The Annual Phenological Cycle

Abstract

The hypothetico-deductive modelling framework introduced in Chap. 2 is applied to the modelling of the annual phenological cycle of boreal and temperate trees. The ecophysiological models of the annual phenological cycle predict the timing of discontinuous developmental events, such as bud burst and height growth cessation. With such events only one, or maximally a few, empirical observations per year are available for testing these models; but as in all other models of the annual cycle, the values of the state variables are nevertheless calculated for each day of the simulation period. The methodological problems caused by this discrepancy are discussed, and an ecophysiological explication of the models is introduced. Most effort is devoted to modelling the springtime developmental events, such as bud burst. The direct environmental regulation by air temperature is first discussed by examining the classical temperature sum (or day degree) models. The ecophysiological interpretation of these models, which use the arbitrary unit of day degree, is explicated, and the experimental research aimed at determining the real ecophysiological air temperature responses of the trees and thus supplementing the day degree approach is discussed. Subsequently, effects of dormancy are introduced into the modelling. Most models for growth cessation are conceptually more straightforward, so that they are not discussed at the same length as the models of springtime development. The chapter is concluded with a discussion of models for the entire annual phenological cycle.