Erle

Overview and range in Europe

Common or black alder, Alnus glutinosa, is a small to medium hardwood of wet ground. EUFORGEN describes it as native to Europe, western Asia and northern Africa with a scattered range, thriving along riverbanks, in woodlands and in marshes, on deep, moist soils with high water levels, in moderate to cold climates. The Wood Database, which gives the distribution simply as "Western Europe", lists a typical tree height of about 20-25 m with a trunk diameter of 0.3-0.6 m. That small diameter matters for a sawmill: it limits board width and log volume (see below).

Ecologically it is a fast-growing, light-demanding pioneer that fixes nitrogen (EUFORGEN). Because many sites were converted to farmland, alder reaches mills as small lots from riparian strips and thinnings (my inference).

Grey alder (Alnus incana) has the same anatomy and essentially the same properties but lower value, according to the LWG article. Red alder (Alnus rubra) of North America is a different species with generally lighter figures; it is used here only where labelled.

Appearance and identification

Alder is a "light-heartwood" species: sapwood and heartwood are not distinguished by colour. The LWG article (Grosser, Germany) describes a reddish-white to light reddish-yellow or light reddish-brown wood that darkens noticeably under light, and darkens to a deeper reddish-brown when steamed. The Wood Database gives light tan to reddish brown, darkening and reddening with age. A characteristic feature of freshly felled logs is a temporary orange-red oxidation colour on the cut end grain, which fades as the wood dries (LWG). Do not confuse this with red heart (discoloured core wood), which Torelli (Slovenia, 2001) identified as the most serious defect of black alder.

The wood is diffuse-porous with fine, evenly distributed vessels, often in radial multiples of two to four, and the grain is usually straight and the texture fine and even (Wood Database; LWG). Two features help identification:

• Aggregate rays. Narrow rays are bundled into false (aggregate) rays up to 0.5 mm wide on the cross-section (LWG). The Wood Database notes that these appear as occasional small streaks on the face grain that can be mistaken for defects, and that, apart from oaks, alder is one of the few temperate northern-hemisphere species with aggregate rays.
• Pith flecks. Brown flecks caused by a leaf-mining fly larva are visible on all cut faces (LWG). Torelli treats pith flecks as a decorative feature rather than a defect.

The Wood Database names pear (Pyrus communis) as a lookalike, which lacks aggregate rays and has smaller, solitary pores.

Properties that matter

The table gives the figures I could read in open sources. Both sources are European-wood descriptions in the sense that they treat Alnus glutinosa, but their test conditions differ and are often not stated. Treat all values as indicative, not as design values.

Property	Value	Source and region
Air-dry density	0.55 g/cm3 at 12-15% MC (range 0.49-0.64)	LWG Merkblatt, Grosser; Germany/Central Europe
Average dried density	535 kg/m3 (conditions not stated)	Wood Database; origin of specimens not stated, species is European
Specific gravity	0.41 basic; 0.53 at 12% MC	Wood Database; origin not stated
Janka hardness	2,890 N (650 lbf) at 12% MC	Wood Database; origin not stated
Modulus of rupture	91.4 MPa (conditions not stated)	Wood Database
Modulus of elasticity	11.01 GPa (Wood Database); 7,700-11,760 N/mm2 bending (LWG)	Wood Database; LWG, Germany
Crushing strength parallel to grain	39.8 MPa (conditions not stated)	Wood Database
Shrinkage R / T / volumetric	3.6 / 7.0 / 11.0%, T/R 1.9	Wood Database; conditions not stated
Volumetric shrinkage, fresh to oven-dry	13.6% average ("moderate")	LWG, Germany
Brinell hardness	16-17 N/mm2 (longitudinal, as listed)	LWG, Germany
Extractive content	about 5%	LWG, Germany
Impact bending work	33-38 kJ/m2	LWG, Germany
Red alder (A. rubra), for comparison	450 kg/m3 average dried; MOR 67.6 MPa; Janka 2,620 N; shrinkage 4.4 / 7.3 / 12.6%	Wood Database; North America, different species

Two cautions apply. First, the LWG article lists bending strength 50-54 N/mm2, compressive 85-97 N/mm2 and tensile 47-55 N/mm2, but column alignment in the text I received was unreliable, so these are not in the table; the bending figure is well below the Wood Database modulus of rupture (91.4 MPa) and I could not reconcile them. Second, both sources agree that alder is a soft, low-strength hardwood; LWG says its strength and elasticity are comparable to lime (linden).

Dimensional behaviour is a positive. LWG reports good stability after drying and little tendency to "work". The Wood Database agrees for the American species (stable once fully dry), and puts the T/R ratio at 1.9 for European alder.

Sawing and log handling

The most important mill rule for alder is speed. The LWG article says to fell in winter, saw up quickly, store the timber in airy stacks protected from the weather, and seal end grain with opaque coatings, adhesive tape or nailed-on protection, because the wood stains easily ("leicht verstockt"). The Wood Database says the same: freshly cut logs should be converted and dried quickly to prevent staining or decay. Neither source gives a number of days or weeks, so I cannot state a safe storage time; this depends on season, temperature and storage conditions, and each mill needs its own experience.

Orange-red cut ends are normal (LWG). Check logs for red heart; I found no source quantifying its frequency or grading effect.

Log size shapes the product. The Wood Database says boards tend to be narrow because the trees are small in diameter. The Teagasc project on Irish alder (20-30 cm thinnings, mostly sold as firewood) planned to compare sawing methods for yield, but the page reports no yield figures. As a labelled inference, small-diameter logs favour narrow boards and blanks.

LWG recommends feeding slowly when sawing logs, otherwise surfaces roughen. I found no open source with tooth geometry or saw settings for alder and give none.

Drying

The sources agree that alder is easy to dry, with the caveat of staining before drying begins. LWG states that natural and technical drying cause no difficulties and the wood does not crack or warp; it gives no schedules or times. The Wood Database comment (for both European and red alder) is that freshly cut logs should be converted and dried quickly to prevent staining or decay.

The only quantitative European drying information I could open is Dzurenda (Zvolen, Slovakia, 2021), specific to steamed alder. Blanks of 38 x 100 x 800 mm from 50-60 year-old Alnus glutinosa were steamed at 125 C for 7.5 hours and then dried from about 50% to 10% moisture in a two-part process: free water removed at 35-40 C and 70-60% relative humidity, then conditioning at 50 C and 84% RH, and bound water removed at 60-80 C. The paper notes that drying at 50-80 C is the normal practice for alder and that its lower-temperature mode lengthened drying time by about 25% compared with conventional alder modes. Final moisture quality was reported as class II for deviation from target, and class I for variation and moisture gradient. The purpose was colour retention, not a commercial schedule; I do not present it as a recommendation for unsteamed lumber.

A second Dzurenda paper (Drvna industrija, 2022) shows that heavier steaming (saturated steam at 135 C against unsteamed) gave a more UV-stable colour, with total colour difference falling from 10.9 to 5.7.

The Teagasc project in Ireland planned to compare air-drying then kiln-drying with "industry Schedule J", a novel schedule, and green kiln-drying, and to assess degrade under BS EN 373, but the page reports no results. The USDA Forest Products Laboratory kiln schedule chapter lists red alder (a North American species) with separate schedules for darker and lighter colour, but the schedule codes did not survive text extraction, so I have no numbers to quote.

Machining, gluing and finishing

Alder machines easily. LWG says it is easy to saw, knife, peel, plane, mill, turn and carve, and easy to split, but planer knives must be kept sharp because nicks leave grooves that remain visible after finishing. The Wood Database calls it very easy to work and especially easy to sand. Torelli's Slovenian tests (planing, shaping, sanding, boring, turning, nail and screw splitting, compared with beech) concluded that alder machines easily; the abstract gives no numbers.

Fastening is mixed: screws hold well and glued joints especially well, but nailing holds poorly and thin stock can split (LWG).

Finishing is a strength. LWG says polishes, lacquers and stains/pigment stains are accepted excellently, and this is why alder has been the traditional substitute for walnut, cherry, mahogany and, formerly, ebony. It notes that polyester-based lacquers may dry more slowly. The Wood Database notes that alder is sometimes used as a lower-cost alternative to black cherry (this is stated on the red alder page).

Two chemical behaviours matter. With extractives of about 5%, damp contact with iron causes faint grey stains and slight corrosion (LWG). The wood also strongly interferes with the setting of cement (LWG). Alnus dust has been reported to irritate eyes, skin and airways (Wood Database).

Durability and treatment

Above ground alder is non-durable to perishable (Wood Database). LWG describes low natural durability when exposed to weather (repeated wetting and drying) or in ground contact, but very high durability permanently under water, said to be not far below oak. The Wood Database makes the same distinction, citing historical underwater piles and supports, notably in Venice.

I did not find a numeric EN 350 durability class for Alnus glutinosa in an open source I could read, so none is given here. It is reasonable to treat alder as not suitable for exterior use above ground without preservation, but that is an inference from the descriptions above, not a classification. I also did not find an open source on treatability or impregnation behaviour for alder.

The practical rule is to sell alder as an interior wood, or an in-water wood where it stays wet, and to flag that it will not perform in weathered exposure (my inference).

Typical uses and market notes

LWG states that alder is not suitable for structural timber or building joinery because of low load capacity, hardness and weather resistance. Uses it names include solid and veneered furniture, blind wood, drawer sides, picture frames, toys, turning and carving, clogs, guitar necks, hive-frame pieces, brush backs, boxes, crates and pallets (cheaper grades), plywood, chipboard and fibreboard, pulp and charcoal. Hydraulic uses (piles, sluice gates, drainage pipes) rely on underwater durability.

The Wood Database lists veneer and plywood, charcoal, clogs, piles and turned items, and says European alder is not commonly available as lumber, with narrow boards and likely moderate prices. I found no open source with European alder prices, volumes or trade statistics, so I give none. LWG says it is traded as round timber, sawn timber and veneer, and used by the chipboard industry alongside poplar and birch. The Teagasc page notes that about 95% of hardwoods used in Irish furniture and woodworking are imported.

What is uncertain or varies

• The Wood Database page does not state the origin of its European alder specimens or the conditions for most strength values. The LWG article cites old DIN standards (1977, 1979) and a handbook.
• LWG column alignment for bending, tensile and compressive strengths was not reliable in the source text; they are omitted from the table, and their relation to the Wood Database values is unresolved.
• No numeric EN 350 class, no preservative treatability data, no storage time limit for logs, no commercial sawing yield, and no kiln schedule for unsteamed lumber were found. The USDA FPL kiln schedule data for red alder (a North American species) were not readable.
• The Dzurenda drying paper concerns small steamed blanks in laboratory conditions in Slovakia, so its times and temperatures should not be copied to production kilns.
• Red heart frequency, and how it is graded, were not quantified in any source I opened.
• Prices and trade volumes: not verified.

Key points

• Common alder is a soft, light-to-medium hardwood: about 535-550 kg/m3 at around 12% moisture in the two sources, from small trees (about 20-25 m tall).
• Heartwood and sapwood are the same colour; end-grain turns orange-red when freshly cut and fades as it dries. Aggregate rays and pith flecks are the identification marks.
• Convert and dry fast, and seal end grain: staining before drying is the main sawmill risk. Once dry it is stable and easy to dry.
• It machines, glues and finishes very well, is a traditional stain-grade substitute for cherry and walnut, but holds nails poorly and reacts with damp iron.
• Not durable in weather or ground contact, yet very durable permanently under water. No EN 350 class was verified.
• Not a structural wood; supply is small and irregular.