Continental-scale phenology: Warming and chilling

With abundant evidence of recent climate warming, most vegetation studies have concentrated on its direct impacts, such as modifications to seasonal plant and animal life cycle events (phenology). The most common examples are indications of earlier onset of spring plant growth and delayed onset of autumn senescence. However, less attention has been paid to the implications of continued warming for plant species' chilling requirements. Many woody plants that grow in temperate areas require a certain amount of winter chilling to break dormancy and prepare to respond to springtime warming. Thus, a comprehensive assessment of plant species' responses to warming must also include the potential impacts of insufficient chilling. When collected at continental scale, plant species phenological data can be used to extract information relating to the combined impacts of warming and reduced chilling on plant species physiology. In this brief study, we demonstrate that common lilac first leaf and first bloom phenology (collected from multiple locations in the western United States and matched with air temperature records) can estimate the species' chilling requirement (1748 chilling hours, base 7.2 °C) and highlight the changing impact of warming on the plant's phenological response in light of that requirement. Specifically, when chilling is above the requirement, lilac first leaf/first bloom dates advance at a rate of ? 5.0/? 4.2 days per 100-h reduction in chilling accumulation, while when chilling is below the requirement, they advance at a much reduced rate of ? 1.6/? 2.2 days per 100-h reduction. With continental-scale phenology data being collected by the USA National Phenology Network (http://www.usanpn.org), these and more complex ecological questions related to warming and chilling can be addressed for other plant species in future studies.