Forest treatment effects on wood production in ponderosa pine

Abstract

Forest treatments reduce wildfire risk and can promote the vigor and production of remaining trees, but they are also a disturbance. Understanding the type, timing, and longevity of tree response to treatment, as well as the potential for interactive effects of treatments and drought, could help managers plan and evaluate forest management practices. Environmental drivers, biological modifiers, and tree capacity to respond to prior disturbances were concurrently tested to predict ponderosa pine basal area increment (BAI) in a lowland and upland dry pine forest in south central Oregon, USA. Environmental drivers included current year and lags or running averages of a drought index, SPEI, and the sum or count of growing degree days >0°C or 10°C. Biological modifiers of environmental drivers considered pre-treatment response to disturbance, tree vigor, and tree-to-tree competition. A model was developed to predict BAI in both topographic positions for applicability to the landscape level, and then was used to test for specific differences in BAI between paired forest treatments differing by one treatment. Forest treatments tested included no management (NM), undercut and even spacing harvest (HE), prescribed fire (Rx), and their combinations. HE significantly increased BAI shortly after treatment. Post-harvest, one or two Rx did not provide additional BAI benefits, nor in the absence of HE, did 2Rx vs. 1Rx treatment. The 1Rx treatment was imposed between multi-year droughts; BAI significantly increased after the treatment and was resistant to droughts. Upland trees were affected by a single year of drought; lowland trees responded only after sequential drought years. A single treatment, HE or 1Rx appeared to be as effective as multiple or mixed treatments in improving BAI in dry pine forest stands. HE appeared to generate the largest effect. Timing of forest treatments relative to site water balance may affect short term (decadal) wood production.