Much of Venice stands on pilings cut from alder trees
Alder wood holds up remarkably well underwater, which is why builders have used it for pier and wharf pilings for thousands of years, and why most of the piles beneath Venice came from alders. Alive, the tree is just as useful: bacteria in its roots pull nitrogen from the air and turn barren ground into fertile soil.
Alders make up the genus Alnus in the birch family, about 35 species of trees and shrubs spread across the northern temperate zone, with a few reaching Central America and the Andes. The biggest, red alder in western North America and black alder in Europe, exceed 30 metres, while green alder rarely tops 5 metres. Unlike birches, alders produce woody female catkins that stay intact and open to release seed much like conifer cones. The English name goes back to Old English alor.
Their superpower is a partnership with Frankia alni, a filamentous bacterium living in root nodules that can grow as large as a human fist. The bacterium captures atmospheric nitrogen for the tree; the tree pays in sugars from photosynthesis. Red alder stands add roughly 130 to 320 kilograms of nitrogen per hectare each year. From Alaska to Oregon, Sitka alder colonises gravel left by retreating glaciers, adding around 60 kilograms per hectare annually and turning sterile ground into soil that can later carry conifer forest.
Alders are classic pioneers after floods, fires, storms and landslides. Their seeds are feather-light, about 1.5 million to the kilogram, so wind scatters them widely. Groves resist fire better than conifers because the leaves and litter burn poorly. Red alder outpaces Douglas fir for its first 25 years but cannot tolerate shade and seldom lives beyond a century, reaching 30 metres or so in about sixty years before heart rot sets in. In effect, it prepares the ground for the giant conifers that replace it.
People and wildlife use alders in other ways too. Many butterflies and moths feed on the leaves. The catkins are edible and may be rich in protein, though bitter enough to count as survival food. Alder wood is a favourite for smoking salmon, other seafood and even coffee. In New Zealand, by contrast, two species are classed as environmental weeds.
Source: Alder
