Landscape structure and climate affect plant invasion in subtropical grasslands
Question How do climate and landscape structure affect plant invasion process in south Brazilian natural grasslands? Location Natural grasslands across Rio Grande do Sul state, southern Brazil. Methods We selected 20 plots (2 km x 2 km each) situated in ten different grassland types in southern Brazil. In each grassland type, one plot was located in a region with remnant grassland cover 60% (conserved landscapes) in a paired design. In each plot, we surveyed the presence and cover of the four most important invasive species of grasslands in Rio Grande do Sul (Cynodon dactylon, Eragrostis plana, Senecio madagascariensis and Ulex europaeus) We used 12 variables related to climate and landscape structure (i.e. composition and configuration) for testing causal models of plant invasion using Akaike's information criterion for path analyses. Results Empirical evidence on how climatic conditions and landscape structure can cause plant invasion in subtropical grasslands was provided through six causal models validated with path analysis. The models indicated that increased water deficit, higher density of roads and losses in natural grassland cover caused an increment of invasive plant cover. Further, invasion differed between plots located in conserved vs not conserved landscapes, resulting in not conserved grasslands having a higher level of invasion. E. plana, a perennial grass from South Africa, was the most important invasive species in the grasslands, in terms of frequency and cover. Conclusions Our results identified the general conditions that promote successful population growth of invasive plants in subtropical grasslands: water deficit, road density and land use, highlighting the effects of climate, landscape structure and human activity in the level of invasion across grassland types.