We investigated the effects of nitrogen inputs on subalpine forb communities in the Rocky Mountains of Colorado to estimate species responses under climate warming scenarios. In this system, nitrogen inputs accumulate in the winter snowpack and are released in a nutrient pulse at snowmelt. Snowmelt date is expected to accelerate with warming, which will shift the nutrient pulse earlier in the season, potentially resulting in fertilization of early-season species at the expense of later-season species. We added ammonium nitrate equivalent to 9 g N m-2 yr-1 to experimental plots at either early-, mid-, or late-season to determine whether the different species cohorts that are dominant at those times respond to elevated nitrogen. We also investigated the effects of warming under an elevated nitrogen scenario by combining a season-long nitrogen addition treatment with a warming treatment, using open top chambers to increase daytime air temperatures by 3°C. Response variables included estimates of species aboveground biomass, reproductive phenology, stomatal conductance, and fluorescence.
Results suggest that a combination of warming and supplemental nitrogen accelerated reproductive phenology and growth in early-season species. Species that develop later in the growing season were generally unaffected by increased nitrogen and warming. There was also evidence that non-nitrogen-fixing species exhibited a brief growth increase following early- and late-season nitrogen pulses, but these size differences were not detectable by the next measurement interval, suggesting a resiliency to nitrogen addition. Soil moisture and stomatal conductance decreased from snowmelt through mid-summer, and the resulting drought stress may explain why most of the sampled subalpine forbs did not appear to capitalize on increased nitrogen availability. Overall, these results suggest that earlier snowmelt may result in increased growth of early-season species while mid- and late-season species suffer increased drought effects.