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COMPETITION FOR RESOURCES IN TREES: PHYSIOLOGICAL VERSUS MORPHOLOGICAL PLASTICITY
Citation:
GRAMS, T. E. AND C. P. ANDERSEN. COMPETITION FOR RESOURCES IN TREES: PHYSIOLOGICAL VERSUS MORPHOLOGICAL PLASTICITY. Progress in Botany. Springer-Verlag, Berlin, Germany, 68:356-381, (2007).
Impact/Purpose:
To examine two hypotheses related to resource acquisition in trees
Description:
In this review we examine two hypotheses related to resource acquisition in trees. The first hypothesis states that when competition is size-asymmetrical, then allocation changes leading to morphological shifts are more important than physiological shifts in obtaining limited resources. For example, the most effective response to competition for light (unidirectional supply) is a change in architectural arrangement of the biomass, i.e. a morphological response, both to maximize light exposure but also to shade neighboring plants. A second hypothesis states that when competition is size-symmetrical, physiological adjustments are more important than morphological adjustments in obtaining limited resources. One example is below-ground competition for nutrients, which is generally considered size-symmetric since resource supply is multi-directional. In this case, architectural arrangement is not important since all biomass is equally effective in accessing resources, and therefore physiological adjustments can lead to greater uptake rates than morphological shifts. However, below ground, resource distribution is often patchy, which may lead to conditions analogous to unidirectional resource supply. If resource supply is patchy below ground, then morphological adjustments may be more effective than physiological adjustments in capturing resources, similar to conditions when resource supply is unidirectional (and hence competition is size-asymmetrical). Despite this possibility, there is little direct evidence to suggest that competition below-ground is size-asymmetric.
Above ground, the literature supports the importance of morphological shifts in response to the primary resource in question, i.e., light. Physiological responses may be essential for tree survival in the shade but hardly relate to their competitive success. Below ground, however, the lack of comprehensive tree studies coupled with complexities associated with the simultaneous limitation of more than one resource makes it difficult to rank the relative importance of physiological vs. morphological plasticity. We conclude that below ground both morphological and physiological adjustments in response to competition for resources are important in trees.