Drivers of understory plant diversity and composition in managed temperate second-growth forests.

다국어 초록 (Multilingual Abstract)

Large forest areas across eastern North America---and increasingly the world---are second-growth, arising after agricultural abandonment or other intense land-uses. In addition, many of these second-growth forests are actively managed for timber. Unfortunately, the long-term consequences of this agricultural legacy, combined with the additional effects of subsequent forest harvesting on plant diversity and composition are unknown. The majority of plant diversity in these forests is found in the understory, and its structure and composition directly influences ecosystem function, animal diversity, and forest development. There is an increasing acceptance of the importance of this stratum for conservation and forest management. Understanding how forest understories vary across topographic and soil types and how site conditions interact with forest harvesting effects and other land-use is critical to biodiversity conservation in these second-growth forests.
My dissertation work examines understory plant community response to human land use through a series of analytic, observational, and manipulative experiments at varying scales. In Chapter 1, I used meta-analysis to test for overall effects and temporal patterns of understory species richness in response to forest harvesting across the entire temperate forest biome and found that no consistent effect of forest harvesting on understory plant species richness. The analysis supported the importance of site and harvest specific characteristics in defining understory plant diversity response to forest harvesting. In Chapters 2 and 3, I examined the relative importance of these site and harvest specific characteristics on understory community response following forest harvesting. By tracking understory plant communities for 15 years following forest harvesting across a series of experimental linear gaps, I was able to isolate the relative importance of gap position (light), ground-disturbance, and site quality on understory diversity and compositional patterns following forest harvesting. Again, results highlighted site specific responses to forest harvesting. Individual soil types retained compositional difference following forest harvesting and showed varying temporal capacity to recover. In Chapter 4, I conducted an observational study across a varied physiographic, soil and disturbance landscape to explore patterns of herb layer diversity and identify the most important environmental drivers influencing herb-layer diversity and compositional patterns in a second-growth managed forest in southern New England. I conducted a survey of vascular plants using 420 fixed area plots across the 3213-hectare Yale-Myers forest and paired the floristic data with over 20 direct environmental measurements. I found that managed second-growth forests can house a significant amount of understory plant diversity, and that patterns are driven more by niche partitioning than dispersal limitation. Edaphic factors were the most influential on species diversity and composition, but environmental controls had greater explanatory power on species richness than composition. In my final Chapter 5, I explore the unmeasured drivers of understory plant composition. A qualitative review of the biotic interactions between woodland herbs and other organisms, I show that these relationships and interactions have the potential to act as strongly as niche factors as commonly measured abiotic factors. Further, that the actions and populations of these biotic drivers may covary with abiotic resource gradients confounding our understanding of the drivers of these understory plant communities.
My dissertation demonstrates the importance of site-specific variables in driving understory plant community dynamics, an assumption commonly encountered in the literature. Further, it highlights important avenues for future research, specifically the interaction between biotic and abiotic variables in driving temperate forest diversity patterns.