Photosynthetic capacity, nutrient and water status following precommercial thinning in Anatolian black pine

Abstract

Changes in gas exchange, photosynthetic pigments, midday xylem water potential (Ψmd) and needle nutrient status of the trees and environmental conditions following precommercial thinning (PCT) in the naturally regenerated Pinus nigra Arn. subsp. pallasiana (Lamb.) Holmboe stand was studied in forest districts of the western Mediterranean region of Turkey. An experiment involving two PCT treatments (spacing varied between 3.0 to 3.5 m and between 2.0 to 2.5 m) and one control (unthinned) was established in late April 2015. Different physiological responses to environmental changes following thinning were observed during the next three growing seasons. The soil water content (SWC) and temperature, Ψmd, chlorophyll pigments, needle N, P, and K concentrations, and gas exchange parameters were affected by PCT and the season. Compared to the unthinned plots, PCT increased the SWC and the soil temperature. The SWC was higher in the 3–3.5 m spaced plots. Increases in soil temperatures were generally similar between PCT treatments. The Ψmd, net photosynthetic rate (A), transpiration rate (E) and stomatal conductance (gs) of the trees in the thinned plots increased significantly compared to unthinned trees. This increase was attributed to increases in the SWC and decreases in soil temperature. The PCT with 3–3.5 m spacing had the highest Ψmd and A or similar to the 2–2.5 m spacing. However, the A, E, and gs was higher at 2–2.5 m spacing in the dry period (July) of the second and third years after PCT. Needle N (May 2015), P (September 2015), and K (July 2016) concentrations increased after the PCT with 3–3.5 m spacing compared with the unthinned and the 2–2.5 m spacing. Needle K concentration was negatively correlated with Ψmd and positively with WUE. A, gs and E were positively correlated with needle P concentration but not correlated with needle N concentration. The chlorophyll content of the unthinned trees remained high in the spring, similar to the 3–3.5 m spacing in the summer. The Ψmd, A, gs, and E varied seasonally in both unthinned and thinned plots, with the higher values in the spring and lower values in summer. According to the short-term results, the PCT with the 2–2.5 m spacing appeared more appropriate when the 3–3.5 m spacing might negatively impact soil, water, and photosynthetic parameters in extreme drought years and management objectives such as improvement of stem quality are considered.