Stochasticity of individual competition and local matchup inequality for saplings in a niche-structured forest

Ecologists have recently accepted the notion that species coexistence involves both niche and neutral processes, but few studies have explained how both of these opposite views can explain coexistence in the same community. Here we focus on competition among sessile organisms and explored first the extent to which species-based niche reflects local "matchups" between nearby individuals, using 726 saplings of 10 temperate tree species, and second the members engaging in the matchups, which have rarely been quantified despite the importance in mixed-species forests. Growth responses to light showed considerable species-level differences, suggesting commonly seen regeneration niches. Outcomes of the individual matchups were basically predictable from the species mean response, but also with substantial contribution of within-species variation. We found strong imbalance in matchup frequencies, such that some individuals meet more individuals of differing species but others meet fewer, as well as many isolated, competition-free ones. The niche and neutral processes appear to reflect, respectively, between- and within-species differences, and our findings suggest that even when niche segregation is discernible, the role of stochasticity for the frequency of local competition, as well as its outcomes, cannot be discounted in species coexistence.