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Within-population variation in phenology of boreal trees indicates their adaptability to climatic variations. Although interannual variations in date of bud burst have been widely discussed, little is known about within-population variation, the key determinants for this variation and the effects of this variation on estimates of trends in bud burst date. Over a period of nine years, we monitored timing of bud burst daily in 30 mature white birch ( Betula pendula  Roth) trees in a naturally regenerated stand. Our results revealed not only large interannual variation but also considerable intraannual variation among individual trees in date of bud burst, the maximum within-population variation being four weeks. Bud burst can be accurately predicted by the date when a threshold value of temperature sum in spring is reached (base temperature +5 °). Based on this temperature sum and past temperature records, we estimated the trend in date of bud burst. The linear trend estimate based on the years 1926–2005 is an advancement of 1.2 days per decade (95% confidence interval, ± 0.7 days), which is much less than that predicted by time series based on coarser time intervals. We conclude that, because of large interannual differences, and large annual within-population variations in bud burst, estimates of bud burst date based on measurements made over a period of only a few decades are unreliable.

tree physiology

Temperature sum accumulation effects on within-population variation and long-term trends in date of bud burst of European white birch ( Betula pendula )