In Irish forests the most damaging frosts are those that occur at the end of spring and beginning of autumn. Young trees are susceptible to frost damage from the early seedling stage until they reach a height of approximately 1.5 – 2 metres. Trees come into leaf in spring when they have experienced a certain accumulated temperature. This mechanism ensures that warm days do not cause the trees to come into growth too early in the season. Similarly, shortening days in autumn is the mechanism that causes the trees to stop growing and become dormant, as a warm period in autumn could cause the trees to grow late into the season making them vulnerable to early autumn frosts. Native species are generally well adapted to the climatic conditions in this country and, with their growth and dormancy cycles synchronised with the seasons, they tend to avoid unfavourable weather conditions. Exotic or introduced species, however, are adapted to the climatic conditions that occur in their native habitat, moving them to Ireland may upset their growth pattern leaving them vulnerable to damage by frosts and winter cold. In introducing new non-native species into Irish forestry it was essential that the early foresters test the species adaptability before planting them on a wide scale.
There are considerable differences in phenology (bud break and dormancy) both between and within species and foresters can use this to their advantage. For example, Norway spruce is a late flushing species which can be planted on low lying midland sites where late spring frosts frequently occur. Its late growth avoids the danger of frosts while Sitka spruce planted on the same sites can be badly affected and a year’s growth lost, or in severe cases killed outright.
Late spring frosts
Late spring frost are a feature of the oceanic climate that Ireland experiences. Frosts can occur as late as early June which cause severe damage to young plantations. At that time the newly flushed shoots are at a vulnerable stage and are readily killed by the freezing temperatures. This can result in trees loosing a year’s growth and repeated frost damage can kill trees or hold them in check for many years. Some trees, however, are able to eventually grow to a height above the frost line and then grow normally.
Late spring frosts are the most damaging frosts to trees in Ireland affecting both nursery stock and young plantations. Their occurrence is very unpredictable and little can be done in the field other than planting frost resistant species such as Norway spruce and pines (Scots, lodgepole, Corsican pines) in low lying areas that are likely to experience late frosts. Many of the main broadleaves such as oak, ash, beech are particularly susceptible to late frosts while some of the minor species such as birch, alder, rowan, sycamore, and hornbeam are frost tolerant.
Early autumn frosts
Early autumn frosts are generally not a problem in young plantations in Ireland. These can occur in early September, but by that stage many species will have completed their year’s growth and are entering dormancy. Southern seed origins of Sitka spruce and Douglas fir, however, can remain growing late into the season can be affected, but these are rarely planted and only in the milder parts of the country.
Early autumn frosts are more a problem for the nurseryman than the forester. Mild weather conditions at the end of the summer can cause young seedlings and transplants to continue growing or produce Lammas growth which is a second flush of growth around late August. This delayed dormancy leaves the plants very vulnerable to early autumn frost and winter cold. However, as plants become older they develop a more predetermined growth pattern which is less affected by prevailing weather conditions at the end of the season. Hence the reason why plantations are less affected than nurseries.
Like late spring frosts, the occurrence of early autumn frosts is unpredictable. Ireland has usually mild weather conditions in the autumn and early autumn frosts are rare. The most severe damage to trees is from late spring frosts.