Tree development alternates between periods of rest (dormancy) shoot growth and flowering. At temperate latitudes the seasonality of tree development (= phenology) is determined mainly by the annual course of temperature. However 100 yr ago Klebs (1914) observed that in a glasshouse saplings of European beech (Fagus sylvatica) were dormant during the short December days but when exposed to continuous illumination in a ‘light chamber’ their resting buds opened within 4 wk and 15 leaves expanded during the subsequent 4-month-long period of continuous shoot growth (Supporting Information Notes S1). In the glasshouse the increase in day length in January induced bud break of dormant saplings in February. Klebs concluded from his many experiments that the autumnal decline in ‘light quantity’ (duration × intensity) induces bud dormancy and in January the increase in light quantity sensed by dormant buds breaks dormancy and triggers bud break of leafless saplings in a glasshouse. He recognized that for any given latitude day length and light intensity are coupled because solar intensity varies significantly through the year as the sun's path in the sky changes with the season. Implicitly ‘light quantity’ is synonymous with ‘daily insolation’ the measure of integrated solar intensity and day length to be used in this paper (Calle et al. 2010). Subsequent studies in which seedlings of many temperate tree species were exposed to experimental variation in day length confirmed Klebs' observations but largely ignored his conclusions (Notes S1; Garner & Allard 1923; Wareing 1956; Romberger 1963). We will use the term ‘photoperiodic control’ when referring to the control of tree development by the seasonality of daily insolation.
Publication year
2015
Authors
Borchert, R.; Calle, Z.; Strahler, A.H.; Baertschi, A.; Magill, R.E.; Broadhead, J.S.; Kamau, J.; Njoroge, J.; Muthuri, C.
Keywords
Phenology, Dormancy, Deciduous forests, Dormancy, Temperature.
