Drivers of productivity differences between Douglas-fir planted within its native range in Oregon and on exotic sites in New Zealand

Abstract

Forest species planted outside their native range sometimes exhibit growth rates that far exceed those in their native range. In species grown for timber production, the improved growth in certain exotic environments has led to the widescale propagation of some outstanding species, but the proposed mechanistic drivers behind this improved growth are little studied. Given the increasing importance of understanding growth response to climate changes, many of these species present ideal test cases, including coast Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), a widely planted timber species native to the Pacific Coast of North America. We identified a set of seven sites in the Oregon Cascades Range and the South Island of New Zealand that were planted with Douglas-fir seedlings originating from a shared seed source in 2004, presenting an opportunity to examine drivers of productivity while reducing the confounding effect of genetics. We sampled plots at each of these sites during the 2019 dormant season by measuring standing trees, collecting increment cores, assessing soil conditions, and installing meteorological instruments. Using these data, we constructed an environmentally sensitive model of annual radial growth of the stem at breast height, accounting for between- and within-location variation controlled by environmental factors. Trees at New Zealand locations had increased diameter growth compared to those in Oregon. Lower vapor pressure deficit during the growing season was the primary cause of this improved productivity in New Zealand through presumed effects on higher stomatal conductance and net photosynthesis. In contrast, other environmental factors were identified as driving productivity differences between sites within Oregon and within New Zealand.