Understory plant communities are dynamic, and play deterministic roles in future forest composition. We examined the patterns of species richness, abundance, and composition of three understory vegetation layers (shrub, herbaceous, and bryoid layers) in the boreal mixedwood forests of central Canada. Our objectives were to: (1) examine the variations of species richness and cover of each layer along an overstory broadleaf compositional gradient; (2) test the role of resource quantity and resource heterogeneity in driving species richness of each layer; and (3) examine the interrelationships of understory layers, and their responses to the same suite of environmental variables. The relationships were determined using linear and quadratic regressions, correlation analyses, and multivariate tests.

Results/Conclusions

Herbaceous species richness and cover, as well as shrub cover increased with increasing proportions of broadleaves in the overstory whereas richness and cover of bryoids declined with increasing broadleaf proportions in the overstory. There were significant, positive linear relationships of herbaceous richness and negative linear relationships of bryoid cover, and significant quadratic relationships of bryoid richness and shrub cover, along the overstory broadleaf compositional gradient. This suggested that resource quantity was the important driver of herbaceous richness, shrub cover, and bryoid cover whereas both resource quantity and resource heterogeneity drive bryoid richness and herbaceous cover. Herbaceous richness increased with increasing shrub richness, and bryoid cover decreased with increasing shrub and herbaceous cover. Species richness, cover, and composition of the three vegetation layers studied responded differently to the same suite of environmental variables. Shrub and herbaceous richness and cover peaked along the broadleaf compositional gradient, while bryoid richness and cover peaked whithin conifer and  mixedwood ranges. We conclude therefore that maintaining a diverse overstory in the boreal forests will ensure diversity of various life forms, because each overstory type, through modification of resources, may favor the establishment of different understory plant communities.