Tree density reduction mitigates the decline of Larix principis-rupprechtii plantations: Evidence from a combination of dendroclimatic and physiological measurements

Drought-induced forest decline and mortality have become increasingly widespread around the globe, while thinning is considered to be an effective measure in forestry to enhance tree growth and forest health particularly in water-limited areas. Nevertheless, there is a keen lack of comprehensive studies combing investigations on patterns of tree growth response to changes in stand density and the underlying physiological mechanisms. In the world-largest Larix forest plantation, we tested the hypothesis that density reduction can mitigate tree decline by alleviating drought-induced hydraulic dysfunction and enhancing tree carbon balance using a combination of dendroclimatic analyses and physiological measurements. We found that density has significant effects on growth performance of larch trees and their responses to drought, with trees of lower density stands exhibited significantly higher radial growth rates, less branch diebacks and greater resilience to extreme drought. Different from our expectation, no significant differences in stem hydraulic conductivity, degree of xylem embolism and embolism vulnerability were found among trees of different stand densities. We did observe strong influences of density on biomass distribution and non-structural carbohydrate (NSC) contents of fine roots, i.e. in stands of high densities fine roots have significantly lower NSC contents and trees have reduced biomass allocation to fine roots at greater depth. The finding that larch trees in high-density stands having higher proportions of dead branches suggests that they maintain hydraulic safety by reducing whole-plant transpirational water demands in the more water-limited conditions, although this passive adjustment is at the expense of carbon assimilation. Particularly, the less favorable plant carbon status in high-density stands showed strong negative impacts on carbon allocation to fine roots that further render plants greater susceptibility to extreme drought events. Results of this investigation provide a theoretical basis for stand density reduction as an effective management measure for forest plantations in water-limited areas.