Long-term experimental manipulation of winter snow regime and summer temperature in arctic and alpine tundra

Three 60 m long, 2.8 m high snowfences have been erected to study long term effects of changing winter snow conditions on arctic and alpine tundra. Th is paper describes the experimental design and short-term effects. Open-top fiberglass warming chambers are placed along the experimental snow gradien ts and in controls areas outside the fences; each warming plot is paired wi th an unwarmed plot. The purpose of the experiment is to examine short- and long-term changes to the integrated physical-biological systems under simu ltaneous changes of winter snow regime and summer temperature, as part of t he Long-Term Ecological Research network and the International Tundra Exper iment. The sites were at Niwot Ridge, Colorado, a temperate high altitude s ite in the Colorado Rockies, and Toolik Lake, Alaska, a high-latitude site. Initial results indicate that although experimental designs are essentiall y identical at the arctic and alpine sites, experimental effects are differ ent. The drift at Niwot Ridge lasts much longer than do the Toolik Lake dri fts, so that the Niwot Ridge fence affects both summer and winter condition s, whereas the Toolik Lake fence affects primarily winter conditions. The t emperature experiment also differs in effect between the sites. Although th e average temperature increase at the two sites is similar (daily increase 1.5 degrees C at Toolik and 1.9 degrees C at Niwot Ridge), at Toolik Lake t here is only minor diurnal variation, whereas at Niwot Ridge the daytime in creases an extreme on sunny days (as much as 7-10 degrees C), and minimum n ighttime temperatures in the chambers are often slightly cooler than ambien t (by about 1 degrees C), The experimental drifts resulted in wintertime in creases in temperature and CO2 flux. Temperatures under the deep drifts wer e much more consistent and warmer than in control areas, and at Niwot Ridge remained very close to 0 degrees C all winter. These increased temperature s were likely responsible for observed increases in system carbon loss. Ini tial changes to the aboveground biotic system included an increase in growt h in response to both snow and warming, despite a reduced growing season. T his is expected to be a transient response that will eventually be replaced by reduced growth. At least one species, Kobresia myosuroides, had almost completely died at Niwot Ridge three years after fence construction, wherea s other species were increasing. We expect in both the short- and long-term to see the strongest effects of snow at the Niwot Ridge site, and stronger effects of temperature at Toolik Lake. Copyright (C) 1999 John Wiley & Son s, Ltd.