Warmer temperatures and longer growing seasons are expected to increase overall plant productivity in alpine ecosystems. Yet, increased temperatures may be coupled with a decrease in growing season soil moisture resulting in distinct effects at different levels of biological organization, from individual species, to life form groups (e.g., graminoids, succulents), to whole communities. Species or groups sensitive to high temperature or low moisture may increase or decrease their relative contributions to total community productivity. The aim of this study was to determine whether increased temperatures and altered moisture regimes alter peak aboveground biomass at community, life form, and species levels in the Alpine-Treeline Warming Experiment at Niwot Ridge, Colorado. Peak aboveground biomass was estimated from aerial cover observations by species over three years (2010-2012) and biomass-cover regressions from a more limited set of destructive harvests. Multiple levels were examined to assess whether a lack of response at one level masks compensating responses at another level.
Results/Conclusions
Heating and growing season watering from 2010-2012 did not significantly affect community-level peak aboveground biomass. Peak biomass differed between years, with lower biomass in 2011, which had later snowmelt than 2010 or 2012. Peak aboveground biomass of cushion plants responded positively to additional water during the growing season. Peak biomass of both graminoids and succulents was insensitive to climate treatments and interannual variability. Forb aboveground biomass decreased with watering treatments in 2010 and decreased with heating treatments in 2011. Species-level models revealed that some individual species were responsive to treatments even when their life form group was not. For example, the graminoid Trisetum spicatum responded positively to warming treatments even though the entire graminoid life form group did not. These results suggest that overall community responses may mask group, as well as individual species responses within the community. At the same time, many species and life form groups were not sensitive to climate manipulations, perhaps because of lagged responses in these largely long-lived perennial species or because of the limited (< 2ºC) average warming.