Frosts can cause damage to the profitability of Sitka spruce (Picea sitchensis [Bong.] Carr.) tree plantations, as frost damage can cause dieback of the leader and loss of apical dominance resulting in a reduction of both wood quality and productivity. Damage to leaders can amount to a loss of productivity of up to seven years. Reducing frost damage could thus be an avenue for breeders to improve the resilience of Sitka spruce plantations.
Frost damage can occur in different forms, as plant tissue is affected by ice crystals and other, harmful effects caused by freezing temperatures. This damage can be counteracted by physiological and metabolic conditioning called frost tolerance. The mechanisms of frost tolerance in woody perennials are complex and controlled by many different genes influenced by abiotic and biotic factors. These adaptations are costly, and the plants’ frost tolerance thus fluctuates throughout the year as temperatures decrease and increase throughout the season.
To understand the risk of frost damage to the forest industry, an investigation into the impact of climate change on the occurrence of frost damage in Sitka spruce was conducted. Modelling scenarios of frost tolerance in plantations throughout GB using temperatures modelled for different climate change scenarios indicated that Sitka spruce is not predicted to suffer an increased risk of early bud burst related frost damage. Conversely, climate change is predicted to improve the growing conditions and reduce the risk of frost damage in Sitka spruce throughout GB.
A systematic map of frost tolerance measuring techniques showed that measurement of frost tolerance involves controlling many factors, such as rate of freezing, thawing, and time of exposure, that affect the estimation of frost tolerance. Varying these factors can change the apparent frost tolerance, with measurements from the same tissue obtaining different estimates of frost tolerance. In order to improve the detection of frost tolerance, it is important to use the appropriate temperatures, with a wide enough range of temperatures to provide an accurate estimate of frost tolerance.
Field samples in the form of branch cuttings of Sitka spruce were collected from six forests in Scotland and one in Wales, from both frost damaged and undamaged trees. These cuttings were then rooted and grown in a controlled environment room, in addition to commercial Sitka spruce varieties. Once rooted, fresh needles grown from these trees were collected and their frost tolerance was tested by freezing treatments at +4 °C (control), -3 °C, -6 °C, -10 °C and -20 °C, and electrolyte leakage was determined by assessing changes in electroconductivity in pure water. No correlation between field observations of frost damage and electrolyte leakage was found.
Genotyping of DNA extracted from phenotyped samples was conducted by an external company, although the data has not yet been received due to the COVID-19 pandemic. A desk-based analysis was conducted to mitigate against the missing dataset. Four conifer studies revealing 165 genes associated with frost tolerance were identified. Of these, 84 possible candidate genes represented in Sitka spruce could be analysed to determine their function in relation to frost tolerance. Stress-related genes were found to be the most common category of genes identified by biological process ontology. Most genes were expressed in the cytoplasm and nucleus, and catalytic activity and protein binding were the main categories of molecular functions of the frost tolerance related genes. It was not possible to determine whether there were underlying genetic differences between the trees that conferred frost resilience among the 93 phenotyped samples. There was, however, evidence to suggest that Sitka spruce frost tolerance is in part genetic, as several genes related with frost tolerance in other conifers have been found to be present in Sitka spruce.