The diversity-invasion resistance theory argues that increased diversity results in increased competitive suppression of establishing plants. Although there is support for the pattern of decreased invasion with increased diversity, empirical demonstrations of increased competition are limited. An experiment was conducted during a severe drought in a native grassland community. The drought resulted in minimal shading among neighbours, and in contrast to prior studies, competition here was exclusively belowground. Neither diversity nor community composition influenced root crowding or competition. It appears that when competition is belowground, it is independent of diversity, likely because of fundamental differences in the mechanisms of above- and belowground competition. This suggests that even at the neighbourhood scale, there is no inherently negative relationship between competition and diversity, and lends support to alternative theories which suggest factors of than diversity may more strongly influence community invasibility.
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