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Tree species interactions in mixed stands are not static - they change temporally and spatially


The spatial and temporal dynamics of species interactions in mixed-species forests: From pattern to process

David Forrester


• Species interactions in mixed-species forests are dynamic, spatially and temporally.
• Complementarity increased with decreasing soil N when interactions increased N.
• Complementarity increased with decreasing water availability when interactions reduced competition for water.
• In some stands complementarity increased with increasing site quality.
• Few studies have examine the processes driving these dynamics.



Mixed-species forests and plantations sometimes have greater levels of ecosystem functions and services, including productivity, than monocultures. However, this is not always the case and there are many examples where mixtures are not more productive. 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. Many studies have examined how species interactions influence the growth of mixtures, but few have examined how spatial and temporal differences in resource availability or climatic conditions can influence these interactions. This review examines these spatial and temporal dynamics. The processes driving the dynamics are discussed using the production ecology equation, where plant growth is a function of resource availability, multiplied by the fraction of resources that are captured by the trees, multiplied by the efficiency with which the resources are used. Relative complementary effects depended on the types of species interactions and how resource availability changed. Complementary effects increased as soil nitrogen or water availability decreased when mixtures contained nitrogen fixing species, or when interactions were assumed to reduce competition for water. In contrast, some studies found that complementary effects increased with increasing site qualities, however in those studies there were no measurements of soil resource availability or any complementarity mechanisms. In those studies it was assumed that as growing conditions improved, competition for light increased and complementary effects resulted from interactions that improved light absorption or light-use efficiency. Multiple types of interactions can occur simultaneously in mixtures (e.g. nitrogen fixation, increased light absorption, and increased water-use efficiency) and so different resource availability-complementarity patterns will probably occur for a given pair of species, depending on the resource being examined. Less than half of the studies actually measured variables of the production ecology equation to indicate the processes driving the patterns. Several questions are listed that cannot yet be answered with confidence. Finally, stand structural characteristics, such as density, have also been shown to strongly increase or decrease complementarity effects and these need to be taken into account when interpreting results, but the mechanisms driving these density patterns were rarely quantified.



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