The third problem relates to a paradox that mild winters may either (1) delay
spring development because of insufficient winter chill and thus higher heat sum
required to bud burst, or (2) may lead to earlier bud-break in photoperiod insensitive
taxa with low chill requirement, with an enhanced risk of frost damage (Cannell &
Smith, 1986; Myking & Heide, 1995). A number of alpine plant species are unresponsive
to temperature, but will not even start leafing in a warmer climate unless
photoperiod requirements are met (Keller & K¨orner, 2003; Colour Plate 6).