Pests and diseases

Longevity in trees varies greatly by individual as well as species, driven by numerous variables – especially the stability of heat, light and moisture levels. When these misalign, there’s always a combo of insects, fungi, bacteria and viruses ready to strike.

Trees maintain energy reserves to fight these threats but suffer when the growth-defence balance is thrown out. For example, a fallen tree enables its neighbours to harvest lots of new sunlight, which they use to grow new branches as quickly as possible. However…

When a tree puts all its energy into new growth, it’s less able to fight pests and diseases. Some trees will get away with this but when an attacker realises a tree has no defensive response, it means trouble.

Incidentally, a tree is likelier to survive another tree falling on it in winter because the bark is drier and tighter fitting. In summer it’s full of water and more easily removed, making it vulnerable to invaders.

Chemicals are a common defence mechanism. For example, phytoncides released by trees, especially conifers like Scots pine, repel invaders by disinfecting the air. They also cause the distinctive ‘forest’ scent that’s more acute in hot, sunny conditions. Walnut is another example, with leaves containing insect-repellent compounds that make its shade a great spot for mosquito avoidance.

You can’t talk about pests and diseases without mentioning biosecurity. Fungi, insects and arachnids can (and do) cross oceans in imported timber or parcels. Less manageably, migrating birds transport small animals, seeds and fungal spores across huge distances and multiple borders.

When fungi reach dry heartwood through a wound, they hollow out the tree, only penetrating the sapwood if the air dries it enough before the tree seals the wound (heart rot).

Ash dieback (Hymenoscyphus fraxineus)  is a great (if unfortunate) example of a fungus that has spread virulently and established itself far from its origin. Proliferating throughout Europe (including Jersey), it has caused a vast proportion of ash trees to die or be destroyed, changing the landscape forever.

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Main Image: Camera man (Pixabay)