While deer may impact tree regeneration, they can also help prevent the spread of severe wildfires. Given increased likelihood of extreme climatic events, such as droughts, Miguel Bugalho explains how we need to consider both the positive and negative effects of wild ungulate grazing. 

Deer mitigate severe wildfires

Wild ungulate herbivores, namely deer, may negatively affect tree regeneration through consumption and damage of young seedlings and saplings. When rubbing their antlers on adult trees during territorial and rutting behaviour, deer can also damage, weaken and eventually cause death of adult trees. Deer, however, also consume woody shrubs and, in doing so, reduce the amount of fuel load and the severity of eventual wildfires. More specifically, through consumption of the shrub understory, deer decrease the likelihood of surface wildfires transitioning into severe canopy wildfires that induce adult tree mortality. Deer may therefore prevent the likelihood of canopy wildfires; consequently contributing to adult tree survival and, in the long-term, ecosystem carbon storage, as, along with soils, adult trees are a major carbon reservoir in forest ecosystems.

fig 1. browsing effects diagram
Browsing by deer decreases understory carbon stocks as well as fine fuel load, therefore decreasing the likelihood of crown fires and of adult oak mortality. By preventing severe wildfires, deer may ultimately contribute to oak survival and enhance long-term ecosystem carbon stocks.

Experimental results

Our research group set up a long-term (more than 14 years) browsing exclusion experiment consisting of five 25m x 25m fenced plots, paired with same number and similarly sized open plots. Fences were over two metres high to exclude deer. The study area was a Mediterranean evergreen oak woodland of cork (Quercus suber) and holm oak (Q. rotundifolia) trees, with a dominant shrub cover of rockrose (Cistus ladanifer), located in Alentejo, southern Portugal. There was no rockrose in the plots, which had been cleared through tillage, when we started the experiment in 2001, but rockrose started colonizing the plots immediately afterwards. Six years later, the average height of rockrose shrubs was significantly higher in fenced plots (approximately 2.5 metres high, compared to less than 1 metre in open plots) but their population density was similar between fenced and open plots (approximately 450 individuals per plot). Fourteen years after the beginning of the experiment, rockrose height (approximately 3 metres high in fenced plots, versus 0.5 metres high in open plots) and density was significantly higher in fenced plots (over 600 individuals per fenced plot, compared to less than 50 individuals in open plots), suggesting that deer were halting ecological succession in open plots. The amount of litter was also significantly higher in fenced plots with no significant differences in the biomass of herbs.

fig 3. shrubs
Deer halt ecological succession and shrub encroachment in Mediterranean evergreen oak woodlands. Although shrubs still occur in open plots (first sight) they are sparser and much smaller shrubs than those occurring in fenced plots where deer browsing had been excluded for over fourteen years (far sight).

Fire modelling results

fig 4. modelled wildfire behaviour
Modelled wildfire behaviour differs between open and fenced plots: Under increasing wind speed and for a constant slope of 5º the Flame length (A), Surface rate of spread (B) and Fireline intensity (C) are significantly higher in fenced plots. Additionally, the probability of a surface wildfire to transit to a severe canopy fire killing the adult trees is higher in fenced plots (A).

We also modelled fire behaviour in the open and fenced plots. Modelling showed that the fire-line intensity, flame length and fire speed and the likelihood of a surface fire to transition to severe canopy fire was higher in fenced plots. Moreover, because canopy fires were more likely in fenced, shrub-encroached plots, the likelihood of adult oak mortality was also higher in these plots. This shows deer to ultimately contribute to the mitigation of wildfire severity and  increase the likelihood of adult oak survival. This is also important because, after soil, adult living trees are major reservoirs of carbon, and thus deer may be indirectly contributing to ecosystem carbon storage.

Management implications

Deer hunting is a widespread practice in several oak woodland estates in the Iberian Peninsula. We show that deer help to halt shrub encroachment and mitigate wildfire severity in Mediterranean evergreen oak woodlands. Under current climate change scenarios, which forecast increased frequency of extreme events, such as droughts and wildfires, it is important to assess trade-offs caused by wild ungulate herbivores such as deer. While, at high population densities, deer may negatively affect tree regeneration, they may also help reduce wildfire severity, and ultimately benefit adult oak survival and therefore oak carbon storage. In the future, our group will assess how increased drought frequency interacts with browsing to shape wildfire behaviour.

Read the full article, Ungulates mediate trade‐offs between carbon storage and wildfire hazard in Mediterranean oak woodlands in Journal of Applied Ecology.