Plant invasion in small forests: regeneration and restoration of native understory species
Date
2022
Authors
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Publisher
University of Delaware
Abstract
Invasive species and habitat loss due to fragmentation threaten biodiversity and ecosystems worldwide. Urban areas are at the intersection of these threats, where natural ecosystems are altered and incorporated into expanding urban environments to meet the demands of population migration. Urban forests often contain problematic invasive plant species and are highly heterogeneous, resulting in changes to plant communities and ecosystem functions that may ultimately affect future forest regeneration. Specifically, the invasive shrub Rosa multiflora (multiflora rose, or “rose”) has become the dominant understory species in northeastern United States forests, where this research was conducted. This dissertation focuses on how plant communities in small urban forests are influenced by invasion, and how they might respond to management intervention. In particular, I investigated how herbaceous and understory plant communities of small urban forests are responding to invasion over two growing seasons. Additionally, I tested several management and restoration strategies (invasive plant removal, removal followed by native seed addition, and removal plus native seed and mulched invasive stem addition) across three levels of rose invasion (low, medium, and high) for their effectiveness at preventing reinvasion, promoting native plant diversity, and slowing soil nitrogen cycling in these forests. I found the herbaceous forest layer contained the most native and least non-native species, though richness declined in more than half of sites. The ground cover layer was dominated by invasive species, and the seedling and sapling layers were dominated by rose, which became more abundant over time. Native shrub abundance declined, and three native tree and shrub species were no longer present in the largest size class (DBH > 2.54 cm and height ≥ 1 m), nor were they present in the seedling or sapling layers. The spread of invasive plants within these forests in two years demonstrates the need to conduct empirically tested management strategies to diminish plant invasion threats to native species. My experimental removal research provided evidence that restoration outcomes were dependent on the amount of rose invasion in forests prior to management implementation. Invasive plant removal alone increased richness, native floristic quality (Native Floristic Quality Assessment Index, FQAIN), and native shrub abundance under medium rose invasion pressure, though addition of a native seed mix effectively increased species richness and FQAIN across all levels of rose invasion. N mineralization was driven by nitrification under low and high rose invasion but by ammonification under medium rose invasion. Moreover, addition of native seed mix and mulched woody stems reduced nitrification under the medium invasion scenario. However, plant community responses to these management strategies remain complex, and the narrow window of investigation in these studies (2-3 years) may not be long enough to capture changes occurring more slowly within these forests. Together, these results demonstrate that urban forest are becoming more invaded over time, and native woody plants and herbs in particular are under threat. The displacement of native woody plants from seedling and sapling layers, and loss of larger native trees and shrubs will undoubtedly affect the ability of small forests to regenerate native overstory canopies. Thus, there is a growing need to study long-term invasion and urbanization dynamics influencing small, urban/suburban forests, and manage invasive species before they pose greater threats to future forest ecosystems.
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Keywords
Forest restoration, Invasive species, Multiflora rose, N cycling, Plant community ecology, Urban forests