Effect of legacy on hydric forest structure in a subtropical urban watershed

Background/Question/Methods

Urban watersheds are heterogeneous, often including complex forest structure and dynamics. Our understanding of the importance of legacy in hydric forest systems is lacking for subtropical urban watersheds. This study focused on hydric sites in the Hillsborough River watershed within the greater Tampa Bay Watershed, Florida. The area was divided into a grid of 0.17 m² hexagons with a random sample point selected in each. A total of 500 plots, each 400 m², were inventoried. Of these, 83 were identified as hydric (within 15 m of a hydric feature such as a stream, river, lake, or wetland). For each plot, diameter at breast height (dbh) for trees ≥ 2.5 cm dbh was measured by species. Based on aerial photography and site visits, each plot was assigned a legacy class—remnant (forest, pre-1948), emergent (forest, post 1948), and managed (actively managed grass on the plot). A hierarchical agglomerative cluster analysis identified community types within a legacy class. Average plot inventory values by legacy classes were compared using one-way AOV (α = .05).

Results/Conclusions

Remnant (43 plots) had the highest species richness (41 species) and only one non-native species. By comparison, managed (18 plots) had the lowest species richness (33 species) and highest richness of non-native species (11). For basal area, remnant had 49 m²/ha while emergent and managed had 28 and 8 m²/ha respectively. Remnant and emergent plots had similar densities for trees ≤ 30 cm dbh, (802 and 820 stems/ha, respectively), whereas managed plots had only 118 stems/ha. For trees > 30 cm dbh, remnant plots had a significantly higher density (196 stems/ha) than emergent (100 stems/ha) and managed (40 stems/ha). Remnant plots were composed of six, natural community types as recognized by the Florida Natural Areas Inventory and no novel communities. Emergent plots, on the other hand, were composed of four natural community types and one novel community, and managed plots were composed of only novel communities. These results suggest that legacy plays a key role in the structure and composition of hydric forest systems in a subtropical-urban watershed. Further, because structure and composition change within novel communities, a different set or quantity of ecosystem services may be derived from those communities as compared to less disturbed community types.