I 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.  I 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.  I 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. 

Results/Conclusions

Preliminary results suggest that primary production and reproductive phenology were generally unaffected by increased nitrogen inputs.  However, both nitrogen-fixing and non-nitrogen-fixing species in the Warming + N treatment had more biomass and reproduced faster than those at ambient temperatures.  There is 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 detectible by the next measurement interval, suggesting a resiliency to nitrogen addition.  Soil moisture decreased from snowmelt date 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.