Long term effects of biotic and biotic disturbances on light properties at understory and forest canopy in a mixed forest in sourthen Pyrenees

Abstract

Strong winds and thinning affect forests worldwide, with strong effects on the forest canopy and light properties. The aim of this work is to study the long-term effect of such abiotic and biotic alterations on the forest canopy of a Mediterranean-type mixedforest in Southern Western Pyrenees. In our study site, an experiment of nine plots with different thinning intensities (0%, 20% and 30%) took place in 1999 and 2009. During 14 years, we took hemispherical pictures to calculate light properties. We applied partial auto-correlation functions to check if changes between years could be attributed to timechanges or to external disturbances. During 2003, 2008 and 2016, we carried out mappings of forest canopy with the aim to calculate the species richness and proportion of canopy covered at increasing buffer areas. We applied generalized linear models to evaluate the effect of light variables (independent variables) on the richness and the proportion of forest canopy (dependent variables). We found that light variables had the higher changes during 2009 and 2010, seemingly explaining by strong winds, but we found consistent effect of thinning treatments. Likewise, we found that the buffer at 5.0 m best explained the species richness of canopy, whereas at 1.0 m that of the proportion of forest canopy. In addition, the light variables, such as diffuse light and leaf area index, were related with species richness of the canopy, whereas variables such as canopy openness and thinning with the proportion of forest canopy. To sum up, our study demonstrates the importance of strong winds in gap formations, causing thinning experiments to be outperformed when both events happen simultaneously.