Soil moisture versus depth-to-water-level: Which is better for predicting plant composition in a restored floodplain wetland?

Depth-to-water-level (DTWL) measurements in shallow groundwater piezometers are commonly used to develop relationships between wetland plant composition and the available water regime. Such relationships can provide useful predictions of plant composition for land managers under potential changing conditions (e.g., climate change, land use change, environmental flow releases, groundwater pumping) when combined with a hydrologic model. These analyses, however, implicitly use DTWL as a surrogate for the water regime within the root zone, which is experienced by plants. Bi-weekly field measurements of both variables (DTWL and SM) were made at a restored floodplain wetland over the 2009 and 2010 growing seasons. Plant species composition and percent cover were also sampled at the same locations (N=62). Nonmetric multidimensional scaling (NMS) and nonparametric multiplicative regression (NPMR) were used to compare how effectively the two hydrologic metrics explain the overall plant community ordination space (NMS) and predict the probability of presence of certain dominant species (NPMR). Both statistical techniques revealed that SM was more successful than DTWL at explaining the overall plant community structure and predicting plant composition for certain dominant species. The predictive modeling results also suggest that hydrologic extremes on a species-specific basis are effective predictors of plant composition. Field evidence based on soil coring and geophysical imaging suggests that the reason for the discrepancy in the efficacy of the two hydrologic variables is the presence of a confining silt-clay layer in some areas of the floodplain that partially decouples soil moisture in the root zone from groundwater in a deeper gravel layer. While DTWL may be adequate as a predictive variable in vegetation modeling at some sites, SM is likely to be more valuable - especially at sites where soil moisture and groundwater are decoupled - at developing robust relationships between the water regime and vegetation.