A comparison of xylem vessel metrics between tropical and temperate Rhododendron species across elevation ranges

Research on tree vascular traits has demonstrated an important trade-off between safety and efficiency of wood, such that tropical trees have more efficient xylem than temperate trees. However, this trade-off is equivocal for plants with non-tree architecture. So as to test the trade-off between safety and efficiency in evergreen shrubs, xylem traits of temperate and tropical Rhododendrons were compared. Rhododendron diversified from the temperate zone of Asia into the Malesian tropical zone, which makes this monophyletic group an excellent subject for testing the trade-off in vessel structure in evergreen shrubs. Wehypothesised that twigwoodof temperate Rhododendron species would have safe twig xylem, whereas that of tropical Rhododendron species would be more efficient. In the present study, safety refers to protection against freeze-thaw-induced embolism. Twig wood anatomy of accessions representing 60 species of Rhododendron was assayed. Most vessel traits of temperate species were significantly safer than those of tropical species, supporting the trade-off in this group. Some safety metrics of the twig xylem increased with an increase in native-range elevation. The adaptive changes in twig wood metrics were small, which means that stomatal conductance is constrained by limited hydraulic conductance in both temperate and tropical species of Rhododendron. This constraint has significant implications to the ecology of Rhododendron.