Community level trait patterns are detectable even along short stress gradients but not readily predictable under very limited soil resources

To date, the stress-dominance hypothesis has not been tested along short gradients of very limited soil resources, like those encountered on rocky cliffs and walls. Under such conditions, the plant trait structure may be primarily driven by either environmental filtering or limiting similarity, depending (among others) on the net outcome of interspecific competition. Therefore, we aimed to reveal community-wide trait patterns induced by the two opposing processes that were hypothesized to govern the assembly of 39 seral chasmophytic communities, while controlling for their regional grouping. Eleven qualitative traits were employed to test their dependency on abiotic variables and variation in their dispersion along the combined (soil nutrient/water) stress gradient. There was strong evidence of single trait dominance - environment relationships, including a significant effect of 'Region'. The traits implying resource investment (i.e., 'vegetative propagation', 'woodiness', and 'tuft formation') displayed decreasing dispersion, whereas the conservative traits (i.e., 'polyploidy', 'succulence', 'ground-lying leaf rosette' and 'short-life span') showed a steady increase in dispersion along the productivity gradient. Unexpectedly, the 'springtime blooming' showed a declining dispersion along the mentioned gradient, while the single trait diversity associated with 'N-fixing', 'underground storage organs' or 'reproduction via spores' displayed no monotonic variation. Also, mixed patterns of convergence and divergence were observed for 'woodiness' and 'springtime blooming'. Despite some inconsistent patterns in trait dispersion, our results demonstrate that the predictions of the stress-dominance hypothesis are applicable even along short environmental gradients, but are also highly dependent on the regional species pool and on the net outcome of interspecific competition for limited resources.