High-intensity thinning treatments in mature Pinus halepensis plantations experiencing prolonged drought
2016
Calev, Ailon | Zoref, Chanoch | Tzukerman, Moshe | Moshe, Yosef | Zangy, Ela | Osem, Yagil
Pinus halepensis plantations are widespread throughout semiarid–subhumid landscapes of the Mediterranean. Recently, drought-induced decline has often been reported raising concerns with regard to the future of these man-made ecosystems. The study was set out to investigate thinning as a means to alleviate water stress and improve performance of mature P. halepensis plantations experiencing prolonged drought. The study was conducted in a 40-year-old P. halepensis forest in the Jerusalem Mountains of Israel (rainfall: 550 mm year⁻¹). Declining stands (stand basal area increment, BAIₛₜₐₙd ≈ −3 % year⁻¹) were treated by thinning: (1) intense thinning—tree density, BAₛₜₐₙd, and leaf area index (LAI) reduced by 81, 68, and 59 %, respectively; (2) moderate—56, 48, and 26 %, respectively; (3) control (~560 tree ha⁻¹). Plots of 70 × 70 m were used in four replicates. Individual tree- to stand-level variables were monitored during 4 years through stem and leaf area metrics alongside with predawn shoot water potential (ψ PD) and tree mortality. Thinning ameliorated drought stress, reduced mortality, and improved individual tree growth (ψ PD = −1.7, −1.8, and −2.0 MPa; mortality = 0.2, 2, and 5 % year⁻¹; BAIₜᵣₑₑ = 3.4, 2.0, and 1.4 % year⁻¹ in intense, moderate, and control treatments, respectively). Thinning effects became more pronounced with time. LAI and individual tree leaf area (LAₜᵣₑₑ) fluctuated with association to annual rainfall. Higher LAₜᵣₑₑ caused by thinning reflected a “selection effect” while increased leaf area efficiency (BAI per unit LAₜᵣₑₑ) was attributed to a “release effect” of thinning.
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