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Alnus

Alnus incana - grey alder

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Growing up to the forest line in the north, grey alder is an important pioneer species in the wild. Growing shrub-like at first, during its relatively short life it can reach almost 20m in fertile soil. By dropping its leaves while still green and fixing atmospheric nitrogen in its roots, grey alder improves the soil of sites like worked-out sand and gravel pits. It also provides an excellent protective canopy for less hardy species.

Grey alder differs from common alder in having dull green leaves downy on both surfaces, and an attractive patchy grey trunk. The species releases pollen from its yellow male catkins starting in February through to late April, before leaf flush. The female flowers, called cones in the vernacular, are an added irritant to fishermen cleaning their nets.

Variation in leaf form has produced several different varieties. Most of them are finely lobed, like that growing at Mustila, which is A. incana ‘Laciniata’.

 

Alnus glutinosa - common (or European) alder

Alnus glutinosa © Susanna

European alder is common in Finland south of the line Kokkola-Iisalmi-Nurmes, though also found on shores and fertile soils as far north as the northern Gulf of Bothnia and southern Lapland. At its best, this species is as impressive in the landscape as the grandest of oaks, developing strong branches and a broad crown when given enough space.

New shoots are somewhat sticky. The species is wind-pollinated and enlivens the landscape in spring (April-May) with its flower catkins about a week before grey alder (A. incana). The seeds fall from the dry “cones” in winter, causing extra work for fishermen emptying their nets on the shores.

Roots extending into lake and sea waters draw attention by their bright red colour. One special characteristic of the alders is their co-operation with Frankia alni-bacteria, which enables them to fix atmospheric nitrogen in their roots. This means alders don’t need to withdraw the chlorophyll from their leaves into storage in the trunk and roots in autumn and the leaves fall while still green, enriching forest soils.

The European alder does not produce root suckers, though it does sucker from the butts of felled trees. The species tolerates trimming and has been used as hedging on the shores of the Gulf of Bothnia. In landscaping use it is rare but beautiful. The species should also be valued in forestry, since the reddish timber has many uses.

 

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