Climate change affects both the annual cycle of tree development and the processes related to tree growth. The annual cycle
of development manifests as observable phenological events such as leaf unfolding, flowering and leaf fall, but also includes
less apparent traits, such as changes in frost hardiness and photosynthetic capacity. Seasonality in these traits can be due
either to a fixed sequence of events that take place even in a constant environment, or to fluctuations in environmental factors.
Thus, in a constant environment, the latter mode of development displays no seasonality. In addition, and depending on the
trait considered, the internal state of development affects the tree’s capacity to respond to environmental factors. Given
that the effects of climate change on the seasonality of a particular phenological trait may depend on interactions between
fixed and fluctuating development traits, in order to explore these effects the entire annual cycle of development must be
modelled. The processes related to tree growth include photosynthesis, respiration and allocation at the level of the individual
tree; at stand level they include resource availability and biotic interactions. In this chapter we present the general theory
of the annual cycle of development of trees, with examples of climate change effects on phenological traits with different
mode of development for tree species in the boreal, temperate and Mediterranean zone of Europe. A process-based model on tree
growth is outlined, with focus on scaling up from the tree to the stand level in time and space. Examples of climate change
are presented, based on a model that couples the annual cycle of development and the growth of trees. Phenological events
are characterized by responses to temperature that are under strong selective pressure. Future lines of development in this
field of research include an assessment of the adaptive potential of phenological events to climate change. An example of
this genetic approach is also presented.