Floodplain Vegetation and Soil Dynamics of the Alabama Piedmont Across a Gradient of Stream Channel Incision

Abstract

Erosion resulting from past land use in cultivated uplands of the Alabama Piedmont has contributed to variable stream conditions throughout the region, with high degrees of channel incision often typifying degraded riparian corridors. Reduced flood frequency and lowered water tables characterize incised headwater streams of the region, thus reducing stream and floodplain function. The objectives of this study were twofold: (i) determine the effects of channel incision upon riparian vegetation assemblages and (ii) quantify the degree to which the aggradation of erosional material that inundated the region’s floodplains has affected soil properties. Stream channel incision was described using bank height ratio (BHR), defined as the ratio of bankfull depth to stream bank height. Shallow groundwater wells were installed at all study sites to examine the effect of channel incision upon floodplain water tables. Study sites were selected across a gradient of BHR values (1.0-5.2); ten sites were chosen for the vegetation component of the study, six sites for the soils component. Results of the floristic component of this study indicate stream incision is correlated with a shift of community type in the ground flora stratum, from wetland-adapted species at low degrees of incision to plants typical of upland habitats at incised sites. Canonical Correspondence Analysis (CCA) was performed to evaluate if select environmental and hydrologic variables were significantly related (α=0.05) to herbaceous species composition. Results of the CCA indicate that species composition of the ground flora layer was significantly related to median groundwater depth, tree stem density, and BHR. Species diversity of the herbaceous layer also showed a distinct unimodal response to the incision gradient, suggesting that diversity may be greatest in riparian areas that receive intermediate amounts of flooding. The results of this study indicate that lowered water table levels and decreased soil moisture due to channel incision are driving compositional changes of herbaceous/ground flora in floodplains of the Alabama Piedmont. Soil organic carbon (SOC), percent nitrogen (%N), and bulk density (BD) were analyzed to determine edaphic differences between reference and incised floodplain soils. At each study site, soils were sampled from three transects established perpendicular to stream flow in transects previously delineated for vegetation sampling. Samples were taken from the following depths: 0-5 cm, 0-15 cm, 15-30 cm, 30-45 cm. Significant differences in SOC, %N, and BD were observed between stream types, with increased amounts of SOC and %N, 58% and 39%, respectively, found at reference sites. In addition, BD of the floodplain soils was significantly greater at incised sites. Significant differences in groundwater levels between stream type were also observed during the study period (p value < 0.0001). The results of this study suggest that alluviation and subsequent channel incision has altered floodplain soil characteristics in the AL Piedmont, potentially limiting the ability of these soils to sequester C in the upper 45 cm. These findings, combined with the results of the vegetation component of this study, suggest that channel alterations due to past erosion in Piedmont uplands has led to significant changes in floodplain vegetation and soils relative to reference conditions.