Samara water absorption and floatation in sheoaks, Casuarina and Allocasuarina (Casuarinaceae)

Diaspores (samaras) of the sheoaks, Casuarina and Allocasuarina, have a mesocarp composed of hydrophilic fibres that, in Allocasuarina, rapidly expand on wetting, capturing significant quantities of water. The ecological function of this water capturing has been the subject of speculation but not quantification or experimentation. Therefore, the rate and quantity of water absorbed, and the floatation properties of samaras of accessions of Casuarina and Allocasuarina were assessed. Casuarina absorbed water slowly (~48 h) with median absorption of 90% (by weight) whereas Allocasuarina absorbed water rapidly (<2 min) with median absorption of 240%. The process was reversible and increasing in Allocasuarina, with median absorption reaching 400% over three to five wetting/drying cycles. The floating half-life of Casuarina and Allocasuarina samaras overlapped, 17-70 and 29-57 h, respectively, so Casuarina samaras were not shown to be better adapted to hydrochory. Based on five accessions of each genus, it does not be appear that water-capturing of sheoak samaras is directly related to habitat aridity, nor to potential hydrochory. Therefore, the ecological implications of differential water-capturing of sheoak samaras remains to be determined.