Frost in a future climate:: modelling interactive effects of warmer temperatures and rising atmospheric [CO2] on the incidence and severity of frost damage in a temperate evergreen (Eucalyptus pauciflora)

Although plants are more susceptible to frost damage under elevated atmospheric [CO2], the importance of frost damage under future, warmer climate scenarios is unknown. Accordingly, we used a model to examine the incidence and severity of frost damage to snow gum (Eucalyptus pauciflora) in a sub-alpine region of Australia for current and future conditions using the A2 IPCC elevated CO2 and climate change scenario. An existing model for predicting frost effects on E. pauciflora seedlings was adapted to include effects of elevated [CO2] on acclimation to freezing temperatures, calibrated with field data, and applied to a study region in Victoria using climate scenario data from CSIRO's Global Climate Model C-CAM for current (1975-2004) and future (2035-2064) 30 years climate sequences. Temperatures below 0 degrees C were predicted to occur less frequently while the coldest temperatures (i.e. those below -8 degrees C) were almost as common in the future as in the current climate. Both elevated [CO2] and climate warming affected the timing and rates of acclimation and de-acclimation of snow gum to freezing temperatures, potentially reducing the length of time that plants are fully frost tolerant and increasing the length of the growing season. Despite fewer days when temperatures fall below 0 degrees C in the future, with consequently fewer damaging frosts with lower average levels of impact, individual weather sequences resulting in widespread plant mortality may still occur. Furthermore, delayed acclimation due to either warming or rising [CO2] combined with an early severe frost could lead to more frost damage and higher mortality than would occur in current conditions. Effects of elevated [CO2] on frost damage were greater in autumn, while warming had more effect in spring. Thus, frost damage will continue to be a management issue for plantation and forest management in regions where frosts persist.