Theoretical and empirical studies have shown climate is a major determinant of species richness, yet the importance of climate relative to local variables, remains unclear. We hypothesized dominant tree species and understory plant species respond differently to climate and local site conditions because they require different resources, depending on position within the forest strata. Using nationwide spatial data and boosted regression tree analysis, we tested the importance of growing degree days (GDD), climate moisture index (CMI), and local soil drainage class on forest plant species richness across Canada’s natural forest ecosystems. Stand age and plot size were included in our models to account for succession and scale dependence of species richness.
Results/Conclusions
Overall, the BRT models explained 41.3 and 33.8% of the total variations in dominant tree richness and total plant richness, respectively. We found diversity of dominant tree species was substantively influenced by GDD and CMI accounted for 48.1 and 22.2% of the explained variation, but diversity of all plant species was mainly controlled by local drainage class, accounting for 75.4% of explained variation. The positively monotonic climate-richness relationship was apparent for dominant tree species, while total plant species richness increased along soil drainage class from rapid to poorly drained, but then slightly decreased on very poorly drained sites. Stand age also affected species richness, but showing different patterns for dominant tree species and total plant species. The climate-richness relationship predicted by species-energy theory is supported for dominant tree species, but not for total plant species. However, the discrepancy of the influences and patterns of climate and local site conditions between dominant trees and total plant species may be reconciled by the resource availability hypothesis. For example, dominant tree species and total plant species, mostly understory non-tree species, have different resource requirements, depending on the positions of plants in the forest strata. Our study indicates that there may not be a universal climate-richness relationship. Thus, future studies to move beyond generalizations of patterns, and to assess the multiple underlying mechanisms simultaneously are needed.