BACKYARD BIODIVERSITY: the Texas Live Oak

Describing a mature live oak as “stately” is a bit of an understatement. They can live several centuries and these older trees command quite a presence. Their trunks can grow 4 feet or more in diameter and their crowns can spread more than 100 feet, sometimes touching the ground in a sprawling display of majesty.

In Texas we have many different species of oak like the Shumard Oak (Quercus shumardii), the Bur Oak (Quercus macrocarpa) and others. This blog will look at the Texas live oak (Quercus fusiformas), which is also known as Escarpment Live Oak. The Texas live oak is nearly identical to the southern live oak (Quercus virginiana) but is more drought tolerant and cold hardy. Its range is southern Oklahoma, through Texas between the Brazos and Pecos rivers, into the northeastern mountains of Mexico. In urban settings, it’s often planted as a landscape tree, which is one reason you might find this species in your neighborhood.

The Austin area is tricky when trying to tell the Texas live oak apart from southern live oak. The ranges of these two trees overlap and they also hybridize. The main differences are in the size and shape of their acorns. Texas live oak acorns are spindle-shaped (fusiform) and narrowed at the base. Southern live oak acorns are oblong in shape (ovoid or oblong-ellipsoid). When hybridization occurs, this makes it even more difficult to tell what species the tree is.  

According to the Big Tree Registry maintained by Texas A&M Forest Service, the largest known Texas live oak in the state is in Bosque county. It is 63 feet high and has a circumference of 342 inches (28.5 feet). The crown spread is 93 feet.

On the UT campus, we have several mature live oaks, some of the most famous being the Battle Oaks at the corner of 24^th Street and Whitis. Varying sources will list these trees as either Texas live oaks or southern live oaks. These three trees are named after Dr. James Battle, UT president from 1914-1916, and professor of classical languages after his term. These three trees were part of a larger thicket of them by the time UT opened in 1883. Legend is such that during the Civil War, when word was received that Union troops had reached Galveston, this hill of oaks was destroyed to erect a fortress and protect the Texas Capitol. Only these three trees remain.

In 1923, the current Biological Laboratories building (BIO) was to be constructed on the site where these trees currently are. However, thanks to Dr. Battle’s efforts to save the trees due to their historical connection, the BIO building was moved further east.

UT actually keeps a website called Treekeeper that is maintained by the landscaping staff. Here, one can note every type of tree on campus, including Texas live oaks. Viewers to the website can also see the ecological benefits they bring to campus through metrics such as energy use savings.

The Texas live oak is the larval host for some Hairstreak butterfly species and Horace’s Duskywing (Erynnis horatius). The acorns are food for several species of birds and mammals. The oak gall wasp is a fascinating insect that induces the tree to create galls that serve as homes for their larvae.
 

Live oaks were often prized for their curved limbs and used by ship builders in the 18^th century to make planking and ribs of sailing vessels. When the naval vessel “Old Ironsides” was refitted in the 1980s, planking was used from Texas live oaks that had been killed by the oak wilt fungus. “Old Ironsides” is a nickname for the warship USS Constitution, constructed in 1794, the oldest commissioned warship in the world still afloat.

Texas live oaks are susceptible to oak wilt and oak decline, particularly during times of drought. Members of the white oak group, such as bur oak, seem to be more immune to oak wilt. The live oaks are part of the red oak group, whose members generally are susceptible. Oak wilt is one of the most destructive tree diseases in the US. It produces a fungus that essentially clogs the water-conducting vessels of an oak. Oak wilt will typically kill a tree within three to six months after symptoms appear. Leaves on diseased live oaks often develop chlorotic (yellow) veins that eventually turn necrotic (brown), a symptom called veinal necrosis. Tip burn, where the leaf edges turn brown, is also another symptom. The tree will lose its leaves somewhat rapidly. Oak wilt is the reason that arborists prefer to prune oaks during the coldest part of the year to minimize the potential for the fungus to invade the tree through the wound site.

Oak decline is a disease that introduces simultaneous stressors like drought, root fungus, and wood-boring insects. Trees that are affected by oak decline do tend to retain their leaves, even after their death, and the decline of the tree is longer than that of oak wilt.

Special thanks to George Yatskievych, Curator in the Billie L. Turner Plant Resources Center, for his assistance with this article.

SOURCES

“Famous Trees of Texas” Texas A&M Forest Service (accessed online: https://tfsweb.tamu.edu/websites/FamousTreesOfTexas/TreeLayout.aspx?pageid=18222)

Kolakawski, Chris."Old Ironsides: History of America's Ship of State" American Battlefield of Trust (accessed online: https://www.battlefields.org/learn/articles/old-ironsides-history-americas-ship-state#:~:text=USS%20Constitution%20is%20the%20oldest,the%20nation's%20first%20100%20years.)

Lady Bird Johnson Plants Database (https://www.wildflower.org/plants/result.php?id_plant=QUFU)

“Trees of Texas” Texas A&M Forest Service (accessed online: http://texastreeid.tamu.edu/content/TreeDetails/?id=109)

“Introduction to Oak Wilt” (accessed online: https://texasoakwilt.org/oakwilt/)