Native-exotic richness ratios in second-growth forests reflect land-use history and environmental gradients

Marion Holmes, Leland D. Bennion, Jessica Poteet and Sara Kuebbing, Biological Sciences, University of Pittsburgh, Pittsburgh, PA, James Whitacre, Powdermill Nature Reserve, Carnegie Museum of Natural History, Rector, PA

Background/Question/Methods
The relationship between native and non-native species richness reflects drivers including disturbance history, resource availability, and environmental character. Disturbance can shift native-exotic richness ratios (NERRs) and result in positive or negative correlations between native and non-native richness, depending on specific impacts and intensity. Land-use history can be used to test hypotheses about how NERRs respond to disturbance and what mechanisms drive richness ratios. We hypothesized that 1) NERRs differ with land-use history: native and non-native richness are positively correlated in less-disturbed forests and negatively correlated with increasing disturbance; 2) NERRs reflect environmental character due to its influence on both native and non-native richness; 3) diversity is most strongly linked to environmental gradients in older forests, reflecting habitat diversification and environmental sorting during succession.
We surveyed forests at Powdermill Nature Reserve in Southwestern Pennsylvania. We selected four forest sites each of 40-70 year old second-growth forests with histories of mining, agriculture, and logging, 14 sites of older second-growth (>100 years), and sampled vegetation and environmental variables using a modified Whittaker plot design. Differences in native richness, non-native richness, and NERRs between land-use histories were tested with generalized linear models, and the influence of environmental variables was assessed with principal component regression.
Results/Conclusions
Native and non-native richness were significantly positively correlated in old forests at every scale; they were not significantly correlated in any of the younger land-use histories except in one-meter mined plots. The ratio of exotic to native species was twice as high in mined and post-agricultural sites than in logged and old forests. This pattern can be attributed to the higher non-native richness found in mined and post-agricultural forests than those with less intense disturbance history. Mined, post-agricultural, and logged forests showed 20%-30% stronger relationships of NERR with site quality than old forests, indicating that the drivers of NERR differ between land-use histories. We did not find evidence that richness and NERR show increasing differentiation along environmental gradients during succession, but instead that diversity-environment relationships are strongest in younger and more severely disturbed forests.