Understory Clearing in Open Grazed Mediterranean Oak Forests: Assessing the Impact on Vegetation

Over recent years, rural abandonment and climate change have challenged grazed wooded landscapes in Mediterranean mountain regions. Fire hazard management procedures such as grazing, prescribed burning, or mechanical clearing must be adjusted to the new socioeconomic and environmental situation and according to the context and circumstances of each territory. This study contributes to adjusting vegetation management techniques in response to low grazing pressure by evaluating the combined effect of mechanical clearing and grazing on the structural and floristic dynamics of understory vegetation in the open-grazed Mediterranean oak forests of northern Portugal. To this end, three treatments were established: mechanical clearing with grazing, mechanical clearing without grazing, and grazing without clearing (the control). The floristic inventories were carried out using the point quadrat method, and the structure was evaluated using line transects. Herbaceous biomass was determined by destructive methods. This study shows that mechanical vegetation clearing effectively reduces phytovolume and, thus, fire hazards. However, for its effects to endure, it must be combined with subsequent grazing, which does not always occur. No consistent pattern was found in the floristic dynamics of the shrub-grassland mosaic (species richness and diversity) related to shrub-clearing or grazing over the short term (24 months).