Complex interactions driven by livestock control the dynamics of the forest-grassland ecotone in highland landscapes of Uruguay

Abstract

Brazeiro, A., Brussa, P., Toranza, C. 2018. Livestock effects on the forest-grassland ecotone dynamics in highland landscapes of Uruguay. Ecosistemas 27(3):14-23. Doi.: 10.7818/ECOS.1470

The expansion of scrubland and native forests over savannas and grasslands has been observed in several parts of the world. The underlying mechanisms of this phenomenon (i.e. woody encroachment) are not yet fully understood, but it is known that it can be multiple factors depending on the ecological context. In the present work, we studied the effects of livestock on forest-grassland ecotone dynamics in a protected area of Uruguay. We found that forest had an expansion of 6-21% between 1966 and 2016 over grasslands, possibly due to the better water availability during summer and the reduction of sheep ranching. Forest expansion was almost four times higher in a cattle exclusion zone of 17 years (21%) than in a continuous grazing zone (5.7%), indicating that grazing constraint the expansion process. We found evidence of two mechanisms driving forest expansion: (1) gradual advance by edge dynamics, and (2) nucleation associated with the facilitation of recruitment of seedlings by woody plants and rocky outcrops (perch and/or nurse effects). Our results indicate that grazing plays an important regulating role in the vegetation dynamics of forest-grassland ecotone in the highlands landscapes of Uruguay. As hypotheses to explore, we propose some direct and indirect explanatory mechanisms of the livestockforest-grassland relationship.