Species interactions facilitate the coexistence, structure, and the presence of permanent spatial heterogeneity in plant communities. In forests, the potential association between the previously-established plants (i.e., canopy, understory) and the plants recruiting beneath them (i.e., tree recruits) could be the consequence of multiple effects, such as stochastic and/or deterministic processes. Here we studied the effect of stochasticity on species associations and community assembly and its potential impact on forest regeneration in Iberian (SW Europe) mixed forests. Our data was gathered from six fully-mapped 0.12 ha plots, in which we surveyed the fine-scale distribution of 20 canopy species ( > 2m), five understory species ( > 0.5 to 2 m), and three tree-recruit species (< 2 m). We calculated the local abundance of species association based on the amount and species composition of the above-ground habitat below the location of each tree recruit. In addition, we determined the spatial consistency of observed species associations by randomly simulating the spatial distribution of each of the tree forest levels. To do so, we considered null models whose hypotheses tested if (i) recruits were independent of the canopy, (ii) recruits were independent of the understory, and (iii) recruits were independent of the above-ground habitat.
Results/Conclusions
We found that the local abundance of recruits was associated with richer areas of recruit species and negatively with the abundance of the most common species of canopy and understory. Comparing observed and simulated data, 33 percent of observed associations were likely predicted, whereas 67 percent were less observed than predicted; our results suggest that our study system was primarily governed by neutral or repulsive associations. From up to 300 potential species associations, we found that 48 percent of them could be explained by neutral associations based on abundance or stochastic processes between species, whereas deterministic associations were related to processes affecting only associations between the three forest levels. Our results highlight the role of species interactions to explain the assembly of plant communities and the maintenance of rich mixed forests as a good predictor of their future regeneration.
