A bounded null model explains juvenile tree community structure along light availability gradients in a temperate rain forest

Ecologists have proposed that tree species may coexist by specialising on light environments associated with gaps of different sizes. Remarkably few community-level studies, however, have actually examined juvenile tree distributions along light availability gradients. Here we describe distributions of juvenile trees in relation to canopy openness in a temperate rainforest, and test the hypothesis that competitive sorting causes coexisting species to overlap less in light environment occupancy than would be expected by chance. Average overlap of species' interquartile ranges on the canopy openness gradient was tested against a bounded domain null model of community structure which used range-size criteria to constrain random placement of species optima. Microsite availability was strongly skewed towards low light, with 43% of microsites occurring at < 5% canopy openness. We therefore transformed canopy openness values to ranks, so that equal intervals on the transformed gradient represented equal areas of microsite availability. We then calculated the interquartile range (25-75%) of sample ranks occupied by juveniles of each species. About half the assemblage was non-randomly distributed in relation to canopy openness, providing evidence of niche expression. Average overlap of species' interquartile ranges did not depart significantly from that predicted by the bounded null model, indicating that community structure in relation to canopy openness was mainly explained by a mid-domain effect. As predicted by the null model, species' interquartile range mid-points were concentrated in the centre of the rank-transformed gradient, and species richness (overlap of interquartile ranges) peaked close to the median light environment. Most species therefore had intermediate light requirements. The apparent lack of constraints on pairwise overlap suggest that differences in light use are not a prerequisite for tree species coexistence. As far as we are aware, this is the first study to identify a mid-domain effect on a resource availability axis.