Modeling tree phenology is highly affected by knowledge gaps, and most research has been targeting spring phenology, more precisely the timing of budburst. Budburst is affected by temperature and photoperiod during winter and spring, and timing of budburst reflects the balance between gaining competitive advantage by earlier growing start and the risk of frost damage which may impair growth. Many temperate and boreal trees require cold temperatures (chilling) to break winter rest before they can respond to warm temperatures (forcing), triggering budburst. In addition, some species are regulated by daylength for which long days can trigger budburst even if the temperature is low.
Phenological model components reflect what happens during winter and early spring when there are no visual signs of development. Temperature is included in the components as units (chilling units and forcing units) which vary depending on the mathematical function used.
Example of a model with three components:
- The accumulation of chilling units for rest completion
- The bud’s growth competence, i.e. the ability of the tree to respond to forcing temperatures which is determined by the definition of rest and amount of chilling at a given time
- The timing of budburst through accumulation of forcing units
With a focus on the five main forest tree species in Sweden, birch (Betula spp.), oak (Quercus spp.), beech (Fagus sylvatica), Scots pine (Pinus sylvestris) and Norway spruce (Picea abies), we are evaluating the models with both ground observations and satellite data.
The objectives of our model development are to:
- Develop a phenological model linking all annual phenological events, thereby including the influence of water and nutrient availability, while expressing geographical adaptations of a species
- Implement the phenological model into an ecosystem model (LPJ-GUESS) to project climate change impact on forest phenology in an ecosystem perspective