Forests in Flux

Hardwood species like this tulip-poplar are shifting inland in response to climate change, according to a new study from Purdue, NC State and the U.S. Forest Service.

Climate change shifted the trees in U.S. forests over the last 30 years, drawing hardwood species westward toward more abundant rainfall and evergreens northward to seek out cooler temperatures, a comprehensive new study shows.

Researchers with Purdue, North Carolina State University and the U.S. Forest Service analyzed data for 86 tree species over three decades, finding differences in tree distribution because of climate change, with precipitation having a more powerful influence than temperature.

Researchers found that evergreen trees such as longleaf pines have been moving north in response to rising temperatures.

Scientists found that evergreen trees such as longleaf pine generally have been moving north in response to rising temperatures. In contrast, most hardwoods like tulip-poplar are shifting westward following changes in rainfall patterns, the team, led by Purdue’s Songlin Fei, finds. Precipitation increased in the central U.S. but dropped in the Southeast, where droughts were more frequent during the study period from 1980 to 2015.

“The surprise was in the hardwood trees wanting to move inland,” says Kevin Potter, an NC State research associate professor and co-author of the research in Science Advances. “In the short term, precipitation had a much stronger effect on abundance of forest tree species than temperature.”

The study uses long-term data from the U.S. Forest Inventory and Analysis program collected from tens of thousands of plots across the country, beginning in 1980. After Forest Service researcher Chris Oswalt assembled the older data in usable form, the team turned to a method Potter developed that uses hexagonal grids to create comparable samples.

“Harvesting data like this is a big challenge, but it’s something we need to do more of to detect broad-scale patterns in the U.S.,” Potter says.

Overall, 73 percent of tree species studied made a shift toward the west, while 62 percent moved toward the north. The westward shift took place 1.4 times faster than the northward movement, showing the importance of available moisture for trees.

“Although many factors are involved in climate change, models tend to be more temperature-driven than precipitation-driven, because temperature is more certain and precipitation is less so,” Potter says.

Does the study mean that forests will change measurably in ways most people will notice?

“Not in the short term,” Potter says. “But it does mean that climate change could have an impact over the lifespan of a tree.

“In the Southeast, we need to think about economically important species such as the loblolly pine, which we grow on a 20- to 30-year rotation in plantations, to make sure that the seed is well-adapted to conditions at the site.”

Pine plantations cover about 15 percent of forested land in the South with almost a billion loblolly seedlings planted each year, according to NC State’s Cooperative Tree Improvement Program, home to tree breeding programs for more than 60 years.

While trees in general are “pretty adaptable, robust organisms,” climate change can affect biodiversity and sustainability of whole ecosystems, Potter says.

“We don’t mean to be alarmist, but this goes to show that the system is dynamic and unpredictable. We need to keep a close eye on what’s happening in our forests.”

5 responses on “Forests in Flux

  1. Donna Newman says:

    Even when communicating with non-scientists, perhaps you should clarify that it is the population distributions of tree species that are moving, not individual trees. I can just imagine someone mistakenly believing that trees are picking up their roots in the middle of the night to quietly creep a few feet north…

    1. Lindsey says:

      Walking trees? Uh-oh, watch out, Macbeth!

    2. D'Lyn Ford says:

      Donna, thanks for the comment. We tried to make that clear in a couple of places, including the second sentence and Kevin Potter’s initial quote: “Researchers with Purdue, North Carolina State University and the U.S. Forest Service analyzed data for 86 tree species over three decades, finding differences in tree distribution because of climate change, with precipitation having a more powerful influence than temperature.”

      “In the short term, precipitation had a much stronger effect on abundance of forest tree species than temperature.”

  2. Robin says:

    I don’t see any reference to this in the study summary. I don’t doubt climate change from whatever cause is a factor. I am however left wondering how much changing land use patterns in regards to industrial scale forest practices (i.e. conversion of “native” poly-specific mixed and hardwood forests to monocultural pine plantations) is affecting the outcome in comparison.

    This is especially evident here when I see “Overall, 73 percent of tree species studied made a shift toward the west, while 62 percent moved toward the north. The westward shift took place 1.4 times faster than the northward movement, showing the importance of available moisture for trees.”; I have to ask what considerations have been applied in regards to method accounting for “geo-political” outcomes as opposed to environmental ones?

    In consideration of this remember North Carolina is much wider E to W than N to S. As such, without consideration made of the effects of things like state land management policy, economics, elevation and development trends along with resulting urban/exurban microclimates I can’t imagine this to represent a meaningful study.

    1. D'Lyn Ford says:

      Hi, Robin: Sound observation. As you said, this article is a brief overview of the research, which appears in the AAAS journal Science Advances. Check the “Discussion” section of the full paper for more details about the issues you raise. The full paper contains source citations, maps, charts and graphics illustrating the findings, as well as supplementary materials. Here’s an excerpt from the “Discussion” section:
      “Given the broad scale at which our analysis was conducted, a variety of indirect and nonclimatic factors (for example, fire regimes, invasive species, forest management, conservation efforts, and land use change) could also have influenced the directional trends observed. The fact that the observed abundance change is positively correlated with the densification of forests, which is often attributed to diminishing fire frequency and severity (26), confirms that nonclimatic factors are also responsible for the observed spatial shift of species abundance. Heavy infestations of invasive insects, plants, and pathogens could contribute to species shifts in other directions (41, 42). In addition, forest conservation and plantation efforts in the study area such as those by the U.S. Department of Agriculture (USDA) Conservation Reserve Program could also influence the observed spatial shift. Nevertheless, we observed clear broad-scale evidence of the impact of climate change on forest tree spatial dynamics, where changes in mean annual precipitation alone explained about 19% of the variability in species abundance change and spatial shift.

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