VEGETATION TEXTURE AS AN APPROACH TO COMMUNITY STRUCTURE - COMMUNITY-LEVEL CONVERGENCE IN A NEW-ZEALAND TEMPERATE RAIN-FOREST

Functional convergence of different communities in similar environment s would be expected as an outcome of the operation of 'assembly rules' . At an ecological level, competitive exclusion would restrict the co- occurrence of species with similar niches. Repetition of competitive s orting on an evolutionary time scale might lead to character displacem ent. Either process would ultimately lead to species niches being more regularly arranged in ecological factor space than expected on a rand om basis, with the consequence that the niche structure of different c ommunities in similar environments would converge. We assessed the app licability of this model of community structure by comparing vegetatio n between study sites spaced in altitude 20 m apart along a continuous gradient in South Westland low-altitude conifer/broad-leaved forest, with respect to seven variates of vegetation texture primarily concern ing the morphology of the photosynthetic unit (PSU). We employed a nul l model that assigns observed species to sites at random, as would be expected in the absence of assembly rules for the communities, compari ng observed variation in texture to variation under the null model to look for convergence or divergence and to determine statistical signif icance. Significant convergence between adjacent sites was found in al l variates when species weighted either by percentage cover or cover r ank were used to calculate site texture means, but convergence was les s pronounced among groups of five or 10 consecutive sites. Significant divergence occurred at the five-site level (three variates) using cov er rank as a weighting factor and at the two-, five- and 10-site level s (five variates) when no weighting factor was used. Overall, divergen ce was more pronounced among sets of sites spanning a wider range in a ltitude, which seemed consistent with the presence of an environmental gradient along the transect, although a DCA ordination of site floris tics failed to reveal a simple altitudinal trend. This study is the fi rst to seek community-level convergence within a local area and the fi rst to find statistically significant convergence between vegetation p atches.