Ancient Cross Timbers

Not many ancient trees are left in Oklahoma. In the 1880s, sawmills were established in the forests of the eastern mountains. The sturdy and gnarly trees of the cross timbers were not considered commercially viable, but were used locally by settlers for fuel and building materials. The cross timbers is a transitional woodland, between the grasslands and the eastern deciduous forest. Trees on the open flats were first to fall to the ax and saw. As the cutting of trees in the cross timbers pushed back to steeper ridges and rockier hillsides, more trees were left uncut. On these less accessible, locations we now find the remnants of the ancient cross timbers forest.

The trees of the cross timbers are not the tall stately trees of our eastern forests, not the shrubby scrub of the western grasslands, but the cross timber trees fall in between – taller than a person, gnarlier than a pine, less showy than a maple in autumn. The cross timber trees have a character all their own. As described above, the cross timbers can be a dense forest of thick undergrowth and stout, low branches. Or it can be a more typical savanna with widely spaced trees and grassy expanses.

The tough, slow growing trees of the cross timbers vary across the ecoregion, but typical dominant species are post oak (Quercus stellata), blackjack oak (Q. marilandica), chinquapin oak (Q. muehlenbergii), hackberry (Celtis occidentalis), and pecan (Carya illinoinensis). The canopy (top of the tallest trees) is not high and can be quite a bit shorter when the woodland is on a dry ridge-top where the wind shears the tree limbs. The trees found in the shade of the tall canopy can be redbud (Cercis canadensis), persimmon (Diospyros virginiana), sumac (Rhus spp.), and eastern red cedar (Juniperus virginiana).

As a transitional woodland, the forested areas are interspersed with pockets of prairie. Four tall grasses reign in these patches: big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), Indian grass (Sorghastrum nutans), and switch grass (Panicum virgatum). On drier sites, the smaller grass species of hairy grama (Bouteloua hirsuta) and buffalo grass (B. dactyloides) can also be found. Showy prairie flowers are also an important element of the habitat: purple coneflower (Echinacea purpurea), foxglove penstemon (Penstemon digitalis), wild bee balm (Monarda spp.), and wild indigo (Baptisia australis).

The cross timbers ecoregion of Oklahoma can be visualized as a belt that stretches from Osage County to the Lake Texoma with a wide buckle in the center of the state. (Interactive Map)

So, how do you determine a tree’s age?

Well, you don’t get a copy of its birth certificate nor do you look at its teeth. You have probably seen tree rings on a cut log or tree stump. In general, each ring represents one year of growth. The rings are visible because the plant cells that form the wood are different sizes. The early spring growth is lighter in color because the cells are larger. The cells grown in late summer are smaller and the ring made up of those cells is darker. As you might guess, rings are most visible in trees growing in areas with distinct seasons that cause cell growth to follow a consistent pattern – hot, dry means small cell growth; warm, moist equals big cell growth. (However, just to warn you, there are trees in North America that are tricky and are known to have “false rings,” but these oaks are pretty faithful ring growers.)

We can observe tree rings in a tree’s cross section, but it wouldn’t be a good idea to start cutting down all the trees we suspect are the oldest. And fortunately there is a more conservative technique to counting tree rings – coring. Using an increment borer, a tool with a long, hollow drill bit, we can extract a sample from the trunk. The sample is a long cylinder of wood representing a horizontal cross section of the trunk.

For our slow growing hardwoods, we cannot easily distinguish the narrow tree rings on the core without a little preparation of the wood. The cores are fragile and may break along any of the rings. For transportation back to the lab, cores are safely stored using the sophisticated scientific materials of kitchen plastic wrap and zippy freezer bags. In the lab, the plastic wrap is removed to allow the cores to dry. Once dry, the cores are glued to wooden mounts for stability. The cores are sanded smooth after the glue is dry. The whole process can take a week or two before you can begin to count the rings. Growth is so minute in some of the cores that magnification is required to see the individual rings.