Climate Change Across Seasons

For many of us, winter in the northeast means cold temperatures and piles of snow, drifting through trees and across fields. It’s hard to imagine that winter here could look different, but climate change has scientists asking just what northeast winters might look like in the future – and how those changes might impact northeast forest ecology.  

At the U.S. Forest Service’s Hubbard Brook Experimental Forest (HBEF) the future researchers are thinking about is the year 2100, when temperatures across the northeast are anticipated to be a full 3-5⁰C (5.5-9⁰F) warmer than today. In the northeast, warmer temperatures could benefit trees with a longer and warmer growing season, but warmer winters actually see more frequent soil freezing, which can damage tree roots.

It seems backwards that soils freeze more during warm winters, but the reason is the snow. The blanket of snow that usually accumulates during a northeast winter acts to insulate the soil below, preventing it from experiencing the full, sub-freezing temperatures of the air. When warmer temperatures leave a thinner blanket of snow, or none at all, the soil is more likely to freeze.

To tease apart the opposing effects of warmer growing season and soil freezing, the HBEF started the “Climate Change Across Seasons Experiment” (CCASE), which investigates cross-season effects and focuses on maple trees. In previous experiments HBEF scientists have seen that maple trees can be more sensitive to soil freezing and they may turn out to be the canary in a coal mine for seasonal climate changes.

To really understand what CCASE is all about, I caught up with Geoff Wilson, a research technician at the HBEF, who explained the history behind the project. It all started in the mid-1950s, when the HBEF began long-term watershed monitoring. Now fast-forward several decades to the 1990s, when climate change researchers noticed something weird in the watershed records: unexplained spikes of high nutrient snowmelt in some years. They suspected the nutrient spikes were linked to the weather, because the spikes happened in years with less snow.

To figure out what was really causing this high-nutrient snowmelt, Geoff was part of the HBEF team that conducted an initial soil freezing study in 1999. They used shoveling to mimic reduced snowpack on 10 x 10 meter plots of sugar maples and yellow birch. I can only imagine an unlucky technician drawing the short straw and trudging out to the study plots after every snowfall. It was worth it, as the research team found some clear consequences of soil freezing.

First, the plots that were shoveled, and therefore experienced soil freezing, had high levels of nutrients in the water collected from them, which in spring could become the high nutrient run-off that the climate change researchers first noticed. Second, these plots had more root damage than plots that stayed insulated with snow. And finally, the team also saw that the trees most affected were the sugar maples.

Like all good science, this first study led to yet more questions, which is why the HBEF started the CCASE project.

Geoff explained that the new project tracks six 11 x 14 meter plots of red maple. In some plots, the soil is warmed 5⁰C during the April-November growing season, and an insulating blanket of snow is left for the winter. In other plots, the soil is warmed for the growing season, but the snow is shoveled off in the winter to let the soil freeze. It took 3 weeks in the summer of 2012 to bury the 2.5 miles of electrical cable that is used to warm the soil. “The electricity bill must be enormous!” Geoff commented.

The CCASE experiment measures nutrient and water uptake by trees, root growth and death, and competition for nutrients between plants and microbes. CCASE is also looking at the impact on soil arthropods, as researchers have seen soil arthropod diversity drop after soil freezing events.

CCASE is a multi-year study that is still in progress, but Geoff shared that HBEF scientists have some early conclusions. They’re seeing that the extra warming in summer plots does increase tree growth. Unfortunately, any increased growth is limited if the trees experience soil freezing in the winter. Studies like CCASE are helping us picture the future for a world experiencing climate change, but there are still many questions ahead before that picture will be complete. 

A version of this article was published in the Northern Woodlands Outside Story series. Check out their website here: https://northernwoodlands.org/