Sugar maples unlikely to move north in a warming world

Sugar maple trees at Coopers Maple Products in Windham, Maine. | Photo by Abigail Curtis

Temperature plays an important role in determining where plants can and can’t grow. In fact, growing zones are determined primarily by minimum average temperatures. With the warming climate, it is logical to assume that the growing range for many plants will shift northwards as minimum temperatures begin to rise in those areas.

New research conducted by the University of Montreal shows, however, that this might not be true for a tree that is central to the economy and culture of northeastern North America. Sugar maples, those hardwood deciduous trees that attract leaf-peepers from around the world come autumn and produce the sap for maple syrup at the outset of spring, are unlikely to move north. 

That is because temperature, while important, is not the only determinant of whether a plant can grow in a certain place. Oft-forgotten soil plays a central role, too. The soils in snowy boreal forests north of sugar maples’ present range may not be able to sustain the trees.

The scientists conducting the study collect soil samples at different altitudes from the eastern slope of Mont Saint-Joseph in Canada’s Mont-Mégantic National Park. Soils collected at lower altitudes modeled the temperate forests that sugar maples currently occupy, while soils collected at high altitudes show the conditions of the boreal forests further north.

“It’s like mimicking the natural range in latitude, but doing it in the same place, we control for a number of other factors, such as the parent material of the soil” explained Alexis Carteron, a Ph.D. candidate at the University of Montreal and lead author on the study. The researchers also collected soil samples from transitional zones between the two types of forest.

Then, the scientists returned to their greenhouse and sprouted maple seeds in the soil samples. They assessed how well the young sugar maples had grown based on their survival rates and biomass.

They found that sugar maples grown in soil from the boreal forest had substantially poorer performance than those grown in the transition zone between temperate and boreal forests and in temperate forest soil.

Among the many factors that could contribute to this finding, the researchers pinpoint fungi — specifically, arbuscular mycorrhizal fungi, which, in exchange for sugar, provide sugar maple trees essential nutrients and help them defend against pathogens — as a likely culprit. Soil from temperate forests allowed for better fungal colonization in the trees’ roots. Trees grown in the boreal forest soil, on the other hand, had very little.

“It’s just a correlation, but it might explain why they grow so little in the boreal forest,” Carteron said.

The findings could be bad news for the range of sugar maples. Presently, sugar maples range from the southeast border of Canada all the way through the eastern interior Midwest like Kentucky and Tennessee. Instead of moving north as some other plants and animals have done to adapt to warming conditions, the range of sugar maples may be squeezed at both ends, north and south.

Unlike many short-lived plants, research shows that plants with long lifespans — trees like the sugar maples, for example — struggle to adapt to changing climatic conditions. The composition of northern soils could change as time goes on to be more suitable to sugar maples, but the researchers predict that the time scale at which soil transforms would not be able to keep up with the rate that the trees need to migrate to cooler climates.

“The fungi could also migrate northward. The biotic and abiotic factors of the soil will change with time and climate change, but it’s the scale that’s different,” Carteron said. “Now it’s already many degrees [warmer], so the tree should be able to grow northward, but those changes are more like dozens of years. The [changes to the] soil would be maybe hundreds [of years].”

Plus, even if the soil becomes more suitable to sugar maples, it would have other adverse effects on boreal forest ecosystems.

“If we artificially change boreal forests to temperate forests, [it’d be] a good thing for sugar maple [because] we can plant it more in the north,” Carteron said. “But first of all, maybe it won’t work, and even if we do so, it’s not so positive because then what do we do with all the biodiversity that’s there naturally?”

Carteron said that scientists are still learning about how different species adapt to changing climatic conditions given factors like soil composition and conditions. Findings like these, however, dispel the notion that even seemingly obvious adaptive solutions to the climate crisis — like running north to escape rising heats — may not be as simple as they seem.

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