Taken together, chilling requirement and photoperiodism represent a dual induction
system, which in combination with actual temperatures (e.g. heat sums) determine
development (Hay, 1990). Needless to say that this complicates predictions of
phenology in a warmer climate; even more so, because species greatly differ in their
chilling and photoperiod requirements. The problem becomes worse, because these
interactions between photoperiod and temperature are not firm, but are in part substitutive,
which means particularly warm temperatures can override photoperiod controls
and particularly long days can override chilling requirements (Heide, 1993a).
A common pattern is that photoperiod and/or a sufficient chill dose releases/induces
a developmental step, and follow-up temperatures determine the speed of progress
(Figure 3.5A). There are domestic tree species that have an evolutionary history
where photoperiodism did not play a significant role in spring,