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The Monday Garden
Alder: The Nitrogen Fix

Issue No. 151 - February 13, 2005
by Sue Sweeney

In North America, we have a good half-dozen native alders, the best know probably being the Western red alder (Alnus rubra), famed for pioneering burned area and for its lovely hardwood. The alders are members of the birch clan and as such are nitrogen fixers. Research shows that alders actually helped change the North American climate back in the Ice Age by pulling nitrogen out of the air and depositing it in the soil in usable form.
picture: The native speckled alder (Alnus incana f/k/a A. rugosa) in the swamp at the Bartlett Arboretum, Stamford CT, August 2004. Note the cute little green cones and the alternate leaves, common to all alders. The speckled alder’s leaves have perfectly parallel, directly opposite veins that run straight to the double- saw toothed leaf edge; the leaves have a quilted appearance and are pointed at the tip but round at the base. Note also the light colored speckles (lenticels) along the twig. In Connecticut, the cold-loving speckled alder is at the southern tip of its range.
Alders are often found along streams and in the fresh water wetlands. In my area, alders grow in mix stands with other swampy trees and shrubs, include the native red maple, winterberry ilex, sweet pepper, and cornus, all of which benefit from the alder’s production of nitrogen.
picture: speckled alder mixed with native cornus (smooth, purplish leaves) Bartlett Arboretum late summer 2004
Alders are very, very tough when in the right temperature zone, with plenty of water and sun. They can handle very acid, heavy, wet, compacted, and disturbed soil. Accordingly, they often the first pioneer (literally the “ground breakers”) after fire or excavation, and are used for reclamation, even at mining sites. The alder grows fast enough that it has value strictly as a producer of bio-mass. Alders make good bonsai, too .
picture: The smooth or hazel alder (alnus serrulata) in the swamp at the Bartlett Arboretum, Stamford CT, September 2004, with next year’s catkins just getting started. The three long catkins in front are the male flowers. Note the dull green leaf color and the wavy leaf edges, the rounded leaf tip and V-shaped leaf base; note that the parallel veins are not quite opposite; and while the leaf edge is toothed, it’s not as jagged as the speckled alder. The bark is smooth without significant markings. The smooth alder is heat-tolerant (for an alder) and grows from Maine to as far south as Florida’s northern border.
Alders can handle intermittent wetness, so do well along streams and marshes where flooding and drought are both likely. Alders are temperature specific and each variety’s range is limited by its tolerance of heat and cold. The babies can get started in deep shade but the adults want full sun or just a touch of shade. The wingless alder seeds can be spread short distances by wind but generally travel by water, buoyed up by internal air floatation sacs, hence the frequency of the alder family along water bodies.

Despite our nice array of native alders, the Euro settlers felt compelled to bring with them the European black alder (alnus glutinosa). I suspect that these settlers didn’t yet know enough about our native flora to know that we already had plenty of good alders. However, they were savvy farmers and knew that planting their alders from home would make nearby plants grow much bigger and faster – a good thing when you grow your own food. I also suspect that the early Euro settlers probably didn’t know why alders had this effect. Since Europeans didn’t isolate nitrogen as an element until the 1770’s, they probably didn’t learn about the workings of nitrogen-fixing plants until sometime later. The European black alder has naturalized throughout Northeastern USA and Southeastern Canada, north to Zone 3 and south to Zone 7, but generally is not at the top the “bad invasives” list. For example, in my home state of Connecticut, the European alder been declared to have “invasive tendencies” which means that, for us, it’s not the worse thug on the block but we should be careful where we put it as it can spread.

picture: close up of an alder’s twig bark, Mill River at Scalzi Park Feb 2005
Now, nitrogen is mega-important to plant growth. On a fertilizer bag, you’ll see three numbers; the first one tells you how many pounds of nitrogen would be found in a 100 bag of the fertilizer (percentage by weight is another way of calculating with the numbers). (FYI: The other two numbers are potassium and phosphorus.) Plants need a bunch of things, like oxygen, that they can grab from the air, and trace minerals that are generally present in most soils. However, not everything that plants need to thrive is always present in a form that they can use.

The reason that most fertilizers contain the “Big Three” is that these elements are often in the shortest supply. In many environments, nitrogen becomes the “growth limiting factor”. This means that nitrogen is the first thing that the plants will run short on, which will then be the factor limiting further growth. (BTW: Natural limits on growth are not necessarily a bad thing, unless you’re a farmer trying to maximize per-acre production). There is plenty of nitrogen in the air but most plants can’t access it. The nitrogen fixers though, can take advantage of nitrogen in the air, often through a symbiotic relationship with bacteria-like micro-organisms. The nitrogen fixers include the pea and birch families.

In the case of alders, the alder benefits from the nitrogen that the mycorrrhizal fungi attached to its roots add to the soil, and the neighboring plants benefit for the extra nitrogen that’s quickly released from the alder’s falling leaves.

IDENTIFICATION: Once you’ve gotten to know one alder, you should be able to identify the family. Alders have distinctive leaves, flowers, seed-cones, bark and buds, so they are pretty easy to spot at any time of year. Like so many woody plants, they cross–breed and have individual variations, so you may see characteristics of more than one species on an individual plant.

LEAVES: Alder leaves are generally elliptical and mid-green with pronounced, straight parallel vines in V-shaped pairs that run right to the leaf edge. The leaves don’t have much color in autumn – usually they drop while green or turn brown and drop

Leaf characteristics that vary between alder species are whether:

• the leaf tip is pointed, round or missing,
• the leaf base is U or V shaped,
• the leaf edge’s teeth are fine or jagged,
• the leaf edge curls under or is wavy
• the leaf underside is lighter or hairy,
• the leaf top is shiny or dull.

The European alder is missing the leaf tip.


pictures: alder leaves, Bartlett Arboretum summer 2004


pictures: catkins in winter, Mill River at Scalzi Park and Bartlett Arboretum February 2005;
look closely at the last picture to see clusters of last year’s brown cones

FLOWER AND SEEDS: Alders start next year’s flowers in summer and, as noted, hold the cones over the following winter. Also, alders start to bear when only a few years old. So it’ll be rare that you’ll see an alder total bereft of the distinctive cones or the birch-family trademark catkins. The pollen floats from tree to tree (and into your sinuses) via the wind in spring for most alders but in fall for the remarkable Seaside Alder (Alnus maritime) which, due to an altercation with the last glacier, is now only found in Maryland, Delaware and parts of Oklahoma.

Alders have separate male and female flowers on the same tree. The male flowers are long (up to 5”) reddish catkins; the females are much shorter (about ½” or less). The female flowers mature into cute little pine cone-like objects that contain tiny wingless nuts with built-in air sacs for floatation. The cones start out green, and ripen to brown in the fall. When ripe, the cones slowly release the nuts through fall and winter. To distinguish between alder species, pay attention to whether cones and catkins droop or stand erect.
BARK: Alder bark tends to be dark and smooth with light colored horizontal markings. The markings and color vary between species. The European alder has gummy twigs and young leaves. The speckled alder has prominent lenticels and the European alder bark tends to have warty patches.

BUDS The fat, blunt, reddish buds sit on top of short stubby stems and have a prominent “seam” where the two side covers (bud scales) meet. The Seaside alder’s bud scales don’t quite meet.


pictures 1 and 2: Alder buds and warty bark of an alder, Bartlett Arboretum February 2005;
picture 3: alder bud, Mill River at Scalzi Park, February 2005. Note that alder buds come on stubby stems.

FORM AND HABIT: Alders have alternating branches. Many alder are shrub-high but some are taller: the big western red alder can reach 100 feet and the European alder can get as tall as 70 feet. Alders are often part of in long lived colonies. While alders don’t root suck very much, they do form colonies via seedlings or stump suckers that extend the colony’s life long beyond than the founding plant’s 100 or so years.
picture: Swamp at the Bartlett Arboretum February 2005; the grassy tuffs host mixed stands of alders, cornus, viburnum, ilex, sweet pepper, swamp rose, swamp azalea, swamp maple and many other species, mostly native.

picture one: young alder on the bank of the Mill River at Scalzi Park Feb 2005;
picture two: warty trunk of an alder, shiny trunk of a winter berry ilex, and red trunk of a native cornus Bartlett Arboretum Feb 2005;
picture three: an alder stretching tall against the sky Bartlett Arboretum Feb 2005

WILDLIFE: Alder seeds are eaten by small songbirds such as goldfinch and the stands provide cover for many birds and other small critters. The American deer like it too. Beavers and possum are said to savor the pollen rich (and therefore protein rich) catkins.
picture: alder flying its catkins like flags in the swamp at the Bartlett Feb 2005. Note last year’s cones and catkins for the coming spring.
picture: mating dragonflies resting on an alder leaf; note the shiny twigs with prominent lenticels. Bartlett Arboretum late summer 2004
From The Monday Garden. Copyright © by Sue Sweeney. Reproduced with permission. 

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