Temperature and plant development phenology and seasonality

Annette Menzel and Tim Sparks

4.1 The origins of phenology

The recording of the timing of life-cycle events has only recently been considered as an area of climate impacts research. For a much longer period, phenology has been recorded by those with an interest in natural history, by those engaged in agriculture and horticulture and where traditional local festivals have been associated with plant phases.

Some plant species and some phases are more apparent than others. Hence the brilliant displays of cherry flowering at the Royal Court in the former Japanese capital of Kyoto or of peach flowering in Shanghai are very obvious and are associated with local festivals. Flowering of forsythia, for example, is much more obvious than that of beech trees. In Europe, religion and folklore may associate some plants with specific calendar dates: for example, daffodil flowering with St David's Day (March 1), snowdrop flowering with Candlemas (February 2) and the Devil spitting on blackberries on the night of October 10. Flowering of other species is of considerable importance for tourism, such as of fruit trees in south-eastern Norway, of crocuses at Husum, Germany, and of tulips in the Netherlands. Given these facts, it is not surprising that the emphasis in traditional plant phenology is biased towards trees and towards plants with obvious flowers, and may have a different emphasis in different countries. At a later date, the importance of phenology to assess environmental conditions for annual and perennial crop cultivation was recognised.

The life cycles of most deciduous plants go through recognisable phases, e.g., leafing, flowering, fruiting, leaf colouration, leaf fall, bare. For some species it is possible to sub-divide these broad categories, for example first flowering, 50% flowering and end of flowering. However, phenology has traditionally been involved with easy-to-record events where fewer opportunities exist for individual interpretation. As a consequence, events such as first leafing and first flowering dates are by far the most popular. This does not mean that there is no room for inconsistency as the time at which complex leaves and flowers open may be subject to interpretation. Species do differ in their dates of phenological phases and the order in which these events occur. For example, in Table 4.1 it is obvious that ash flowers before leafing, and loses its leaves early, in contrast to oak.

The oldest known phenological series is that of the Kyoto cherry (Prunus jamasakura) flowering, mentioned above (Menzel, 2002b). Data on this series stretch back to the year 705 ad. Some Chinese series stretch back to the sixteenth

Table 4.1 Average dates of phenological phases of ash (Fraxinus excelsior) and oak (Quercus robur) in Worcestershire, UK, in the early twentieth century*
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