Understorey vegetation in boreal Picea mariana and Populus tremuloides stands in British Columbia

Abstract

We compared the species composition and species density of vascular plants in the understorey vegetation of boreal forest between Picea mariana (Black spruce) and Populus tremuloides (Trembling aspen) stands in British Co- lumbia, Canada, and related differences in species composi- tion and species density between the two forest types to dominant canopy tree species as well as a wide variety of environmental factors. We analysed 231 stands, distributed in three different climatic regions representing drier, wetter, and milder variations of montane boreal climate. Of these stands 118 were dominated by P. mariana and 113 by P. tremuloides. P. tremuloides stands had higher species density than P. mariana stands in all climatic regions, but species density of each domi- nance type varied among climatic regions. The floristic compo- sition of the understorey vegetation was markedly different for P. mariana and P. tremuloides dominated stands. A detailed study on the effect of canopy dominance and local environmental factors on the understorey vegetation of the boreal forest was conducted using 88 stands from one of the three climatic regions. Using a combination of ordination and variation partitioning by constrained ordination we dem- onstrated a small but unique effect of canopy dominance type on the understorey vegetation, while a larger amount of com- positional variation was shared with other factors. Our results accord with a scenario in which differences in primary envi- ronmental factors and humus form properties, the latter accen- tuated by the canopy dominants themselves, are the most important causes of higher species density in P. tremuloides stands than in P. mariana stands, as well as differences in species composition among the two canopy dominance types. Processes and time scales involved in the small but significant direct and indirect effects of the canopy dominant on understo- rey species composition are discussed.