In warmer Minnesota forests, drier trees will grow slower
On the same day an international panel of climate experts predicted dire consequences if human-caused global warming continues unabated, scientists at the University of Minnesota added northern forests to the list of potential victims.
Scientists looked at 11 species of trees growing in two northern Minnesota forests and said predicted temperatures will cause drier soils and reduce tree growth even as temperatures warm.
Scientists had hoped that trees might grow faster in warmer conditions. But when they added temperatures predicted to occur in northern Minnesota in future decades, they found the opposite occurred. The expected dry spells will outweigh the impact of short-term, less-frequent but heavier rainfall events, at least for tree photosynthesis.
The results were published in the journal Nature. The forest study plots were located at the university's Cloquet Forestry Center and the Hubachek Wilderness Research Center on Fall Lake near Ely.
"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," said Peter Reich, a professor of forest resources in the College of Food, Agricultural and Natural Resource Sciences and the study's lead author.
"These results show that low soil moisture will slow down or eliminate any potential benefits of climate warming on tree photosynthesis even in moist, cold climates like Minnesota, Canada and Siberia," Reich added.
Researchers looked at more than 2,000 young trees from 11 different species — including birch, maple, oak, pine and spruce — growing in 48 plots in two forests in northern Minnesota.
During the three-year study, researchers increased temperatures at the test plots by 3.4 degrees Celsius, or 6 degrees Fahrenheit, an increase possible in Minnesota by the end of the century. They routinely measured photosynthesis at the plots to see how fast leaves were taking carbon dioxide out of the air to make sugars for the trees. Researchers found that, in moist soils, photosynthesis was indeed higher in plants growing at warmer temperatures. But in moderately to severely dry soils, which occurred during two-thirds of the growing season, warmer temperatures reduced photosynthesis.
While bigger flood events are expected in warmer conditions, along with longer dry spells in between, both will impact the region in different ways.
"Depending on what part of nature or human infrastructure we are talking about, one may be more important than the other," Reich told the News Tribune. "For roads, floods will be worse. For stream ecology, both will be bad. For forests on floodplains, both will be bad. For an upland forest, droughts will be more influential."
The report on Minnesota forests comes as the latest report from the United Nations Intergovernmental Panel on Climate Change was released. The U.N. report released Monday explains the likely dire global impacts of climate change as soon as 2040 — if warming isn't held to 1.5 degrees Celsius or under — and highlights the multiple ways climate change will impact nature and humanity, including sustainability of food, water and biodiversity.
"Our work, alongside that of the community of scientists worldwide, can inform decisions that can put the world on a path toward a sustainable future," Reich said. "This will be the largest challenge humanity has ever faced and unless we shift gears to effectively tackle it, future generations will view us as having completely failed in our responsibility as stewards of the earth."
The Minnesota forest study was based on a multiyear project funded primarily by the U.S. Department of Energy.