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Changes in Wetland Forest Structure, Basal Growth, and Composition across a Tidal Gradient

Abstract Because of their proximity to oceanic waters, freshwater tidal forests are susceptible to impacts from future climate change and sea level rise. These wetlands are historically understudied and we conducted our study to improve the understanding of structural changes in forested wetlands as they become tidally influenced. Using 20 forested stands across a tidal gradient on the lower Apalachicola River, Florida we examined changes in forest structure (tree density, size classes), basal growth, sapling/shrub cover and richness, understory cover/richness, and coarse woody debris. Tidal wetlands had greater tree (>2.5 cm DBH) density compared to nontidal forests (1446 ± 159 and 962 ± 100 stems ha−1, respectively) and more small trees (2.5 to 5.0 cm DBH) (699 and 359 stems ha−1, respectively). Over a 3 y period (2007 to 2010), basal area increment ranged from −0.07 to 0.30 m2 ha−1 y−1 for tidal wetlands and 0.07 to 0.55 m2 ha−1 y−1 for nontidal wetlands. Both forests experienced tree mortality during the study that influenced basal area. Tidal wetland tree mortality appeared to result from saltwater intrusion while several nontidal forest plots were affected by downed trees during a winter wind storm. Mean sapling/shrub (<2.5 cm DBH) density and richness in tidal forests were more than twice that estimated for nontidal forests. Mean herbaceous cover in tidal wetlands (72 ± 4%) was significantly higher than nontidal forests (19 ± 4%) and dominated by perennial herbaceous plants as opposed to nontidal wetlands that were often dominated by tree seedlings. Mean, large, coarse, woody, debris biomass (>7.62 cm diameter) was significantly lower in tidal wetlands compared to nontidal wetlands. Results demonstrated that riparian forests can have large and sudden shifts in stand structure and species composition related to tidal influences where rivers approach the coast.