Winter hydraulic conductivity end xylem cavitation in coniferous trees from upper and lower treeline

Current hypotheses suggest xylem conduits of conifers cavitate as a result of low xylem water potentials during the growing season and as the combined result of low xylem water potentials and repeated freezing and thawing of the xylem during the winter. To investigate variation in resistance to xylem cavitation among evergreen and deciduous coniferous trees from different environments, we analyzed conifers from upper and lower treeline. Species from lower treeline (Larix occidentalis and Pinus contorta) exhibited higher resistances to xylem cavitation compared to species from upper treeline (Larix lyallii and Pinus albicaulis). At lower treeline, the deciduous conifer Larix occidentalis was more resistant to xylem cavitation than the evergreen Pinus contorta. None of the four species experienced losses in stem hydraulic conductivity greater than 5% during the growing season. During the winter, lower treeline species experienced between 8 and 15% loss in xylem conductivity compared to greater than 30% for upper treeline species. At upper treeline, losses in stem hydraulic conductivity were correlated with freeze-thaw events, mid-winter warming periods and decreased stem water contents. Wintertime xylem cavitation in the upper treeline environment was related to freezing and thawing events and the total time the xylem was above freezing.