Climatic warming, spring budburst and frost damage on trees.
Year-to-year variation in the risk of spring frost damage during the 20th century was examined with reference to variation in the date when a fixed thermal time was reached, and in the recorded dates of full bloom of Cox's Orange Pippin apples in Kent and the estimated dates of budburst on young Picea sitchensis trees in the Scottish uplands. The data indicate that if future CO2- induced warming of 2°C increased the incidence of warm springs, of the type that have occurred in Britain during this century, then warming would induce earlier blossoming and budburst in many temperate trees with an increase in the risk of subsequent damaging frosts. If budburst occurred after a constant thermal time (e.g. 100 day °C > 5° after mid-January), then budburst would occur so much earlier in the spring that, on average, the temperature on the date of budburst would be lower than at present. However, in many trees there is an increase in the thermal time to budburst with decreased chilling. This increase prevents very early budburst in warm springs and lessens the risk of frost damage. Theoretically, warming could delay or advance budburst, depending upon the extent to which a tree's chilling requirements are currently met. Empirical thermal time-chilling models suggested that, on average, Cox's apples in Kent would blossom 18-24 days earlier than at present following 2° warming, but that P. sitchensis in the Scottish uplands would burst its buds only 5 days earlier than at present.