Effects of winter frosts on xylem embolism and the consequences for growth and phenology across provenances of silver birch in Scotland

Silver birch has the potential to become an important component of upland forestry throughout Great Britain, However, to realize this potential, more knowledge is necessary on the ecology of growth and reproduction of this species and, especially, about its silvicultural requirements. Particularly, worries have been expressed in the past with regard to the frequent occurrence of phenotypes characterized by poor stem and crown form. Winter damage to young twigs by frost has been suggested as an important potential reason for such occurrence. Silver birch requires protection from low temperatures and drying winds: the death of terminal buds and shoot cambium is often observed in North Scotland (Blackburn. & Brown 1988). Frost damage to the new shoots of birch have also been observed to occur in some years in early spring in Southern Scotland. Environmental differences across sites and annual climatic variations may have profound effects on the regulation of the timing of bud-burst and flowering during spring and the timing of growth cessation and dormancy development during late summer and fall, as well as on the adaptations to resist to frost damage. Frost may therefore be a major factor responsible for reductions in stem growth, and in the quality of the timber of silver birch in Scotland. This study examined the variations in stem form and phenology of several provenances of silver birch established in three planted trials in Scotland, and investigated the frequency, extent and duration of sap freezing events at one site and the consequent variations in the hydraulic supply capacity of the shoots (i.e. presence of xylem embolism) in winter and early spring. The results show that the investigated provenances differed significantly in height and in time of bud-flushing in spring. Provenances from the South-East of Scotland tended to be taller and flush earlier. The first bud on the top leader flushed earlier than the four subtended buds, i.e., it exerted an apical dominance on the other buds. I attempted the construction of a selection index, in order to detect which provenance gave the best overall performance by combining the scores for height, bud-flushing, and stem shape. Observations of shoot dieback during the summer 2000 showed little mortality of the apical shoots due to frost, in agreement with the temperatures recorded in the previous winter and spring, which were seldom below —5 °C. Most of the dieback was localised in the upper and thinner part of the leader stems. The provenances showing the highest frequency of damage were also characterised by earlier flushing. Thermocouples were inserted in the xylem of trees to monitor freezing cycles in the field. Exotherms revealing events of sap freezing were detected at temperatures as high as —0.8/-1 °C. All the investigated provenances developed high values of xylem embolism during the winter. The peak of embolism was -50 % for 3-year-old shoots and -P71 % for one-year-old shoots. Water content of the twigs was also found to decline during winter. During spring (bud-bursting phase), all provenances recovered from winter embolism, with values dropping from the high values of the winter months to the minimum annual values of April, in association with positive xylem pressures measured at 40 cm from the base of the trees. Experimental freeze-thaw cycles in the laboratory on branches of silver birch confirmed that the development of xylem embolism was linked with changes in temperature from below to above the threshold of zero degrees. Younger shoots were found to be more vulnerable to loss of hydraulic conductivity than older shoots, suggesting that the location of greatest damage by frost is the apical twigs. This is consistent with the qualitative observations of shoot mortality in the field. Differences in flushing and in stem height growth across the investigated provenances are likely to be related to the water relations of the shoots during winter.