Successional Trajectories of Constructed Forested Wetlands


This research addresses the development and application of the trajectory approach to assessing adequate site progress toward ecological goals for created forested wetlands on phosphate mined land. I address natural variation in these processes using both space-for-time substitution and initiating the formulation of trajectories for a number of individual wetlands. The main objectives within the scope of this research are to: (1) identify parameters exhibiting clear and statistically significant trajectories; (2) calculate 95% confidence intervals around the trajectories; and (3) estimate sampling intensity necessary to be confident in the site mean for each indicator. This work differs from previous performance trajectory efforts by comparing the current condition of a created wetland to the anticipated condition based on developmental trends of wetlands created under similar conditions. Statistically significant trajectories (defined as r2 > 0.75 and p < 0.10) were identified for four canopy parameters: height (r 2 = 0.81; p < 0.05); diameter at breast height (r2 = 0.80; p < 0.05); stand basal area (r2 = 0.75; p < 0.05); and canopy cover (r2 = 0.77; p < 0.05). Ilex cassine, a subcanopy species, demonstrated increased stand basal area with time (r2 = 0.75; p < 0.05). No statistically significant trends, as defined by this study, were found in shrubs or understory across all wetlands types combined. Several understory parameters exhibited statistically significant trends when wetlands were grouped by hydrology. In stream floodplain wetlands, vine species richness increased with time (r2 = 0.90; p < 0.05); in depressional wetlands, understory species richness (r2 = 0.76; p < 0.05) and Shannon-Weaver diversity (r2 = 0.94; p < 0.05) declined with time. A decreasing temporal trend in grass, sedge and rush species richness (r2 = 0.76; p < 0.05) was detected in depressional wetlands. Moderately significant increasing trends (0.30 < r2 < 0.75; p < 0.05) were identified in subcanopy diameter at breast height and subcanopy basal area (cm2 m-2). Moderately significant decreasing trends were noted for understory species richness, understory species diversity and for a range of plant functional groups: grass, sedge and rush species richness; herbaceous species richness; the ratio of woody to herbaceous species in the understory, and the percent of the understory comprised of an herbaceous, woody, and vine functional groups. These data represent successful restoration to date and can therefore be used to assess whether future projects are progressing at a minimum, no worse and, ideally, better than was done in the past.

Mark T. Brown and Susan M. Carstenn, University of Florida Center for Wetlands. April 2009.