Montana State University

MSU scientist part of international study showing more efficient forests in 20th century

May 11, 2015 -- MSU News Service

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BOZEMAN – European forests became nearly 20 percent more efficient at using water during the 20th century because of increases in atmospheric carbon dioxide, although, surprisingly, this didn’t result in regional water savings, according to an international consortium that involved Montana State University.

The research team published its results May 11 in Nature Climate Change, a monthly journal dedicated to publishing the most significant and cutting-edge research on the science of climate change, its impacts and wider implications for the economy, society and policy. Ben Poulter, an MSU faculty member with a dual appointment in the ecology department and Montana Institute on Ecosystems, was one of the co-authors.

The researchers discovered the increased efficiency after investigating the consequences of greater amounts of atmospheric carbon dioxide on plant functioning and feedbacks to the environment. They noted that leaves play a major role in global terrestrial carbon and water cycles, with more than 100 kilograms of water passing through the tiny stomatal pores to photosynthesize one kilogram of sugars. Changes in how ecosystems cycle water under increasing atmospheric carbon dioxide concentrations have implications for flooding, soil moisture and climate.

The team began its study by collecting tree rings from Morocco to Norway and measuring carbon isotopes to determine variations in water efficiency, which is defined as the amount of water needed to assimilate a given amount of carbon. They also used statistical techniques and model simulations to determine how trees and forests responded to climate variations and increased carbon dioxide in the atmosphere. Climate variations, including regional warming and increases in carbon dioxide, affect isotopes and water use efficiency.

The researchers determined that the water-use efficiency in temperate forests increased by 14 percent in broadleaf species and 22 percent in needleleaf species.

Interestingly, that increase didn’t translate into a reduction in transpiration and the regional water balance, however. Computer simulation modeling showed instead that any net savings from increased water efficiency was countered by a longer growing season, increases in leaf area and greater transpiration.

“It thus seems unlikely that plants will reduce the surface-to-atmosphere flux of water vapor -- a strong greenhouse gas,” the researchers said. “It is also unlikely that plant responses to increased carbon dioxide will substantially increase soil moisture or river run-off.”

By accounting for climate change and its influence on the behavior of stomata – microscopic openings in the leaves -- the researchers said the study provides benchmark information about how trees respond to increased carbon dioxide. They added that one of the key uncertainties in projecting global climate is how the models represent the carbon cycle, but they were pleased with how their study handled that.

“We were able to compare the tree-ring-based estimates with various vegetation models and were pleased to see tight agreement. This sort of testing helps us understand where we can further improve models, or in this case gives us confidence in the model projections for these ecosystem metrics,” said co-author Chris Huntingford, a climate modeler at the Center of Ecology and Hydrology in the United Kingdom.

David Frank, lead author of the study and a dendroclimatologist at the Swiss Federal Research Institute WSL, noted that “Tree-ring data provide one of the unique opportunities to obtain long-term records of ecosystem responses to climate change.”

Poulter said the research also showed the importance of studying processes on a variety of scales, from the stomata to ecosystems.

“Deep insights into the complex array of earth system feedbacks are only possible by coordinating large interdisciplinary research teams and approaches integrating both empirical data and model results,” Poulter said.

Evelyn Boswell, (406) 994-5135 or evelynb@montana.edu