Insolation and photoperiodic control of tree development near the equator.

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 openedwithin 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’ (duration9 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).Wewill 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 correlations were not recognized in the past (Van Schaik et al., 1993; Thomas & Vince-Prue, 1997; Jackson, 2008). InCentral American semi-deciduous forests, a dry-season lasting from December to May alternates with a 6-month-long rainy season. Tree phenology is determined mainly by photoperiod or rainfall seasonality. Photoperiodic control is indicated by synchronous bud break or flowering of conspecific trees at the same time each year. The increase in insolation around the spring equinox induces flushing or flowering of leafless trees during the dry season (Rivera et al., 2002; Elliott et al., 2006) and its autumnal decline causes flowering of trees with mature foliage during the late rainy season (Rivera & Borchert, 2001). In other deciduous species, the timing of synchronous bud break of leafless trees varies from year to year with the first rains of the wet season, and during the dry season irrigation causes bud break within a few days (Borchert, 1984; Williams et al., 1997). At the equator, where day length is constant, many trees leaf out at irregular intervals, but others do so at the same time each year (Borchert et al., 2005a). There is no consensus regarding the environmental signal that causes synchronous greening. Seasonal variation in irradiance (Van Schaik et al., 1993), sunrise or sunset time (Borchert et al., 2005a), sunlight (Huete et al., 2006), solar radiation (Myneni et al., 2007; Morton et al., 2014) and high solar intensity (Renner, 2007; Yeang, 2007) have been considered.Most studies ignore that bud break preceding synchronous greening of tropical forests must be caused by an environmental signal perceived some weeks before leaf emergence (Klebs, 1914). The only environmental variable known to induce synchronous tree development near the equator is daily insolation, in short ‘insolation’ (Calle et al., 2009, 2010; L€ uttge, 2009; Guan et al., 2013).