Frost hardiness vs. growth performance in trembling aspen: an experimental test of assisted migration
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- According to the range limit hypothesis, the distribution of many temperate species is restricted by a trade-off between their capacity to survive winter extremes in the north (or high elevation) and their ability to compete with better-adapted species in the south (or low elevation range limits). This trade-off has important implications in forestry, particularly in the context of managed seed movement under climate change.
- In this study, we aim to quantify trade-offs among growth, frost hardiness and timing of leaf senescence and bud break in populations of trembling aspen, Populus tremuloides Michx., which were observed in a large reciprocal transplant experiment across five planting sites in western Canada, including additional provenances from Minnesota.
- After 10 years, we found pronounced increases in productivity as a result of long-distance transfers in a north-west direction. For example, provenances moved 1600 km north-west from Minnesota to central Alberta (a shift of 7° latitude to the north) produced almost twice the biomass of local sources. Similarly, provenances moved 800 km from central Alberta to north-east British Columbia (4° latitude north) also produced twice the biomass of local sources.
- We further found that increased growth was not associated with lower survival rates. Bud break in provenances transferred north-west generally occurred slightly later than in local sources, suggesting decreased risk of spring frost injury. Leaf abscission was later in provenances transferred in a north-west direction, but they appeared to be very frost hardy, well ahead of very rare early fall frost events.
- Synthesis and applications. This study demonstrated that assisted migration prescriptions have considerable potential to enhance forest productivity. In the case of aspen, even long-distance seed transfers in a north-west direction were successful. We conclude that benefits in productivity outweigh potential risks associated with northward transfer of aspen planting stock under both current and projected future climate conditions.