Declining insolation induces synchronous flowering of Montanoa and Simsia (Asteraceae) between Mexico and the Equator

We analyze the latitudinal shift in the onset of synchronous flowering in the woody genera Montanoa and Simsia (Asteraceae) between Mexico (28A degrees A N) and the Equator, where it cannot be caused by declining day length. Synchronous flowering of > 100 Montanoa quadrangularis trees was observed during two consecutive years near Cali, Colombia (4A degrees A N). Analysis of herbarium specimens yielded flowering periods for 21 Montanoa species and 18 Simsia species between 4 and 28A degrees A N. Daily insolation is a function of day length and the angle at which the sun's rays strike the earth. Between Mexico and Colombia (4A degrees A N), the maximum of insolation gradually shifts from the summer solstice to the autumn equinox. In parallel, flowering of Montanoa and Simsia starts progressively later between July and November, during the period of declining insolation. Near the Equator, there are two periods of declining insolation, and correspondingly, two flowering periods. Thus, at all tropical latitudes, flowering time of Montanoa and Simsia is highly correlated with declining insolation. The seasonal decline in daily insolation, rather than in photoperiod, apparently induces synchronous flowering of Montanoa and Simsia at the same time each year.