Global Warming’s Not-So-Hot Impact on Trees

How will climate change impact trees? Scientists are exploring the possibility as we speak. (D. Simmons)

Trees help make us cool in the face of global warming*. But, sturdy and steadfast as they may be, some species are showing vulnerability in our increasingly warming world.

“The effects of climate change on trees will be complex,” says Robert Fahey, PhD, of the Morton Arboretum, as they will face more frequent, increasingly severe storms, more instances of drought, and the potential for an increase in pest and pathogen populations.

The signs of impact are already clear in some areas. A study from the Canadian Forest Service on the forests of Siberia, Canada, and Alaska found that many of the modeled predictions of forest change are now taking place, including a decline of certain species and a migration of some trees further north and upslope (like, literally, up the mountain, which, you guessed it, does have to end somewhere).

You Win Some…

Whether trees can, ahem, weather the storm will vary significantly based on geography and specific nature of the threat. For example, on the East Coast, temperature is not likely to pose a particular threat to trees anytime soon, points out Charles Canham, PhD, of the Cary Institute. This is in large part because eastern U.S. species are suited for a broad range of temperatures, so a few extra degrees of warmth isn’t likely to bother them. Take the beech tree, for instance, which grows from Nova Scotia to Mexico. It already deals well with 22-26 degrees Fahrenheit variation—so, by itself, the potential global increase of 2.5-10 degrees in the next century probably won’t make a real difference.

In other words, as Canham says, “the word ‘temperate’ means what it means.” The forests of eastern North America, Europe, and temperate Asia—occur across the broadest range of temperature and precipitation of any biome, so it makes sense that these would be one of last places to expect rapid response. “Trees are also slow to change, with long life spans,” he explains. “Big trees on the East Coast are relatively buffered from climate change, at least for the next 100 years or so.”

This also rings true for some species in the Midwest, where Fahey says the region’s most “charismatic” trees, oaks, will likely be among the region’s climate “winners.”

Oak saplings can take up to 50 years to reach their full-grown glory. (Photos by the Morton Arboretum.)

Oak saplings can take up to 50 years to reach their full-grown glory. (Photos by the Morton Arboretum.)

“Most tree species in the Midwest, like the Northeast also have extremely broad ranges. In terms of their range limits, they can exist in places with much hotter, drier climates than they’re going to experience in the next 50 years,” says Fahey.

For example, most of the oaks that dominate in Illinois exist all the way to Arkansas and Texas. Since those Southern states currently model what Midwestern climate could look like 100 years from now, to some degree, Illinois oaks will be a-okay in terms of temperature variation.

…You Lose Some

Unfortunately, many tree species, in the Western and Southern states as well as further afoot, are much more vulnerable to global warming. According to an article in Science, warming and related increases in water shortage has already contributed to a rapid increase in tree mortality in the West within the last few decades.

In California, for example, the famed Joshua tree, which is especially vulnerable to temperature change, could disappear from 90 percent of its current range in 60-90 years due to warmer temperatures, according to a USGS study.

This little emerald ash borer is causing some big problems...(Pic via USDA/Flickr)

This little emerald ash borer is causing some big problems…(Pic via USDA/Flickr)

Warming temperatures are also very likely to enhance the viability of non-native pests and pathogens, according to an article published in Forestry. Already, oaks in the South are being severely threatened by an increase in pests, which thrive in warmer conditions. The Fraser fir in the southern Appalachians has declined by as much as 91 percent, in large part because of an increase in pests.

According to Fahey, pests like the hemlock woolly adelgid and emerald ash borer are quick to adapt—especially when the climate is changing in their favor. Many of these pests wouldn’t likely survive the current Midwestern winter, but they could potentially withstand the milder winters of the predicted future. “Because these pests generally adapt very quickly, we’re more likely to see an increase in those types of interactions, wherein a pest comes in and takes down entire tree populations,” he explains.

In the West, mountain pine beetles are devastating whitebark pine populations, causing whole forests to die. This is due to longer life cycles of the beetle and an increase in what amounts to birth cycles per year (hello, overpopulation) spurred by unusually high temperatures. And, in turn, these still-standing but wholly dried out trees contribute to longer, more severe fire…a vicious cycle for sure.

“The role of fire out West also makes this whole issue a lot trickier,” notes Canham. Because forests in Western states are much more on the cusp between prairie and woodland, he explains that “changes in fire there could lead to big changes in forest cover and the very nature of forests.” Forest Service research concludes that the increasing occurrence of large fires across the western U.S. from the ’80s on is strongly linked to rising temperatures, beetle kill, and earlier spring snowmelt.

Further afoot, the iconic baobab of Madagascar and multiple species in Mexico are also reeling from the effects of climate-related challenges.

It’s No Longer “Just” a Prediction

The predicted effects of climate change on trees are happening already, as was highly visible in the nightly news throughout 2020 – featuring huge wildfires across huge swaths of the American West. 

According to the Colorado State Forest Service report in 2017, there were already 834 million standing-dead trees, threatening watersheds and “worsening risk of ruinous fires”.  One in 14 trees were already dead in Colorado forests and the number of gray-brown standing-dead trees had increased 30 percent since 2010 to 834 million, according to the state’s annual survey. Unfortunately, in the 4 years since that report, the number of dead trees in Colorado has continued to grow. Colorado experienced by far its worst wildfire season in history in 2020, with the three largest fires in its history all occurring in 2020.  

The record-breaking wildfires in the West have burned huge tracts of land (and homes and businesses), and taken lives. According to the National Interagency Fire Center, By December 18, 2020 there were about 57,000 wildfires compared with 50,477 in 2019. More than 10.3 million acres were burned in 2020, compared with 4.7 million acres in 2019. Five of the top 20 largest California wildfires fires occurred in 2020, according to CalFire’s list. Wildfires in California have burned a record 4.2 million acres, damaging or destroying 10,500 structures and killing 31 people.”

Speak for the Trees

So, can we just let the winners win and not worry about the losers? It’s complicated (what else is new?), but Fahey says that generally, any one tree species can be very important for animal habitats and interdependent ecosystems, and losing it can make the entire system less resilient.

To put a more personal spin on the matter, healthy forests are good for all of us. From Canham’s perspective, “the regrowth of forests in the Eastern U.S. [after over-logging in the 1800s, and again in the 1950s and ’60s] has been part of what’s kept us from cooking, and there is still a lot of potential for forests to continue to grow and offset our emissions.” Importantly, he clarifies that promoting healthy forests doesn’t mean that we’re off the hook for curbing CO2 emissions more directly. “We do also have to radically reduce emissions—but trees are playing a part—and not an insignificant part helping offset our emissions,” he declares.

Wanna play the Lorax? Along with cutting your own energy use to help curb polluting energy emissions, you can specifically help trees by doing One Green Thing: Suck up more CO2 by planting a tree.


*This is a companion piece to the myth that treehugging isn’t cool. #ReadtheMyth to learn why trees help us all stay cool in the face of global warming.