Alders belong to the birch family Betulaceae and belong to the genus Alnus. Around 35 monoecious species of trees and shrubs are found in this genus. The leaves of alders are alternate, simple, serrated, and deciduous with a few exceptions. Usually, before leaves appear, the flower buds develop into catkins with elongated males and females on one plant. The flowers are mostly pollinated by the wind, but some bees also visit.
History and origin
Alders are vigorous deciduous trees and shrubs that are native to northern regions from the Mediterranean to northern Europe. They are also found in most of North America. Riverside trees, best known for their moisture-loving characteristics, have been used for timber for a long time, mostly to resist water damage. Nitrogen-fixing trees are usually used in productive tree-based systems.
The actinorhizal plants, like alders, fix nitrogen through their root systems but are not legumes (like Acacias). Sea buckthorn (Hippophae) and Elaeagnus also belong to this group. The nitrogen-fixation process takes place much the same way as with legumes, but with different bacteria (Frankia species), which are more tolerant of wet and cool conditions.
It is not uncommon for alders to grow at a rate of more than 1m (3ft) per year due to their nitrogen-fixing abilities. Their rapid colonization of damp areas in the wild and ability to tolerate substantial competition as young trees make them pioneer trees. Their lifespans are typically between 60 and 100 years, similar to other pioneers.
Except for the Sitka alder, all of the species listed here are single-trunked tall trees, sometimes reaching 100 feet in height. There are only a few Sitka alder trees that reach heights of more than 30 feet. These trees are much smaller and shrubby in shape. It is known for its sucking nature, but the other alders don’t. As alders age, they become rounded and less narrow (about 6m/20ft in diameter), compared to Italian and red alder.
In general, alder leaves are oval in shape (heart-shaped on A. cordata) and are high in nutrients when they fall, which is one-way nitrogen is returned to the system for use by other plants (the other is through root turnover every year).
When leaves drop late in autumn, they typically contain 3% nitrogen plus a significant amount of phosphorus. Early to mid-spring is the time when catkins are born. As autumn and winter approach, small cones with many flattish seeds are released.
In spite of the fact that timber has been used for many different purposes, it is not durable outside of water. Traditionally, river and pier piles have been used when immersion in water maximizes benefits. Windbreaks made from alders are excellent. When using them, shading needs to be considered since most will grow very large in time.
In England, European alder trees are often used as windbreaks around apple orchards with a height of 20ft or more. Windbreak lines should be made with Italian and red alder, as they remain narrow and upright and do not sucker out. It is their deliberate use as nitrogen-fixing plants that makes alders particularly useful in this article.
The introduction of nitrogen into the system is largely achieved by using them in hedges or by scattering them throughout gardens and orchards. In the north end of the land, where the trees shade the least, large hedges of unpruned trees may be most appropriate. Trees with narrow and upright growth can be scattered throughout a system best by Italian or red alder. When trees grow, prune their side branches to allow more light to penetrate beneath their canopy. One tree can supply enough nitrogen to feed several fruit trees if pruned like this every two to three years.
Pollarding and coppicing alder trees regularly (or even shredding them if you’re keen) is another way to utilize alders as fertility trees. The alder is often used as a nurse tree in forestry (planted among the main crop and removed before it begins to shade it). It is traditional to use most species of plants for medicinal purposes and for dyeing.
The edible sap of several alders, such as Italian and red alder, can also be collected in late winter (as with maple syrup from maple trees). Riverbank stabilization is a helpful property of all species. Traditional uses of European alder bark include tanning leather and making fishing nets due to its high tannin content (16-20%). In the woodworking industry, alder is used to make furniture, cabinets, and other wood products. For centuries, leather has been tanned with alder bark and wood (like oak and sweet chestnut).
The European and gray alder has a few ornamental cultivars, but these are not particularly useful.
The cultivation process
Almost all alders like a sunny location and well-drained soil. Italian alder, on the other hand, tolerates drier soils in summer, so if your soil dries out a lot in summer, this alder might be a better choice. Wind exposure is also tolerated by all species, even by species that live near the coast. Alders are usually coppiced and pollarded well (Italian alders are less so as they age).
The most common method of propagation is by seed. It is either dormant (Italian alder) or lightly dormant – in this case, 4-6 weeks of pre-chilling at fridge temperature combined with moist sand improves germination. Eight to sixteen inches of growth can be expected in the first year.
Diseases and pests
A fungus called alder phytophthora attacks roots in the lower part of the trunk, causing black ‘ink’ to ooze out. Trees along waterways are more likely to be infected than trees in gardens and orchards because of this fungus’s propensity to spread via water. Alder foliage is not very attractive to grazers like rabbits and deer.
Species that are similar
As small as Sitka alder, green alder is a shrubby species native to mainland Europe. It is obtained from forest tree nurseries (many states have their own nurseries).