The Red Alder tree (Alnus rubra) is a fast-growing, deciduous hardwood native to the Pacific Northwest, extending from southern Alaska down to central California. This species plays a transformative role in ecological succession—the predictable change in a biological community over time. Red Alder is a significant agent in modifying the environment, altering the foundation of the forest ecosystem to set the stage for the next generation of plant life.
The Mechanism: Nitrogen Fixation
The Red Alder’s ability to drive successional change stems primarily from a unique biological partnership that allows it to enrich the soil with nitrogen. Red Alder forms a symbiotic relationship, known as actinorhizal symbiosis, with a filamentous bacterium of the genus Frankia. This microbe resides within specialized growths, called root nodules, that develop on the tree’s root system.
Inside these root nodules, the Frankia bacteria possess an enzyme called nitrogenase, which breaks the triple bond of atmospheric nitrogen gas (N₂). This process, known as nitrogen fixation, converts the inert atmospheric N₂ into a biologically usable form, specifically ammonia (NH₃). The energy required to fuel this chemical reaction is supplied by the alder tree in the form of sugars produced through photosynthesis.
The bacteria benefit from a safe, carbohydrate-rich habitat, while the tree gains a consistent supply of fixed nitrogen, which is often a limiting nutrient in forest soils. The fixed ammonia is incorporated into the tree’s tissues, allowing Red Alder to thrive in nutrient-poor environments. This mechanism is the foundation for the environmental changes that follow the tree’s establishment. Red Alder stands supply substantial amounts of nitrogen to the soil, with fixation rates reported up to 320 kilograms per hectare annually in pure stands.
Soil and Ecosystem Modification
The constant input of fixed nitrogen from the Frankia symbiosis fundamentally alters the soil’s chemical composition. The high levels of nitrogen enrich total nitrogen stocks, which are released into the surrounding soil through various pathways.
Nitrogen Release Pathways
- Decomposition of dead roots and nodules.
- Direct excretion from living tissues.
- Leaching of nitrogen-rich compounds from the foliage.
A consequence of this nitrogen cycling is the effect on soil acidity. The excess fixed nitrogen stimulates microbial processes like nitrification, which produces nitric acid, leading to a decrease in soil solution pH. However, compared to the highly acidic conditions often found under conifer forests, Red Alder often makes the soil less acidic and more fertile for subsequent species.
The tree’s rapid growth and deciduous nature also contribute to a fast turnover of organic matter. Red Alder leaves contain a high concentration of nitrogen and can drop them while still green in the autumn. This nitrogen-rich leaf litter decomposes quickly, adding organic material to the forest floor at an accelerated rate. This rapid decomposition increases the speed of nutrient cycling, which helps build a rich topsoil layer, improving soil structure and water-holding capacity for later-successional species.
Pioneer Role in Successional Timelines
Red Alder is considered a classic pioneer species, meaning it is one of the first trees to colonize a disturbed site. It quickly establishes itself where the soil has been exposed and the canopy removed, such as clear-cuts, landslides, or following a forest fire. The tree’s small, winged seeds are easily dispersed by wind, allowing for rapid colonization of these open, newly exposed mineral soils.
Its success is also due to its fast rate of growth. Under favorable conditions, young Red Alder saplings can grow several feet per year, quickly reaching a height that dominates the site. This rapid vertical growth is coupled with a high light requirement, as Red Alder is intolerant of shade. This allows it to outcompete slower-growing, shade-tolerant species in the initial, sun-filled phase.
By forming a dense canopy, the Red Alder stand creates the initial conditions necessary for the next stage of succession. The canopy provides shade and wind protection, which moderates the microclimate near the forest floor. The improved soil fertility and accumulated organic matter create a hospitable seedbed for shade-tolerant conifers and other late-successional plants to germinate and survive. Red Alder acts as a biological nurse, preparing the site for the long-lived forest that will eventually replace it.
The Shift: Decline and Conifer Establishment
The Red Alder’s role as a successional driver is temporary, dictated by its relatively short lifespan and shade intolerance. While fast-growing, the tree is considered mature at 60 to 70 years and rarely survives beyond 100 years, especially compared to the centuries-long lifespans of many Pacific Northwest conifers. Height growth slows substantially after age 20, allowing slower, more persistent conifers to begin catching up.
The dense canopy prevents Red Alder seedlings from regenerating beneath it, as the species cannot grow in the shade. As the trees age, their crowns begin to break apart and decline. This decline creates small gaps in the canopy, allowing light to reach the forest floor, which is now rich in nutrients from decades of nitrogen fixation and organic matter accumulation.
These modified environmental conditions favor the growth of shade-tolerant, late-successional conifers, such as Western Hemlock, Western Redcedar, and Douglas-fir. These conifers, which have been growing slowly in the understory, are released from suppression and begin to dominate the site. Even after the Red Alder dies, its successional influence continues, as the decay of its nitrogen-rich wood and roots releases the final stores of fixed nitrogen back into the soil, sustaining the growth of the newly established conifer forest.