Patterns in forest soil chemistry across Eastern U.S. national parks

Background/Question/Methods

Since 2006, the Northeast Temperate Network (NETN) and Eastern Rivers and Mountains Network (ERMN) have been sampling forest soils in permanent plots as part of long-term forest monitoring programs for the National Park Service Inventory and Monitoring Program. In addition to collecting soil samples in each plot, both networks sample a suite of metrics that assess the ecological integrity of forest resources, with special attention to structure, composition and function of forested systems. By 2012, 695 plots have been sampled across 16 national parks from West Virginia to Maine, totaling over 400 O horizon and 500 A horizon soil samples collected. These parks represent a diverse range of geology, land-use history, forest type and stressors.  Relating these factors and indicators of forest condition to patterns in soil chemistry is the focus of our analysis.

Non-metric Multidimensional Scaling (NMDS) ordinations were used to examine major gradients in forest soil chemistry across network parks. To examine potential relationships between soil chemistry and other gradients, a matrix of environmental variables was assembled that included plot level information such as elevation, aspect, topographic position, slope, location (X and Y coordinates), vegetation type, bedrock geology, native plant species richness, tree regeneration, and average cover of invasive plant species. Additional variables compiled at the park level were average pH, acid neutralizing capacity and nitrate concentration of park streams, and total N and S atmospheric deposition rates. NMDS ordinations were performed on 1) the full soil chemical dataset, 2) by horizon (O and A horizons), and 3) by network (ERMN and NETN).

Results/Conclusions

Soil horizon was the first major gradient in the ordinations, with a clear separation between O and A horizon samples. After the influence of soil horizon was removed, the next major gradient related to soil pH and cations, with acidic soils containing higher aluminum and acidic cations on one end of the spectrum, and higher pH soils with more base cations and higher Ca:Al ratio on the other end of the spectrum. Plots on the higher pH side of the ordination also tended to have greater acid neutralizing capacity and higher pH in park water bodies. These results suggested that parks differ in their vulnerability to acidification, with parks comprised of acidic, poorly buffered soils and in areas receiving relatively high rates of acid deposition the most at risk.