The impact of adaptive forest management on water fluxes and growth dynamics in a water-limited low-biomass oak coppice

Marginal semi-arid forests in areas currently affected by climate change are a challenge to forest management, which has to focus on key functional traits that can effectively contribute to resistance under extreme drought. We studied the effects of thinning in a marginal forest by quantifying functional responses relating to growth, carbon and water fluxes. Two experimental plots were established, one thinned in 2012 and the other one left as a control. The environmental conditions varied substantially during the 4-year study period, although dry years predominated. There were signs of dieback in the control with a decreasing inter-annual trend in LAI, as opposed to the treated plots, where LAI by the end of the study almost reached pre-thinning levels. Sap flow and transpiration were greatly enhanced by the treatment, with thinned trees transpiring 22.41 tree(-1) day(-1) in the growing season, about twice the control figures. The seasonal patterns of transpiration and soil moisture were uncoupled, indicating a contribution of deep groundwater to the former flux. In the control, limitations to water and carbon dynamics (canopy conductance) occurred at soil moisture values below 16%, whereas in the thinned trees these limitations appeared when soil moisture dropped below 10%. Overall, oaks' transpiration was enhanced with thinning to the point that stand-water use surpassed that of the control by the second half of the study period, averaging 24% of gross rainfall in both plots. Soil evaporation increased from 12 to 20% of gross rainfall after treatment in the overall period. The treatment had a profound watering effect in this marginal forest, led by fewer trees using the same amount of water as those in the untreated overstocked plot. This research may provide guidelines for ecohydrology-oriented silviculture in stands experiencing tree encroachment and transformation into shrublands that are more prone to global change-induced disturbances.