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. 

Fall Color

I love the leaf colors on the Chinese tallow tree in the fall.  Bright red leaves with yellow edges, burgandy leaves with receding green edges, yellow leaves with red veins, and a few almost-orange leaves can be found on one tree in November.  The bright yellow highlights on many of the leaves look sponge-painted and, taken together with the bunches of white seeds held on some of the trees in the fall, give the trees an even more highlighted appearance.  In east Austin, the Chinese tallow tree is the clear winner of the fall foliage competition.

Chinese tallow leaves (Sapium sebiferum) in fall

The bad news about the Chinese tallow tree (Sapium sebiferum) is that it is non-native and invasive.  And it's not simply invasive as in occasionally escapes from cultivation, but invasive as in a real problem.  The Chinese tallow tree was imported into South Carolina in the 1700's, where it quickly naturalized and spread up the Atlantic Coast and across the Gulf Coast to Texas.  The invasive tree has also spread to California, where it is listed as a "red alert" pest plant.  In Florida, where the Chinese tallow tree clogs waterways and ditches, planting or moving the trees is prohibited.  Nonetheless, the tree is still available at nurseries throughout the southern United States, where it is sold as an ornamental and promoted for its fall foliage.

Chinese tallow tree (Sapium sebiferum) in fall

In Texas, Chinese tallow tree has taken over the coastal plains, replacing the grasses and wildflowers that once supported a diversity of insects and animals with single-species forests.  Almost a quarter (23%) of the trees in the Houston area are now Chinese tallow trees.  In the United States, the Chinese tallow tree is very fast growing, which contributes to its ability to take over pastures and disturbed areas.  Ecologists studying the tallow trees believe that trees are able to grow so quickly because they lack insect pests here in the US, and therefore do not have to manufacture insect-repelling compounds but can instead direct all of their energy into growth.  In Asia, where the Chinese tallow tree is native and has insect pests, the trees grow much more slowly.

Another non-native, invasive tree that contributes to the fall-color landscape is the Tree of Heaven, the colors of which can be seen along the shores of Ladybird Lake downtown or in vacant lots throughout town.  While the Tree of Heaven (Alianthus altissima) does not have leaves that turn colors, the female trees are covered in red-tinted samaras in the fall.  Samaras are flattened, aerodynamic fruits, like the flyers of maple trees, that carry the seeds away from the parent tree.  Huge clumps of reddish samaras, in contrast with the green foliage of the trees, create striking fall color.

Tree of Heaven (Alianthus altissima) with samaras

Unfortunately, each of those pretty samaras carries a seed for an invasive pest of a tree that, like the Chinese tallow tree, is fast-growing, soft-wooded, and does not feed or protect any of our native birds and butterflies.  I have always wished that the Tree of Heaven had a different, less positive sounding name.  In reading about the tree I learned that some people call it the "ghetto palm," which sounds like a perfect name to me.  I imagine that it would be much easier to cut down and poison, as is suggested on many native plant websites, a Ghetto Palm than do the same to a Tree of Heaven.

So where is the native display of Texas fall color?

One option is to travel to Lost Maples State Park, which protects a relic maple forest left from a cooler and/or wetter time.  Which points to the reason why we don't have a lot of fall color in the first place - aside from a few maples in a park, we don't have the right trees.  Fall color is produced by temperate, deciduous trees.  In central Texas we have a mix of semi-tropical trees (like mesquite and acacia), evergreen trees (like cedar and live oak), and temperate, deciduous trees (like cedar elm and pecan). 

Most of our temperate, deciduous trees turn yellow, not red, in the fall.  This is not because the conditions aren't right for red leaves here because, actually, our sunny fall days are perfect for red-leaf production.  The leaves of cedar elms, pecans, hackberries, and mulberries turn yellow because those trees don't have the genetic ability to make red leaves.  In fact, only about 10% of temperate trees do have the ability to make anthocyanins, or the red to purple pigments that are so prized in fall foliage.  New England, famous for its fall color, just happens to have a very high concentration (up to 70%) of trees that have the ability to make red leaves.

The leaves of temperate trees are green throughout the spring, summer, and, in Texas, much of the fall because the leaves are full of green-pigmented chloroplasts, which are busy capturing the energy of the sun to make sugar for the plant.  Auxiliary pigments that help in light capturing or protect the leaves from certain parts of light, such as yellow xanthophylls and orange carotenoids, are also present in the leaves throughout the season.  In the fall, shorter days or longer nights cue the tree to store sugar in its roots and to stop auxin production, which causes a layer of cork cells to form between each leaf and tree branch, so that the leaf no longer gets water and nutrients.  The chloroplasts in the leaf are broken down and, as the green pigment of the leaf disappears, the yellows, oranges, and browns of the auxiliary pigments become visible, producing fall leaf color.  In a few species of trees, such as maples and oaks, sunny fall days also stimulate the production of red to purple pigments, which, unlike the auxiliary pigments, are not present throughout the season.

Virginia creeper (Parthenocissus quinquefolia), a vine that grows throughout the eastern United States and is native to the streamside forests of east and central Texas, has leaves-of-five that turn deep red in the fall.  I found this example of striking, and native!, fall foliage in downtown Austin.

Virginia creeper (Parthenocissus quinquefolia) leaves in fall

Another native vine, poison ivy, has leaves-of-three that also turn yellow, orange, and red in the fall.  Poison ivy (Toxicodendron radicans), though native, is invasive in the sense that it is a species that thrives in disturbed habitats.  According to urban legend, over-enthusiastic suburban housewives occasionally make holiday centerpieces out of the attractive fall-colored vines.  I imagine it's not a mistake that is made twice in any given neighborhood.

Poison Ivy (Toxicodendron radicans) on a pecan tree in fall

Finally, we do have a magnificent tree that the New Englanders can't claim that gives us four-season beauty - the Bald Cypress tree.  Unique among the conifers, Bald Cypress (Taxodium distichum) is not evergreen but drops its leaves each fall.  The feathery leaves turn yellow, or orange, or rust-colored before falling, and the different rates of the trees in turning color and dropping their leaves create variegated fall color along the rivers and streams of our area.

Bald Cypress (Taxodium distichum) in fall