Effects of warmer temperatures and a smaller winter snowpack on nitrogen retention in a northern hardwood forest
Climate models project an increase in mean annual air temperatures and a reduction in the depth and duration of winter snowpack for the northeastern United States by the year 2100. The combined effects of these changes in climate will likely lead to warmer soils in the growing season, but increased frequency of soil freeze-thaw cycles in winter due to the reduction of a continuous, insulating snowpack. Past research has shown that warmer soil temperatures accelerate growth of trees, leading to enhanced ecosystem nitrogen retention and increased nitrogen uptake by plants. In contrast, a smaller snowpack and increased soil freezing in winter reduces plant uptake of nitrogen due to root injury and mortality and leads to greater ecosystem nitrogen loss. However, the combined effects of climate change in the growing season and winter on the ability of forest ecosystems to retain nitrogen is unknown.
We conducted a field-based experiment at Hubbard Brook Experimental Forest to assess the combined effects of climate change across seasons on ecosystem nitrogen retention of northern hardwood forest trees. We established six 11 x 14 m plots in a mixed red maple-beech-birch forest in 2012. Two plots experience growing season warming, two experience growing season warming coupled with freeze-thaw cycles in winter, and two plots experience ambient temperature year-round (reference). Freeze-thaw cycles in winter are achieved by manual snow removal to induce soil freezing followed by warming with buried heating cables to trigger thaws. During the growing season, the same buried heating cables are used to warm the soil 5 degrees C above ambient soil temperatures. We measured root nitrogen uptake, foliar nitrogen content, and ecosystem nitrogen retention throughout the growing season.
Snow removal and soil heating cables were successful at inducing freeze-thaw cycles in winter. The results from the first year of this multi-year study indicate that mature red maple trees are affected by changing soil temperatures in winter and the early growing season. Therefore, we conclude that the effects of climate change across seasons on the ability of trees to retain nitrogen should be considered when projecting the nitrogen retention capacity of northern hardwood forests in a changing climate.