Alder

• 1. Introduction

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  Most of the foundation beneath Venice rests on alder. The pilings driven into the lagoon mud, holding up that city for centuries, were cut from these trees of the genus Alnus, members of the birch family Betulaceae. Alder is an unremarkable sight along a streambank. It is deciduous, modest, the sort of tree a walker passes without a second glance. Yet it does work that few trees can match. It pulls nitrogen from the air and feeds it into barren ground. It refuses to rot when sunk underwater. It carries a painkilling compound in its bark. Some 35 species are spread across the north temperate zone, with a few reaching into Central America and the northern and southern Andes. Why does a tree that seldom lives a century shape forests, cities, and even the body of an electric guitar? The answers run from a glacier's edge to a coat of arms in an Austrian town.

• 2. Woody Catkins

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  The female catkins of an alder turn woody and stay shut. This single trait separates Alnus from the birches, Betula, in the same family. Birch catkins fall apart at maturity. Alder catkins hold together, then open to release their seeds much like the cones of many conifers. The flowers themselves come as catkins, with longer male catkins and shorter female ones on the same plant, since alders are monoecious. They appear often before the leaves do. Wind carries most of the pollen, though bees visit to a small extent. The leaves are alternate, simple, and serrated, with a few exceptions to the deciduous habit. Size varies enormously across the genus. Red alder, Alnus rubra, grows on the west coast of North America. Black alder, Alnus glutinosa, is native to most of Europe and has been introduced widely elsewhere. Both push past 30 metres. The green alder, Alnus alnobetula, stays a shrub rarely more than 5 metres tall. That gap between giant and shrub is mapped by how botanists have carved the genus into groups.

• 3. Three Subgenera

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  Botanists divide Alnus into three subgenera by the timing of their catkins. Subgenus Alnus holds trees with stalked shoot buds whose male and female catkins form in autumn, stay closed over winter, and pollinate in late winter or early spring. It is the largest group, with about 15 to 25 species, among them Alnus glutinosa, Alnus rubra, and Alnus cordata. Subgenus Clethropsis contains just three species, including Alnus maritima and Alnus nitida. Its trees and shrubs also carry stalked shoot buds, but their catkins form in autumn and expand and pollinate then rather than waiting through winter. Subgenus Alnobetula breaks the pattern. Its shrubs have shoot buds that are not stalked, and their catkins appear in late spring after the leaves, expanding and pollinating then. This is the group of Alnus alnobetula, a species also known by the synonym Alnus viridis. Beyond these tidy boxes sit names that resist them: species not assigned to any subgenus, others whose taxonomic status is unresolved, including Alnus djavanshirii and Alnus dolichocarpa from Iran, and a roster of described hybrids such as Alnus × spaethii. The deeper past of the genus is read not from living catkins but from grains of pollen turned to stone.

• 4. Stone Pollen

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  The oldest fossil pollen identifiable as Alnus comes from northern Bohemia. It dates to the late Paleocene, around 58 million years ago. That makes the genus old, present long before the forests it now pioneers took their modern shape. Other fossils trace its spread across deep time. †Alnus heterodonta is an Oligocene species from Fossil, Oregon. †Alnus parvifolia comes from the Ypresian of the Okanagan Highlands. Several Miocene species, including †Alnus fairi, †Alnus hollandiana, †Alnus largei, and †Alnus relatus, turn up across western North America. The name itself carries a different kind of record. The common word alder evolved from the Old English alor, which traces back to the Proto-Germanic root aliso. The Latin equivalent Alnus gave French aulne and the Spanish Alamo, the Spanish term for poplar. The tree's hold on language is matched only by its hold on the soil.

• 5. Feeding The Soil

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  Inside an alder's roots live nodules that can grow as large as a human fist, light brown and split into many small lobes. They house Frankia alni, an actinomycete, filamentous, nitrogen-fixing bacterium. The bacterium takes nitrogen from the air and hands it to the tree. The tree returns sugars made through photosynthesis. This exchange lets alder enrich the ground where it stands and act as a pioneer species, leaving nitrogen behind for whatever grows next. The numbers are large. Red alder stands have been found to supply between 130 and 320 kilograms per hectare of nitrogen to the soil each year. From Alaska to Oregon, Sitka alder, the shrubbier Alnus viridis subsp. sinuata, colonizes fresh gravelly sites at the foot of retreating glaciers. Studies show it adds nitrogen at an average rate of 60 kilograms per hectare per year, turning sterile glacial terrain into ground that can hold a conifer forest. Alders are among the first to seize disturbed land after floods, windstorms, fires, and landslides. Their seeds are featherlight, numbering 1.5 million to the kilogram, so the wind scatters them with ease. Alder groves often work as natural firebreaks, since these broad-leaved trees burn far less readily than conifers. Their foliage and leaf litter do not carry fire well, and their thin bark resists light surface burns. Red alder outgrows coastal Douglas-fir for the first 25 years. It is very shade intolerant and seldom lives more than 100 years, eventually giving way to the giant conifers it helped make possible.

• 6. Bark And Body

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  Alder bark carries salicin, an anti-inflammatory compound that the body turns into salicylic acid. Some Native American cultures use red alder bark, from Alnus rubra, to treat poison oak, insect bites, and skin irritations. The Blackfeet have traditionally brewed an infusion from red alder bark to treat lymphatic disorders and tuberculosis. Clinical studies have since found that red alder contains betulin and lupeol, compounds shown to work against a range of tumors. The inner bark also enters smoking mixtures known as kinnikinnick, used by some Indigenous peoples of the Americas, alongside red osier dogwood or chokecherry, to improve the taste of bearberry leaf. The wood reaches into kitchens and craft. Certain alder species are burned to smoke coffee, salmon, and other seafood. The catkins of some species are edible and can be rich in protein, though they are reported to taste bitter and unpleasant, making them more useful for survival than for the table. Alder bark and wood, like oak and sweet chestnut, hold tannin and have long been used to tan leather. The outer bark yields a red dye and the inner bark a yellow one. One more property sets alder apart from almost every other timber: what it does underwater.

• 7. Underwater And Onstage

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  Submerged alder does not give way. The wood is notably stable when immersed, and people have used it for millennia as material for the pilings of piers and wharves. That stability is why most of the timber holding up Venice was cut from alder. The same wood found a second life in music. Electric guitars, most famously those built by the Fender Musical Instruments Corporation, have carried alder bodies since the 1950s. The wood is prized for a tone called tight and evenly balanced, especially next to mahogany, and many guitar makers have taken it up. Because alder has no prominent grain, it is usually finished in opaque lacquer, whether nitrocellulose, polyurethane, or polyester. As a hardwood it also serves furniture, cabinets, and other woodworking. Its plain grain means it is often veneered, either with stained light woods like oak, ash, or figured maple, or with darker teak or walnut. The tree's reach extends even into heraldry and film. Alder appears on the coat of arms of the Austrian town of Grossarl. Ermanno Olmi's 1978 film The Tree of Wooden Clogs, in the original L'Albero degli Zoccoli, names alder in its title, the wood typically used to make the clogs at the heart of its plot.