Landscape patterns of understory composition and richness across a moisture and nitrogen mineralization gradient in Ohio (U.S.A.) Quercus forests

This study quantified relationships of understory vascular plant species composition and richness along environmental gradients over a broad spatial scale in second-growth oak forests in eastern North America. Species frequencies were recorded in 108 25 × 25 m plots in four study sites extending over 70 km in southern Ohio, U.S.A.. The plots were stratified into three long-term soil moisture classes with a GIS-derived integrated moisture index (IMI). In addition to the IMI, the environmental data matrix included eight soil and three overstory variables. Canonical correspondence analysis (CCA) indicated that variations in understory species composition were most strongly related to topographic variations in predicted moisture (IMI), N mineralization rate, nitrification rate, and soil pH. In addition, floristic variation at the regional scale was correlated with variations in soil texture, nitrification, pH, and PO₄ ^–, resulting from differences in the soil parent material complexes among sites. Species richness averaged 65 species/plot, and increased with moisture and fertility. Stepwise regression indicated that richness was positively correlated with N mineralization rate and nitrification rate, and inversely correlated with tree basal area. Greater richness on fertile plots was the largely the result of increasing forb richness. Forb richness per quadrat (2 m²) was most strongly and positively related to N mineralization rate. Conversely, richness of understory individuals of tree species was greatest on xeric, less-fertile plots. Our results describe general, broad-scale species-environment relationships that occurred at both the topographic scale (long-term moisture status and fertility) and the regional scale (geomorphological differences among the sites). Strong species richness-N mineralization correlations indicate an important link between below-ground processes and above-ground biodiversity. Because N availability was a strong correlate to vegetation patterns at a broad-scale, our results suggest that the increasing rates of atmospheric N deposition in the region could have a major impact on understory vegetation dynamics.