Long-term phenological changes in tundra plants in response to experimental warming and observed changes in climate

Arctic regions are predicted to undergo strong warming in response to increasing concentrations of atmospheric greenhouse gases, and they have already undergone measurable warming within recent decades. To directly measure the ways in which tundra plants and communities respond to consistent, low-level increases in temperature across the tundra biome, a coordinated international experiment on the effects of warming on individual tundra species and plant communities, the International Tundra Experiment (ITEX), was initiated in 1990. The experiment is centered around a small-scale passive-warming treatment using open top chambers that increase temperatures about 2 °C during the short growing season. The ITEX experiment and others have shown that tundra systems are clearly capable of responding to climate warming fairly rapidly, and these changes will have significant impacts regionally and globally. One aspect of tundra plants that is highly sensitive to slight changes in temperature is phenology, the timing of key life-history events such as bud burst, leaf growth, flowering etc. In highly seasonal environments, such as the Arctic, timing key phenological events with the appropriate climate is essential for plant success. Plants whose phenology is regulated by unchanging internal factors or daylength will be unable to respond to changes in season length and will be replaced by species responding to other cues. The ITEX phenology, growth and community data analyzed to date show responses that in many ways mirror patterns seen along latitudinal temperature gradients. However, changes are too subtle and too variable to be detected by most individual-site studies, some after more than a decade of warming. ITEX as an observing network provides a unique opportunity to evaluate plant phenology changes at a global scale across the tundra biome. In this working group we will evaluate changes in plant phenology across the tundra biome over the past 10-15 years in two ways: 1) comparing ambient-temperature plots versus plots subject to long-term experimental warming, and 2) comparing measurements of ambient-temperature plots taken during the mid 1990's International Polar Year. The aim of these analyses is to help us answer one of the driving questions of arctic biology in the face of climate warming, "What form will new arctic plant communities take and what are the processes driving these changes?"