Impact of winter climate change on nutrient and water uptake in a northern forest ecosystem
Socci, Anne M.
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Plants are significant drivers of ecosystem nutrient and water retention and loss, and plant responses to climate change could impact ecosystem function. Forests of the northeastern United States are projected to experience a shorter duration and a smaller depth of the winter snowpack over the next century, which is expected to lower soil temperatures and increase soil freezing. To determine the effects of a reduced snowpack and increased soil freezing on plant function, I conducted a two-year snow removal experiment at the Hubbard Brook Experimental Forest, New Hampshire. Prior to snow removal, to address challenges inherent to measuring fine root nutrient uptake and better understand seasonal uptake patterns, I measured rates of inorganic nitrogen uptake by fine roots of mature sugar maple (Acer saccharum) and red spruce (Picea rubens) trees using two approaches. During each growing season following snow removal, I measured rates of nitrogen uptake and xylem sap flux density by mature sugar maple trees. Snow removal delayed the onset of a continuous snowpack and increased the depth and duration of soil frost, which persisted into the early growing season in both years. A smaller snowpack and increased soil frost reduced fine root nitrogen uptake by sugar maple during the early and mid-growing season, which overlapped with increased rates of soil nitrogen leaching in both years. Responses of sap flux density to snow removal were variable, and suggest that sugar maple trees may be able to compensate for impaired water uptake capacity during the early growing season such that there is little or no net effect on whole-season water uptake. Results of this dissertation provide direct evidence that reduced nitrogen uptake by plants is an important mechanism for reduced forest nitrogen retention following years of a reduced snowpack and increased soil frost. Impacts of winter climate change on water uptake by trees remain unclear. Results demonstrate that projected changes to winter climate in northern forests may impair the ability of plants to take up nitrogen and water, which could yield greater ecosystem nitrogen losses, an effect that is associated with nutrient imbalances in trees and acidification of streamwater.
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