The ability of plants to minimise the risk of freezing damage is conferred by sychronising their phenology with the growing environment. The indigenous flora generally has a good safety margin between its frost hardened status and minimum temperatures, unless the growth environment changes. A negative link between acid deposition and reduced frost hardiness was shown to be a casual factor in the observed decline of red spruce in the nineteen eighties (Fowler et al., 1989; DeHayes et al. 1991). Cape et al. (1991) showed that the causal agent was sulphate ions, rather than nitrate ions, and that effects were exacerbated by acidity and ammonium ions. Subsequent research showed similar responses for Norway (Sheppard et al., 1998) and Sitka spruce (Sheppard et al. ,1994).
The bioassay is conducted on detached shoots, which are frozen to a range of temperatures in a purpose built frosting chamber using a cooling rate of 5 °C per hour, and kept for 3 hours at the target temperature. The shoots are then thawed at 10 °C increase per hour back to ambient temperature. Damage is assessed by electrolyte leakage, by immersing shoots into de-ionized water and analysing conductivity.
Suitability to indicate atmospheric concentrations:
Various indicators, including frost hardiness, have been used to estimate Critical Levels for exposure to acidic cloud water (Cape, 1993). This was set at an air concentration of sulphate particles of 1 mg S m-3 for sites where cloud occurred for > 10% of time.
Suitability to indicate atmospheric depositions:
While there may be a link between acid deposition and increased frost sensitivity the evidence is often inconclusive implying a large N excess may be a prerequisite.
Suitability to indicate environmental impacts:
Sensitivity to other factors:
Frost hardiness is also sensitive to water availability and nitrogen deposition, and expected to depend on ozone exposure. A limitation is that the frost hardiness response depends on local climatic conditions and may vary between years.
Available evidence suggests that the method is more suited for assessing short term changes.
Natural variation is high so that effects must be large to be detectable. The analysis is destructive and requires large numbers of samples especially as assessments may need to be made at different times - early autumn, winter and spring. Effects are also difficult to quantify in the absence of controls.
Expertise in field:
Collection of material in the field requires trained, experienced personnel.
Expertise in laboratory:
The assessment of frost hardiness in the laboratory requires specialist equipment and trained, experienced personnel.
Cost (per unit sample): £100-500
Cost Comment: The method would need to be carried out in a specialist laboratory. The assessment of frost hardiness at one site would probably take 3 days work at the cost £300 per day + equipment running costs. Total costs per site - £920