Drought and cool temperature cue general flowering synergistically in the aseasonal tropical forests of Southeast Asia

The family Dipterocarpaceae, along with species from many other families, flower synchronously at irregular intervals of several years in the extensive humid forests of Southeast Asia. The intermittent mast seeding in these forests satiates seed predators whose populations remain low due to starvation in non masting periods, effectively increasing seed survival and seedling recruitment. A previous study demonstrated that a synergistic interaction between drought and cool temperatures, instead of either climate factor alone, best explains the occurrence of general flowering events observed over 14 years at the Pasoh Research Forest, Malaysia. However, several measures of drought were proposed and which drought measure should be used to explain general flowering remains untested. Here we examined the relationship between flowering of five Shorea species and three drought indices, the difference between the mean daily accumulation of precipitation and a threshold level of precipitation, water deficit and antecedent precipitation index. Our analyses confirmed that for most of Shorea species we studied the synergistic model that considers both cool temperature and drought performs the best regardless of the choice of drought index. In addition, a simplest drought measure using cumulative precipitation is sufficient to explain flowering activities in Shorea species.