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?
Final Report Abstract
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. When mixtures are more productive than monocultures, higher light absorption is often named as a cause. Few studies have compared the light absorption or light-use efficiency in mixed and monospecific forests and even fewer have examined which light-related interactions are most important, such as the effects of species interactions on tree allometric relationships and crown architecture, differences in vertical or horizontal canopy structure, phenology of deciduous species or the mixing effects on tree size and stand density. This project will contribute to each of these aspects by adding several new cases studies and synthesising this new information and previous work in the form of a literature review. The first case study focused on mixed and monospecific Pseudotsuga menziesii and Fagus sylvatica forests at several sites in Bavaria, Germany. This case study also included planting density experiments of P. menziesii (on two sites) to enable the density effects to be examined in more detail. While the mixtures and P. menziesii were more productive and absorbed more light than F. sylvatica monocultures, the differences were not significant, despite large differences in stand structures. There were also few intra-specific differences in tree allometry. The second case study was a planted F. sylvatica, Alnus glutinosa and Betula pendula experiment in Bangor, Wales. Here, the effects of species interactions and reductions in density due to mortality were examined. On average, the monocultures grew less and had lower APAR and LUE than the 2-species mixtures. Mortality that resulted in reductions in basal area of 8.3% had no significant influence on tree or stand APAR or LUE. These case studies reiterate the patterns that are emerging from recent work that shows that there are many different light-related interactions and the importance of each appears to depend on the species composition, stand structure and site characteristics.
