Alder trees, members of the genus Alnus within the birch family Betulaceae, are deciduous flowering trees and shrubs with a broad natural distribution. The genus encompasses approximately 35 species, ranging from tall trees to smaller shrubs. Alders are notable for their unique reproductive structures, which consist of male and female catkins found on the same plant, and woody, cone-like female fruits. This widespread genus is recognized for its adaptability and ability to thrive in environments that challenge many other woody plant species.
Global and Continental Range
The distribution of the Alnus genus is largely confined to the cold and temperate regions of the Northern Hemisphere, spanning a vast latitudinal band. They are native across North America, Europe, Asia, and extend into parts of North Africa and the mountainous regions of Central America. The genus is considered the most widely distributed group of actinorhizal plants, which are non-leguminous species that fix nitrogen.
The range extends into northern temperate forests, with some species found in subarctic zones. While the majority of species are concentrated north of the equator, the genus crosses into the Southern Hemisphere along the mountainous spine of the Andes. Species like the Andean alder (Alnus acuminata) reach as far south as northern Argentina, highlighting the alders’ preference for cooler climates and high-elevation environments.
Specific Habitat Requirements
Alder trees are well-known for their strong association with high moisture levels, often localizing their growth in areas near water. They frequently inhabit riparian zones, lake shores, wetlands, and floodplains. This water-demanding nature is partly attributed to the leaves of many species lacking sufficient mechanisms for controlling water loss.
While alders thrive in moist, even saturated, environments, they prefer well-draining soil that avoids becoming completely waterlogged for prolonged periods. They exhibit a high tolerance for various soil types, including loamy, sandy, and clay compositions. Optimal growth occurs in slightly acidic to neutral soil conditions, typically within a pH range of 5.5 to 7.2.
Alders require ample sunlight for robust growth, generally performing best when exposed to full sun. Although they can tolerate partial shade, they are often considered shade-intolerant. This preference for open conditions is reflected in their common presence on disturbed sites, such as recently cleared areas or gravelly riverbanks. Alders can be found from sea level up to significant altitudes, with some species thriving at elevations reaching 3,800 meters in tropical mountain regions.
Dominant Species and Regional Distribution
The broad continental range of alders is composed of several distinct species, each with its own specific geographic distribution and habitat preferences.
Red Alder (Alnus rubra)
Red alder is the largest species in North America, found almost exclusively along the Pacific Coast. Its range extends from Southeast Alaska down to central coastal California, typically remaining within 200 kilometers of the Pacific shoreline.
European Alder (Alnus glutinosa)
Also known as black alder, this species is widespread across Europe, reaching up to 66°N in Scandinavia and extending south into North Africa. It is common in gallery forests along rivers and streams and has been widely naturalized in other regions, including parts of the northeastern United States. European alder is generally found at elevations below 1,000 meters, though it can occasionally be found along watercourses up to 1,800 meters in Central European mountains.
Green Alder (Alnus viridis)
Green alder exhibits a circumpolar distribution in subalpine and subarctic zones across North America and Eurasia. This species is typically a shrub, rarely growing taller than 5 meters, and is often found in high-altitude environments and along the margins of retreating glaciers.
Speckled Alder (Alnus incana)
Speckled alder has a vast distribution spanning both North America and Eurasia, with various subspecies occupying different regions. For instance, the thinleaf alder subspecies dominates stream margins in mountainous areas across the western United States and Canada.
Ecological Importance in Their Environment
The ability of alders to thrive in nutrient-poor or disturbed soils is directly linked to nitrogen fixation. Alders form a mutually beneficial relationship with the filamentous bacterium Frankia, which colonizes specialized root nodules. This symbiotic association allows the bacterium to convert atmospheric nitrogen gas (N₂) into ammonia (NH₃), a usable form of nitrogen for the plant.
The root nodules create an environment that limits oxygen diffusion, protecting the nitrogenase enzyme within the bacteria. By fixing nitrogen, alders effectively create their own fertilizer, allowing them to grow successfully where other trees struggle due to insufficient soil nitrogen. This capability establishes alders as pioneer species, frequently being the first woody plants to colonize barren mountains, floodplains, or areas disturbed by fire or logging. As the alder leaves and debris decompose, the fixed nitrogen is released into the soil, improving its fertility and enabling the establishment and growth of later successional species.