These familiar shrubs often form dense thickets, especially northward. Nodules on the roots harbor nitrogen-fixing microorganisms, a feature of relatively few plants other than the Fabaceae, Myricaceae, and Ceanothus.
Alnus serrulata (Aiton) Willd., smooth alder, has been collected in Indiana near the Michigan border and ranges southward. It might be expected in southwestern Michigan. It differs from A. incana in its more elongate, elliptic or slightly obovate, usually rather acute leaves with regular finely serrate margins (in this aspect, as also in slightly glutinous surface, resembling the northern A. viridis).
1. Leaf blades all very broadly rounded to truncate or notched at apex, obovate to suborbicular in outline; plant an erect, usually single-trunked tree spread from cultivation.
1. Leaf blades all or mostly acute (barely obtuse) to short-acuminate at apex, roughly ovate to elliptic in outline; tall native shrub, usually many-stemmed.
2. Staminate aments expanding to maturity in early spring, before the leaves begin to open; filaments almost fully adnate to the perianth, the anthers appearing sessile; leaves not glutinous, both finely serrate and more coarsely dentate or obscurely lobed, the appearance being one of teeth quite varying in size and regularity; nut not winged at maturity (but so thin when immature as sometimes to be misleading).
2. Staminate aments expanding at the same time as the leaves; filaments free, ca. 0.5–1 mm long; leaves glutinous (with shiny sticky dots and veinlets) especially beneath and when young, finely and regularly serrate but not basically dentate or lobed; nut with a broad membranous wing on each side.
All species found in Alnus
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. October 17, 2017. http://michiganflora.net/genus.aspx?id=Alnus.