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 Zealand, to document their texture and to look for convergence between carrs in two hemispheres. One of the New Zealand carrs was predominantly native, and one mixed native/exotic. Plant community texture was evaluated using the variate: (1) support fraction (proportion of the ultimate twig that is non‐photosynthetic); and four variates relating to the PSU (Photosynthetic Unit): (2) SLW (specific leaf weight: PSU weight/area), (3) length:width ratio, (4) width, (5) area. The modular construction of species was measured by determining the density of modules at different hierarchical levels. A null model for texture convergence is presented, in which the species present are assigned to sites with no constraint on the co‐occurrence of species similar in morphology. The test is one for coevolutionary convergence and coecological sorting, not for similarity of adaptation to the environment. There was no indication of convergence of the four carrs to a common texture, compared to this null model, when species presences were considered; on the contrary there was considerable divergence between sites for all variates, 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 as photosynthetic biomass), convergence was seen for PSU width and possibly for area. Since this gives considerable weight to the canopy trees, it may reflect mainly their adaptation to canopy conditions, compared with that of the understorey species. It is concluded that there is some convergence, at least in the attributes of the canopy species, but convergence cannot be shown for the whole community.