Ungulates mediate trade-offs between carbon storage and wildfire hazard in Mediterranean oak woodlands.
Ungulates influence ecosystem services in important ways, including by altering the amount of standing plant biomass and species composition. Browsing, for example, removes biomass and, in consequence, can decrease the risk of wildfires. The influence of ungulates on carbon storage is more complicated. Browsers reduce carbon stocks directly by consuming biomass, but if browsing reduces fine fuel loads, then long-term carbon storage may increase. We investigated how wild ungulates mediate trade-offs between carbon stocks and wildfire hazard in a Mediterranean oak woodland. We assessed the effects of deer (Cervus elaphus and Dama dama) on the colonization by gum cistus (Cistus ladanifer), a highly flammable Mediterranean shrub, through a long-term ungulate exclusion experiment. We established fenced (unbrowsed) and open (browsed) plots in areas where shrubs were previously cleared, and compared shrub density, height, biomass, and above-ground carbon stocks 6 and 14 years after fencing. There were no significant differences in shrub density between treatments after 6 years of browsing exclusion but, by this point, height was already significantly greater in fenced plots. At the end of the experiment, shrub density and height were both significantly greater in fenced plots. Biomass and carbon stocks of gum cistus increased and reached a plateau in fenced plots, while in open plots biomass and carbon stocks increased initially but then decreased. Ungulates also prevented accumulation of litter carbon stocks, which was significantly higher in fenced plots after 14 years of browsing exclusion. We also modelled fire behaviour in fenced and open plots. Browsing reduced fine fuel load, an important contributor to fire spread, by 80%. In browsed plots, modelled wildfire rate of spread, flame length, and fireline intensity decreased by 50%, 65%, and 90%, respectively, which also decreased the probability of canopy fire and of oak mortality. Synthesis and applications. By decreasing fire hazard and the probability of canopy fire, ungulates may ultimately increase ecosystem carbon stocks through improved adult oak survival. These critical, indirect trade-offs need to be assessed when addressing the effects of ungulates on ecosystems and the management of their populations.