The TACA (Tree and Climate Assessment) model is an effective tool for identifying tree species sensitivity and vulnerability to changes in climate within the most sensitive stage of plant development, the regeneration phase.  TACA determines the probability of tree species establishment in past, present, and/or future climate scenarios based on empirically derived parameters for the following seven variables:

  • Growing degree day (GDD) thresholds: GDD is a measure of heat accumulation, important because plants depend on the accumulation of specific quantities of heat for development and growth.  Below a baseline GDD a plant will not grow and above it a plant will respire more than photosynthesize.  In TACA, if GDD exceeds the minimum or maximum threshold within which a specific tree species can regenerate, then it is assumed that it is absent.  If maximum and minimum temperature accumulation requirements are not met, then it is assumed energy requirements for growth are not met (minimum GDD) or energy requirements are too high resulting in greater respiration of carbohydrates over photosynthetic gain, resulting in mortality (maximum GDD).
  • Minimum temperature: Minimum temperature thresholds are used to determine if winter killing frosts occur.
  • Chilling requirement: Sufficient chilling time is required for a tree to become frost hardy and for bud-break to take place.  Therefore, species-specific chilling requirements are incorporated into TACA, accounting for the differing sensitivity of each species.  In TACA, if a chilling requirement is not met for a particular species, it is assumed to be absent.
  • Bud Break: In the model, accumulation of GDD occurs above a baseline temperature until a threshold is reached, triggering bud-break.  Climate change is expected to cause bud break to occur at earlier dates, increasing the risk of early spring frosts.  Tree species that have been frost hardened, even under increased temperatures from climate change, will have the date of mean bud break occurring at a lower mean temperature than is presently the case.  This will increase the risk of frost damage from late spring cold events and is accounted for in TACA.
  • Drought: A major limiting factor for determining tree species range limits; it can cause mortality and increase susceptibility to insect attack.  Tree species thresholds to drought are incorporated into TACA.
  • Frost Free Period: This is the period of time from the last spring frost to the first fall frost event.  This is a major factor for determining tree species range limits, as frost can cause mortality and limit the length of the growing season. These thresholds, for a number of tree species, are incorporated into TACA.
  • Frost: Growing season frosts can kill buds and terminal twigs or even the entire plant.  Frost damage even makes trees more susceptible to damage by insects and disease.  In TACA, the more a species is subjected to frost events, the more its presence probability will be modified.

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