Monday, September 20, 2010

Fall Elm

I have a mild, though occasionally annoying, case of Austin allergies.  The intensity of my congestion seems to correlate with the density of mold spores in the air, so I've concluded unscientifically that I suffer from mold allergies.  When I'm sneezing more than usual, or when I think to, I look up the pollen counts.  Mold spores always make the list, along with a few pollen-producing plants that vary with the season.  In August and September, ragweed and "fall elm" are the major pollen producers.

I've always wondered why "fall elm" isn't called by its actual name, cedar elm, given that cedar elm is the elm that blooms in the late summer here.  Maybe it's a local preference, like calling Ashe juniper "cedar," or maybe it's because cedar elms, though common and widespread in central Texas, are not much known.  Dismissed as one of the "three unimportant elms found in the Southeast" by my very-dated guide to trees of North America, cedar elms obviously aren't among the trees, like pecans, live oaks, and bald cypresses, for which our region is known.

Cedar Elm (Ulmus crassifolia) is the most widespread native elm of Texas, and is found throughout east Texas, except for the southeastern corner of the state.  Though they grow to their largest sizes in moist, bottom-land soils, cedar elms can also grow in the alkaline, compacted-clay soils or the rocky, limestone soils that are common in central Texas.  The medium- to large-sized trees provide food, cover, and nesting sites for birds, and host the larval stages of two species of butterflies.  Cedar elms can tolerate part shade or full sun and, once grown, provide dense shade for houses and yards.

Cedar elm leaves are miniature versions of the leaves of the better-known American elm (Ulmus americana), which also grows along waterways or in yards in Austin.  Cedar elm leaves are small, just one to two and a half inches long, but thick, asymmetric ovals.  The edges of cedar-elm leaves are doubly serrate, meaning that there are smaller notches within larger notches along the edges of each leaf.  The tops of the leaves feel like sandpaper, while the leaf bottoms are soft like felt.  Altogether, the small, leathery leaves are well-adapted to the heat- and drought-stresses of central Texas.

Cedar Elm (Ulmus crassifolia)

Cedar elm blooms in July, August, or September, depending when, or if, late-summer rains occur each year.  Because they are wind pollinated and do not need to attract insect pollinators, the flowers of cedar elm trees are small and inconspicuous.  (I rarely notice them until they fall from the trees and get carried into the house in the treads of my shoes, littering my kitchen floor with tiny, brown crunches.)  The tiny flowers release pollen into the air, which, ideally, will carry the pollen to another cedar elm tree that is downwind.  But wind is an unreliable messenger, so some of the pollen reaches our noses instead, creating the potential for allergies.

Pollinated elm flowers turn into samaras, or simple, dry fruits with wing-like outgrowths that are adapted to riding on the wind.  Light green and round with a notch on the bottom side, each fruit has a distinct, more-darkly colored seed in its center.  Following solid late-summer rains, the cedar elms in Austin are covered in samaras this year.  Later in the season, the samaras will turn yellowish-brown and fall from the trees along with the leaves.  Next spring I will be reminded of this year's fall rains, and the large crop of cedar elm fruits, when hundreds of elm seedlings sprout in my backyard.

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