Temperature and precipitation impact ecosystem functioning in every ecosystem, and are also two factors changing most dramatically due to anthropogenic climate change.  High altitude and high latitude ecosystems dominated by snowfall are experiencing rapid changes in temperature, precipitation and water availability regimes.  Functioning in these ecosystems is thought to be temperature sensitive, but the impacts of changing water availability on net ecosystem exchange (NEE) are less well known.  A long term warming experiment at the Rocky Mountain Biological Laboratory (RMBL, Gothic, CO, USA), resulted in significant changes in plant community composition, as well as the timing and magnitude of carbon uptake and soil respiration.  The objective of this study was to determine whether the observed changes in ecosystem functioning with warming are primarily due to changes in temperature, or due to changes in water availability and timing of snowmelt.  We combined observations of NEE along a natural climate gradient with measurements of NEE in a water addition experiment and in the warming experiment to determine the relative impact of temperature and water on ecosystem functioning in subalpine meadows.
Results/Conclusions

A comparison of seasonal diurnal NEE in 2006 and measurements from the third year of the heating experiment in 1993 show similar early season patterns of NEE, with heated plots having higher above ground uptake at the beginning of the season, but lower peak uptake.  The two years differed significantly in late season NEE patterns, however, with 2006 NEE continuing to increase through the end of the growing season and the 1993 NEE approaching zero by the same time.  We attribute this to a difference in precipitation:  1993 was an exceptionally dry summer, whereas 2006 was exceptionally wet.  Examination of seasonal patterns of NEE along an elevational gradient at the site over multiple years shows no relationship with temperature, but a positive correlation with soil moisture.  Across elevation, adding water extends the growing season and increases late season NEE.  These results combine to suggests that water availability, not temperature, has the strongest impact on patterns of subalpine NEE, particularly in above ground production.  Therefore, predicting subalpine ecosystem response to climate change depends critically on our ability to predict shifts in summer precipitation patterns in the Rocky Mountains.