CARR TEXTURE IN BRITAIN AND NEW-ZEALAND - COMMUNITY CONVERGENCE COMPARED WITH A NULL MODEL

Four carrs (wooded fens) were sampled, two in Britain and two in New Z ealand, to document their texture and to look for convergence between carrs in two hemispheres. One of the New Zealand carrs was predominant ly native, and one mixed native/exotic. Plant community texture was ev aluated using the variate: (1) support fraction (proportion of the ult imate twig that is non-photosynthetic); and four variates relating to the PSU (Photosynthetic Unit): (2) SLW (specific leaf weight: PSU weig ht/area), (3) length:width ratio, (4) width, (5) area. The modular con struction of species was measured by determining the density of module s at different hierarchical levels. A null model for texture convergen ce is presented, in which the species present are assigned to sites wi th no constraint on the co-occurrence of species similar in morphology . The test is one for coevolutionary convergence and coecological sort ing, not for similarity of adaptation to the environment. There was no indication of convergence of the four carrs to a common texture, comp ared to this null model, when species presences were considered; on th e contrary there was considerable divergence between sites for all var iates, significant for three of them. The native New Zealand carr was atypical in having high SLW, and the N.Z. mixed exotic/native carr in its PSU width and area. Weighting the species by abundance (measured a s photosynthetic biomass), convergence was seen for PSU width and poss ibly for area. Since this gives considerable weight to the canopy tree s, it may reflect mainly their adaptation to canopy conditions, compar ed with that of the understorey species. It is concluded that there is some convergence, at least in the attributes of the canopy species, b ut convergence cannot be shown for the whole community.