Cun-Yang Niu, Frederick C. Meinzer & Guang-You Hao

Keeping the integrity of water transport via the water-conducting xylem vessels is crucial for trees. But a great challenge to temperate trees is to maintain hydraulic function in overwintering organs, due to freeze-thaw cycles that they normally experience. As the air-saturated sap in xylem conduits freezes, air bubbles are formed due to the insolubility of gases in ice. Upon thawing, small bubbles would collapse due to surface tension but large bubbles may expand to fill the whole conduits and hence block the water transport through these conduits, especially if the water columns in the conduits are under a relatively large tension. This process is known as freeze-thaw induced embolism and the sensitivity of xylem to such dysfunction is positively correlated with xylem conduit size. In stems of some tree species with relatively large conduits, a single frost can cause almost total loss of water transport function. Although embolism may not harm trees during the dormant season, it may cause severe damage or even tree death during the following growing season. Some tree species depend on strong resistance to freeze-thaw induced embolism by producing conduits of smaller size, but that will unavoidably compromise their efficiency of water transport. Some other trees simply abandon old conduits for water transport and almost totally depend on new conduits formed in the early spring. In many temperate trees species, the ability to generate positive xylem pressures, including root pressure and stem pressure, has evolved to facilitate the dissolving of air bubbles in winter-embolized conduits. It has been shown to be an effective mechanism to refill winter-embolized vessels in some tree species; however, the potential impact of such a dramatic change of life history on adaptation of temperate trees is under studied. Using a common garden setup, we specifically compared the water-transport-related characteristics of species having positive xylem pressure with those that don’t have this ability. Contrasts observed between functional groups in these functional traits indicate that positive xylem pressure may have partially freed some temperate tree species from adaptive constraints imposed by winter embolism formation.

Image caption: Photo by Miao Wang.
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