Sediment accretion rates and radial growth in natural levee and backswamp riparian forests in southwestern Alabama, USA

Abstract

Riparian forested wetlands improve downstream water quality by trapping suspended sediment from adjacent waterways. Waters that transport sediment and nutrients into adjacent wetlands also create wet and dry hydrologic periods and thus, have the potential to impact site productivity. In this study, we used a dendrogeomorphic technique with green ash (Fraxinus pennsylvanica Marsh.) to estimate sediment accretion for two time periods (1881–2012 and 1987–2012) along a natural levee (35m from river) and backswamp (75m from river) and identified the influence of hydrology and climate on radial growth in green ash and water tupelo (Nyssa aquatica L.) along the Tensaw River in southwestern Alabama. We detected significantly higher sediment accretion rates for the 1987–2012 time period along the natural levee (p=0.00; 1.6cmyr−1) and backswamp (p=0.03; 1.2cmyr−1) than for the 1881 to 2012 period (0.4 and 0.5cmyr−1). Using previously measured (2010) soil bulk density for the site, estimated mass of sediment trapped per unit area ranged from 55–135Mgha−1yr−1 for the 1987–2012 period and 17–61Mgha−1yr−1 for the 1881–2012 period. We identified positive, significant correlations between green ash radial growth and the number of days the backswamp was flooded (1.4m stage), days the Tensaw River was at bankfull (2.1m), and average daily river stage during the overall growing season (April to August) and for the month of April. Green ash radial growth also illustrated a positive, significant response to April precipitation totals demonstrating the overall role of moisture availability just prior to the onset of xylem formation. Green ash trees along the natural levee and backswamp were more responsive to hydrology and climate than water tupelo trees located further in the backswamp, illustrating the potential resistance of water tupelo to perturbations. This study illustrates the important role forested wetlands play in improving water quality through quantification of sediment accretion rates and the potential impact that introduced disturbances (i.e., forest harvest-related disturbances) can have on ecosystem services.