Print PageClimate warming is predicted to decrease snowpacks, increase winter flooding, increase summer water use by vegetation, and reduce summer streamflow. Using up to 60-yr records of climate and stream flow from long-term ecological research (LTER) sites, we assess the sensitivity of water yield from ecosystems to climate variability. We quantified changes in air temperature, precipitation, and streamflow at eight headwater, undisturbed, forest ecosystem sites with long-term streamflow and climate records: Andrews, Coweeta, Hubbard Brook, Fernow, Fraser, Luqillo, Marcell, and Niwot Ridge, using linear regression and Mann-Kendall tests on annual, seasonal, and daily data.
Results/Conclusions
Annual minimum temperatures have increased significantly for all sites except for Niwot Ridge. Increased minimum temperatures were most frequent in summer (5 sites), followed by spring (4 sites), winter (3 sites), and fall (2 sites). Annual precipitation changed significantly at only two sites (increases at Hubbard Brook and Niwot Ridge). Despite warming, few sites display expected responses of streamflow to climate change at annual or seasonal timescales. Runoff ratios (discharge divided by precipitation) measure ecosystem water use and facilitate comparisons among sites over time. Spring runoff ratios declined significantly at three sites (Andrews, Fraser, and Hubbard Brook), and winter runoff ratios increased at two sites (Fernow, Hubbard Brook). No sites experienced decreases in summer runoff ratios. Analysis of daily records showed an advance of a couple of weeks in the snowmelt-streamflow pulse at sites with seasonal snowpacks (Andrews, Fraser, Hubbard Brook, Marcell). However, sites and/or seasons with actively transpiring vegetation have not experienced expected trends, apparently as a result of vegetation succession or altered transpiration in response to changes in climate.
Experience and results emerging from up to six decades of studies at LTER sites can help assess responses, adaptation and resilience to climate variability and change. Although water yield from headwater ecosystems has responded to climate change, many sites display evidence of ecological resilience to climate change, and hydrologic responses to climate change are smaller than responses to experimental vegetation treatments. Work is needed to examine how far downstream climate change effects on hydrology can be detected. These findings demonstrate the power of long-term ecological research to synthesize ecosystem process understanding to respond to new issues and provide relevant input to ecosystem management in the face of climate change.
