Tree structure, diversity and spatial patterns were quantified in the highly urbanized, tropical, coastal urban forest of Miami-Dade County (Florida, U.S.A.) with randomly located 0.04 ha plots. We obtained information on urban forests under different management and land tenure regimes in the 1273 km2 study area, which encompassed the urbanized portion of a major metropolitan area adjacent to ecologically important areas (e.g., Everglades National Park) as well as commercially important agricultural areas. We were interested in describing how woody invasive species are distributed across urban landscapes to understand the potentially damaging spread of these species into sensitive areas, as well as in quantifying the ecosystem services provided by such species. Using a spatially-explicit general linear mixed effects model, we tested hypotheses about structure and diversity across this urban ecosystem. Variogram analysis allowed us to investigate spatial patterns. Forest structure was described by basal area, tree height, crown area, and a crown light exposure index; we quantified diversity using Shannon and Simpson indices, and Evenness, and Species Importance values. Leaf area and air pollution values were estimated by the USDA Forest Service’s Urban Forest Effects model, which provided insights into the ecosystem services of urban trees.
Results/Conclusions
Our sample consisted of 90 plots in private residential, 28 plots in public non-residential, and 21 in private non-residential areas; 36 plots contained Category I or II invasive woody plants. We measured a total of 1087 trees in 107 species and 85 genera; 111 trees were categorized as invasive, representing 16 species and 16 genera. Most of the sampled trees were located in residential areas, where structural characteristics and species diversity were statistically different than those in non-residential land tenure regimes. Patterns of urban forest structure differed across the study area; tropical, broadleaf evergreen trees were the most common growth form and 10% of all trees were exotic and invasive tree species. We analyzed the spatial distribution of invasive species, and discuss the contributions these species make in terms of ecosystem services. Results indicate that, Malaleuca quinquenervia – a highly invasive tree – contributes significantly to air pollution removal and carbon sequestration due to its size, abundance, contribution to total leaf area, and low maintenance requirements. Since urban forests influence a substantial portion of the world’s population and contain a large proportion of exotic invasive species, we discuss a sustainable approach to management and maintenance of their ecosystem services.
