Native-exotic richness relationships in second-growth forests differ along a gradient of land-use history

Context The relationship between native and non-native species richness is influenced by drivers including disturbance history and environmental character. Disturbance influences native-exotic richness relationships (NERRs) and results in positive or negative relationships, depending on its intensity. Land-use history can be used to understand how NERRs respond to disturbance and what mechanisms drive diversity. Objectives We test the hypotheses that (1) native and nonnative plant species richness differ between land-use histories; (2) diversity is more strongly linked to environmental gradients in older forests than young stands, reflecting environmental sorting; (3) Native and non-native richness are positively correlated in older forests and negatively correlated in young forests. Methods We surveyed forests at Powdermill Nature Reserve in Southwestern Pennsylvania. We selected four replicates each of 40 to 70 year old second-growth forests with histories of mining, agriculture, and logging, and 14 older second-growth sites (>100 years) without recent human disturbance and collected presence data in a modified Whittaker plot design that included 1000, 100, 10, and 1-m(2) subplots. Results Native and non-native richness were positively correlated in older forests, but uncorrelated in young stands. Proportions of non-native species were higher in mined and post-agricultural sites than in logged and old forests. Diversity-environment relationships were strongest in younger forests. Conclusions Differences in native and non-native richness between histories were strong enough to shift the direction of richness relationships in young forests. We conclude that differing impacts of land use on native (positive) and non-native (neutral) richness drive contrasting NERRs in forests with different histories.