The response of light, water, and nutrient availability to pre-commercial thinning in dry inland Douglas-fir forests

Fire suppression and limited forest management have caused overstocking in many forests across the western United States. Overstocked stands have higher competition for limiting resources and causes tree stress. The amount of stress a tree experiences is related to the current availability of resources (site productivity), and the competition for those resources (stand density). Stressed trees are more susceptible to insects, disease, and mortality, which cause fuel buildup and increase wildfire risk. Pre-commercial thinning (PCT) can alleviate stress by decreasing the amount of competition in younger stands. The objective of this study was to determine how reducing competition through PCT might improve resource availability to trees at a range of initial stand conditions. We used a triplet-plot approach including 4.3 m and 5.5 m spacing and compared those to unthinned controls in stands varying in site productivity (height growth) and density (initial stand basal area) throughout northern Idaho and northeastern Washington. Stands were dominated by Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), grand fir (Abies grandis (Douglas ex D. Don) Lindley), and western red cedar (Thuja plicata Donn ex D. Don). As expected, higher density stands intercepted greater amounts of light. Thinned stands intercepted 33-58% less light than controls, depending on tree spacing. Site productivity was positively correlated with soil moisture in the spring and soil temperature in both the spring and summer. Pre-commercial thinning increased soil moisture by 6% in the spring and 10% in the summer, and soil temperature increased by 0.5 degrees C in spring and 1 degrees C in summer. Douglas-fir foliar N, Ca, Zn concentration decreased after thinning, while P and B increased and S, K, and Cu were unchanged. Thinning had the greatest relative impact on summer soil moisture, followed by soil N availability, and light interception. Thinning response studies frequently focus on light availability; our results demonstrate that the response of soil moisture and nutrient availability exceeded that of light availability for the studied forest types. (C) 2015 Elsevier B.V. All rights reserved.