COS 57-2 - Local, spatial, and temporal variability in vegetation species composition in relation to environmental heterogeneity in the Everglades ecosystem

Abstract: The ridge and slough (R&S) landscape in the Everglades ecosystem comprises a mosaic of flow-oriented sawgrass ridges, sloughs, and tree islands. The unique landscape patterning is crucial for ecosystem functioning and ecological, economic, recreational, and cultural habitat values. However, the ecosystem has been modified and is threatened by managed-induced changes in the hydrologic regime and other anthropogenic disturbances. Restoration efforts are underway aimed to reduce and reverse ecosystem degradation. Past research in this ecosystem has evaluated the extent to which surface patterning is regulated and the factors associated with species composition dynamics. However, most works focus on a subset of the ecosystem, hindering our understanding of system-wide patterns and processes at local and landscape scales.
In this study, we assessed the variation in hydrology-based microtopographic patterns and vegetation species composition. We aimed to evaluate how species composition dynamics vary at local and landscape scales in association with management practice, fire frequency, and hydrology (long-term water depth). We conducted our study over ten water years (2009-2020), and during those years, we sampled twice, 2009-2015 (cycle 1, hereafter) and 2015-2020 (cycle 2, hereafter). We recorded species cover, water depth, and soil depth in fifty-eight 2×5 km permanent sampling units (PSUs), each containing up to 135 1×1 m quadrat. We calculated fire frequency using the data from Everglades National Park and Fish and Wildlife Conservation. We used model-based clustering techniques to examine the presence of bimodal distributions in elevation values as a proxy for conserved patterning. We analyzed data at local (primary sampling unit, PSU) and landscape (management blocks) levels to understand whether the association between landscape patterning, species composition dynamics, and environmental variables operate similarly or differently. We also compared the species composition indices at the PSU level between cycle 1 and cycle 2 to assess changes due to the ongoing restoration efforts and site conditions.
Our results suggest variability in maintenance and loss of elevation bimodal distribution among PSUs across the landscape. Species composition dissimilarity strongly correlated with hydrology but varied among PSUs. While the vegetation community became more dissimilar (based on the distance between vegetation clusters) in some PSUs, it became homogenized in others. The relationship between species composition and environmental variables also varied between management blocks. This study provides baseline measures related to the association between vegetation communities and microtopographic structures valuable for detecting changes/trends in the local and landscape patterns due to water management operations and restoration initiatives.