Effects of experimental summer fires on litterfall biomass in Mediterranean Pinus nigra forests

This study analyzes the effects of prescribed summer fires on the quantity, fractions and temporal patterns of litterfall biomass in Pinus nigra stands in the long-term. Although a number of studies have demonstrated the effects of low intensity (spring and autumn) burning on P. nigra litterfall production, composition and fall patterns, information about the effects of summer prescribed fires on these biomass input characteristics is scarce. We conducted summer experimental fires in June 2019 within an experimental design framework whereby spring and autumn prescribed burning had been carried out in 2016. Litterfall was collected monthly for 43 months. An immediate strong effect was recorded ensuing the shedding of scorched foliage, and the mean annual litterfall biomass was slightly higher in experimental summer fire plots than in unburned plots, with differences tending to decrease across the study period. The experimental summer fire did not alter the typical seasonal distribution of litterfall patterns, with a peak in summer and a minimum in winter. The proportion of different fractions of litterfall and monthly and annual trends of needles, branches and bark did not appear to be altered by the experimental summer fires. Besides, a Bayesian analysis conducted to assess the influence in the short-term of variables related to fire treatments, including spring, autumn and summer burns, showed basal area of the plots to be the most important variable determining the amount of litterfall, whereas meteorological variables were not influent. As summer experimental fires increase litterfall at the onset of the wildfire season, increasing fire hazard, forest managers should take this one-time event into account when deciding when to burn.