Colder soils in a warmer world? It doesn’t sound intuitive, but it is a possible consequence of global climate change here in the Northeast, and one that might have interesting effects on the composition of our forests, their ability to cycle nutrients, and the water quality of our lakes and streams.
Soils in Vermont and New Hampshire forests do not typically freeze in winter because of the insulating effects of snow, which acts as a blanket to shield soil warmed during the growing season from the cold air of winter. Climate change scenarios for the Northeast, however, predict a less reliable snowpack – one that would likely develop later and melt earlier. This would expose the soil to more cold air and, ironically, increase the likelihood of it freezing.
Several soil-freezing events have already been studied in our Northeastern forests, and the changes in stream water chemistry that accompanied these events have piqued the curiosity of researchers working at the U.S. Forest Service’s Hubbard Brook Experimental Forest (HBEF) in Woodstock, New Hampshire. Watershed monitoring at the HBEF, combined with similar watershed monitoring in Maine, the Catskills, and the Adirondacks, recorded unusually high concentrations of nitrate in spring snowmelt after the winter of 1989-1990, in which there was widespread soil freezing due to cold temperatures and a thin blanket of snow.
Researchers in Canada have also linked soil freezing to sugar maple decline. This suggests that something different is happening underground when soils freeze, and this change could be influencing both how our forests look and how they cycle nutrients. Because soil freezing affects streams, soils, and trees, and is likely to occur more often, Hubbard Brook scientists decided they needed to know more. Just what was changing? Was it tree roots, soil microbes, or both?
Approaching this problem was not easy. For one, studying anything belowground is difficult without disturbing it. Secondly, freezing soils in the forest is easier said than done. For good results, the researchers needed paired measurements from frozen and unfrozen forest plots, in which each conceivable link in the nutrient cycles could be measured, to determine where the changes were taking place. In addition, they needed detailed weather, snowpack, and soil temperature measurements in order to place their results in the context of future climate predictions. With these challenges in mind, a group of snowpack experts, microbial ecologists, forest ecologists, and soil chemists came together and confronted their first problem: how would they freeze the soil in an area large enough to hold all of their research equipment.
With support from the National Science Foundation, the research team was able to design a study with four pairs of plots distributed in stands of sugar maple and yellow birch. Each plot was 10 meters by 10 meters in size, and one plot of each pair was kept snow free until early February in order to deprive the soils of the insulating effects of snow. Plenty of high-tech equipment was used in this study, but snow removal was done the old-fashioned way: with snow shovels. Within each plot, roots (growth and death), microbes (populations and activity), soil frost (depth), soil water chemistry, and gases leaving the soil were measured.
This study was conducted in the winters of 1999-2000 and 2000-2001 in yellow birch and sugar maple plots and is currently taking place in pairs of high- and low-elevation, mixed-species plots, all at the HBEF. From the first study year, elevated nitrate in the soil water in the frozen plots was observed, indicating that it was in fact soil freezing that was causing the excess nitrate in the waterways. There was also significantly more root death in the frozen plots, but no difference was observed between tree species. The microbial community did not seem directly affected by the frost, although it did respond to the increase in dead roots, which are a food source for some microbes.
The second round of this study is still underway, and it will no doubt reveal more about the changes in forest dynamics when the soils freeze. For now, though, it looks as if the colder soils brought on by a warmer world will affect forests by damaging or killing tree roots. This means that there will be fewer roots in the spring to take up available nitrate, and thus more nitrate will enter streams. This change in the nutrient dynamics of the soil could affect both streams and forests alike. Just how large the effect will be is unknown, because for now, these regional soil-freezing events remain the exception, not the rule.