Woody encroachment processes are driven by different ecological factors whose relative importance varies throughout tree life stages. Classical ecological models suggest tree-grass niche separation to explain coexistence. Instead, demographic models suggest that tree establishment is strongly limited by one or several demographic bottlenecks that may be altered by disturbance regime. Climatic variability, fire or grazing regime modify vital rates of tree populations and also could modulate the intensity of tree-grass interactions in savannas.
We studied woody encroachment from a demographic perspective in a mesic Argentinean savanna dominated by Acacia caven. This study aims to evaluate the effect of cattle grazing and grass competition over early tree establishment (0-18 months). Trough factorial designed experiments, we studied the effect of cattle grazing (exclosure and grazing) and grass removal (with and without grass removal) on 1) seed predation, 2) seed germination and seedling emergence, 3) seedling survival, and 4) the probability of a seed to recruit an individual, across the two main factors (grazing and grass removal), by combining the results of emergence and survival experiments.
Results/Conclusions
We found no evidence of the role of grazing or grass removal or interaction between them on seed predation rates (mean exclosure: 0.05, SE: 0.011; mean grazing: 0.03, SE: 0.008). Germination - seedling emergence doubled under grazing (p < 0.05) but there was no evidence of grass removal effect. Conversely, we found marginal effect of grazing decreasing seedling survival after 18 months (mean exclosure: 0.19, SE: 0.1; mean grazing 0.07, SE: 0.04; p = 0.07). Besides, grass removal decreased seedling survival (mean grass: 0.15; SE: 0.08; mean grass removal: 0.09, SE: 0.05; p <0.05). Taken together, these processes indicated that the probability of a seed to recruit an individual was not affected by the manipulation of grazing or grass removal; this probability did not change between treatments. Overall, the net effect of grazing on A. caven recruitment was compensated as a result of the positive effect on seed germination and seedling emergence and the negative effect on seedling survival. We concluded cattle grazing and tree-grass interaction have contrast effects over A.caven depending on the tree life stage. These results highlight the importance of considering seed - seedling conflicts during the early establishment of woody plants in savannas.