Crown bulk density and fuel moisture dynamics in Pinus pinaster stands are neither modified by thinning nor captured by the Forest Fire Weather Index

Abstract

Key message. No temporal change was recorded during summer in fuel availability in Pinus pinaster stands, contrary to predictions from the Forest Fire Weather Index. Also, thinning had no mid-term effect on fuel moisture or canopy structure. Context. Forest fires are a major problem in Mediterranean countries. Management actions, such as fuel reductions, are one of the main tools to diminish fire risk, but the midterm efficacy of such tools remains largely untested with empirical data. Aims. Here, we test for midterm effects of thinning on fuel moisture and crown bulk density in P. pinaster stands and whether temporal variations in fuel moisture correlated with predictions from the Fire Weather Index, a commonly used index on fire risk, and its components. Methods. We compared fuel moisture over a fire season and crown bulk density in nine pairs of thinned/unthinned plots 7 years after treatments were applied. Results. We observed that fuel moisture remained stable during a fire season, as a likely result of drought-induced physiological adjustments, including stomatal regulation and others, which allow leaves to maintain a large humidity even during drought, and that thinning had no midterm effect on fuel moisture or crown bulk density. Moreover, the Fire Weather Index and its components displayed different temporal dynamics than those observed in fuel moisture. Conclusion. These results are important as they indicate that thinning may only have a limited, short-term impact towards diminishing the potential for crown fire spread in these stands and that current indices to evaluate fire risk may require a re-evaluation.

This study is funded by the Spanish Government (RYC-2012-10970, AGL2015-69151-R, AGL2015-68274-C3-3-R).