Vascular performance of woody plants in a temperate rain forest: lianas suffer higher levels of freeze-thaw embolism than associated trees

Abstract

The success of lianas in tropical rain forests has been linked to their wide xylem vessels that confer much higher hydraulic conductivity than tropical trees. In contrast, the poorer representation of lianas in temperate forests and high elevations might reflect lower competitiveness there, because of the vulnerability of high-performance xylem to freeze–thaw embolism. Nevertheless, the empirical evidence that support this idea is surprisingly scarce. We assessed this hypothesis comparing the hydraulic performance of five lianas and nine trees coexisting in cold weather, by measuring wood traits, hydraulic conductivity, xylem embolism and root pressures. Vessel diameters of lianas were on average twice those of trees (51·3 vs. 23·2 μm), resulting in an 18-fold difference in mean specific hydraulic conductivity (15·5 vs. 1·05 kg s−1 m−1 MPa−1). Lianas on average suffered higher levels of freeze–thaw embolism than trees (29·9 vs. 12·8%), reducing their potential hydraulically supported competitive advantage; however, partially embolized lianas still had greater water transport capacity than trees. Specific hydraulic conductivity (Ks) was well correlated with vessel size in lianas but not in trees; however, plants with big vessels were more vulnerable to freeze embolism. Although all species of lianas developed root pressures and these tended to be higher than trees, root pressure were insufficient to repair embolism in plants > c. 7 m tall. The magnitudes of root pressure were well correlated with vessel size in lianas, but this relationship was not observed in trees. Synthesis: Our results show empirical evidence that hydraulic performance of temperate lianas exceed the performance of coexisting trees, in similar magnitude than its tropical counterparts. Nevertheless, this hydraulic advantage is reduced as a consequence of freeze embolism. Temperate lianas appear to employ a bet-hedging strategy entailing significant embolism in winter, but likely enabling rapid carbon gain during frost-free periods. Loss of winter carbon gain opportunities because of embolism may explain the low success of lianas in temperate rain forest. However, predicted future climatic scenarios could are likely to increase liana abundance in mid-latitudes and allow lianas to expand their ranges towards higher latitudes and elevations.