UMaine scientists find that trees are out of equilibrium with climate, posing new challenges in a warming world
Marcus Wolf
Under climate change, there’s a
growing interest in assessing whether trees and other species can keep pace
with changing temperatures and rainfall, shifting where they are found, also
known as their ranges, to track their suitable climates.
To test this, a University of
Maine-led research team studied the current ranges of hundreds of North
American trees and shrubs, assessing the degree to which species are growing in
all of the places that are climatically suitable.
Researchers found evidence of
widespread “underfilling” of these potential climatic habitats — only 50% on
average — which could mean that trees already have disadvantage as the world
continues to warm.
Benjamin Seliger, a then UMaine Ph.D. student with the Climate Change Institute, spearheaded the study with his doctoral adviser, Jacquelyn Gill, a UMaine associate professor of paleoecology and plant ecology. Brain McGill, a UMaine professor of biological sciences, and Jens-Christian Svenning, a macroecologist and biogeographer from Aarhus University in Denmark also contributed.
The team used species distribution
models to assess the degree to which 447 North American trees’ and shrubs’
“fill” their potential climatic ranges by comparing regions that are
climatically suitable, known as potential ranges, against where trees are
actually found, or their realized ranges.
The Journal of Biogeography published the
team’s research paper for the study.
Seliger, now a postdoctoral
researcher at the Center of Geospatial Analytics at North Carolina State
University, and co-authors discovered a significant difference between where
the trees they studied could grow, and where they actually grow, also known as
range filling. The average range filling value across all 447 species equalled
48.6%, indicating that on average, trees are not found in about half of the
areas that are climatically suitable for them, according to
researchers.
“We found tree ranges are more
limited by non-climatic factors than expected, suggesting trees may not simply
track warming climates.” Seliger says.
Species distribution models (SDMs)
are a common tool to predict how climate change will affect biodiversity and
the future ranges of plants and animals. Various studies, including the one
from the UMaine-led group, however, caution that because this tool assumes that
species live in all areas that are climatically suitable, known as experiencing
climatic equilibrium, it may not provide an accurate prediction of where
species will be found in the future.
An SDM relies on what has been
considered a foundational principle, “that geographic ranges generally appear
to be in equilibrium with contemporary climate,” according to researchers.
Growing evidence suggests otherwise for many species, which experience climatic
disequilibrium.
Seliger and his team found that
North American trees and shrubs with large ranges tended to show much stronger
evidence of climatic equilibrium, meaning they had high range filling.
Small-ranged species, however, had much lower range filling overall, performing
worse than predicted by a null model. According to researchers, that means
small-ranged tree species, including many rare trees and species the
International Union for the Conservation of Nature (IUCN) lists as vulnerable,
will face additional challenges as they try to track their climates into the
future.
The group also found that small-rage
species may be more limited by nonclimatic influences, such as soils or
pathogens. Conservation efforts for these plants and animals, therefore, should
“account for a complex interplay of factors in addition to climate when
preparing for the next century of global change,” according to
researchers.
Their findings support a growing
body of evidence that for a climatic disequilibrium among various flora. As to
what causes the disequilibrium could be due to two factors, according to
researchers: dispersal lags that date back to the time when glaciers covered
large portions of North American 21,000 years ago, or by non-climatic factors
that may influence ranges more than previously appreciated, such as soil,
competition with other plants, or symbiosis.
“It’s been thought that if you zoom
out to the scale of North America, climate was the most important factor in
determining where species would be found. This study reveals some striking gaps
in our knowledge; even at the scale of an entire continent, soils or other
plants and animals may be playing an important role too. We used to think those
were more important at the more local scale — think of how the trees might
change across two areas of your favorite park,” Gill says. “All of this means
that when it comes to plants, our predictive tools need to get a lot more
sophisticated, if they’re going to be useful for conservation.”
