The impact of fire recurrence on populations of Quercus suber in southeastern France
Résumé
Cork oak (Quercus suber) is known as one of the most fire-resistant and fire-resilient tree species in French Mediterranean ecosystems. However, the repetition of high intensity wildfires is hypothesized to entail direct or delayed mortality in the populations of the Maures massif (southern France). We have installed a network of 90 permanent plots in an area affected by the intense 2003 wildfire. The plots were sampled along a gradient of fire recurrence since 1959, with one to four fires, and unburned control plots. Trees were individually surveyed between 2004 and 2007. Field survey included tree mortality, damage intensity, and individual tree growth and allometry. Results indicate that the total tree mortality (i.e. root and stem mortality) ranged from 3% to 8% one year after the fire. This confirms the high survival ability of Quercus suber, especially in comparison to the other tree species of the study area. However, delayed mortality has been observed from 2004 to 2007, probably in relation to repetitive summer droughts. The mortality of cork oak did not vary linearly with fire recurrence: it was maximal after one first fire and 3-4 fires. The best-fit logistic model predicted the mortality of cork oaks as a function of their dendrometric characteristics (crown and stem diameter), which are hypothesized to express the ability of trees to escape fire and to resprout. At population scale the fires have exerted a size-dependent selection with higher mortality for small and large trees. As a consequence, the fire recurrence tends to regularize and to open the cork oak stands. Our findings suggest that the mortality rate should increase and become a major concern for forest and land managers of this area in a context of global change, with presumably increasing fire recurrence and summer droughts
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