Tree seedling banks in forest understories affect future forest composition and they are thus one of the key components of sustainable and healthy forest ecosystems. However, tree seedlings compete and otherwise interact with other understory plants. Niche theory and resource partitioning suggest that species-rich understories should use resources more fully and thus be potentially more resistant to colonization by tree seedlings than species-poor understories. On the other hand, more diverse understories may reflect greater heterogeneity of resources, which should positively affect the abundance and richness of forest tree seedlings. Non-native species invasions provided insights into these diversity-invasibility questions, but little work has been done to link tree seedling colonization of forest understories to species richness of resident understory plant communities. We surveyed forest understories on 750 subplots (1x1 m in size) spread across 50 plots (20 x 50 m, 15 subplots per plot) in northern hardwood forests in the Adirondack Mountains of the northeastern United States. We used generalized linear mixed effects models (GLMMs) with a Poisson distribution to tease apart the effects of understory diversity, soil conditions, and other factors (e.g., canopy openness) on tree seedling richness and abundance.
Results/Conclusions
The most consistent ecological drivers of tree seedling species richness and abundance on subplot scales were understory plant cover (not including tree seedling cover) and richness of herbaceous species. The best and most parsimonious models contained both of these predictor variables together, with negative effects of understory vascular cover and positive effects of herbaceous richness, and these relationships held for both of the examined tree seedling size classes (5-20 cm and 20-100 cm tall). Models for seedling abundance and richness were nearly identical. Some models contained additional variables (e.g., shrub richness, canopy openness), but these tended to be much less important. Our study is one of the first to investigate the effects of understory plant diversity on the diversity and abundance of forest tree seedling banks. Our results suggest that species-rich understory plant communities may be conducive to colonization by tree seedlings as they were associated with more diverse and more abundant forest regeneration than species-poor understory communities. Thus, long-term forest management and conservation may require conserving non-woody plant diversity in forest understories.