The influence of structural, functional, and species diversity to temporal stability of productivity and efficiency of resource use in a tropical tree diversity experiment

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Owing to past deforestation and ecosystem degradation there is a large need and potential for afforestation in tropical and subtropical regions. Mixed-species plantations offer an opportunity to establish forests that may be more productive and offer ecosystem services at a higher level compared to monocultures. However, whether diverse tree plantations are in fact more resilient to droughts and whether they use nutrients and water more efficiently than monocultures has not been sufficiently studied, in particular not in tropical regions. Here we intend to use the unique opportunity to analyse the recently harvested “Sardinilla” tree diversity experiment in Panama, with 16 years the oldest one of its kind. The overall objective of the proposed project is to determine whether there is a stabilizing effect of tree neighbourhood diversity, the scale at which tree-tree interactions occur, on the productivity and resource use dynamics (water and nutrients) of trees over time. Specifically we hypothesize that: a) complementarity of productivity and resource use increases with increasing diversity (species, structural, functional) of tree neighbourhoods and over time, b) the temporal variation in productivity and resource use declines and stability increases with diversity, c) structural diversity and species richness independently and directly affect the productivity and resource use of individual trees, and d) during drought periods, complementarity regarding woody biomass production, water-use efficiency, and nutrition increases with increasing diversity. To test these hypotheses, we will analyse stem discs from almost 150 trees to determine the biomass increment, element concentrations, and C and O isotopes in individual tree growth rings. For these trees, which originate from experimental plots with 1, 2, 3, or 5 tree species, we will determine the influence of neigbourhood diversity on wood biomass production, water use and nutrition of trees during normal or wet and dry years. This will be the first study that assesses these response variables in combination, at an annual resolution, and over a relatively long time period (16 years) in tropical trees. The proposed analyses of me-dium-term tree diversity effects during wet and very dry periods is particularly relevant for future plantations against the background of projected climate change. Results from this study will indicate whether mixed tree plantations can produce wood with lower nutrient and water costs and at lower risks with regard to droughts.