What it does
Alder fixes nitrogen. It does this through Frankia, a filamentous actinobacterium that lives in orange-brown nodules on the tree's roots.
The bacteria pull dinitrogen from the air and convert it to ammonium. The tree feeds them carbon. In return, alder grows on soils that would starve almost any other hardwood.
The numbers are serious. A healthy stand fixes 40 to 300 kg of nitrogen per hectare per year. Most settle around 100 to 150 kg. That matches what conventional cereal farming applies as synthetic fertiliser, powered by sunlight instead of natural gas.
The nitrogen does not stay in the tree. Alder leaves run 2.5 to 3 percent nitrogen by dry weight, against 1 to 1.5 percent for most deciduous species. They drop, they rot fast, and the nutrients hit the topsoil within months. Root turnover and exudates add more across the growing season.
Over decades, the result is measurable: higher total soil nitrogen, more organic matter, livelier soil biology. Ground that started impoverished can carry demanding species. See nitrogen fixers for the wider group.
Where it grows
Alder is a tree of wet ground. Stream banks. Floodplains. Lake edges. The boggy hollows at the foot of a hillside where groundwater seeps.
Its roots form a dense fibrous mat that tolerates saturation. The Frankia nodules work fine in low oxygen. That combination makes alder the dominant tree along temperate riparian corridors across the Northern Hemisphere.
Common alder (Alnus glutinosa) runs from Scotland to Turkey. Dark fissured bark, rounded notched leaves, pendulous male catkins. It forms the carr woodlands that line European rivers.
Red alder (Alnus rubra) takes the Pacific Northwest. It hits 25 m on logged ground from Alaska to California and supplies furniture timber and smoking wood.
Italian alder (Alnus cordata) handles drier, warmer sites, heavy clay, and chalk. It is the alder to plant where common alder gives up.
Grey alder (Alnus incana) is the hard one. It ranges into Siberia, takes cold, exposure, and the nutrient-poor glacial soils nothing else wants.
In the southern hemisphere, Italian and red alder get planted on mine tailings and industrial wasteland where native nitrogen fixers are absent.
Why it matters in succession
Alder is a pioneer, but the useful kind. It does not just colonise bare ground. It rebuilds the ground.
The pattern is consistent. Alder takes wet, nutrient-poor soil. It fixes nitrogen for 30 to 60 years. It drops rich leaf litter. Organic matter climbs. Soil biology wakes up. By the time the alder declines at 60 to 100 years, the site can carry oak, beech, ash, Douglas fir, hemlock. Whatever the regional climax species are.
In the Pacific Northwest, the classic sequence runs red alder to Douglas fir and western hemlock. In Europe, common alder hands off to oak and beech as the soil deepens and shade-tolerant species push through the canopy.
Alder is moderately shade-intolerant. It bows out willingly once taller, longer-lived trees overtop it. That cooperation is what makes it a good nurse tree, and far cheaper than fertilising bare ground for decades.
How to plant it
Use alder as nurse stock. Mix it into restoration plantings at 10 to 30 percent of the total stems, spread evenly through the site.
Pair it with the species you actually want long-term. Native oaks. Walnut. Beech. The alder grows fast, breaks the wind, builds soil, and steps aside.
Three concrete uses.
Riparian buffers. Plant along streams and rivers, especially where livestock have trampled the banks or the channel has been straightened. Roots hold the bank. Canopy cools the water by several degrees in summer, holding dissolved oxygen for fish and invertebrates. Leaf litter feeds the aquatic food web. This is the standard prescription in wetland restoration and rain garden design.
Brownfield reclamation. On mine tailings, quarry spoil, and compacted industrial ground, alder is one of the first trees worth trying. Expect measurable gains in organic matter, microbial activity, and earthworm counts within 10 to 15 years. Tony Rinaudo's farmer-managed regeneration work in the Sahel runs on the same logic with different species: protect the pioneer, the rest follows.
Mixed woodland nurse stock. At 40 to 60 years, coppice the alder to extend its working life, or leave standing dead stems for dead wood habitat. Cavity-nesting birds, beetles, and fungi all use them.
See also
- Nitrogen Fixers
- Pioneer Species
- Native Oaks
- Wetland Restoration
- Reforestation Techniques
- Nurse Trees
- Tony Rinaudo