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-m² 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. [ABSTRACT FROM AUTHOR]