Winter climate change and the effect of soil frost on nitrogen uptake by mature trees in a northern hardwood forest of the United States

Background/Question/Methods

Forests of the northeastern United States are predicted to experience a decrease in the depth and duration of the winter snow pack over the next 100 years. A reduction in the winter snow pack could lead to an altered soil frost depth and duration due to loss of insulation of soils, even when coupled with warmer winter temperatures. Past research has shown that soil frost leads to increased root mortality and soil nitrate leaching in stands dominated by sugar maple (Acer saccharum), a dominant tree species of northern forests. We hypothesized that greater nitrogen losses from forests may be due to reduced nitrogen uptake by plant roots. To determine the impact of increased soil freezing on fine root uptake of nitrogen, we established a snow manipulation experiment in mixed sugar maple/American beech (Acer saccharum/Fagus grandifolia) forests at the Hubbard Brook Experimental Forest in New Hampshire (n=4 paired snow-removal and reference plots; each 13m X 13m). Snow removal occurred during the first six weeks of winter over two years. During each growing season following snow removal, we used the nitrogen depletion technique to measure in situ rates of uptake of ammonium and nitrate by live fine roots of sugar maple during the early, peak and late growing season. We also incubated root ingrowth and exclusion cores in each of the 4 paired plots for one year. Resin bags were placed at the bottom of each core to measure rates of nitrogen leaching during the incubation period. Using the ingrowth and exclusion cores, we determined the impacts of a smaller snow pack on relative rates of belowground productivity and the role of plant nitrogen uptake in forest nitrogen retention and loss.

Results/Conclusions

During  the first year, we observed significantly less uptake of ammonium by fine roots of sugar maple in the snow removal plots relative to the reference plots during the early growing season (April-May), with no significant differences in uptake of ammonium during the peak (July) and late (September) growing season. We observed no differences in uptake of nitrate among the snow removal and references plots. Results of our study suggest that the observed increase in nitrogen losses following periods of a smaller snow pack and greater soil frost could be due to a decrease in ammonium uptake by fine roots of mature trees, especially in the early growing season.