Response of semi-desert grasslands invaded by non-native grasses to altered disturbance regimes

Aim Using a long-term data set we investigated the response of semi-desert grasslands to altered disturbance regimes in conjunction with climate patterns. Specifically, we were interested in the response of a non-native grass (Eragrostis lehmanniana), mesquite (Prosopis velutina), and native species to the reintroduction of fire and removal of livestock. Location The study site is located on the 45,360-ha Buenos Aires National Wildlife Refuge (31°32' N, 110°30' W) in southern Arizona, USA. In 1985, livestock were removed and prescribed fires were reintroduced to this semi-desert grassland dominated by non-native grasses and encroaching mesquite trees. Methods Plant species cover was monitored along 38, 30-m transects five times over a period of 15 years. Data were analysed using principal components analysis on the variance-covariance and correlation matrix, multivariate analysis of variance for changes over time in relation to environmental data, and analysis of variance for altered disturbance regimes. Results Reintroduction of fire and removal of livestock have not led to an increase in native species diversity or a decrease in non-native grasses or mesquite. The cover of non-native grass was influenced by soil type in 1993. Main conclusions Variability of plant community richness, diversity, and cover over time appear to be most closely linked to fluctuations in precipitation rather than human-altered disturbance regimes. The effects of altered grazing and fire regimes are likely confounded by complex interactions with climatic factors in systems significantly altered from their original physiognomy.