The Annual Cycle of Frost Hardiness

The hypothetico-deductive modelling framework introduced in Chap. 2 is applied to modelling the annual cycle of frost hardiness in boreal and temperate trees. The concept of frost hardiness is straightforward as such, denoting the threshold temperature that distinguishes between the damaging low-temperature range and the undamaging one. In practice, however, the concept of frost hardiness needs to be specified in regard to the tree tissue addressed and the methods used in assessing it. Similarly to the models of the annual cycle of photosynthetic capacity discussed in Chap. 4, the models of the annual cycle of frost hardiness can be tested continuously on a daily basis. The main environmental factors regulating the annual cycle of frost hardiness are air temperature and night length. These environmental factors regulate the annual cycle of frost hardiness directly and also indirectly by affecting the annual phenological cycle, which in turn changes the direct response. This complexity calls for development of integrated models that address both the fixed-sequence aspects of the phenological cycle and the fluctuating aspects of frost hardiness as such. Integrated models have been developed since the 1970s, and two of them are discussed in detail here. Both models are quite accurate, but there is still room for considerable improvement in their realism. Novel simulations using long-term air temperature records as input for an integrated model revealed large year-to-year variation in the annual cycle of frost hardiness.