More than half of eastern U.S. tree species examined in a
massive new Duke University-led study aren’t adapting to climate change as
quickly or consistently as predicted.
“Many models have suggested that trees will migrate
rapidly to higher latitudes and elevations in response to warming temperatures,
but evidence for a consistent, climate-driven northward migration is
essentially absent in this large analysis,” says James S. Clark, H.L.
Blomquist Professor of Environment at Duke’s Nicholas School
of the Environment.
Nearly 59% of the species examined by Clark and his
colleagues showed signs that their geographic ranges are contracting from both
the north and south.
Fewer species—only about 21%—appeared to be shifting
northward as predicted. About 16% seemed to be advancing southward, and around
4% appeared to be expanding in both directions.
The scientists analyzed data on 92 species in more than
43,000 forest plots in 31 states. They published their findings in Global
The study found no consistent evidence that population
spread is greatest in areas where climate has changed the most; nor do the
species’ response patterns appear to be related to seed size or dispersal characteristics.
“Warm zones have shifted northward by up to 100 km in
some parts of the eastern United States, but our results do not inspire
confidence that tree populations are tracking those changes,” says Clark,
who also holds appointments at Duke as a professor of biology and statistics.
“This increases the risk of serious lags in tree migrations.”
The concept of climate-driven migration is based on the
assumption that as temperatures warm, the southern edge of some tree species’
ranges could begin to erode as adult trees die and the seeds they leave behind
in the soil can no longer sprout. At the same time, the species could spread to
higher latitudes as seedlings dispersed on their northern boundaries are able
to take root in newly favorable climates there.
To test whether this predicted response was occurring in
real life, Clark and his colleagues pored through decades of data compiled by
the U.S. Forest Service’s Forest Inventory and Analysis Program. They compared
the relative distributions of seedlings, saplings, and adult trees of 92 widely
distributed eastern U.S.
species at 43,334 plots in 30 different longitudinal bands, and factored in
things like seed characteristics, and changes in climate and precipitation.
“The patterns of tree responses we were able to
document using this seedling-versus-tree analysis are more consistent with
range contraction than with northward migration, although there are signs some
species are shifting to higher elevations,” Clark
The fact that the majority of the northernmost latitudes
documented for seedlings was lower than those for adult trees of the same species
indicates “a lack of evidence for climate-mediated migration, and should
increase concern for the risks posed by climate change,” he says.