Illustration by Adelaide Tyrol
Some stands of Scots pine in Europe (that’s what it’s called in its native range) are gorgeous, with straight, unbranching trunks that self-prune nicely, revealing the lovely orange bark that is characteristic of mid-sized stems. But the Scotch pine plantations growing in the U.S. rarely live up to that ideal. Some planted stands are successful, but the species is susceptible to a great variety of insects and diseases. When it was first attempted here, nursery stock was grown very densely, so when the spindly seedlings were transplanted, they were easily knocked over by snow. As they matured, many of these stems became s-shaped and only slowly resumed vertical growth. Nowadays, nurseries grow sturdier seedlings.
Scotch pine has the largest geographic range of any pine, from Great Britain, Ireland, and Portugal east to eastern Siberia. It grows above the Arctic Circle in Scandinavia and south to the Mediterranean. There is significant genetic variation across that vast expanse, and Scotch pine seeds from some areas seem to be particularly mismatched when grown in North America. All of the plantations I have seen have been hopeless for timber, though if you can put that goal aside, the irregular, flat-topped shape of the tree, the blue-green needles, and the orange bark are quite picturesque.
Many introduced plant species are oblivious to our native insect and disease pests – in this way, the exotic species have a competitive advantage over native plants. Scotch pine proves that this is not always the case. The white pine weevil, for one, has found Scotch pine to its liking and causes the same distorted growth on a Scotch pine as it does on a white pine. Although Scotch pine has modest nutritional requirements, lop-sided growth seems to be more pronounced in trees grown on poor soils. Because the tree is often grown on poor soils in land-reclamation and windbreak plantings, its reputation for being hopelessly crooked is reinforced.
Christmas tree growers appreciate Scotch pine because it survives transplanting easily and seems well adapted to a plantation existence. Like most other Christmas tree species, Scotch pines need to be sheared. Insect and disease control is a normal and accepted part of the Christmas tree business, and this keeps crookedness from developing. Scotch pines hold their needles very well and still look good three or four weeks after being cut. Even dried-out trees keep most of their needles.
The 1- to 2-inch-long needles are in pairs and have a characteristic twist. They stay on the tree for two to four years. The brown cones have the pine family habit of taking two years to mature.
Once in a while, Scotch pines spread from their plantations out to adjacent fields and forest. Like most pines, the seedlings will die out in a dark forest, and on fertile sites, they are likely to be overgrown by many other trees. In some places, however, it is considered to be an invasive species.
We don’t normally think of plants as being able to call for help, but a study of Scotch pine’s reaction when the eggs of a sawfly (Diprion pini) were laid in its tissues suggests that the tree is quite capable of using the plant version of 9-1-1 to defend itself against at least one insect pest.
Soon after sawfly eggs have been deposited in Scotch pine needles – and well before they hatch – the tree reacts by releasing volatile compounds that attract a tiny parasitoid wasp, Chrysonotomyia ruforum, that specializes in sawflies. The wasp flies in and lays its own eggs, one in each sawfly egg. A gruesome drama unfolds as the eggs of both insects hatch and the wasp larva proceeds to slowly consume its sawfly host. The combatants both consume nutrients from their mutual host, the Scotch pine, but the damage is minimal, and the sawflies that are parasitized do not survive to make more sawflies.
Many plants have been found to release chemical calls for help in response to caterpillar feeding, but this preventive defensive strategy – before a bite has been taken – is less well known. It has also been discovered in elm trees; when eggs of the elm leaf beetle are deposited on its leaves, an elm leaf beetle parasitoid is summoned. Knowing that a plant can call in an insect specialist to kill an insect that’s bugging it makes you wonder what else trees might be capable of doing.