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 9 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.

While knowledge of photoperiodic control of dormancy in small experimental trees expanded, photoperiodic responses of mature trees received little attention (Romberger, 1963). Many years later, we observed in tropical forests at low latitudes that the phenology of many trees is highly correlated with seasonal variation in insolation (Rivera & Borchert, 2001; Rivera et al., 2002). The large size of trees precludes experimental confirmation of such field observations, but the strong correlations between seasonal variation in insolation and synchronous development of many tree species are, in fact, the results of the large-scale, long-term natural experiment conducted during the evolution of tropical trees (Calle et al., 2010). These