Mid-term effects of thinning on canopy variables related to crown fire hazard on pure, even-aged pine stands

One of the main objectives of forest stand management in Mediterranean areas of Europe is the mitigation of crown fire hazard. Effective silvicultural strategies for reducing the likelihood and intensity of crown fires include reducing surface fuels, increasing canopy base height (CBH) and reducing canopy bulk density (CBD). All these fuel variables depend to a certain degree on stand structure and are therefore responsive to stand density management through thinning treatments. It is well known that canopy fuel load (CFL) and CBD diminish and CBH increases immediately after thinning from below. Nevertheless, for how long the thinning effect be sustained and whether it could be species-specific is less clear. We examined how different levels of thinning intensity altered canopy fuel variables in the mid-term in Pinus pinaster and Pinus radiata stands. We used data from 41 thinning trial locations installed in pure, even-aged stands of P. pinaster (22 locations) and P. radiata (19 locations). At each location, three rectangular plots were established and a different treatment was applied to each plot: control (unthinned), low thinning (20% of the basal area removed), and heavy thinning (40% of the basal area removed). Six years after treatments, CBD and CFL in treated plots were still significantly lower than in untreated plots for both species. Moreover, for radiata pine, CBD values were even lower than in the pre-thinning situation. On the other hand, CBH did not show any change following thinning in P. pinaster, whereas for P. radiata, heavy thinning resulted in significantly lower CBH values since four years after the treatment. The results highlight that thinning may only have a limited impact towards diminising the potential for crown initation, especially in maritime pine stands, altough they could significantly influence the crown fire rate of spread in stands of both species.