American Trees Are on the Move But No One Knows Why

[DIGEST: Scientific American, The Atlantic, CBS, Great Lakes Echo, Guardian]

North American forests are moving out. Deciduous, broadleaf species are moving west. Evergreen, needle-leaf species are moving north. The reason? Climate change.

A study of forest populations published in Science Advances has found that the mix of trees long associated with each region is changing rapidly. Trees that once thrived in specific areas are dying in those regions, and are appearing in new areas that now feature the rainfall amounts and temperatures they require. Tree species, like animals species, are on the move in search of suitable habitat as conditions change.

As equatorial zones increase in temperature, growing seasons are extending northwards. Trees that favor cooler conditions are finding their traditional zones too hot, and their seeds are now taking hold in northern regions that were formerly too cold — but are now just right.

Climatologists and backyard gardeners alike have noted this change in recent years, and in 2012, the U.S. Department of Agriculture updated its Plant Hardiness Map to reflect the shifts in growing zones to match the current warmer reality. However, scientists were surprised by data indicating that tree species are not just moving north, but moving west. As former dry zones receive greater rainfall amounts, trees that flourished in southern and eastern regions are finding new purchase in soils to the west.

Credit: Source.

“That was a huge surprise for us,” said Songlin Fei, a professor of forestry at Purdue University and one of the authors of the study.

“Different species are responding to climate change differently. Most of the broadleaf species—deciduous trees—are following moisture moving westward. The evergreen trees—the needle species—are primarily moving northward,” said Fei. Hardly any types of trees are moving south or east.

The study found that about three-quarters of tree species common to eastern American forests — including white oaks, sugar maples, and American hollies — have moved west since 1980. More than half of the species studied also moved northward during the same

period. The shifts are occurring at a rate of about 7 miles per decade — a gradual, yet significant change to the appearance and ecosystems of the continent.

On a local level, these shifts mean that existing trees will find the ecosystem that supported them when they were planted no longer offers optimal conditions for those species, leading to vulnerability to pests and disease, damage from more frequent storms, and early die-offs due to temperature shifts.

"Some species will become more vulnerable to limbs breaking from high winds" during more frequent storms, said Leslie Brandt, a Forest Service climate change specialist at the Northern Institute of Applied Climate Science in St. Paul. "Some will be stressed by heat and drought, and then become vulnerable to insects and fungal diseases."

Evergreen trees like white spruce and broadleaf species like paper birch, pin cherry, and white oak will not survive the new warmer temperatures. Trees currently considered southern species, like the Kentucky coffeetree, will thrive in northern climates.

In Wisconsin, the U.S. Forest Service has identified about a third of 85 current tree species as vulnerable to the effects of climate change. Cities like Chicago are anticipating a 20 percent loss of its urban forest due to climate change.

Credit: Source.

“We’re anticipating, by the end of this century, to be between two and eight degrees warmer here in the Chicago region,” said Lydia Scott, Director of Chicago Region Tree Initiative.

The global temperature rose one degree in the past century. Planting a greater diversity of tree species, so that regions don’t lose a substantial number of mature trees in a short time period, is one way to protect the urban tree canopy. “We are encouraging that people expand their species palette so that they have a broader species diversity in the region,” Scott said. 

Since trees take decades to mature and can lives for hundreds of years, creating a healthy future forest is a challenge, since the effects of climate change can damage a tree very quickly. In Canada, which is largely forested with cool-weather species, shifts in climate conditions are expected to outpace tree species migration capacity. The rate of projected climate change is expected to be 10 to 100 times faster than the ability of trees to migrate.

Ironically, one strategy to slow the rate of climate change involves planting trees. Reforestation is included in the Paris Climate Agreement and has been embraced as a way to capture carbon and create cooler microclimates. So plant a tree for Arbor Day — just make sure to plant a species that can withstand the climate to come.