Flowering while leafless in the seasonal tropics need not be cued by leaf drop: Evidence from the woody genus Brachychiton (Malvaceae)

Background and aims: A suite of woody plants inhabiting the seasonally-dry tropics flower while leafless during the dry season. A functional hypothesis for this phenomenon is that leaf drop early in the dry season pre-empts dehydration, and that flowering is triggered by the improved water status caused by leaf drop. If true, this would strongly constrain adaption to optimise flowering times.

Methods: I examine phenological patterns in the Australasian genus Brachychiton based primarily on summary descriptions in a published taxonomic treatise, along with relevant morphological, anatomical and ecophysiological evidence, in order to evaluate the role of leaf drop, and identify other sources of hydration that may initiate or support flowering while leafless.

Key results: Most of the 31 Brachychiton species are deciduous and many flower during the dry season following leaf drop, but some flower later in the dry season, at leaf flush, or when leafy. Many have enlarged stems, branchlets or tap roots, and the limited ecophysiological evidence suggests that these organs are associated with enhanced water storage potential. Brachychiton displays both considerable evidence of divergence, and a degree of phylogenetic conservatism in these traits.

Conclusions: Rehydration following leaf drop is not a necessary cue to initiate flowering even when leafless. Flowering time is likely to be adaptive and species-specific in a wide range of woody plants in the seasonally-dry tropics. Care is needed to disentangle phenological correlates and the proximal and ultimate factors driving flowering.