| The primary purpose of this study is to systematically test whether shoot dieback in birch trees is related to thawing and re-freezing events in winter that lead—in turn—to shoot and/or root freezing injuries, to reductions in spring root pressure and to a subsequent recovery from the winter-induced xylem cavitation. For that reason, extent of shoot dieback, shoot xylem cavitation, shoot and root freezing injury and root pressure were measured in yellow birch (Betula alleghaniensis Britt.) under simulated- and natural winter conditions. Shoot xylem cavitation was determined as percent loss of hydraulic conductivity. Shoot and root freezing injuries were measured by way of relative electrolyte leakage (REL) and triphenyl tetrazolium chloride (TTC) reduction. It was found that formation of winter shoot xylem cavitation was related to thaw duration, to freezing temperature, and to shoot dessication. For seedlings and mature trees, shoots lost 75 to 100% of their hydraulic conductivity, thereby indicating that shoot xylem cavitation is an unavoidable winter phenomenon. Springtime root pressure in this species, however, was observed to eliminate winter cavitation almost completely. While REL and TTC approaches were found to be suitable to diagnose freezing injury of roots, only REL was deemed good enough to diagnose shoot injury. A soil freezing temperature of −10°C was found to cause a 10% increase of root REL in potted yellow birch seedlings. This soil temperature also caused a considerable reduction of root pressure, hence leaving a large amount of post-spring xylem cavitation intact. The residual cavitation was—in turn—associated with shoot dieback of potted yellow birch.; A process-based model was formulated to calculate shoot water content, extent of xylem cavitation and extent of shoot and root freezing injuries from generally available weather records such as air and soil temperatures, relative humidity, solar radiation and wind speed. In this model, shoot dieback was empirically linked to extent of shoot and root freezing injuries and residual cavitation. Laboratory controlled experiments were used to generate the model-relevant process parameters, i.e., the parameters that were needed to compute the cumulative, weather-affected loss of moisture from the shoots, based on an experimentally defmed relationships between xylem moisture content, xylem water potential, and xylem cavitation, and between the extent of soil/air freezing to root/shoot injuries. Special investigations were done for mature yellow birch trees to compare extent of xylem cavitation, shoot freezing injuries, and shoot dieback with field observations. With special model calibrations it was possible to have all model predicted values fall within the 95% confidence interval of the mean observed field values for most of the consecutive measurement dates. |
