Defending Alders

Alder trees are part of the birch family.

Take a moment and imagine a Puget Sound lowland forest. Likely, you see layers of gray-green firs, fading into a bank of fog.  But were alder trees part of your picture?

Native red alders (Alnus rubra) are frequently dismissed and often villainized as “weed trees” here in the Pacific Northwest.  Yet most people don’t realize they fulfill important functions vital for a healthy forest.

Alders establish quickly in a new clearing, stabilizing soils and competing against noxious weeds like scotch broom and Canada thistle. Unlike most plants, alders extract nitrogen from the air, thanks to a type of bacteria that colonizes on alder roots. The nitrogen is then converted into a form essential for plant growth and photosynthesis.  Nitrogen also enriches the forest soil encouraging plant biodiversity.

Alder is a deciduous hardwood particularly beneficial to insects. That’s because deciduous leaves are much more edible to moths and butterflies and their caterpillar young than conifer needles. In fact, red alders host almost twice the number of these species than Douglas fir.  And while other tree species in our region including Garry oaks, cottonwoods and willows may support more beneficial insects, none of these trees are as abundant locally as alder.

Alders are also a favorite hangout for birds, thanks to the juicy caterpillars and seeds they provide, attracting a huge variety of species.  Young, vigorous alders support large breeding populations of Black-throated Grey Warblers and Swainson’s Thrushes, and even an occasional Red-eyed Vireo.  Mature alders are frequented by Black-headed Grosbeaks, Yellow Warblers, Cedar Waxwings, and Warbling Vireos.  And most woodpeckers nest in dead or dying alders which rot much more rapidly than conifers.

So next time you think of native forests, don’t forget to include patches of hardwoods, particularly alders, as important members of the forest web that support the Island wildlife we love.

A grove of island alders.