations we will further (3) evaluate the role Si for plant drought resistance in a common garden drought-irrigation experiment in the forest understory, and (4) assess the effect of Si on herbivory in the natural habitat, and in feeding trials with a model herbivore. The proposed study will thus substantially contribute to closing the vast knowledge gap on the ecological role of this abundant element in tropical forests, and improve their understanding under current and future conditions.