overcome their winter rest, warmer temperatures are all they need to start sprouting
or flowering. The resultant relationship with temperature is often almost linear (see
Figures 4.14 and 4.2b). Spring temperatures also play a decisive part in determining
the time at which the fruit ripens in summer and autumn, as well as the duration of the
entire growing season. In order to prevent too early a break of dormancy or too late an
induction of dormancy in autumn, photoperiodism (day length) plays an additional
role in triggering phenological events. As discussed in Chapter 3, photoperiodism
constrains the influence of temperature on development to ‘safe periods’. However,
it seems that in many regions dormancy is broken by photoperiod and sufficient
chilling, and only the subsequent warmer conditions trigger the onset dates of bud
burst or other spring phases. Chilling requirements are quite modest relative to heat
requirements so that most of the delay in phenology caused by reducing chilling
under climate warming will likely be swamped by advanced phenology arising from
spring warming.