Large, near-term increases in warming-induced tree growth may help offset impacts of increasing disturbance across the Canadian boreal forest

Background/Question/Methods

Large projected increases in forest disturbance pose a major threat to future wood fibre supply and carbon sequestration in the cold-limited, Canadian boreal forest ecosystem. Given the large sensitivity of tree growth to temperature, warming-induced increases in forest productivity have the potential to reduce these threats, but research efforts to date have yielded contradictory results attributed to limited data availability, methodological biases, and regional variability in forest dynamics. Here we apply a novel machine-learning algorithm (i.e. boosted regression tree) to an unprecedented network of over 1 million tree growth records from 18,715 permanent sample plots distributed across both Canada and the US, spanning a 16.5°C climatic gradient, to 1) disentangle non-linear and interacting climatic controls on tree growth for the six most abundant boreal tree species in North America, including balsam fir (Abies balsamea), black spruce (Picea mariana), white spruce (Picea glauca), jack pine (Pinus banksiana), white birch (Betula papyrifera) and trembling aspen (Populus tremuloides); and 2) project the near-term (2050s time period) growth of Canadian boreal forest under different climate forcing scenarios.

Results/Conclusions

Our results reveal a large, positive effect of increasing thermal energy on tree growth for most of the target species, leading to 40-52% projected gains in growth with climate change under RCP 4.5 and 8.5 for the 2050s time period. The large overall positive growth responses varied substantially among regions that the eastern Canadian boreal area displayed the highest gains in growth while the southern region the least. These growth trends also varied significantly among species that balsam fir and jack pine displaying the highest median gains in growth and white spruce the least with regional growth decline. The magnitude of these gains, which peak in the colder and wetter regions of the boreal forest, suggests warming-induced growth increases should no longer be considered marginal, but may in fact significantly offset some of the negative impacts of projected increases in drought and wildfire on wood supply and carbon sequestration and have major implications on ecological forecasts and the global economy.