An analysis of 35 headwater basins in the United States and Canada found that the impact of
warmer air temperatures on streamflow rates was less than expected in many
locations, suggesting that some ecosystems may be resilient to certain aspects
of climate change.
The study was published in BioScience, in which the Long-Term Ecological Research (LTER)
network of 26 sites around the country funded by the National Science
Foundation is featured.
Lead author Julia Jones, an Oregon State
University geoscientist,
said that air temperatures increased significantly at 17 of the 19 sites that
had 20- to 60-year climate records, but streamflow changes correlated with
temperature changes in only seven of those study sites. In fact, water flow
decreased only at sites with winter snow and ice, and there was less impact in
warmer, more arid ecosystems.
“It appears that ecosystems may have some capacity for
resilience and adapt to changing conditions,” said Jones, a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “Various ecosystem processes may contribute to that resilience. In Pacific Northwest forests, for example, one hypothesis is
that trees control the stomatal openings on their leaves and adjust their water
use in response to the amount of water in the soil.”
“So when presented with warmer and drier conditions, trees
in the Pacific Northwest appear to use less
water and therefore the impact on streamflow is reduced,” she added. “In other
parts of the country, forest regrowth after past logging and hurricanes thus
far has a more definitive signal in streamflow reduction than have warming
temperatures.”
LTER sites were established to investigate ecological
processes over long temporal and broad spatial scales throughout North America,
including the H.J. Andrews Experimental
Forest in Oregon,
as well as sites in Alaska, New
Mexico, Minnesota, New Hampshire, Georgia,
Puerto Rico, Antarctica, and the island
of Moorea. Not all were
part of the BioScience study.
In that study, warming temperatures at some of the headwater
basins analyzed have indeed resulted in reduced streamflow due to higher
transpiration and evaporation to the atmosphere. But these changes may be
difficult to perceive, Jones said, given other influences on streamflow,
including municipal and agricultural water usage, forest management, wildfire,
hurricanes, and natural climate cycles.
“When you look at an individual watershed over a short
period of time, it is difficult to disentangle the natural and human-induced
variations,” Jones said, “because hydrologic systems can be quite complex. But
when you look at dozens of systems over several decades, you can begin to gauge
the impact of changing vegetation, climate cycles, and climate trends.”
“That is the beauty of these long-term research sites,” she
said. “They can provide nuanced insights that are crucial to effective
management of water supplies in a changing world.”
Jones said the important message in the research is that the
impacts of climate change are not simple and straightforward. Through
continuing study of how ecosystems adapt to changing conditions, resource
managers may be able to adapt policies or mimic natural processes that offer
the most favorable conditions for humans and ecosystems to thrive.
