Warming-induced tree growth may help offset increasing disturbance across the Canadian boreal forest

Significance

Warmer temperatures have the potential to increase productivity in the cold-limited, Canadian boreal forest, but evidence remains controversial. We explored the climatic sensitivity of growth of the six most abundant boreal tree species in North America using an unprecedented network of permanent sample plot records distributed across both Canada and the United States. Our results indicate an overall positive effect of warming on tree growth under several climate change scenarios by midcentury, peaking in the colder, wetter regions of the boreal forest. Despite substantial variations among regions and species, such higher growth rates may help offset some of the negative impacts of projected increases in forest disturbance on future wood supply and carbon sequestration.

Abstract

Large projected increases in forest disturbance pose a major threat to future wood fiber 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 machine learning algorithm to an unprecedented network of over 1 million tree growth records (1958 to 2018) from 20,089 permanent sample plots distributed across both Canada and the United States, spanning a 16.5 °C climatic gradient. Fitted models were then used to project the near-term (2050 s time period) growth of the six most abundant tree species in the Canadian boreal forest. Our results reveal a large, positive effect of increasing thermal energy on tree growth for most of the target species, leading to 20.5 to 22.7% projected gains in growth with climate change under RCP 4.5 and 8.5. The magnitude of these gains, which peak in the colder and wetter regions of the boreal forest, suggests that 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.