Hydraulic Strategies of Cool-Temperate Trees and Lianas Against Freeze-Thaw Embolism: Is Cold Winter More Severe on Lianas?

Lianas are abundant and influential in tropical forests but decline sharply in cooler regions. This pattern has been attributed to their reliance on a small number of large vessels, which are vulnerable to freeze-thaw embolism. However, the hydraulic functioning of lianas in cooler regions remains poorly understood. We examined the hydraulic properties of nine tree and four liana species in a cool-temperate forest. Compared with tree branches of similar diameter and age, liana branches had substantially wider vessels and, on average, 10 times greater specific conductivity (Ks), supporting approximately six times more leaf area. While they experienced a nearly complete loss of Ks during winter, conductivity was fully restored before or during leaf expansion. This recovery was achieved either by refilling embolised vessels using positive xylem pressure or by reconstructing the hydraulic pathway with newly formed vessels, similar to mechanisms observed in trees. Seasonal sap-flow patterns during the growing season fell within the range observed for trees. Thus, cool-temperate lianas maintained hydraulic function at least as effectively as trees. Our results do not support a simple freeze-vulnerability hypothesis but instead suggest that the ecological success of cool-temperate lianas depends strongly on mechanisms that restore stem hydraulic function after winter.