Multiple drivers of plant diversity in forest ecosystems

Aim Theoretical and empirical studies have shown that climate is a major determinant of species richness, yet the importance of climate relative to local site conditions remains unclear in forest ecosystems across large regions. Furthermore, little is known about how climate and local site conditions affect species evenness and understorey plant diversity. Location Canadian forest (42 degrees 37' to 68 degrees 14' N; 53 degrees 25' to 134 degrees 46' W; 4 to 2170 m elevation). Methods Using national, broad-scale forest inventory data, we tested the influence of growing degree days (GDD), climate moisture index (CMI), soil drainage class (SDC) and stand age (SA) on plant diversity across Canada's forests. Diversity was assessed on variable plot sizes from 125 to 500 m(2) for canopy tree and total plant species, and from 100 to 400 m(2) for understorey plants. Results Our models accounted for 47, 30 and 49% of total variation in canopy tree species richness, Simpson's dominance index and total plant species richness, respectively. After accounting for the effects of plot size and management history, canopy richness increased with GDD and CMI and peaked at intermediate SDC and SA. Simpson's dominance index decreased with increasing GDD and CMI and was lowest at intermediate SDC and SA. Total species richness was most strongly related to SDC and was highest on imperfectly drained soils. Climatic controls on species richness of understorey strata were generally weaker than those on canopy richness. Main conclusions Our results for canopy richness along a mainly longitudinal gradient demonstrate the positive effects of energy on tree species diversity and show evidence for the effects of local site conditions and secondary succession on diversity. Furthermore, our results show that the extent of the influence of climate and local site conditions on plant species diversity can strongly differ between vegetation strata.