Water Availability, Climate Change and the Growth of Douglas-Fir in the Georgia Basin

Abstract

Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is an important species for timber production in the Georgia Basin with growth dependent on moisture availability during the summer. The potential effect of climate change on water availability and the productivity of coastal Douglas-fir was evaluated using relationships between available water and Douglas-fir yield developed from field measurements and the Ministry of Forests' timber production and recovery analysis system. Summer water availability (May to July rainfall plus 60% of the water storage capacity of the root zone) ranged from 175 to 295 mm for the sites investigated. The May to July potential evaporation for 1951 to 2002 calculated using the Penman-Monteith equation and a field-derived canopy resistance function varied from 153 to 279 mm. The sites had a sensitivity of 6.1 m3 of merchantable volume ha-1 per mm of available water. Summer potential evaporation for a range of climate change scenarios was calculated. Change in water availability through a 10% reduction in rainfall or a 6% increase in potential evaporation over the life of the stand could result in a reduction in stand volume by up to 80 m3 ha-1 at harvest, 10 to 30% of current merchantable volume.