Testing associations between tree species and nitrate availability: do consistent patterns exist across spatial scales?

Abstract

In forest ecosystems there are numerous factors that influence nitrate (NO₃) availability and retention in ways that can significantly affect receiving waters. Unfortunately these factors often co-exist and interact making it difficult to establish the importance of each individually. Three reference watersheds at the Fernow Experimental Forest (FEF) provide a unique opportunity to evaluate the influence of tree species on soil NO₃ availability across spatial scales because they differ in stream-water NO₃ concentrations despite sharing the same macro-climate, geology, hydrology, dominant soils series, stand age, pollution regime, and land-use history. To test the strength of plant/soil associations inferred from previous work in two watersheds in the FEF, we measured NO₃ availability and the composition of tree species in a third watershed that has intermediate stream-water NO₃ concentrations. We also examined plant/soil relationships in these watersheds at the scale of individual trees. Across the spatial scales examined – from individual trees to small watersheds – there were consistent associations between tree species and soil NO₃ availability. For individual trees, species with high underlying soil NO₃ availability included Acer saccharum and Lireodendron tulipifera, whereas species with low underlying soil NO₃ availability included Acer rubrum, Nyssa sylvatica, and Quercus prinus. For small plots we found plant/soil relationships that were consistent with those made apparent by sampling under individual trees of A. saccharum, A. rubrum, N. sylvatica, and Q. prinus. And at the scale of entire watersheds, those with higher stream-water NO₃ had a greater importance of tree species associated with high NO₃ availability at finer spatial scales than did the watershed with low stream-water NO₃. The spatially-robust relationships between tree species and soil NO₃ availability at the FEF suggest that well-characterized relationships between dominant tree species and soil properties should improve our understanding of how changes in the species composition of forest trees due to human activities, or natural causes, will alter rates of NO₃ production and loss.

Citation

Peterjohn, William T.; Harlacher, Margaret A.; Christ, Martin J.; Adams, Mary Beth. 2015. Testing associations between tree species and nitrate availability: do consistent patterns exist across spatial scales? Forest Ecology and Management. 358: 335-343.