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 enhanced N deposition and reduced frost hardiness was widely suspected to be a casual factor in the observed decline of red spruce in the nineteen eighties (Eagar & Adams 1992).
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 ºC per hour, and kept for 3 hours at the target temperature. The shoots are then thawed at 10 ºC increase per hour back to ambient temperature. Damage is assessed by electrolyte leakage, by immersing shoots into de-ionized water and analysing conductivity.
Nihlgard (1985) postulated that increasing N deposition prolonged the growing season and led to carbon assimilate being diverted away from the production of 'cryoprotectants', which protect from frost damage. Subsequent studies using N fertilizers which have significantly raised foliar N concentrations, failed to support this hypothesis showing that the addition of N to N-deficient trees improved frost hardiness (DeHayes et al. 1990, Klein et al. 1989).
Nursery studies with conifers also suggested improved frost hardiness with N fertilisation although the effect was dependent on the timing of the fertilizer application (Benzian and Freeman 1967). Christersson (1973) observed higher levels of freezing damage in field plots treated with N, which he attributed to nutrient imbalances with respect to N.
Shoots of spruce taken from three N manipulation experiments showed a positive but non-significant improvement in winter hardiness in response to N additions (Sheppard et al. submitted). When foliar N status is very significantly raised there does appear to be some loss of hardiness.
Suitability to indicate atmospheric concentrations:
Recent evidence from the GANE Whim experiment measuring the response of Calluna vulgaris (heather) suggests the plants respond to peak NH3 concentrations rather than average NH3 concentrations and deposition (L. Sheppard, pers. comm.).
Suitability to indicate atmospheric depositions:
While there may be a link between N deposition and increased frost sensitivity the evidence is often inconclusive implying a large N excess may be a prerequisite. Ammonium appears to have a stronger effect than nitrate when applied as wet deposition.
Suitability to indicate environmental impacts:
Reduction in frost hardiness represents a direct ecological response to high N gaseous concentrations and deposition.
Sensitivity to other factors:
Frost hardiness is also sensitive to water availability and sulphur 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