Loss of spatial variability and complexity in headwater wetland complexes

Jessica Beth Moon, Penn State University

October 21, 2009 @ 01:20 pm to 02:10 pm

10 Tyson

Natural wetlands tend to exhibit high levels of spatial and temporal heterogeneity, particularly in open systems such as headwater complexes. In these systems heterogeneity regulates important ecosystem functions. For example, the combination of microtopography and seasonal changes in hydrology plays a key role in creating both aerobic and anaerobic zones for microbial processes such as denitrification. It also creates a mosaic of soil patches with substrates that differ structurally, hydrologically, and chemically, leading to high levels of floral and faunal diversity. In recent years, however, investigations have shown that chronic disturbances, such as surrounding land cover change, can lead to hydrologic modification, accelerated sedimentation, and shifts in vegetative and invertebrate community composition. Less is understood about how these disturbances affect heterogeneity in soils and the linkages between soil and vegetation. The objective of this study was to characterize variability and complexity in the soil and litter microhabitats at two spatial scales within eight headwater wetlands--four that are surrounded by a matrix of anthropogenic land cover classes (i.e., disturbed sites) and four located in intact forested landscapes (i.e., forested sites). Soil and litter samples were collected from forty-five 1m2 sampling blocks within each 1600 m2 site on four sampling dates between August of 2006 and 2007. Additional soil cores were collected at 32 random points within six 1m2 plots in June of 2008. Results show that disturbed sites lack a significant soil organic horizon, while forested sites have high variability in the depth of their organic horizons. Site-level variability in soil moisture and soil organic matter content was higher in forested wetlands, while site level variability in pH was not significantly different between disturbances levels. This result was consistent across sampling dates and spatial scales.Spatial complexity was higher for all three soil parameters in forested wetlands, but soil organic matter content was more spatially complex only when the organic horizon was taken into account. That is, mineral horizon soils did not differ between disturbance levels Finally, stronger correlations were found between soil habitat parameters and litter quality measures (i.e., TC and TN) at forested sites, suggesting that a disconnection might occur between soils and associate litter at high levels of disturbance. Over time, the homogenization of these wetland landscapes could lead to a loss in their resilience and, consequently, a loss in key ecosystem functions.