Forest protection: the Douglas fir

Introduced tree species such as the Douglas fir are very attractive to game and are therefore particularly at risk. Douglas firs are most heavily damaged by thrashing. The males of the red deer (Cervus elaphus), roe deer (Capreolus capreolus) and sika deer (Cervus nippon) remove the velvet from their newly formed antlers by thrashing them against Douglas firs in the thicket stage, thus marking their territory and tearing the bark of the tree in the process.

Browsing of Douglas firs mainly occurs in winter. The tree's popularity and the resulting loss of increment are somewhat lower than those of the silver fir. However, repeated browsing of the top shoots also results in permanent loss of quality. This is particularly significant in the case of planted trees, as these are individual investments.

Red and sika deer also damage Douglas firs in the pole wood stage by stripping the bark off trees. The hazard is less serious than that faced by the spruce and the hazard period is also shorter because Douglas fir bark brittles earlier. The loss of quality is low because the Douglas fir is able to heal such wounds relatively well. Damage is significant from the young stand to pole wood stage, especially in trees between 8 and 25 years old.

Fungi pose a major problem for Douglas firs when it comes to forest protection, especially needle cast fungi (such as Swiss needle cast and Rhabdocline needle cast).

Swiss needle cast (Nothophaeocryptopus gäumannii, syn. Phaeocryptopus gäumannii) mainly causes significant damage in dense stands where humidity is high, with the damage being considerably greater in years with wet early summers. Depending on the intensity of the infestation, needles are shed after one to three years and frost resistance is reduced. The combination of frost and fungal attack accelerates and exacerbates needle loss, resulting in substantial increment loss. However, even a severe infestation does not usually cause the tree to die. In contrast, secondary damage caused by the dark honey fungus (Armillaria ostoyae) or insects such as the six-toothed spruce bark beetle (Pityogenes chalcographus) or the fir bark beetle (Pityophthorus pityographus) will occur in the case of major needle cast infestations lasting several years. Trees can also be infested with the Cooley spruce gall adelgid (Gilletteella cooleyi) at the same time.

Rhabdocline needle cast (Rhabdocline pseudotsugae) often leads to the death of the tree. Compared with Swiss needle cast, Rhabdocline needle cast progresses much more quickly and intensively. However, our most common Douglas fir variety (Pseudotsuga menziesii var. menziesii) has proven to be highly resistant, whereas the varieties P. menziesii var. glauca and var. caesia are only suitable for cultivation in clearly continental climates due to their vulnerability to such infestations.

Dothistroma needle blight (Dothistroma sp.) can also affect Douglas firs. This fungal disease is mainly seen on pines and is considered a particularly dangerous organism. Very serious outbreaks of this disease can jump to Douglas firs from neighbouring pines.

Diseases affecting the shoots of the Douglas fir can be caused by Allantophomopsiella pseudotsugae (syn. Phacidium coniferarum, Phomopsis pseudotsugae), Botrytis cinerea or Sirococcus conigenus. A. pseudotsugae, which causes phomopsis disease in Douglas firs, is probably the most concerning. It attacks the bark of twigs and trunks, causing it to die. This interrupts the supply to the higher parts of the tree, causing the needles of the affected branches to turn red and the top to die.

The fungus occurs on the bark of various conifers, but only damages Douglas fir and occasionally the Japanese larch. Young Douglas firs weakened by frost, lack of water, plant shock, damage to bark, hail damage, severe needle cast or other stress factors are particularly at risk. The greatest outbreaks often occur in the first year after planting, when weakening due to lack of water and winter desiccation facilitates the spread of the fungus in the leaves.

While Douglas firs in North America are home to a large number of insect species, including several that could cause damage, the situation in Switzerland and Europe is still largely unproblematic. Aside from the American utilizable wood bark beetle (Gnathotrichus materiarius), which was first observed in Switzerland in 1984 and prefers to attack Scots pines, no other American bark beetles of the Douglas fir have been introduced to Europe so far. Various insect species of the Douglas fir appear in Switzerland mainly in combination with other influences such as needle fungi or winter frost damage.

The native insects that use the Douglas fir as a host tree include the large pine weevil, native green leaf weevil species, the pine processionary moth and the fir bark beetle.

The adults of the large pine weevil (Hylobius abietis) gnaw the bark of young trunks near the ground during maturation feeding. If the pockmarked feeding pattern covers the entire trunk, the young tree dies. Since the beetle larvae develop in the roots of fresh stumps, significant damage mainly occurs in large-scale plantings following clear cutting or storm damage.

Native green leaf weevil species (e.g. Phyllobius spp., Polydrusus spp.) sometimes feed on young Douglas fir needles. Feeding on the shoots causes damage similar to that of late frost due to the brown colouring of the youngest needle class and is sometimes conspicuous, but is usually minor. Noticeable increment loss mostly occurs in forest nurseries.

The pine processionary moth (Thaumetopoea pityocampa) can infest Douglas firs as well as pines. It is found on the southern side of the Alps as well as in Valais and the Lake Geneva area. From the pole wood stage onwards, the distinct, fist-sized hibernation nests of the caterpillars can then be observed in the tree crowns. However, feeding in the crowns is mostly insignificant and damage is usually negligible; to date, major feeding damage has only been observed on younger pines or in monocultures. The pine processionary moth could become a serious danger to the Douglas fir, however, should the caterpillars' preference for Douglas fir needles observed in the laboratory replicate itself in the field.

Bark beetles are also found on Douglas firs. The fir bark beetle (Pityophthorus pityographus) is by far the most common insect to nest in pre-weakened young Douglas firs, causing them to die. The star-shaped breeding gallery of this small bark beetle species is recognisable by the way the mating chambers clearly sink into the sapwood.

The fir bark beetle occasionally also appears with the six-toothed spruce bark beetle (Pityogenes chalcographus). Sometimes both species are seen on the same Douglas fir. Other native softwood bark beetles can also be found on Douglas fir from time to time, mainly in horizontal infestations. In Switzerland, the larger eight-toothed European spruce bark beetle (Ips typographus), the large larch bark beetle (Ips cembrae), the grained spruce bark beetle (Cryphalus abietis), the striped ambrosia beetle (Xyloterus lineatus) and the black timber bark beetle (Xylosandrus germanus), which has become naturalised from East Asia, have already been found on Douglas firs. In addition, several other native forest insects, mainly butterflies and beetles, have taken Douglas fir as their host tree in Europe. So far, however, there have been no reports of damage in Switzerland in this respect.

Some insect species migrated with their host tree in the 19th century and have become established here. The Douglas fir seed chalcid (Megastigmus spermotrophus) is one example. Unlike in America, however, the chalcid does not cause significant seed loss here. As in North America, the chalcid is only found in the Douglas fir. Other newcomers are the Cooley spruce gall adelgid (Gilletteella cooleyi), the western conifer seed bug (Leptoglossus occidentalis) and the Douglas fir needle midge (Contarinia pseudotsugae).