A multi-level approach to assessing alpine productivity responses to climate change

Future, warmer temperatures are predicted to increase alpine productivity, but few studies have addressed the role of water in constraining such responses. We tested the hypothesis that, in the absence of additional water during the growing season, warming may not increase community-level productivity by warming plots March-November and providing supplemental water during the snow-free growing season in an alpine plant community at Niwot Ridge, Colorado. We measured productivity responses to treatments at three levels of biological organization: community-, life form-, and species-levels in 2010-2012. Heating advanced snowmelt 9.4 ± 0.14 days (x ̅ ± sd) and subsequently decreased cumulative soil temperatures and increased cumulative soil moisture. Warming alone did not alter community-level productivity during any years of the experiment but warming with watering increased community-level productivity by 20% during the first year of treatments. Forb productivity increased with both warming and watering in all years of the experiment, while cushion productivity only increased with watering treatments after the first year of treatments. Graminoid productivity was insensitive to treatments and year, and while succulent productivity varied idiosyncratically by year and treatment, there were no overriding changes between treatments and controls. Responses at the species-level did not always follow the responses of their respective life form group in all or even a single year of the experiment, nor did they reveal compensatory responses that could fully explain productivity at higher levels of biological organization. For example, when the forb life form group responded to treatments in 2010, A. fendleri was the only species to exhibit a response. In 2011, both A. fendleri and G. rossii responded to treatments while, in 2012, G. rossii was the only species to respond to treatments. In our experiment, warming appears to indirectly increase soil moisture by advancing snowmelt, allowing species to break dormancy earlier and take advantage of early season moisture from a prolonged snowmelt. Our results also suggest that community-level responses mask life form group responses, as well as individual species responses within the community under future climate changes. Further, interannual climate variability strongly influences productivity at all levels of organization and is essential to account for when studying how climate change will influence alpine species, life forms, and communities