Wednesday, December 8, 2010

The Greenbelt in December

The wildflowers of fall, now faded, frozen, and dried, have disappeared into the underbrush along the Greenbelt trail.  Their broken stems and stiff seed heads were barely distinct from the brown grasses and leafless shrubs lining the trail, except that I remembered the dense stands of bright yellow Golden-eye that filled every clearing in October and, in remembering the flowers, was able to find their dried stalks.  My attention did not linger with the dead wildflowers for long, though, because soon I found myself looking up as I walked, turning and gazing and staring across the treetops to the bright red leaves of the oak trees above.

I have to apologize to the oak trees for dismissing them.  A few weeks ago, when I set out to document what I thought was the peak of our fall-color season, I was disappointed to find that the most brightly-colored, stunning examples of fall foliage being displayed in east Austin were on naturalized Chinese tallow trees.  I couldn't help but admire their colors as the Chinese tallow trees in my neighborhood turned from multi-colored, yellow and red and burgundy, to deep scarlet in recent weeks, yet I was also saddened each time I realized that the beautiful ball of red in my neighbor's yard, or down the street, or on the horizon, was being produced by an invasive pest of a tree.  So it was a real treat to find myself staring upward into the bright red leaves of native oak trees.

Two species of red oak trees are native to the Austin area, Spanish Oak and Shumard Oak.  Shumard Oak (Quercus shumardii) grows throughout the southern United States, from Virginia to north Florida and west to Texas.  The Shumard Oak is at the east end of its range in Texas, and therefore is restricted to streamside or bottomland sites with deep, moist soils.  Shumard Oaks grow to be large (80 to 100 feet) trees that often have buttressing around the base of the trunk.  The leaves on Shumard Oak trees are large (up to 8 inches), with the pointy-lobed pattern characteristic of red oak trees, and turn yellow to scarlet in the fall.

The Spanish Oak (Quercus buckleyi), which is also called the Texas Oak, is endemic to north, central Texas and southern Oklahoma.  Once considered to be variety of the Shumard Oak, Spanish Oak is now recognized as a distinct species by ecologists.  The leaves on a Spanish Oak tree tend to be smaller (up to 6 inches), with narrower lobes than those on the Shumard Oak, and turn yellow to red in the fall.  While most oak trees prefer slightly acidic soils, the Spanish Oak is adapted to the alkaline soils of the Edwards Plateau, where it is often found growing on limestone ridges, slopes, or creek bottoms with live oak and juniper (aka cedar) trees.

Juvenile red oak tree in fall

The oak trees turning red along the Greenbelt this month are probably Spanish Oak trees, given their habitat (limestone slopes above the creek), their associates (Ashe Juniper, Cedar Elm, and Plateau Live Oak), and their smaller-tree-with-smaller-leaves appearance.  But I can't be sure whether I was admiring the red leaves of Spanish Oak or Shumard Oak trees for two reasons.  One is simply that red oaks are tricky to identify because they vary greatly in their leaf shape and size.  The second reason is that here, on the edge of the Edwards Plateau, where Spanish Oak is at the eastern edge of its range and Shumard Oak is at the western end of its range, trees of the two species hybridize (interbreed).  So the native red oak trees that we have in Austin are not necessarily either Spanish Oak or Shumard Oak but, more likely, are some combination of both.

Since this weekend's trip to the Greenbelt, I am noticing red oaks, displaying leaves of yellow, orange, red, scarlet, and brown, all around Austin.  I wish that I could give each tree a name, rather than simply thinking of it as a "red oak," which is rather vague, given how many species of red oak there are.  But in town, the problem of oak tree identification is further complicated by the fact that oak trees are frequently planted for their beauty, shade, or color.  Northern Red Oak (Quercus rubra or Q. borealis) is widely planted in the United States and Europe for its beauty and wood.  Scarlet Oak (Quercus coccinea) and Pin Oak (Quercus palustris), two other northern species prized for their foliage, are also sold by nurseries, as are Black Oak (Quercus velutina), Water Oak (Quercus nigra), and Southern Red Oak (Quercus falcata), three southern species that have ranges into eastern Texas. 

In town, a red oak could be any number of species, or a hybrid of two of those species, and will have to simply be a Red Oak.  East of town, call a botanist and expect a lecture on the phenotypic plasticity within and between individuals and species of red oak trees.  West of town, though, if it wasn't planted by a human, then it is a Spanish Oak tree, a beautiful red oak that is unique to central Texas.  The Spanish Oaks have figured out how to grow in the rocky, limestone hills of central Texas, farther west than any of their east-coast, red-oak relatives and, in doing so, bring us native, red foliage in December. 

No comments:

Post a Comment