Disturbance types, herbaceous composition, and rainfall season determine exotic tree invasion in novel grassland

Novel grasslands embedded in anthropogenic landscapes are increasingly threatened due to invasion by exotic woody species. This potential for tree invasion in grasslands can be either facilitated or hindered by disturbance agents and/or herbaceous composition. Furthermore, disturbances may depend on inter-annual rainfall modifying the chance of woody establishment. Here we determine how large- and small-scale disturbances influence the establishment of the exotic tree Gleditsia triacanthos across different herbaceous patches in an old-field pampa grassland in Argentina. Tree seeds were copiously added in all study plots and were then monitored over 4 years for seedling recruitment and growth rates in a fully factorial disturbance x patch type experimental design. We used burning (large scale) and armadillo-like burrowing (small scale) as the two levels of disturbance, and grass dominated and forb dominated patches as the two types of patches. After seed sowing, two seedling cohorts emerged during the drier and wetter summers, in March 2000 (xeric cohort) and January 2001(mesic cohort), respectively. Tree recruitment was twice as high for the mesic cohort in comparison to the xeric cohort, but for both cohorts tree recruitment was significantly higher in the forb patches than in the grass patches. For the mesic cohort, tree recruitment was enhanced for both types of disturbance, while for the xeric cohort, recruitment was depressed by the small-scale disturbance. Lastly, sapling growth rates were influenced by disturbance, with xeric cohort saplings being larger in the small-scale disturbance and mesic cohort saplings being smaller in the large-scale disturbance. Taken together, tree seedling invasion was enhanced in forb-dominated patches during an early dry season, but it was mainly enhanced by disturbance agents during a wetter season. Hence, while disturbances can promote exotic plant species, our findings emphasize the spatiotemporal aspects of woody invasions in novel grasslands.