Tree of the month: Alder
Common alder - Alnus glutinosa
A tree which is quite at home in wet conditions, alder is known to be very rot resistant, but only when kept moist - making it an ideal timber for boats and sluice gates. Alder is a key component of wet woodland, one of the UK’s rarest but most biodiverse habitats, supporting several rare plants and invertebrates.
In ancient wet woodlands, alder would have been coppiced for charcoal and gunpowder production, this would prevent trees becoming too large and drying-out the land. However, today’s lack of traditional management, along with drought and water extraction, means these valuable habitats are becoming more threatened.
Because of alder’s ability to thrive in damp conditions and soak up large amounts of water, it is sometimes planted in flood plains, or to slow the flow of fast-moving floodwater in a river catchment. They can also intercept surface water run-off which trap pollutants before they reach the waterway, and help stabilise the soils of riverbanks to prevent erosion.
Alder is extremely tolerant of poor, low nutrient soils. This is because it can absorb nitrogen – a key component of plant health - directly from the air. It does this by having a symbiotic relationship with a bacterium called Frankia alni. This forms nodules on the roots, which absorb nitrogen and convert it to ammonia - which the tree can then use. Newly fixed nitrogen is also released back into the soil, improving conditions for other plants and trees in the vicinity.
Alder’s main identifying feature in winter are the female cone-like catkins – which remain on the tree year-round and are easily spotted in the leafless branches. As spring comes around, fresh green ones will grow alongside their more elongated male counterparts. Once pollinated, the cones will turn brown and open to disperse their seed – either in the wind, or in the waterways they overhang, settling on riverbanks downstream.
You can also identify the young twigs which have pale spots (called lenticels) and large purple alternate buds. They’re sticky - which is where its Latin name, meaning glutinous or ‘glue-like’, comes from.
Unfortunately, it can suffer from a disease known as Phytophthora alni which causes legions in the bark and dieback - this can be spread through alder populations along waterways.
Read our other Tree of the Month blogs...
