They finally hypothesized that it had something to do with a very cold winter the year before, when there was a hard freeze before snow covered the ground. Groffman and Templer's experiments to date have borne this out.
Watching tree roots with a video camera
In various places in the forest, they trekked in during early winter and shoveled the first snow off some plots, so the soils there froze. Right next to those plots, they left the snow alone.
In the areas where they shoveled the snow and the soil froze, "nutrients came pouring out of the soil the following summer," Groffman said.
The freezing soil, it turns out, kills a lot of tree roots. One researcher, Geraldine Tierney, was able to see this by inserting a video camera into clear plastic tubes in the ground so she could watch what happened to the roots over the winter.
When soil freezes, said Groffman, the water in it turns into big balls of ice. The dynamic is similar to how a crack in the pavement, filled with water, gets ripped up in the winter when it freezes.
This happens with roots, too. The ice that forms in the frozen soil cuts the roots, damaging them so they can't take up nutrients like nitrogen, phosphorus and calcium.
"We've shown that early in the growing season, when the trees would normally be waking up and taking up water, or taking up nitrogen, they are doing much less of that. And we believe it is from damage caused by freezing in the previous winter," Templer said.
'Really pushing the frontiers'
The freezing can hurt animals that live in the soil, too. Templer found that the diversity and numbers of arthropods -- things like beetles and spiders -- were also reduced in areas where the soil froze because of lack of snow cover.
With her new experiment, Templer is finally able to put two parts of climate change -- warming summer soils and frozen winter ones -- together for the next five years. What she finds will perhaps paint one of the most reliable portraits to date of how climate change will affect the forests of New England.
Lindsey Rustad, a U.S. Forest Service forest ecologist who tracks more than 200 warming experiments worldwide, said Templer's experiment "is really pushing the frontiers."
"As far as I know, there is no other combination of warming and frost, so it is a very unique experiment," said Rustad.
Because of climate change, the forest is likely to see more extremes, like very hot summers and winters with a lot less snow.
"She is setting up a system that will allow her research group to look at extreme events, whether it is frost cycles or extreme temperature treatments," Rustad said.
Finding a solution for an 'iconic' species
Templer had to create her forest plots with red maples, rather than sugar maples, because she couldn't get electricity high enough into the forest to make warming plots that included sugar maples.
That is why 70 newly planted sugar maple and 70 red maple saplings sit just a short walk down the road from the wired cabin and forest plots, off to the side in a freshly dug patch of light-brown dirt.
Each of the saplings is buried in a pot in the ground, a self-contained unit. Templer plans to warm the soil around these, as well, and is also adding another variable to this set of experimental trees.
Researchers have predicted that sugar maples may move north as the climate warms. Yet another of the unknowns about how climate change will affect these trees is how changes to the freeze-thaw cycles in winter, with more periods of alternating warm and cold spells, could affect the roots, she said.
"If you think about the middle of winter, when we have these little hot spells, do the roots get damaged right away when they freeze, or is there some sort of threshold?" she said.