The role of down-slope water and nutrient fluxes in the response of Arctic hill slopes to climate change

The down-slope movement of water and nutrients should link plant and soil processes along hill slopes. This linkage ought to be particularly strong in Arctic ecosystems where permafrost confines flowing water near the surface. We examined whether these hill-slope processes are important in assessments of the responses of Arctic tundra to changes in CO₂ and climate using the Marine Biological Laboratory–General Ecosystem Model. Because higher rates of water flow decrease the distance over which nutrients must diffuse to the roots, down-slope vegetation is more productive under current conditions. In response to elevated CO₂ and a warmer, wetter climate, the relative increase in carbon stored in vegetation and soils was higher uphill, but the absolute increase was higher downhill. Very little of the increase in carbon anywhere on the hill slope resulted from an increase in total ecosystem nitrogen. Instead, the increases were associated with increases in vegetation C:N ratio (woodiness) and with the redistribution of nitrogen from soils (low C:N) to vegetation (high C:N). Because these changes are fueled by nitrogen already in place, the down-slope movement of nitrogen does not appear to be a major determinant of the responses of Arctic tundra to changes in CO₂ and climate.