Effects of Climate and Thinning on Coastal Douglas-fir Annual Biomass Growth at Four Sites

Issues related to global warming and human releases of CO2, primarily from the use of fossil fuels, have simulated interest in using forest biomass and forest products to remove store carbon through photosynthesis which removes atmospheric CO2 and using forest biomass for energy as a fossil fuel substitute. Unfortunately, current methods for estimating forest biomass and the amount of carbon and energy it contains are crude and inaccurate and relatively little research has been done to understand how silviculture and climate affect a tree's annual biomass growth and its distribution within the tree. This study uses a unique set of x-ray densitometer data obtained from five positions along the stems of Douglas-fir trees from a thinning trial on four sites in western Oregon and Washington to examine how thinning and climate affect the biomass increment and its distribution. The x-ray densitometer data, combined with local climate data over the life of each stand permitted examination of how variations in climate at each site and imposition of thinning between 1987 and 1991 and harvested in 2006 at age 33--50. Not surprisingly responses differed greatly between the sites due to great geographic dispersion and soil differences which affect water deficit conditions. However, it was found that, compared to unthinned controls, thinning increased annual biomass increment. Immediately following thinning, there was greater ring mass and relatively more latewood produced toward the lower stem and relatively less toward the upper stem. This is consistent with stresses due to wind sway and heavier crowns after thinning and the reduction of competition for water by the residual stand. Although ring mass gains due to thinning were still evident 12 years, later response was lower and there was a reversal of the mass distribution patterns along the stem. Ring mass production and distribution were also affected by climate. Higher summer temperatures worsened drought and water stress conditions and lowered ring mass growth; likely a reduction in growth when dense latewood would typically be produced. This did not occur on one site seemed to always be in drought-induced summer dormancy. It was also apparent that the an early return of cold winter-like storms in the fall, the continuation of such storms into the spring, or winter flooding can reduce ring mass growth and alter the relative production and distribution of its earlywood and latewood components.