Effects of warmer temperatures and a smaller winter snowpack on nitrogen uptake and retention in a northern hardwood forest

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We conducted a field-based experiment at Hubbard Brook Experimental Forest to assess the combined effects of climate change across seasons on ecosystem nitrogen uptake and retention by northern hardwood forest trees.  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. We established six 11 x 14 m plots in a mixed red maple-beech-birch stand 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. Rates of nitrogen uptake were greater in the reference and warmed plots compared to the plots that experienced soil freeze/thaw cycles in winter, which corresponded to greater root damage (as measured by root electrolyte leakage). Rates of nitrogen leaching were also greater in the plots experiencing freeze/thaw cycles in winter, suggesting that freeze damage to roots impairs the ability of maple trees to take up nitrogen, which in turn leads to greater nitrogen loss from these forests.