Arctic regions have undergone measurable warming within recent decades and warming effects are predicted to strengthen. A coordinated international experiment on the effects of warming on the phenology and growth of tundra species and on plant communities, the International Tundra Experiment (ITEX), started in 1990 to directly measure how tundra plants and communities respond to consistent, low-level increases in temperature across the tundra biome. Previous meta-analyses utilizing the ITEX program evaluated phenological and growth responses and this study reported on the first 4 years of warming of the ITEX. Here, we will present results of a second meta-analysis, focusing on the phenological responses of plants. The primary objective of this synthesis is to evaluate changes in plant phenology across the tundra biome over the past 10-15 years in response to climate change. We compared our comprehensive historical data sets on timing of green-up, flowering, seed maturation and senescence, with new phenological measurements to evaluate temporal changes in plant phenology using open-top-chambers (OTC).
Our results suggest that warming effects differ across and within tundra types, across plant functional groups, and plant communities. Low arctic sites had the greatest initial responses to warming in early and late flowering variables, suggesting a reversal of the effects of the experimental OTCs toward smaller effect sizes over time. In high arctic sites significant effect sizes suggested earlier flowering throughout the duration of the experiment. Seed maturation in low and high arctic sites indicated a similar but dampened response as compared with flowering events. In contrast, alpine sites had smaller initial effect sizes that increased over time in early flowering, although represented by only two alpine sites. Overall, sedges responded with effect sizes indicating relatively earlier end of flowering events in high arctic sites. Evaluating responses over time and within plant functional groups, green-up of deciduous shrubs in the high arctic was significantly earlier in the initial years of the experiment with smaller to non-significant effect sizes in later treatment years. Evergreen shrubs showed a more consistent response over time. Responses over time in the high arctic indicated that timing of early and late flowering events was earlier through the duration of the experiment and differed little among plant communities. In the low arctic, early flowering events were significantly earlier in dry sites in the initial years of the experiment with significant effect sizes indicating earlier flowering in moist and wet sites in later years.