Phenotypic plasticity in plant phenology: a hierarchy of two levels

Phenotypic plasticity is defined as the variation of a trait of a given genotype caused by variation in the environmental conditions. Phenotypic plasticity is present both in animals and plants, but it is generally more pronounced in the traits of plants than in those of animals. The size of the organism is probably the best-known example of phenotypic plasticity in plants: under limited availability of growth resources plants remain small, whereas the body size of animals under limited availability of resources is generally less plastic. The timing of phenological events, such as spring leaf-out, is another good example of a plant trait with a high degree of phenotypic plasticity. Rather than taking place on a given constant calendar day every year, as a result of year-to-year variation of environmental factors the timing of leaf-out may vary by one or two months between years. We put forward a hierarchical concept stating that in addition to the phenological timing as such (classical plasticity of phenology), also the environmental responses regulating phenological timing can be plastic (ecophysiological plasticity of phenology). For instance, the depth of dormancy in trees varies according to the environmental factors prevailing during dormancy induction. This indicates that the responses to chilling and forcing temperatures during dormancy are plastic. Similarly, it has been shown with several crop cultivars that rather than being always the same, the optimal model predicting the timing of phenological stages in the development of the cultivar varies among geographical locations. This shows that the modelled phenological responses of the cultivar are plastic. In this paper we review the studies supporting this hierarchical two-level concept of plasticity in plant phenology and discuss its implications to process-based plant phenology modelling.