How do inter- or intra-specific differences in crown architecture, light absorption and light-use efficiency determine thinning responses and complementary effects in mixed-species forests?

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Very little of the information from ecological research on the effects of tree species diversity on ecosystem functioning has been translated into forest management. This has been hampered by the lack of information to scale up the results from case studies to real forests with their large variation in site conditions and stand densities. Whether or not mixtures are more productive depends on the net effects of different types of interactions, and these are dynamic, changing through space and time. Few studies have examined how spatial and temporal differences in resource availabilities or climatic conditions can influence these interactions, and there appears to be a general perception that complementary effects in mixtures usually increase as resource availability declines. This is often true, but about 30% of the studies in the forest or plantation literature show the opposite trend, where complementary effects for a given species combination increase as resource availability increases or climatic conditions become more favourable, but no studies have actually measured the processes that drive this pattern. Increasing complementarity with increasing productivity could occur if species interactions increase the absorption of photosynthetically active radiation (APAR) or light-use efficiency (LUE). As productivity and stand leaf area increase, competition for light will also increase and any interaction that improves APAR or LUE could become increasingly useful. This study will examine the APAR and LUE of individual trees in mono-specific and mixed-species neighbourhoods and how this is influenced by canopy structure and inter- or intra-specific variability in crown architecture. Relationships between tree growth, APAR and LUE will probably be influenced by stand density. Therefore analyses in planted biodiversity experiments and mixed-species forests along productivity gradients will be accompanied by analyses using data from thinning experiments, also along productivity gradients. These results will be used to test a new conceptual model describing the spatial dynamics of species interactions.