Climatic Warming, Spring Budburst and Forest Damage on Trees

SUMMARY (1) If future C02-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. There would, for example, be an increase in the already high incidence of frost damage to apple blossom (Malus pumila Mill.) cv. Cox's Orange Pippin, in Kent and to new vegetative shoots of Picea sitchensis (Bong.) Carr. in the Scottish uplands. (2) If budburst occurred after a constant thermal time (e.g. 100 day 'C > 5 0C 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. Consequently, the risk of damaging frosts occurring after budburst would be much increased. (3) 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. (4) Theoretically, warming could delay or advance budburst, depending upon the extent to which a tree's chilling requirements are currently met. (5) Empirical thermal time-chilling models suggested that, on average, Cox's apple in Kent would blossom 18-24 days earlier than at present following 2 'C warming, but that P. sitchensis in the Scottish uplands would burst its buds only 5 days earlier than at present.