Phenology is defined as "the study of cyclic and seasonal natural phenomenon, especially in relation to climate, plant, and animal life." Whether its deciding when to plant certain crops or when to start taking your allergy medication, our lives are intricately tied to such cycles. The study of phenology has other applications as well. By and large, it is one of the best methods we have in understanding the effects of climate change on ecosystems around the globe.
For plants, phenology can be applied to a variety of things. We use it every time we take note of the first signs of leaf out, the first flowers to open, or the emergence of insect herbivores. In the temperate zones of the world, phenology plays a considerable role in helping us track the emergence of spring and the onset of fall. As we collect more and more data on how global climates are changing, phenology is confirming what many climate change models have predicted - spring is starting earlier and fall is lasting longer.
Researchers at the USA National Phenology Network have created a series of maps that illustrate the early onset of spring by using decades worth of data on leaf out. Leaf out is controlled by a variety of factors such as the length of chilling temperatures in winter, the rate of heat accumulation in the spring, and photoperiod. Still, for woody species, the timing of leaf out is strongly tied to changes in local climate. And, although it varies from year to year and from species to species, the overall trend has been one in which plants are emerging much earlier than they have in the past.