Alder

Alders are trees and shrubs that belong to the genus Alnus in the birch family Betulaceae. There are about 35 different types of alders, and they grow mostly in cool areas of the world. Some of these trees can grow very tall.

These trees are found in many places, from the north temperate zone to parts of Central America and the Andes mountains. Alders are interesting because they can help clean water and support many kinds of animals and plants. They are a very important part of nature in many regions.

Description

Alder trees are mostly deciduous, meaning they lose their leaves each year. Their leaves grow alternately on the stem, are simple in shape, and have jagged edges. These trees produce flowers in the form of catkins—long, thin clusters. Male catkins are longer and appear on the same plant as shorter female catkins, often before the leaves grow. While mostly pollinated by wind, bees sometimes visit them too.

Unlike birches, alder trees have female catkins that become woody and stay intact when mature, opening to release seeds like some conifer cones do. The largest alder species include the red alder, found on North America's west coast, and the black alder, native to Europe and introduced elsewhere, both growing over 30 metres tall. In contrast, the green alder is usually just a small shrub, rarely taller than 5 metres.

Phylogeny

Classification

A young bull moose browsing on alder trees in Homer, Alaska, in 2010 grew to be about 3.7–4.6 metres (12–15 ft) tall by 2021.

The genus of alder trees is divided into three subgenera.

Subgenus Alnus

This subgenus includes trees with special buds and flowers that appear in the autumn but stay closed until late winter or early spring to pollinate. There are about 15–25 species in this group, such as:

• Alnus acuminata Kunth
• Alnus cordata (Loisel.) Duby
• Alnus cremastogyne Burkill
• Alnus firma Siebold & Zucc.
• Alnus glutinosa (L.) Gaertn.
• Alnus hirsuta (Spach) Rupr.
• Alnus incana (L.) Moench
• Alnus japonica (Thunb.) Steud.
• Alnus jorullensis Kunth
• Alnus lusitanica Vít, Douda, & Mandák
• Alnus matsumurae Callier
• Alnus nepalensis D.Don
• Alnus oblongifolia Torr.
• Alnus orientalis Decne.
• Alnus rhombifolia Nutt.
• Alnus rohlenae Vít, Douda, & Mandák
• Alnus rubra Bong.
• Alnus serrulata (Aiton) Willd.
• Alnus subcordata C.A.Mey.
• Alnus tenuifolia Nutt.
• Alnus trabeculosa Hand.-Mazz.

Subgenus Clethropsis

This subgenus has trees or shrubs with special buds and flowers that appear and pollinate in the autumn. It includes three species, like:

• Alnus formosana (Burkill) Makino
• Alnus maritima (Marshall) Muhl. ex Nutt.
• Alnus nitida (Spach) Endl.

Subgenus Alnobetula

This subgenus consists of shrubs with buds that are not on stalks. Their flowers appear in late spring after the leaves grow and pollinate at that time. It includes one to four species, such as:

• Alnus alnobetula (Ehrh.) K.Koch (synonym-Alnus viridis)
• Alnus firma Siebold & Zucc.
• Alnus mandshurica (Callier) Hand.-Mazz.
• Alnus maximowiczii Callier
• Alnus pendula Matsum.
• Alnus sieboldiana Matsum.

Not assigned to a subgenus

Some species of alder have not been placed into a specific subgenus yet, including:

• Alnus fauriei H.Lév. & Vaniot
• Alnus ferdinandi-coburgii C.K.Schneid.
• Alnus glutipes (Jarm. ex Czerpek) Vorosch.
• Alnus hakkodensis Hayashi
• Alnus henryi C.K.Schneid.
• Alnus lanata Duthie ex Bean
• Alnus mairei H.Lév.
• Alnus paniculata Nakai
• Alnus serrulatoides Callier
• Alnus vermicularis Nakai

Species names with uncertain taxonomic status

The exact classification of some species is still unclear. Examples include:

• Alnus balatonialis Borbás
• Alnus cuneata Geyer ex Walp.
• Alnus dimitrovii Jordanov & Kitanov
• Alnus djavanshirii H.Zare – Iran
• Alnus dolichocarpa H.Zare, Amini & Assadi – Iran
• Alnus figerti Callier
• Alnus frangula L. ex Huth
• Alnus gigantea Nakai
• Alnus glandulosa Sarg.
• Alnus henedae Sugim.
• Alnus hybrida Rchb.
• Alnus laciniata Ehrh.
• Alnus lobata Nyman
• Alnus microphylla Arv.-Touv.
• Alnus obtusifolia Mert. ex Regel
• Alnus oxyacantha Lavalle
• Alnus subrotunda Desf.
• Alnus vilmoriana Lebas
• Alnus washingtonia Wetzel

Hybrids

Scientists have identified several hybrid plants made from crossing different alder species, such as:

• Alnus × elliptica Req. (A. cordata × A. glutinosa)
• Alnus × fallacina Callier (A. incana subsp. rugosa × A. serrulata)
• Alnus × hanedae Suyinata (A. firma × A. sieboldiana)
• Alnus × hosoii Mizush. (A. maximowiczii × A. pendula)
• Alnus × mayrii Callier (A. hirsuta × A. japonica)
• Alnus × peculiaris Hiyama (A. firma × A. pendula)
• Alnus × pubescens Tausch. (A. glutinosa × A. incana)
• Alnus × suginoi Sugim.

The exact identity of some hybrids is still being studied, including:

• Alnus × aschersoniana Callier
• Alnus × koehnei Callier
• Alnus × ljungeri Murai
• Alnus × purpusii Callier
• Alnus × silesiaca Fiek
• Alnus × spaethii Callier (A. japonica × A. subcordata)

Fossil record

The oldest known pollen from alder trees comes from northern Bohemia, dating back to around 58 million years ago during the late Paleocene period.

Etymology

The word "alder" comes from the Old English word alor, which traces back to a Proto-Germanic root aliso. The scientific name Alnus is the Latin version of this name. It also influenced words in other languages, like French aulne and Spanish Alamo, which means "poplar".

Ecology

Alders grow close to streams, rivers, and wet areas. In some places, areas filled with alders are called alder carrs. In the Pacific Northwest of North America, the white alder grows in warm, dry places near water, like along rivers.

Alder leaves and flowers are eaten by many butterflies and moths. Alder roots and leaves help the soil by adding important nutrients like nitrogen, which helps plants grow.

Alders have a special friendship with a tiny organism called Frankia alni. This organism lives in nodules on alder roots. It takes nitrogen from the air and gives it to the tree, and the tree gives it sugars made through photosynthesis. This helps make the soil richer and better for other plants to grow. Alders are often among the first plants to grow in places damaged by floods, fires, or landslides. Their thin bark helps protect them from small fires, and their lightweight seeds can be carried far by the wind.

Uses

Some types of alder trees have catkins that can be eaten in emergencies. Though they taste bitter, they have protein that can help keep someone alive. The wood from certain alder trees is great for smoking food like coffee, salmon, and other seafood.

Alder wood stays strong even when it gets wet, which is why it has been used for thousands of years to build supports for docks and buildings. Many of the supports holding up the city of Venice are made from alder.

The bark of alder trees has natural substances that can help reduce swelling and irritation. Some Native American groups used the bark from red alder trees, called Alnus rubra, to treat itchy rashes and bites. Scientists have found that this bark includes special compounds that may help fight certain sicknesses.

Alder wood is also used to make electric guitars, especially by a company called Fender Musical Instruments Corporation. Musicians like this wood because it gives the guitars a clear and balanced sound. It is also used to make furniture and cabinets, where it is often covered with thin layers of other woods to give it different looks.

The bark and wood of alder, like oak and sweet chestnut, have natural substances used to treat leather. People can also get red and yellow colors from the bark to use as dye.

Culture

The movie The Tree of Wooden Clogs by Ermanno Olmi, made in 1978, is called L' Albero Degli Zoccoli in its original language. The title of the movie refers to alder trees, which were often used to make wooden clogs, just like in the story of the movie.