Changes in Understory Composition of Rural North American Temperate Forests after a 14-Year Period with Focus on Exotic and Sensitive Plant Species

Abstract

A better understanding of the mechanisms influencing compositional changes in understory plant communities is crucial to protect temperate forests against global change stressors, including anthropogenic disturbances and invasion by exotic species. We assessed changes in species composition after a 14-year period in 20 rural temperate forest stands located in the northern hardwood biome of eastern Canada. We identified species that underwent the largest changes in relative occurrence during that period and assessed the influence of biotic and anthropogenic filters on the trajectory of those understory communities. We found small but significant compositional changes after 14 years, mostly related to a decrease in species diversity in the younger forest stands originating from abandoned pasture. The largest occurrence gains for understory species were observed in these stands, but also in stands with lower understory community diversity. Understory species occurrence losses could not be linked to any of the biotic and anthropogenic filters considered in this study presumably because they were difficult to isolate from secondary successional dynamics. Shade tolerant exotic species showed a small but significant increase in relative occurrence over 14 years, with notable gains in only a few stands. We observed generalized occurrence gains for sensitive spring geophytes during the same period throughout most sites. Overall, the understory vegetation community of rural temperate forests in this region was found to be relatively stable and seems to retain its potential for natural recovery after disturbance.