Published today in Nature Ecology & Evolution, the study projects that if global greenhouse gas emissions continue on their current trajectory, rising temperatures and shifting rainfall patterns could amplify tree pest and disease pressure across more than 80 percent of North American forests by mid-century. The implications stretch far beyond the timber industry: these forests serve as some of the planet’s most important carbon sinks, filter the air billions of people breathe, and anchor entire regional economies. Their transformation, the researchers warn, is not a distant possibility but a process already underway.

“The forests that exist today aren’t going to be ones existing in 2040,” said Dr. Jonathan Davies, professor in the departments of forest and conservation sciences and botany at the University of British Columbia and senior author of the study. “The process is happening already. I think we’ve got to put everything on the table because the status quo is no longer tenable.”

To arrive at these projections, the research team assembled an extraordinarily rich dataset: observations from more than one million individual trees across the United States and parts of Canada, combined with both current and historical climate records. By analyzing where tree damage from insects and pathogens has occurred and how those occurrences correlate with climatic variables, the team built statistical models capable of forecasting where future risks are most likely to emerge under continued warming. The result is a set of continent-scale risk maps that reveal, in unprecedented detail, which forests face the greatest threats and from which agents.

The findings paint a picture that is anything but uniform. Climate change, Dr. Davies explains, is creating a complex mosaic of winners and losers rather than affecting every forest in the same way. Perhaps counterintuitively, the forests most likely to suffer are those that have historically been coolest. As temperatures rise, previously cold-limited insect populations and pathogens are expanding their ranges into ecosystems that never evolved defenses against them. In some historically warmer regions, the effects may be smaller—or even reversed—as conditions become less favorable for certain pests and pathogens already at the thermal limits of their tolerance.

Among the species facing the steepest projected increases in pressure are northern and mountain-dwelling trees: gray willow, Rocky Mountain fir, and—perhaps most iconically—black spruce, the slow-growing conifer that dominates vast stretches of the boreal forest and stores enormous quantities of carbon in its soils. The prospect of losing these trees carries global significance. Boreal forests are among the largest terrestrial carbon reservoirs on Earth, and their degradation would not merely eliminate a carbon sink; it could actively convert these ecosystems into carbon sources, creating a feedback loop that accelerates the very warming driving the problem.

The insects themselves tell a compelling story about what warming makes possible. More than 60 percent of the areas examined are projected to experience 30 percent more insect pressure by mid-century, with insects posing the more immediate threat compared with pathogens. Among the most notorious beneficiaries of a warming climate is the spongy moth, an invasive defoliator already wreaking havoc in Canadian forests, which is projected to expand significantly across eastern North America. Another is the hemlock woolly adelgid, a tiny sap-sucking insect that has already devastated hemlock forests throughout parts of eastern North America, killing mature trees and transforming the composition of entire forest stands. As winters warm, cold snaps that once killed off overwintering pests fail to arrive, and insect generations that once required two years to complete their life cycles now manage it in one, allowing populations to explode.

For Canada specifically, the projections reveal a fascinating east-west divide. In western British Columbia, the models indicate elevated insect pressure but potentially fewer pathogens, while the pattern reverses in parts of eastern British Columbia—a reflection of the profound differences in climate and forest composition between the two regions. Species of particular ecological concern include limber pine and whitebark pine, keystone trees of the province’s mountain ecosystems that may be especially vulnerable to the combined effects of a changing climate and shifting pest pressure. These five-needle pines already face existential threats from white pine blister rust and mountain pine beetle; the addition of climate-driven stress could push them past a threshold from which recovery becomes nearly impossible.

Yet the study’s authors are careful to acknowledge the limits of their models. Because the underlying damage data is drawn primarily from the United States, projections become less certain further north, where forest conditions diverge from those represented in the training data. “With more local forest health monitoring data, it might be possible for future work to refine these projections for B.C. forests,” said Dr. Zihui Wang, a postdoctoral researcher in UBC’s department of botany and lead author of the study. This data gap represents a genuine vulnerability for a country whose forests cover nearly nine million square kilometers—more than a third of its landmass—and whose forest products sector supports hundreds of communities.

Not every forecast is grim. In a twist that underscores the complexity of ecological responses to climate change, some tree species may actually benefit. Tulip trees and American sycamores in the southeastern United States could see reduced pest and disease pressure as warming conditions become less favorable for the agents that currently attack them. These relative “winners” may expand their dominance in forests that are simultaneously losing other species, reshaping the structure and function of eastern woodlands in ways that are difficult to fully anticipate. But even for these apparent beneficiaries, the long-term picture remains uncertain, as ecological communities reorganize under conditions without historical precedent.

What distinguishes this study from previous work on forest pests is its predictive utility. Rather than documenting damage after the fact, the risk maps produced by Wang, Davies, and colleagues give forest managers a genuine window into the future—and, critically, time to act. “Our maps can help forest managers to identify where additional monitoring and prevention efforts should be focussed,” said Dr. Wang. “We can also project which tree species may be particularly vulnerable and which pests and pathogens could pose the biggest threat, giving us a window into the future and hopefully, a headstart on protecting our future forests.”

The practical interventions that follow from such foresight are diverse. Governments and forest managers can prioritize planting hardier tree species in vulnerable regions. They can create physical barriers to pest or pathogen spread by strategically removing specific trees or entire forest sections, disrupting the continuity that allows outbreaks to sweep unimpeded across the landscape. And they can maintain and enhance tree diversity—a form of ecological insurance, since forests composed of many species are far less likely to be completely destroyed by any single pest or pathogen than monoculture stands. Assisted migration, in which foresters deliberately plant species better suited to future conditions, represents a more controversial option that some researchers argue deserves serious consideration.

Underlying all of these strategies, however, is a more fundamental point that Dr. Davies is eager to emphasize: the findings are a reminder that climate change is not merely a story about weather. Its effects cascade through biological systems in ways that reshape entire ecosystems, and pest dynamics are one of the most potent and least visible vectors of that transformation. “Forests are a fundamental part of our lives, but climate change is reshaping these ecosystems,” he said. “This research is another early warning sign of how we’re altering the climate system and the impact it’s going to have.”

The research was partly funded by the Natural Sciences and Engineering Research Council of Canada. As emissions trajectories continue to point toward a warmer world, the study’s message is unambiguous: the forests that define North America’s landscapes, economies, and carbon balance are being rewritten in real time. Whether the continent’s forests of 2050 resemble those of today depends, in large measure, on decisions made in the next few years—about emissions, about monitoring, and about how boldly forest management adapts to a future that is no longer hypothetical.