20-24 November 2017


NOTE: This document is a repeat of that appearing as a Supplementary Information File for last week.

For centuries, various peoples in Europe and the Orient have kept track of recurring biological events such as the budding and flowering of plants or migration of wildlife that appeared related to the seasonal cycle. More recently, people across North America have become aware of phenology, which represents an organized and systematic effort to monitor the well-defined seasonal phases in plant and animal life. In addition to these biotic events, several abiotic or non-living events would also fall within the realm of phenological observations, such as watching for the freezing and "ice out" (or opening) of lakes in northern latitudes. Thus, phenology can be used in the scientific study of the influence of climate upon the periodic annual phenomena of plant and animal life. In the addition, phenology can be employed to see how long-term changes in these seasonal events change in response to long-term changes in climate.

The origin of the term "phenology" is either from the contraction of the word phenomenology or from the Greek words "phaino" (to show or appear) and "logos" (to study). The origins of phenology came from observations of recurring phenological events made by early agrarian cultures that attempted to monitor the progress of the natural calendar. Some of these observations resulted in proverbs and sayings that were meant to forecast future weather events or trends, especially for those that could affect the planting and harvesting of crops. Some historians suggest that the Chinese may have had written records dating back to approximately 974 BC. The Japanese were making observations of the timing of the peak in the blossoming of cherry trees at the Royal Court in Kyoto since approximately 705 AD. While some phenological societies in Europe have been around for more than a century, interest in phenology has developed in this country. The USA National Phenology Network, which is sponsored by NOAA, NASA and the US Fish and Wildlife Service, is a nationwide network consisting of citizen scientists, government agencies, non-profit groups, educators and students willing to monitor the impacts of climate change on plants and animals in the United States. Stimulated by the National Phenological Network, a program called Project BudBurst has been developed at the National Center for Atmospheric Research (NCAR) and represents "a national phenology and climate change campaign for citizen scientists."

So how are recognizable phenological events related to climate? Consider the onset of spring growth, which usually depends upon various climates and weather factors. Invariably, the lengthening daylight and higher sun angles should warm the air and ground, permitting the reemergence of spring foliage across the northern sections of the country. The Swedish botanist Carolus Linnaeus (1707-1778) compiled annual calendars of leaf opening, flowering and fruiting together with concurrent weather conditions for 18 locations in Sweden over many years. He was one of the first scientists to make a connection between phenological events and climate. Furthermore, by comparing these phenological observations over a wide area, we can also monitor how these events move geographically over the year, responding primarily to the seasonal changes in the sunlight with latitude. Andrew D. Hopkins, an entomologist with the U.S. Department of Agriculture during the early 20th century (and no known relation to this author), attempted to correlate phenological phenomena of plants and animals with the various climate elements across the nation. According to the relationship that he proposed (now known as Hopkins Law), most phenological events tend to progress northward and upward in altitude during spring, while southward and downward in fall. A delay is also noted in the eastward direction. This empirical relationship suggests that the time for the peak flowering of a certain plant species, would progress northeastward at a rate of 4 days per 1 degree of latitude and upward at 1 day for 100 feet of altitude. We should note that this relationship was valid for the United States during the early 20th century.

Since some biotic and abiotic events are sensitive to small variations in temperature or other atmospheric factors, climate scientists and historians have attempted to use phenological records as a proxy indicator of past climates prior to the instrumental records. For example, researchers studying the records of grape harvests in Europe were able to reconstruct growing season temperatures extending back for more than 500 years, well before the invention of the thermometer. Inspection of the long-term phenological records has also revealed a phenomenon that some people refer to as "season creep," which represents the earlier observed timing of seasonal events in spring. The famous cherry blossoms around the Tidal Basin in Washington. DC currently tend to bloom at the start of April, earlier than the mid-April peak blooms of earlier decades. This season creep appears to be associated with the general increase in temperature across the globe.

A continuation of phenological observations should help track the effect of climate change on organisms and help scientists make predictions concerning the future health of the ecosystems. If springtime temperatures across a region increase over the years, the timing of specific phenological events, such as the budding, leafing and fruiting of plants could occur earlier. A drastic change in the timing of these events could result in fewer seeds and insects, which could also have an impact on other animals that depend upon the seeds and insects.

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Prepared by Edward J. Hopkins, Ph.D., email
© Copyright, 2017, The American Meteorological Society.