Springing to life
If you’ve been following the news this past spring, you’ve undoubtedly learned that spring came remarkably early in much of the U.S., marked by record-breaking temperatures and plant leaf-out and flowering occurring weeks ahead of schedule across much of the country. These plant responses are examples of phenology, defined as life-cycle events in plants and animals.
Though we may see an early spring one year, like in 2012, we may see a very late spring in another year. However, the overall trend across decades is that spring is coming sooner than it used to. For approximately the past decade, researchers around the globe have investigated plant and animal response to earlier springs. In many temperate locations, a clear trend toward earlier leaf-out and flowering in plants has been documented. This trend has been linked with warm temperatures occurring earlier in the year than in previous decades (Menzel et al., 2006). Following suit, many animals are also showing trends toward earlier initiation of key life cycle events such as emergence from hibernation, foraging, and mating in the spring.
Changes in the timing of plant and animal phenological events can have clear and direct effects as well as cascading effects throughout ecosystems. Whether these impacts are positive or negative depends on the species and system under investigation. Humans are generally happy to avoid the treacherous roads, endless piles of snow to shovel, and long, gray days associated with long, cold winters. However, the flip side of early springs and a longer growing season can mean a requirement for greater volumes of irrigation water—which is not always available in abundance—as well as extended and intensified allergy seasons. Cultural events such as the famous Cherry Blossom Festival in Washington DC and Tulip Time in Holland, Michigan can suddenly seem irrelevant if their focal phenological stages have long since passed by the time the event occurs.
Similarly for plants and animals, earlier springs are advantageous for some species – for example, at Walden Pond in Massachusetts, researchers have documented that species such as campions (a type of flower native to North America) and certain species of rose have increased in abundance since Thoreau’s time ~150 years ago. The plants that are faring best are those that are able to “match step” with the increasing temperatures and can initiate their phenological events earlier in the season. In other cases, changing conditions are having clearly detrimental effects. For example, pied flycatchers, migratory birds that overwinter in the southern hemisphere, are exhibiting steep declines in population. Their springtime migration north is cued by sun angle, which is static from year to year. However, their primary food source, winter moth caterpillars, are emerging earlier and earlier in the spring, directly in response to the earlier onset of warm spring temperatures. By the time the flycatchers are arriving at their breeding grounds in Europe in the spring, many of the caterpillars have already metamorphosed out of their larval stages, leaving the birds with little to eat.
Here in the Southwest, we are less constrained by temperatures in the spring. Rather, many phenological events in plants and animals are closely tied to water availability or a combination of temperature conditions and moisture availability. Trends in precipitation patterns are unclear, though we know for certain that temperatures are increasing. Given the mixed drivers for species in this region, predicting changes is tricky. The multifaceted topography of the Sky Islands adds another dimension of complexity.
Despite this variability, some clear changes in plant and animal phenology have recently been observed and appear to be the result of a changing climate. In the Catalina Mountains, dozens of species are being observed in flower at higher elevations than in previous decades. Similarly, the diversity of plants flowering at high elevations in these mountains has been increasing over the past three decades; these changes are believed to be the result of increasing summer temperatures. Further, in contrast to temperate communities, few spring-flowering species appear to be advancing the onset of flowering, as they are influenced primarily by rainfall in the previous autumn season, which has been decreasing in recent years. The mixed phenological cues and mixed responses being observed in the Southwest across species makes it difficult to predict what our plant communities may look like in the future.
One of the ways we can improve our understanding of how species are responding to changing climate conditions is by making careful observations of both plants and animals in many locations. The USA National Phenology Network (USA-NPN) offers a wonderful structure for doing this – complete with easy-to-follow observation protocols, an online interface, smartphone apps for data collection, and an engaging map-based online tool for viewing patterns among species and observation locations. My friend and co-worker, Echo Surina, wrote about this in a recent blog post. We enthusiastically invite you to check out the resources the USA-NPN has to offer – both the observation collection side of things, as well as the data resource we are rapidly amassing. Our hope is that we can all work together to better understand how earlier springs – and other shifts in climate patterns – are impacting our world.