Intensification pathways modulate Eragrostis plana invasion trajectories in natural grassland

Júlio Kuhn da Trindade , Régis Maximiliano Roos de Carvalho, Diego Bitencourt de David, Eduardo Bohrer de Azevedo, Carolina Bremm, Fernando Luiz Ferreira de Quadros

Management pathways aimed at increasing forage supply and livestock productivity are often proposed in natural grasslands, but their consequences for established invasive grasses remain uncertain. We evaluated trajectories of an established Eragrostis plana Nees (Poaceae) invasion under contrasting pathways in the Brazilian Pampa. From August 2014 to May 2017, vegetation was monitored in eight paddocks arranged in two blocks, two per pathway: managed natural grassland under grazing control, natural grassland with liming and basal fertility correction, natural grassland with fertility correction, Italian ryegrass and forage legumes, and natural grassland with fertility correction, Italian ryegrass and additional urea-based mineral nitrogen. We analysed Eragrostis plana frequency using beta-binomial mixed models and evaluated total cover and conditional cover where the species was detected as complementary response layers. Dynamic reference-adjusted summaries described trajectories relative to paddock-specific operational baselines and the contemporaneous reference trajectory, while formal inference was based on the fitted models. Responses did not indicate uniform suppression or promotion across pathways. The clearest pathway-dependent signal occurred in frequency: the reference-adjusted contrast between the Italian ryegrass + additional urea-based mineral-nitrogen pathway and the Italian ryegrass + legume pathway was +9.19 percentage points in the first post-intensification grazing-use year and +5.91 percentage points in the second, and planned contrasts supported greater Eragrostis plana frequency under the former in both years. The fertility-correction pathway remained close to the managed natural-grassland trajectory. Total cover showed the same contrast direction but with smaller magnitude and weaker statistical support, whereas conditional cover showed no corresponding supported difference. Thus, pathway divergence was expressed more clearly through detectable occurrence than through local cover where Eragrostis plana was detected. In invaded natural grasslands, intensification pathways should therefore be evaluated not only for production objectives but also for their association with the spatial expression of established invasive grasses.