On two days each year the length of daylight and darkness are very nearly the same. The first day of spring, the Vernal Equinox , occurs about the 21st of March and the first day of autumn, the Autumnal Equinox, about the 22nd of September. The word equinox means “equal night.” The beginning of spring (when days get progressively longer than nights) and the beginning of autumn (when days get progressively shorter than nights) occur at the point at which the sun’s path crosses the celestial equator. The summer solstice on 21 June is the longest day of the year, about 15 hours of daylight, and the winter solstice on December 21 is the shortest day with some nine hours of daylight.
The expression ‘phenology’ is applied to the timing of biological events. Although such celebrated observers as Lewis and Clark and Henry David Thoreau have left records of seasonal biotic changes, the Belgian botanists Charles Morren introduced the term phenology in 1853 as the science of observing and measuring biological events in time. Events such as bird migration, seed germination, egg laying, and leaf fall are good examples. The celebrated author Aldo Leopold wrote eloquently of Midwestern biological cycles in his environmental classic A Sand County Almanac.
Depending on day length and associated temperature and precipitation, plants may bloom and birds may migrate on different dates, varying from days to weeks, across time. During some years of the last decade early spring has been getting warmer and some spring flowering plants such as bloodroots and spring beauties have flowered a week or so earlier than the long term average. The calling of frogs too can vary from late March to early April depending on specific conditions. I once heard a forlorn bull frog calling at Augustana’s main field station, Green Wing Environmental Laboratory near Dixon, IL, on a sunny March day while the ponds were still mostly ice covered. That was exceptionally early and most large and smaller frogs do not become active till later in spring. In a ‘Floral Calendar for Rock Island County’ published in the Augustana Observer newspaper in April, 1912 Professors Udden and Beausang list (among many other records) Hepatica triloba, the liverleaf plant, as having a blooming date of April 6. I have observed the same species in my own backyard blooming in late March for many years.
Across the Midwest it appears that blooming dates and other spring activities are moving up in time as global temperatures slowly warm up. Such events bear careful watching as harbingers of longer term changes in global weather patterns. An excellent web site to explore data about plants, animals and climate change is the USA National Phenology Network, www.usanpn.org .
Bohdan Dziadyk, PhD
Professor of Biology
Director of Field Stations