Alders are trees comprising the genus Alnus in the birch family Betulaceae. The genus comprises about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few species extending into Central America, as well as the northern and southern Andes.
Alnus glutinosa, the common alder, black alder, European alder, European black alder, or just alder, is a species of tree in the family Betulaceae, native to most of Europe, southwest Asia and northern Africa. It thrives in wet locations where its association with the bacterium Frankia alni enables it to grow in poor quality soils. It is a medium-sized, short-lived tree growing to a height of up to 30 metres (98 feet). It has short-stalked rounded leaves and separate male and female flowers in the form of catkins. The small, rounded fruits are cone-like and the seeds are dispersed by wind and water.
Alnus rubra, the red alder, is a deciduous broadleaf tree native to western North America.
Betula nigra, the black birch, river birch or water birch, is a species of birch native to the Eastern United States from New Hampshire west to southern Minnesota, and south to northern Florida and west to Texas. It is one of the few heat-tolerant birches in a family of mostly cold-weather trees which do not thrive in USDA Zone 6 and up. B. nigra commonly occurs in floodplains and swamps.
Alnus incana, the grey alder or speckled alder, is a species of multi-stemmed, shrubby tree in the birch family, with a wide range across the cooler parts of the Northern Hemisphere. Tolerant of wetter soils, it can slowly spread with runners and is a common sight in swamps and wetlands. It is easily distinguished by its small cones, speckled bark and broad leaves.
Alnus jorullensis, commonly known as Mexican alder, is an evergreen or semi-evergreen alder, native to eastern and southern Mexico, Guatemala and Honduras. Although previously reported from the Andes, further collections showed these to be the similar species Alnus acuminata, commonly found in South America.
Alnus cordata, the Italian alder, is a tree or shrub species belonging to the family Betulaceae, and native to the southern Apennine Mountains and the north-eastern mountains of Corsica. It has been introduced in Sicily, Sardinia, and more recently in Central-Northern Italy, other European countries and extra-European countries, where it has become naturalised.
Corylus cornuta, the beaked hazelnut, is a deciduous shrubby hazel with two subspecies found throughout most of North America.
Alnus nepalensis is a large alder tree found in the subtropical highlands of the Himalayas. The tree is called Utis in Nepali and Nepalese alder in English. It is used in land reclamation, as firewood and for making charcoal.
Carpinus caroliniana, the American hornbeam, is a small hardwood tree in the genus Carpinus. American hornbeam is also known as blue-beech, ironwood, musclewood and muscle beech. It is native to eastern North America, from Minnesota and southern Ontario east to Maine, and south to eastern Texas and northern Florida. It also grows in Canada. It occurs naturally in shaded areas with moist soil, particularly near the banks of streams or rivers, and is often a natural constituent understory species of the riverine and maritime forests of eastern temperate North America.
Betula glandulosa, the American dwarf birch, also known as resin birch or shrub birch, is a species of birch native to North America.
Ostrya virginiana, the American hophornbeam, is a species of Ostrya native to eastern North America, from Nova Scotia west to southern Manitoba and eastern Wyoming, southeast to northern Florida and southwest to eastern Texas. Populations from Mexico and Central America are also regarded as the same species, although some authors prefer to separate them as a distinct species, Ostrya guatemalensis. Other names include eastern hophornbeam, hardhack, ironwood, and leverwood.
Alnus rhombifolia, the white alder, is an alder tree native to western North America, from British Columbia and Washington east to western Montana, southeast to the Sierra Nevada, and south through the Peninsular Ranges and Colorado Desert oases in Southern California. It occurs in riparian zone habitats at an altitudes range of 100–2,400 metres (330–7,870 ft). While not reported in northern Baja California, it has been predicted on the basis of its climatic adaptation to occur there also. Alnus rhombifolia is primarily found in the chaparral and woodlands, montane, and temperate forests ecoregions.
Alnus acuminata is a species of deciduous tree in the Betulaceae family. It is found in montane forests from central Mexico to Argentina.
Frangula alnus, commonly known as alder buckthorn, glossy buckthorn, or breaking buckthorn, is a tall deciduous shrub in the family Rhamnaceae. Unlike other "buckthorns", alder buckthorn does not have thorns. It is native to Europe, northernmost Africa, and western Asia, from Ireland and Great Britain north to the 68th parallel in Scandinavia, east to central Siberia and Xinjiang in western China, and south to northern Morocco, Turkey, and the Alborz in Iran and the Caucasus Mountains; in the northwest of its range, it is rare and scattered. It is also introduced and naturalised in eastern North America.
Corylus americana, the American hazelnut or American hazel, is a species of deciduous shrub in the genus Corylus, native to the eastern and central United States and extreme southern parts of eastern and central Canada.
Frankia alni is a Gram-positive species of actinomycete filamentous bacterium that lives in symbiosis with actinorhizal plants in the genus Alnus. It is a nitrogen-fixing bacterium and forms nodules on the roots of alder trees.
Alnus serrulata, the hazel alder or smooth alder, is a thicket-forming shrub in the family Betulaceae. It is native to eastern North America and can be found from western Nova Scotia and southern New Brunswick south to Florida and Texas.
Salix arbusculoides is a species of flowering plant in the willow family known by the common name little tree willow. It is native to northern North America, where its distribution extends across Alaska and most of Canada.
Toensbergia blastidiata is a species of crustose lichen in the family Sporastatiaceae. Found in northwestern North America, it was described as a new species in 2020 by lichenologists Toby Spribille and Tor Tønsberg. The type specimen was collected in Glacier Bay National Park at the base of Marble Mountain (Alaska). Here the lichen was found growing on the bark of Alnus viridis subsp. crispa. The specific epithet blastidiata refers to the "blastidiate thallus surface"; blastidia are vegetative propagules containing both mycobiont and photobiont, which are produced by yeast-like "budding".
