Close-to-nature management effects on tree growth and soil moisture in Mediterranean mixed forests

Given the rising frequency of drought events, close-to-nature principles appear to be a suitable management option for promoting more adapted forests by increasing diversity and vitality (e.g., enhancing tree growth), while preserving soil moisture. In this study, we evaluate individual tree growth from selected trees (including future crop trees) and soil moisture responses to close-to-nature silvicultural treatments in sub-humid Mediterranean mixed forests in Catalonia (Northeastern Spain). We relied on 18 permanent sample plots (i.e., 9 paired-plots: managed / control) representing a competition intensity gradient and different forest types (dominated by either Quercus ilex, Castanea sativa, Quercus pubescens/petraea, Pinus pinea and Pinus sylvestris). From them, 8 plots were subject to intense monitoring in terms of intra-annual tree radial growth, monitored by manual band dendrometers, and hourly soil water content. Basal area increment, and annual and seasonal (May, June, July) radial increment rates were computed as a proxy of tree growth for all plots and for those plots with dendrometers, respectively. We conducted Spearman correlation analyses between tree growth, competition intensity, soil water content and climatic variables to detect significant relations between forest management and variables of interest (e.g., basal area increment, radial increment rates, soil water content). To investigate further into the effect of this silviculture on tree growth, we used mixed-effects models using as predictors variables related to competition intensity and forest management (dummy variable) in combination with monthly soil water content variables. We observed that basal area increment and radial increment rates had a positive (albeit weak) correlation with forest management, while competition intensity had a negative correlation with both variables. The models were not capable of detecting clear species-specific tree growth responses to competition. Compared to control plots, on average, the treatments boosted soil water content by about a 10% in managed plots in Q. ilex, P. sylvestris and P. pinea stands, and this increase was particularly significant during the growing season. Moreover, the best-fitted model predicted seasonal radial increment rates using not only forest management and competition variables, but also monthly soil water content factors. Our results suggest that close-to-nature-forest management may reduce the vulnerability to drought by increasing tree vitality and soil moisture, at least in the short term (<3 years).