Geographic variation in adaptative traits in Douglas-fir and responses to climate change
Douglas-fir (Pseudotsuga menziesii) has considerable geographic genetic variation with one of the widest natural ranges of any tree species. Douglas-fir populations are generally regarded as being closely adapted to their native environments with relatively steep clines in adaptive traits associated with steep environmental gradients. Genecology studies have found considerable population variation in adaptive traits associated with climatic gradients at seed-source locations, particularly with winter temperatures and summer aridity. A relatively strong relationship between adaptive traits and seed-source climates implies that rapid climate change will result in native populations that are increasingly maladapted. Genecology studies, however, necessarily make assumptions that local populations are best adapted to local climates. Moreover, the direct consequences of maladaptation with respect to long-term survival, growth and reproduction are not quantified. Reciprocal transplant studies directly test the long-term adaptation of different populations including directly testing assumptions about local adaptation. We established a reciprocal transplant study in which 120 open-pollinated families from 60 populations from 12 regions representing a diversity of environments in Washington, Oregon and California were planted back into sites in 9 of those regions. Survival, heights and diameters were recorded at the end of each growing season. Adaptive traits of vegetative phenology, cambial phenology, drought hardiness, and cold hardiness were recorded on a subset of populations and years.
One management option to respond to concerns about maladaptation of local populations is to move populations to locations where they may be expected to be adapted to future climates. Results from genecology studies indicate that transfer distances of up to 1,000 m in elevation may be required to maintain adapted populations in the future assuming that local populations are currently best adapted to local climates. Early results from the reciprocal transplant study provide evidence of local adaptation, and indicate that large deviations from the local climate would result in decreases in productivity. Thus, there are likely to be trade-offs between short- and long-term adaptation. Traits associated with drought resistance and cold hardiness were related indicating that a general stress-tolerant genotype may be found in populations from locations with either cold winters or arid summers. This implies that population movements from warmer, drier environments may allow for increased cold tolerance to near-term cold events while allowing for greater drought resistance in future climates, but perhaps with a possible decrease in near-term productivity.