Spring Phenology and Global Climate Change

Yellow-rumped warbler in spring red maple woods
Yellow-rumped warbler. Johann Schumacher/Photolibrary/Getty Images

As spring arrives we notice the changing of seasons by the weather, but also by a host of natural events. Depending on where you live, the crocuses may poke through the snow, the killdeer may be back, or the cherry trees may bloom. There is an orderly sequence of events that seem to occur, with various spring flowers appearing in order, red maple buds bursting into new leaves, or the old lilac by the barn scenting the air. This seasonal cycle of natural phenomena is called phenology. Global climate change appears to be interfering with the phenology of many species, at the very heart of species interactions.

What Is Phenology?

In temperate regions like the northern half of the United States, there is comparatively little biological activity in winter. Most plants are dormant, and so are the insects feeding on them. In turn, animals that rely on these insects such as bats and birds are hibernating or spending the cold months in more southerly locations. Ectotherms like reptiles and amphibians, which take their body warmth from their environment, also have active phases tied to the seasons. This long winter period constrains all the growing, breeding, and dispersing activities that plants and animals do to a short favorable window. That’s what makes spring so vibrant, with plants flowering and putting on new growth, insects emerging and breeding, and birds flying back to take advantage of this short-lived bounty. The onsets of each of these activities add up to so many phenological markers.

What Triggers Phenological Events?

Different organisms respond to different cues to initiate seasonal activities. Many plants will start growing leaves again after a set period of dormancy, which very roughly dictates the leaf-out window. Cue that more precisely determine when the buds break can be soil temperature, air temperature, or water availability. Similarly, temperature cues can promote the beginning of insect activity. Day length itself can be the operative trigger for some seasonal events. It is only when there are a sufficient number of daylight hours that reproductive hormones will be produced in many bird species.

Why Are Scientists Concerned with Phenology?

The most energy-demanding period in the life of most animals is when they reproduce. For that reason, it is to their advantage to coincide breeding (and for many, the raising of young) during a period when food is most abundant. Caterpillars should hatch just as the young tender leaves of oak tree emerge, before they harden and become less nutritious. Breeding songbirds need to time the hatching of their young just during that peak in caterpillar activity, so they can take advantage of this rich source of protein to feed their offspring. Many species have evolved to exploit peaks in resource availability, so all these seemingly independent phenological events are indeed part of a complex web of precise interactions. Disruptions in seasonal events can have profound effects on ecosystems.

How Is Climate Change Affecting Phenology?

The Intergovernmental Panel on Climate Change, in a 2007 report, estimated that spring arrived earlier by 2.3 to 5.2 days per decade in the previous 30 years. Among hundreds of observed changes, the leafing out of ginkgo trees in Japan, the flowering of lilacs, and the arrival of warblers have all shifted earlier in the year. The problem is that not all these shifts happen at the same rate, if at all. For example:

  • Winter moths have been timed to hatch just when the young oak leaves burst from their buds. With climate change, both have been happening earlier in the year, but significantly more so for the winter moth hatch. The young emerging caterpillars then starve and die.
  • Some North American migratory songbirds have advanced their arrival data. However, at least one of the main tree species they forage on has shifted its leafing out even earlier. Birds might then be missing the peak in availability of insects that are found on these trees and provide the energy and protein needed by the birds at the beginning of their nesting season.

These types of misalignment of important events in nature are called phenological mismatches. There is much research underway currently to recognize where these mismatches might be occurring.