An Abundance of Acorns

Texas Red Oak displaying fall color.

Texas is famous for its oak trees, with over 50 different species found in the Lone Star State.  Each of these oak species produce acorns, also called oaknuts or mast, which is a collective term for fruits or nuts.  The word acorn is related to Gothic term akran which has been interpreted to mean ‘the fruit of the unenclosed land.’  When oaks are dominant in the landscape, as they are here in central Texas, they play an important role in the ecology of the forest.

Acorns usually contain one seed enclosed in a tough, leathery shell, and have a cup-shaped cap or cupule. Depending on the oak species, acorns can take from 5 to 24 months to mature.  As a general rule, acorns mature in late summer, turning green to brown, and start falling from oak trees in September and October.  Acorns produced by trees in the red oak family (such as Texas Red Oak, Quercus buckleyi) take two years to mature, while acorns produced by trees in the white oak family (such as Bur Oak, Quercus macrocarpa) only take one year. 

Texas Red Oak Acorns

Some years, known as mast years, trees produce copious amounts of acorns with smaller crops in the years between.  Scientists have proposed a range of explanations for the mystery of what might trigger a mast year, but they do know that it is not resource-driven as annual rainfall and temperature fluctuations are generally much smaller in magnitude than the variation in the crop sizes of acorns.  Some scientists hypothesize that masting trees are trying to maximize pollination efficiency.  If these trees flower and release pollen at the same time in order to increase their chances of reproduction, since large amounts of pollen correlate with larger amounts of germination, they ultimately produce more acorns.  Large, occasional outputs of seeds like acorns appears to be more favorable than frequent, smaller outputs.

Bur Oak acorns are our largest acorns.

Boom and bust years of acorn production actually benefit oak trees from an evolutionary perspective, because acorns are an important, highly nutritious food source for many animals including squirrels, mice, turkeys, blue jays, pigeons, ducks, deer, and bears.  In a mast year, these animals can’t consume all of the acorns produced, so some are left to germinate and grow into future oak trees.  In leaner years, animal populations are kept in check so there are fewer animals to eat the acorns in the mast years.  Over time, a higher proportion of acorns survive to become oak trees.

Copious amounts of acorns are produced in a mast year.

Too heavy for wind dispersal, acorns need other ways to spread beyond the mother tree into a suitable area for germination.  Jays, squirrels, and some woodpeckers serve as the main dispersal agents, as they gather and hoard acorns in caches.  While they are remarkable in creating mental maps of their cache locations, these animals rarely eat every single acorn, so a small number manage to germinate and produce the next generation of oaks.

The Blue Jay is one of many species that cache acorns.

As autumn arrives and acorns mature and fall from our oak trees, think of the potential in the abundance of acorns produced.  As Henry David Thoreau once wrote, “Every oak tree started out as a couple of nuts who stood their ground.”  

Quercus with a Purpose

All oaks, like this Post Oak, are members of the genus Quercus.

Texas is home to dozens of native species of oaks, all of which are in the genus Quercus.  These trees provide humans with ample shade in the summer and beautiful color in the fall, and they sustain many mammals and birds with their acorn fruit.  But did you know that they are native host plants for dozens of butterflies and moth species, or the plants the female adults lay their eggs on for their caterpillars to eat?  In turn, the caterpillars of these butterfly and moth species provide a critical food source for almost all of the songbirds raising broods in the spring. Three of the most productive native oak species in central Texas are the Texas Live Oak (Quercus fusiformis), Post Oak (Quercus stellata), and Bur Oak (Quercus macrocarpa).

Texas Live Oak

The Texas Live Oak is also known as Escarpment Live Oak, Hill Country, and Plateau Live Oak.  Considered a semi-evergreen tree, the previous year’s leaves fall from the tree only when pushed out by newly emerging ones in early spring.  It has a stately mature form and unparalleled longevity, reaching to 40 feet in height with large limbs that over time spread an appreciable distance from the main trunk.  Firm textured leaves are oval to elliptical, 1 to 3 inches long, with young leaves having pointed lobes.  Its acorns are spindle-shaped or fusiform, narrowed at the base and ¾ to 1 inch long.  

Juvenal's Duskywing

Grote's Buckmoth

The Texas Live Oak is a host plant for Oak Hairstreak (Northern form), Juvenal’s Duskywing, and Meridian Duskywing butterflies, as well as Grote’s Buckmoth, Eastern Buckmoth, and Delilah Underwing moths.

Post Oak

Also known Iron Oak and Cross Oak, Post Oak is a deciduous oak to 50 feet, coarsely-branched with a dense, oval crown.  Its leaves are typically 3 to 5 inches long, with 4 pairs of lobes on each side, and the upper pair are often larger than the others, resembling a cross.  Acorns are ¾ to 1.25 inches long.  Post Oak is the most common oak throughout Texas, and its hard or iron wood is used for railroad ties as well as construction posts and timbers.  

'Northern' Oak Hairstreak

Polyphemus Moth

It is a host plant for the Oak Hairstreak (Northern form) and White M Hairstreak butterflies, and the Polyphemus, Eastern Buckmoth, Scarlet Underwing, and Little Nymph Underwing moths.

Bur Oak Acorn

Bur Oak is one of our largest oaks, also known as Savanna Oak, Overcup Oak, Prairie Oak, and Mossy-cup Oak.  A deciduous tree that can exceed 100 feet in height, its massive trunk supports heavy, horizontal limbs and lobed leaves up to 9 inches long.  Its acorns are the largest of all native oaks, up to 1.5 inches wide, with much of the acorn enclosed in a coarsely scaled cup with a heavily fringed margin. Sometimes spelled Burr Oak, it is the northern most oak in the New World, extending farther north than any other oak species.  

Banded Hairstreak 

Imperial Moth

Bur Oak is the host plant for the Banded Hairstreak and Juvenal’s Duskywing butterflies, in addition to the Ilia Underwing, Imperial, and Greater Oak Dagger moths.

While Texas is known for its oaks, care must be taken in identifying and maintaining oak trees.  Most all of the species can hybridize, occasionally making exact identification difficult,  and several of them are susceptible to oak wilt disease. However, they are worth the effort from a human and wildlife standpoint, as they are Quercus with a purpose!