The Winteraceae are regarded as among the oldest and most primitive families of flowering plants. These ~65 species lack xylem vessels, which are thought to be a critical adaptation permitting angiosperms to achieve their current ecological domination. The Winteraceae have been viewed as 'living fossils' that survive in wet forest pockets where their tracheid-based wood does not impose hydraulic constraints. However, phylogenetic analysis implies that the common ancestor of extant angiosperms had vessels and that these were lost in Winteraceae. Preliminary studies on Winteraceae hydraulics indicate that vessel-less wood does not impose a hydraulic constraint. Small xylem conduits, such as those found in conifers and the Winteraceae, may confer a functional advantage for evergreen woody plants experiencing freeze-thaw cycles. This hypothesis will be examined using physiological data on the water balance of ten Winteraceae species in the sub-alpine rainforest of Tasmania and tropical montane forests of New Caledonia compared to co-occurring trees with and without vessels.
