The effects of partial harvest on the understory vegetation of southern Ontario woodlands

We studied the effects of partial cutting on understory vegetation communities within 19 mixed maple forests in an agriculture-dominated landscape in southwestern Ontario. Woodlots that had been recently harvested were grouped according to provincial silviculture guidelines (standard and heavy cuts) and compared to woodlots that had been uncut for at least 24 years (reference stands). We found significant differences in richness, diversity, and quality of understory vegetation in response to harvest indices. More intensive harvesting resulted in increased richness and diversity, but mostly through the addition of habitat generalists and weedy species. However, partial harvest does not appear to drive vegetation community composition, as ordination methods found no clear community differences between the treatments. Use of the single-tree selection system based on basal area and harvest intensity targets will have an effect on the understory plants, but other factors including past management, disturbance history, and site level microclimate features will also play an important role in shaping vegetation communities. We caution against the removal of large volumes of trees ≥38 cm in diameter, and large reductions in canopy cover, as this can reduce the presence of “conservative” (forest dependent specialist) species, despite a general increase in species richness and diversity. Furthermore, we recommend additional research to investigate the potential for incremental degradation to occur on sites with a long-term history of harvesting, as we found that richness of exotics increased on sites with a history of forest management.