Effects of climate change across seasons on ecosystem nitrogen retention and carbon uptake by maple saplings

Background/Question/Methods

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 greater carbon and nitrogen uptake by plants and enhanced ecosystem retention of these elements. 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 and sequester carbon 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 and carbon uptake of maple saplings.  We planted 70 Acer saccharum (sugar maple) and 70 Acer rubrum (red maple) saplings in buried pots filled with homogenized soil in an open field.  For each species, ten saplings were assigned to one of seven treatments: ambient temperatures (reference), growing season warming, growing season warming coupled with two, four, six, or eight freeze-thaw cycles, and snow removal in winter with ambient temperatures in the growing season.  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 ecosystem nitrogen retention and leaf-level photosynthesis at the beginning of the growing season.

 Results/Conclusions

Snow removal and soil heating cables were successful at inducing a range of freeze-thaw cycle frequency across treatments in winter. The results from the first year of this multi-year study indicate that both sugar maple and red maple saplings 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 saplings to retain nitrogen and take up carbon should be considered when projecting the ability of these plants to retain nutrients and grow.