Ireland being an island on the edge of the Atlantic Ocean experiences some of the windiest and wettest weather in Europe. Irish forests are regularly subjected to high winds and when these are accompanied by heavy rainfall they pose a real threat to the stability of forest stands. Windthrow, of all the potential damaging agents, is perhaps the greatest threat to forests and economic forestry in this country.
Windthrow affects plantations at nearly all stages, from the later establishment stage through to the pole stage and mature high forest.
Some species which have rapid early growth such as lodgepole pine, Douglas fir, larches, ash and sycamore produce large crowns in relation to their root systems. As a result, they are vulnerable to toppling in high winds before their roots systems are extensive enough to provide good stability. Poor planting practice where the roots are badly deformed can compound this effect.
Once toppled, trees attempt to grow upright and in doing so the main stem develops a curve in the lower section known as basal sweep. This causes difficulties in harvesting; and also, as the lower section is the most valuable part of the tree, the timber value of a crop can be significantly reduced.
In the later stages of a crop, windthrow can lead to increased harvesting costs due to difficulties in harvesting flattened and tangled trees. Also, shattering and breakage reduces the timber quality and therefore the value of the material harvested. Windthrow often requires shortening rotations resulting in stands being clearfelled well in advance of the economic rotation length.
There are two types of windthrow in older stands:
- Insipient windthrow – this is gradual windthrow that can occur at the edges of a stand or breaks in the canopy. It is often local to a stand and occurs over a period of years. On vulnerable sites the crop is often left to mature without thinning.
- Catastrophic windthrow – This type of windthrow usually occurs over large areas of forest and as the name implies has a major effect on the forest landscape. Periodically Ireland experiences severe storms, usually the tail end of hurricanes that sweep in from the Atlantic. These occur irregularly but can be expected at 10-15 year intervals. Extensive damage is caused by these storms such as the storm of 1974.
Large volumes of blown timber cause huge logistical difficulties for both the forest and the mill manager as harvested timber can degrade in time unless processed. This is particularly the case with pines and storage under sprinklers is often used to prevent the timber drying out or becoming infected by the blue stain fungus which discolours the wood.
While little can be done to avoid catastrophic windthrow there are a number of ways in which forester can reduce the risk of forest stands being blown:
- Cultivation and drainage – using an appropriate cultivation technique for the soil and site conditions and ensuring that drainage will lower the watertable sufficiently to provide adequate rooting depth.
- Planting stock and planting – ensuring that planting stock has good fibrous root systems and that these are not badly deformed during the planting operation.
- Thinning – careful thinning on wet and exposed sites or, employing a no-thin regime on sites vulnerable to windthrow.
- Clearfelling stands to windfirm edges where the trees have become more adapted to exposed conditions.
- Roading – avoiding excessive openings for roads, entrances and turning points.