Effects of thinning and understory removal on water use efficiency of Pinus massoniana: evidence from photosynthetic capacity and stable carbon isotope analyses

Understanding the relationship between forest management and water use efficiency (WUE) is important for evaluating forest adaptability to climate change. However, the effects of thinning and understory removal on WUE and its key controlling processes are not well understood, which limits our comprehension of the physiological mechanisms of various management practices. In this study, four forest management measures (no thinning: NT; understory removal: UR; light thinning: LT; and heavy thinning: HT) were carried out in Pinus massoniana plantations in a subtropical region of China. Photosynthetic capacity and needle stable carbon isotope composition (δ¹³C) were measured to assess instantaneous water use efficiency (WUE_inst) and long-term water use efficiency (WUE_i). Multiple regression models and structural equation modelling (SEM) identified the effects of soil properties and physiological performances on WUE_inst and WUE_i. The results show that WUE_inst values among the four treatments were insignificant. However, compared with the NT stand (35.8 μmol·mol⁻¹), WUE_i values significantly increased to 41.7 μmol·mol⁻¹ in the UR, 50.1 μmol·mol⁻¹ in the LT and 46.6 μmol·mol⁻¹ in HT treatments, largely explained by photosynthetic capacity and soil water content. Understory removal did not change physiological performance (needle water potential and photosynthetic capacity). Thinning increased the net photosynthetic rate (A_n) but not stomatal conductance (g_s) or predawn needle water potential (ψ_pd), implying that the improvement in water use efficiency for thinned stands was largely driven by radiation interception than by soil water availability. In general, thinning may be an appropriate management measure to promote P. massoniana WUE to cope with seasonal droughts under future extreme climates.