Warmer climate will also be a drier climate, with negative impacts on forests
Because cool summers slow growth of forests in cold places, like much of North America, Europe and Asia, scientists have speculated that a warming climate might speed up their photosynthesis and growth. However, a new University of Minnesota study with more than 2,000 young trees growing in northern forests has found that drier soils, which will occur with warmer temperatures, will markedly reduce tree photosynthesis under the climate change expected later this century.
The study, published today in the journal Nature, is based on a multi-year project, known as “B4Warmed,” that simulated the effects of climate change on 11 tree species, including birch, maple, oak, pine, and spruce, grown in an open-air setting in 48 plots in two forests in northern Minnesota.
“These results have important implications for the future,” says Peter Reich, CFANS professor of forest resources at the University of Minnesota, who led the project and is the paper’s lead author.
“Typical dry spells already occur frequently enough to erase most of the potential benefits to tree growth of warmer summer temperatures. In a warmer future, the extra evaporation from warmer plants and soils will make those dry spells drier, further suppressing photosynthesis. As a result, low soil moisture will exert a powerful braking effect on, or even reverse, potential benefits of climate warming on tree photosynthesis even in moist, cold climates like Minnesota, Canada, and Siberia.”
Researchers increased temperatures at the test plots by 3.4 degrees celsius, an increase that might happen by the end of the 21st century. Over three years, scientists repeatedly measured photosynthesis, to see how fast leaves were taking carbon dioxide out of the air to make sugars. When soils were moist, photosynthesis was higher in plants growing at warmer temperatures. But, in moderately to severely dry soils, which occurred two-thirds of the growing season, warmer temperatures reduced photosynthesis. As a result, photosynthesis was reduced on average by the experimental climate warming. Similar effects of climate change on northern forests are likely to occur in much of the northern U.S., Canada, Europe and northern Asia.
B4Warmed was funded primarily by the U.S. Department of Energy; the research team included scientists from the University of Minnesota, as well as institutions in Georgia and Maryland.