Abstract
In the Mediterranean region, wildfires are a major disturbance, determined by ecosystem and forest species characteristics. Both the flammability and resistance to fire of a mixed forest may vary from those of the individual species. Two mixed Mediterranean woodlands, a Cupressus sempervirens and Quercus ilex stand in Italy; and a Juniperus thurifera and Quercus faginea stand in Spain were investigated. Laboratory flammability tests were conducted on live foliage, litter samples and on litter beds from individual and mixed species to evaluate: (i) the flammability traits of the mixtures of live foliage and litter samples; (ii) whether the flammability of the two-species mixtures are non-additive, i.e., differ from expected flammability based on arithmetic sum of the single effects of each components species in monospecific fuel; (iii) the ignition success and initial fire propagation in litter beds. Flammability tests were also conducted on bark samples to estimate the resistance of the tree species to fire. The ignitibility of live foliage was lower and the combustibility was higher in Cupressaceae than in Quercus. Non-additive effects were observed in some flammability components of live foliage and litter, especially in the mixtures of C. sempervirens and Q. ilex. Ignitability and combustibility were higher and lower than expected, respectively, and tended to be driven by Quercus), while the consumability was lowered more than expected by both Cupressaceae. The ignition success in the litter beds was low, especially for the presence of Cupressaceae that increase the bulk density of the mixtures. Cupressaceae, which have a thinner bark, suffered more damage to the cambium after shorter exposure to the heat source than Quercus species. In all the species studied, time to reach lethal temperatures in the cambium was dependent on thickness rather than on flammability of the bark. The study findings revealed that tree species may influence flammability of mixed fuels disproportionately to their load. The studied species showed to exert a contrasted effect on flammability of the mixtures, increasing ignitability and decreasing combustibility and consumability well out of their proportion in the mixture. This may potentially influence fire dynamics in mixed forests.
Highlights
Mixed forests may be more resistant and resilient than monospecific forests to anthropic and environmental disturbance, with a more intense and quicker response (Loreau et al, 2001; Jactel et al, 2009)
Live Foliage (See Table 2 for Acronyms) Flammability tests on live foliage showed that ignitability of C. sempervirens (Cs) and J. thurifera (Jt) was lower than that of Q. ilex (Qi)and Q. faginea (Qf), respectively (Figure 3)
Pure live foliage of Cs showed the highest TTI (77.3 s) and the corresponding value was significantly lower in mixed samples with 50% or more Quercus ilex subsp. ilex (Qi), while the lowest TTI was recorded in pure live foliage of Qi and in the 75% Qi mixture (18 and 18.3 s respectively)
Summary
Mixed forests may be more resistant and resilient than monospecific forests to anthropic and environmental disturbance, with a more intense and quicker response (Loreau et al, 2001; Jactel et al, 2009). They generally have higher biodiversity levels and higher carbon storage capacity. This is reflected in better adaptation to global change, greater potential for mitigation strategies (Wagner et al, 2014), higher water and nutrient regulation and higher productivity and support for ecosystem services than in monospecific woodlands (Knoke et al, 2008). Information about the effects of wildfires on mixed and monospecific forests, i.e., different degree of damages caused, different mortality of species, rate of fire spread, duration and intensity that are affected by the stand composition could provide a better knowledge of forest management strategies aimed at fire control and prevention
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