July 11, 2013
Trees Use Water More Efficiently Due To CO2 Rise
redOrbit Staff & Wire Reports - Your Universe OnlineDave Hollinger from the US Forest Service's Northern Research Station (NRS) and Trevor Keenan of Harvard University analyzed direct, long-term measurements of whole-ecosystem carbon and water exchange.
According to Hollinger, his team's analysis "suggests that rising atmospheric carbon dioxide is having a direct and unexpectedly strong influence on ecosystem processes and biosphere-atmosphere interactions in temperate and boreal forests." He and Keenan were joined on the study by colleagues from The Ohio State University, Indiana University and the Institute of Meteorology and Climate in Germany.
They found a substantial increase in water-use efficiency in both temperate and boreal forests of the Northern Hemisphere over the past two decades. Water-use efficiency is the ratio of water loss to carbon gain and it is a key characteristic of ecosystem function central to the global water, energy and carbon cycles.
Plants lose water vapor from leaves while removing CO2 from the atmosphere through photosynthesis, the researchers explained. How efficiently trees use water has implications for ecosystem function, services and feedback to the climate system. They added that implications, which include enhanced timber yields and improved availability of water, could help offset the potential effects of future droughts.
"However, reduced evapotranspiration, or the combination of evaporation and plant transpiration from the land to the atmosphere, resulting from higher water-use efficiency could lead to higher air temperatures, decreased humidity, and decreased recycling of continental precipitation. This could cause increased continental freshwater runoff, along with drought in parts of the world that rely on water transpired in other regions," the NRS said in a statement.
Hollinger, Keenan and their colleagues looked at data from seven locations in the Midwest and Northeastern US, including the Bartlett Experimental Forest in Bartlett, New Hampshire and the Howland Cooperating Experimental Forest 36 miles north of Bangor, Maine. The analysis was then expanded to include 14 additional sites in temperate and boreal regions.
Carbon dioxide uptake and water loss at those locations were measured using data collection stations known as flux towers. The researchers reported a "substantial" increase in water-use efficiency in temperate and boreal forests over the past two decades. Next, they assessed various competing hypotheses to explain the train, and discovered that the increase was most consistent with a strong carbon dioxide fertilization effect.
"The results suggest a partial closure of stomata -- small pores on the leaf surface that regulate gas exchange -- to maintain a near-constant concentration of CO2 inside the leaf even under continually increasing atmospheric CO2 levels," the study authors wrote. "The observed increase in forest water-use efficiency is larger than that predicted by existing theory and 13 terrestrial biosphere models."
"The increase is associated with trends of increasing ecosystem-level photosynthesis and net carbon uptake, and decreasing evapotranspiration," they added.
"Our findings suggest a shift in the carbon- and water-based economics of terrestrial vegetation, which may require a reassessment of the role of stomatal control in regulating interactions between forests and climate change, and a re-evaluation of coupled vegetation--climate models," the researchers concluded.