Root competition: beyond resource depletion

1 Root competition is defined as a reduction in the availability of a soil resource to roots that is caused by other roots. Resource availability to competitors can be affected through resource depletion (scramble competition) and by mechanisms that inhibit access of other roots to resources (contest competition, such as allelopathy). 2 It has been proposed that soil heterogeneity can cause size-asymmetric root competition. Support for this hypothesis is limited and contradictory, possibly because resource uptake is affected more by the amount and spatial distribution of resource-acquiring organs, relative to the spatial distribution of resources, than by root system size per se. 3 Root competition intensity between individual plants generally decreases as resource availability (but not necessarily habitat productivity) increases, but the importance of root competition relative to other factors that structure communities may increase with resource availability. 4 Soil organisms play important, and often species-specific, roles in root interactions. 5 The findings that some roots can detect other roots, or inert objects, before they are contacted and can distinguish between self and non-self roots create experimental challenges for those attempting to untangle the effects of self/non-self root recognition, self-inhibition and root segregation or proliferation in response to competition. Recent studies suggesting that root competition may represent a 'tragedy-of-the-commons' may have failed to account for this complexity. 6 Theories about potential effects of root competition on plant diversity (and vice versa) appear to be ahead of the experimental evidence, with only one study documenting different effects of root competition on plant diversity under different levels of resource availability. 7 Roots can interact with their biotic and abiotic environments using a large variety of often species-specific mechanisms, far beyond the traditional view that plants interact mainly through resource depletion. Research on root interactions between exotic invasives and native species holds great promise for a better understanding of the way in which root competition may affect community structure and plant diversity, and may create new insights into coevolution of plants, their competitors and the soil community.