Impacts of precommercial thinning on gas exchange, midday water potential, and chlorophyll content in Pinus nigra subsp. pallasiana stand from the semiarid region

The net photosynthetic rate (A), stomatal conductance (g s ), and transpiration rate (E), chlorophyll content, and midday xylem water potential (Ψ md ) of Pinus nigra Arn. subsp. pallasiana (Lamb.) Holmboe trees were studied in a natural stand (Burdur-Aziziye, Turkey) for the first (2015), second (2016) and third (2017) years after precommercial thinning (PCT) to better understand the physiological and biochemical responses of trees in a semiarid region. In this study, three treatments (control, 2–2.5 × 2–2.5 m and 3–3.5 × 3–3.5 m spacing) were applied. Soil water content (SWC) and soil temperature (ST) was greater in both PCT treatments than in the control treatments. In general, the control treatment had a lower Ψ md compared with PCT treatments. A, g s , and E are generally higher than PCT treatments applied with 3–3.5 m spacing. The lowest g s and E were determined in the control trees. Ψ md and A were positively correlated with SWC, but negatively correlated with ST. The decrease in stress level, improvement in resources, and reduced competition for resources by fewer trees increased the A and g s in trees in the PCT treatments. Chlorophyll a, chlorophyll b, and total chlorophyll content are higher in the control treatments as compared to PCT treatments. SWC, ST, Ψ md , A, g s , E, and photosynthetic pigment showed seasonal changes. The two PCT treatments were generally similar in the ecophysiological changes in plots and a longer time is needed to decide for PCT treatments applied with 3–3.5 m spacing. According to the results obtained, it is recommended that PCT be applied with spacings of 2–2.5 × 2–2.5 m.