Plant Invasion in Forests Understories: A Native Community Perspective and Implications for Management

Abstract

Forest ecosystems provide humans with a wide array of ecosystem services, e.g., water, clean air, extreme weather mitigation, flood and landslide prevention, products, and aesthetic, spiritual, and recreational values. Still, in the face of global change, these systems are at increasing risk of losing their functionality, with one major threat coming from the introduction of harmful invasive species. As a result, numerous resources, in research and management, have been allocated towards studying their ecology and promoting their control. These efforts, however, are rarely successful, and positive outcomes only last for a short period of time before the forest understories are invaded again. In an effort to address these shortcomings, my research deviates from the conventional perspective on management and research of biological invasions by switching the focus from the invader to the invaded community. By focusing on the invaded community, research can investigate means of ensuring this community resists and recovers from invasion instead of attempting to predict which species will be the next invader. I adopted a three-pronged approach to investigate the processes by which ecosystem functionality changes with invasion and to assess how management could enhance the native community's resistance to and recovery from invasion. First, I synthesized the existing knowledge on the mechanisms underlying shrub invasion in forest understories across the globe and their impacts on the native community by using a meta-analytical approach. Despite no differences in invasive species performance across mechanisms of invasion, the native community was significantly more impacted when the invasion was driven by low biotic resistance. Additionally, the results of this chapter underscore the importance of a wide representation of resource-use strategies and competitive abilities among the native community to resist shrub invasion. Second, I carried out a four-year-long field experiment in southeast Michigan forest understories to investigate whether priority effects, i.e., the ability to arrive first, could be a successful mechanism of native community recovery in terms of coverage percentage after invasive species removal. I also assessed which characteristics of the native community led to faster native coverage recovery rates. Contrary to expectations, priority effects were not the primary mechanism driving community assembly post-invasive removal. However, rich native understory communities displaying higher values of acquisitive leaf traits related to rapid carbon processing strategies showed the fastest recovery rates. Third, I collected observational data on the plant community and composition of invaded forest understories at a local forest in Michigan. There, I compared the role of increasing levels of invasion, in relation to leaf trait distributions which I used a proxy to understand potential changes in ecosystem functions. The results revealed that invasion led to a significant increase in community-level leaf nitrogen. The native community leaf trait values, however, remained unchanged across the entire range of invasion levels. These findings have important management implications. Given the complementary effect of invasion rather than the displacement of trait values and functions, the removal of invasives will likely restore function to levels similar to pre-invasion conditions. My research takes a novel approach to the study of biological invasions by focusing on the native community rather than on the species invading, which provides further and unique insights into the field of invasion ecology. My research additionally addresses how we can use ecological principles to make native forests more resistant to invasive species.