Impacts of clear-cut logging on floodplain sediment characteristics in a southeastern Coastal Plain bottomland hardwood forest

Background/Question/Methods

Floodplain forests once characterized most major alluvial river valleys in the southeastern U.S. Due to widespread clearing for agriculture and timber harvest, these bottomland hardwood forests now cover a fraction of their potential extent. Management has become important for maintaining ecologically functioning floodplains and successful mitigation or restoration of degraded and fragmented floodplain forests rests on understanding the processes that shape these extensively modified ecosystems. Large river floodplains are functionally connected to river sources via flood pulses. In particular, periodic flooding supplies sediment and nutrients which are important for structuring floodplain ecosystem dynamics. The objective of this project was to test the hypothesis that clear-cut logging has had lasting negative influences on floodplain nutrient levels. Soil samples were collected at clear-cut and intact floodplain forest sites with varying hydrologic exposures. These samples were processed for macronutrients, micronutrients, organic matter, pH, CEC, and soil texture. The data were analyzed using principal components analysis (PCA) to test for correlation between nutrient levels and biotic and abiotic environmental characteristics (i.e., hydrologic exposure, soil texture, vegetation community type, and logging history) at different floodplain positions.  

Results/Conclusions

PCA results demonstrated that soils from intact forest sites with more flood exposure grouped with higher macro- and micro-nutrient levels, more diverse nutrient compositions, and a dominant cypress-tupelo vegetation type (i.e., an assemblage with high tolerance for flooding). Sites with most hydrologic exposure were structured along a dominant nutrient gradient characterized by high phosphorus. Phosphorus attaches to eroding mineral particles and research shows phosphorus levels increasing with accumulating mineral soils. Sites with high phosphorus correlated with dominant cypress-tupelo forest type, which is typical of floodplain drainage systems because of its tolerance for frequently flooded conditions. Logged floodplain sites grouped in ordination space with less exposure to flooding, lower nutrients, and uniform nutrient compositions. These results suggest that flooding is the primary driver of nutrient replenishment, but suggest that logging could have had a significant negative and homogenizing effect on nutrient levels. To verify possible lasting impacts of logging, clear-cut sites were removed and the data were re-ordinated. These results preserved the correlation of high flood exposure and higher nutrient levels, but also reinforced that logging was focused on sites with less flood exposure, thereby complicating interpretations of logging impacts.