Deterministic processes drive functional and phylogenetic temporal changes of woody species in temperate forests in Northeast China

Key Message: Deterministic processes drive functional and phylogenetic temporal changes of woody species in temperate forest, depending on successional stage and tree size classes. The dominant influential factors changed from abiotic (especially topographic factors) to biotic (e.g., basal area), with both increasing successional stage and tree size, suggesting that deterministic processes shifted from habitat filtering to biotic interactions. Context: Disentangling how deterministic and stochastic processes govern the temporal change of community composition is critical for understanding community assembly and predicting long-term ecosystem change. However, rare studies have examined the distinctive imprint of these two processes on functional and phylogenetic temporal changes of woody species in temperate forests. Aims: The objectives of this study are to detect (1) the relative importance of deterministic vs. stochastic process in driving the functional and phylogenetic temporal changes at different successional stages and tree size classes in temperate forests and (2) the relative influence of abiotic vs. biotic factors on temporal change. Methods: We analyzed 10 years of detailed species composition, phylogenetic information, and 14 functional traits from 66 woody species in young-growth (5-ha) and old-growth (25-ha) forest plots in Northeast China. We devised a null model approach to determine the relative importance of deterministic and stochastic processes in driving functional and phylogenetic temporal changes. Then, we investigated the influence of abiotic (soil and topography) and biotic (basal area, stem density, and species richness) factors on temporal change using boosted regression tree (BRT) models. All analyses were conducted for different tree sizes (all trees, small trees [diameter at breast height (DBH) < 10 cm], and large trees [DBH ≥ 10 cm]). Results: In young-growth forest, functional and phylogenetic temporal changes were basically higher than expected for all trees, small trees, and large trees. Conversely, in the old-growth forest, functional and phylogenetic temporal changes were lower than expected for all trees, but opposite patterns were found for two size classes; i.e., change was higher than expected for large trees and lower than expected for small trees. BRT models showed that the most influential factors underlying the temporal change of all trees shifted from abiotic (e.g., topography) to biotic (e.g., basal area) factors with increasing succession stage. Regardless of successional stage, the relevant factors changed from abiotic factors for small trees to biotic factors for large trees. Conclusion: Deterministic processes govern tree functional and phylogenetic temporal changes, and the underlying deterministic processes change from habitat filtering to biotic interaction over succession and with increasing tree size.