Exploring the role of anthropogenic and biophysical factors in forest dynamics of native and invasive legumes in St. John, US Virgin Islands

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Matt, Jesse Mansfield
Major Professor
Zimmerman, Emily
Beck, William
Nonnecke, Gail
Wolter, Peter
Committee Member
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Natural Resource Ecology and Management
More than 90% of global tropical dry forests are degraded in the Caribbean region, including on the island of St. John in the US Virgin Islands (USVI). On the island of St. John, the total area of dry forest has steadily decreased since 2009, but the relative abundance of nonnative leguminous trees such as Leucaena leucocephala (common name, Tantan) has increased. In turn, the frequency and relative abundance of native legumes such as Parasenegalia muricata (common name, Spineless Wattle) have decreased in the same period. Plant succession is often shaped by disturbance. Land-use history such as deforestation for agricultural purposes is a common anthropogenic disturbance in tropical dry forests. Plants also cause disturbance via colonization and influencing soil nutrient profiles in a process known as plant-soil feedback (PSF). Both land-use history and plant-soil feedback may be responsible for shaping native and nonnative forests in places such as St. John, USVI. Disturbed and cleared forests typically allow for the succession of shade-intolerant leguminous trees. Many of these trees have high seed viability and the ability to increase soil nutrients via litterfall and nitrogen fixation, which can lead to rapid spread and the formation of monodominant stands. Approximately 63% of the forests on the island of St. John, US Virgin Islands (USVI), are dry tropical forests. Many of St. John’s forests were disturbed historically by plantation agriculture and livestock grazing in the 18th and 20th centuries, respectively. Despite protection by the establishment of the Virgin Islands National Park (VINP) in 1956, forest cover has declined since 1994, while the relative abundance of nonnative leguminous trees such as Leucaena leucocephala, has increased. In two separate studies, this thesis explored how anthropogenic and ecological factors affected forest composition and nonnative leguminous shrubs in dry forests on St. John, USVI. To assess the role of anthropogenic factors on forest composition, historical and present-day land-use data and infrastructure data (i.e., roads and trails) were obtained, and data from forest inventory plots were collected in 2021 and 2022. One- and two-way ANOVAs were used to evaluate these relationships. Results suggested that the presence of nonnative L. leucocephala is significantly correlated with forests that were disturbed by grazing in the 1940s (P < 0.01), and that the presence of native legumes, such as P. muricata, was significantly correlated with land that remained forested both in 1780 and 1947 (P < 0.01). Both L. leucocephala and P. muricata were significantly associated with the presence of historical and present roads (P < 0.01), though those relationships varied. To assess potential ecological factors associated with forest composition, specifically of a nonnative legume, L. leucocephala, and a native legume, P. muricata, a PSF experiment was conducted on St. John using locally collected seed and soil conditioned by local native and nonnative trees. Results indicated that germination of L. leucocephala was significantly lower (P < 0.001) in soil conditioned by P. muricata relative to soils conditioned by L. leucocephala. This suggests that P. muricata may inhibit the growth of L. leucocephala via PSF on Southgate-rock soils. However, while soils conditioned by P. muricata versus those conditioned by L. leucocephala trended differently in soil tests for acidity, electrical conductivity, and total active fungi/bacteria, those tests were not statistically significant. This suggests that further research is needed to understand the mechanisms driving the differences in germination observed in this experiment. Taken cumulatively, this research suggests that P. muricata is highly sensitive to land-use disturbance. However, where undisturbed stands of P. muricata exist, they may inhibit the encroachment of nonnative L. leucocephala. With further research this may be a valuable tool when managing the spread of invasive species.