Different approaches to the study of the annual rhythm of forest trees are described and compared by analysing the concepts and theories presented in the literature. The seasonality varying morphological and physiological state of forest trees is referred to as the annual rhythm s. lat., from which the annual ontogenetic rhythm is separated as a distinct type. The dormancy phenomena of the trees are grouped into four categories. Theories concerning the regulation of the annual rhythm are divided into two main types, the most common examples of which are the photoperiod theory and the temperature sum theory. Recent efforts towards a synthetic theory are described.
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