We evaluate genetic test plantations of North American Douglas-fir provenances in Europe to quantify how tree populations respond when subjected to climate regime shifts, and we examined whether bioclimate envelope models developed for North America to guide assisted migration under climate change can retrospectively predict the success of these provenance transfers to Europe. The meta-analysis is based on long-term growth data of 2800 provenances transferred to 120 European test sites. The model was generally well suited to predict the best performing provenances along north-south gradients in Western Europe, but failed to predict superior performance of coastal North American populations under continental climate conditions in Eastern Europe. However, model projections appear appropriate when considering additional information regarding adaptation of Douglas-fir provenances to withstand frost and drought, even though the model partially fails in a validation against growth traits alone. We conclude by applying the partially validated model to climate change scenarios for Europe, demonstrating that climate trends observed over the last three decades warrant changes to current use of Douglas-fir provenances in plantation forestry throughout Western and Central Europe.

With lengthening growing seasons but increased temperature variability under climate change, frost damage to plants may remain a risk and could be exacerbated by poleward planting of warm-adapted seed sources. Here, we study cold adaptation of tree populations in a wide-ranging coniferous species in western North America to inform limits to seed transfer. Using tree-ring signatures of cold damage from common garden trials designed to study genetic population differentiation, we find opposing geographic clines for spring frost and fall frost damage. Provenances from northern regions are sensitive to spring frosts, while the more productive provenances from central and southern regions are more susceptible to fall frosts. Transferring the southern, warm-adapted genotypes northward causes a significant loss of growth and a permanent rank change after a spring frost event. We conclude that cold adaptation should remain an important consideration when implementing seed transfers designed to mitigate harmful effects of climate change.

Northern forests at the leading edge of their distributions may not show increased primary productivity under climate warming, being limited by climatic extremes such as drought. Looking beyond tree growth to underlying physiological mechanisms is fundamental for accurate predictions of forest responses to climate warming and drought stress. Within a 32-year genetic field trial, we analyze relative contributions of xylem plasticity and inferred stomatal response to drought tolerance in regional populations of a widespread conifer. Genetic adaptation leads to varying responses under drought. Trailing-edge tree populations produce fewer tracheids with thicker cell walls, characteristic of drought-tolerance. Stomatal response explains the moderate drought tolerance of tree populations in central areas of the species range. Growth loss of the northern population is linked to low stomatal responsiveness combined with the production of tracheids with thinner cell walls. Forests of the western boreal may therefore lack physiological adaptations necessary to tolerate drier conditions.

Choosing drought-tolerant planting stock in reforestation programs may help adapt forests to climate change. To inform such reforestation strategies, we test lodgepole pine (Pinus contorta Doug. ex Loud. var latifolia Englm.) population response to drought and infer potential benefits of a northward transfer of seeds from drier, southern environments. The objective is addressed by combining dendroecological growth analysis with long-term genetic field trials. Over 500 trees originating from 23 populations across western North America were destructively sampled in three experimental sites in southern British Columbia, representing a climate warming scenario. Growth after 32 years from provenances transferred southward or northward over long distances was significantly lower than growth of local populations. All populations were affected by a severe natural drought event in 2002. The provenances from the most southern locations showed the highest drought tolerance but low productivity. Local provenances were productive and drought tolerant. Provenances from the boreal north showed lower productivity and less drought tolerance on southern test sites than all other sources, implying that maladaptation to drought may prevent boreal populations from taking full advantage of more favorable growing conditions under projected climate change.

Understanding the magnitude and cause of variation in tree growth and forest productivity is central to sustainable forest management. Measurements of annual growth rings allow assessments of individual tree, tree population and forest ecosystem vulnerabilities to drought stress or other changing forest disturbance regimes (insects, diseases, fire), which can be used to identify areas at greatest risk of forest losses. Given a heightened demand for tree-ring data, we consolidated and synthesized tree-ring studies and datasets gathered over the past 30 years in Canada by scientists with the Canadian Forest Service and research partners. We incorporated these datasets into a data repository that currently contains tree-ring measurements from 40,206 tree samples from 4,594 sites and 62 tree species from all Canadian provinces and territories. Through our synthesis, we demonstrate the value of such large ensembles of tree-ring data for identifying patterns in tree growth over large spatial scales by mapping pan-Canadian drought sensitivity. Overall, we found high coherence in the samples analysed; low coherence was generally limited to data- poor regions and species. Drought sensitivity was widespread across species and regions: 34% of sampled trees displayed a significant positive relationship between annual growth increment and summer soil moisture index. Dependence upon water availability in species Picea mariana, Pinus banksiana, Pinus contorta, and Pseudotsuga menziesii was more strongly expressed in the warmest regions of the species’ range; for species Picea glauca and Populus tremuloides, drought sensitivity was stronger in the driest regions. This unprecedented consolidation and synthesis of tree-ring data will enable new research initiatives (e.g., meta-analyses) aimed at improved understanding of the drivers, patterns, and implications of changes in tree growth, as well as facilitating new research collaborations in earth and environmental sciences. Amongst other things, there is a need for expanding the spatial distribution of sites across Canada’s northern regions, increasing the number of samples collected from older stands and angiosperm species, and integrate datasets from studies that evaluate the effects of silvicultural experiments, including provenance and progeny trials, on tree growth.