Betulaceae

Wind-pollinated trees and shrubs of the northern temperate and boreal zone, recognised by their hanging catkins, doubly toothed leaves and — in Betula — bark that peels in sheets. They are pioneer species: first onto burned ground, gravel and abandoned field, which is why they are so widespread and so heavily used.

Two quite different medical stories run through this family, and they point in opposite directions.

The first is betulin. Birch outer bark is unusual among plant materials in being roughly 10–35% betulin by dry weight — a pentacyclic triterpene present at a concentration that makes extraction genuinely economic rather than merely possible. A standardised birch-bark dry extract formulated as an oleogel has been through phase II and III trials in split-thickness graft donor sites, partial-thickness burns and epidermolysis bullosa, and is approved in Europe. That is a family-level fact worth knowing, with a species-level catch attached: the medicine is made from European B. pendula and B. pubescens, not from North American paper birch, which has never been trialled even though its bark carries the same molecule.

The second is allergy. Birch pollen is one of the principal spring aeroallergens of northern Europe and North America, and its major allergen, Bet v 1, cross-reacts with proteins in apple, cherry, peach, carrot, celery and hazelnut — the pollen-food (oral allergy) syndrome. For a large number of people the most consequential encounter with this family is a seasonal one they never chose.

Alongside both sits the older herbal use: birch leaf as a “spring cure” diuretic, carried by the European monographs, and birch sap tapped in late winter as a mineral drink across northern Europe and Siberia.

• Triterpenes — betulin and betulinic acid dominate the bark; lupeol and betulinic aldehyde accompany them. Betulinic acid has an extensive anticancer and antiviral literature of its own.
• Salicylates — methyl salicylate is concentrated in the bark of the sweet birches (B. lenta, B. alleghaniensis), which is where the wintergreen smell comes from and where the aspirin-interaction caution belongs.
• Flavonoids — hyperoside, quercitrin and myricitrin in the leaf; these carry the diuretic monographs rather than the triterpenes do.
• Diarylheptanoids — platyphylloside and the papyriferosides, a phenolic class the birches share with alder and, distantly, with turmeric’s curcuminoids.
• Papyriferic acid, a birch-specific triterpene ester and a succinate-dehydrogenase inhibitor — a defence compound against browsing mammals, and a reminder that “birch bark” is not uniformly benign.