Diversity begets more diversity: Species-rich understories support diverse tree seedling banks at fine-scales across a broad range of environmental conditions

Martin Dovciak and Jordon Tourville, Department of Environmental Biology, State University of New York College of Environmental Science and Forestry, Syracuse, NY, Jay W. Wason, School of Forest Resources, University of Maine, Orono, ME, Monica B. Berdugo, Ecological Plant Geography, Faculty of Geography, University of Marburg, Marburg, Germany

Background/Question/Methods
Biodiversity loss is one of the most pervasive aspects of global environmental change. In forest ecosystems, diverse tree seedling banks indicate resilience to disturbance and environmental change (e.g., climate change). While previous work suggested that diverse understories in temperate deciduous forests tend to support species-rich and abundant tree seedling banks, it is not clear if this relationship holds across broad gradients in environmental conditions and forest composition. To address this knowledge gap, we studied the effects of understory diversity and environmental conditions on tree seedlings across a range of forest types (from low-elevation broadleaf to high-elevation conifer forests) and canopy conditions (from closed canopy to large gaps) across four states in the northeastern United States. We collected vegetation and abiotic data along elevational gradients (500 to 1000 m above sea level) on 1,710 plots (each 1x1m) distributed across 57 sites on ten study mountains. We used generalized linear models to relate tree seedling richness (for seedlings <0.5 m tall) to understory diversity and cover (of both herbs and shrubs, pooled and separately), forest canopy openness, and seedling substrate along the elevational gradients reflecting changes in forest composition, soil, and climate.
Results/Conclusions
Our study suggests that diverse tree seedling banks were positively associated with species-rich understory communities even across broad environmental gradients. Tree seedling species richness (at fine-scales, 1x1m) was positively associated with species richness of other understory vascular plants (both shrubs and herbs), moss cover, and canopy openness. On the other hand, tree seedling species richness was negatively associated with understory vascular plant cover and elevation. Besides these main effects, we observed that several interactions also influenced tree seedling species richness. Our models suggest that high moss cover compensated for the decline in seedling species richness associated with high elevations and high understory vascular plant cover. In addition, high understory vascular plant cover negated the positive effects of canopy gaps, while low understory cover enhanced them. Our results suggest that the observed positive relationship between understory vascular species richness and tree seedling richness is of a general nature and holds at fine scales across large forest landscapes encompassing various forest types. Since species-rich forest understories seem to support more diverse tree seedling banks than species-poor ones do, long-term forest conservation and management may be enhanced by conserving forest understory plant diversity.