The Ancient Cross Timbers

 Large tracts of ancient deciduous forest still grace the ridges and rugged ecarpments of southeast Kansas, Oklahoma, and central Texas.These interesting woodlands are dominated by centuries-old post oak (Quercus stellata) and are part of the Cross Timbers ecosystem. The Cross Timbers are a complex mosaic of upland forest, savanna, and glade which form the broad ecotone between the eastern deciduous forests and the grasslands of the southern Great Plains. The presettlement Cross Timbers are believed to have covered some 30,526 square miles (7,909,700 hectares), extending from central Texas across Oklahoma into southeastern Kansas (Figure 1). The short, stout oaks of the Cross Timbers were not ideal for lumber production, so the original Cross Timbers have often survived on steep terrain that was unsuitable for farming. Literally thousands of ancient post oak can still be found in this region, and there is no doubt that the Cross Timbers is one of the least disturbed forest types left in the eastern United States.

    Many public and private land managers do not realize that ancient forests survive extensively across the rugged terrain of the Southern Plains, but in their defense, the Cross Timbers do not satisfy the stereotype for ancient forests, which remains fixated on giant redwoods or massive hardwoods. The Cross Timbers are drought-stressed woodlands, populated by low-stature, slow-growing trees, many of which predate not only statehood, but also the birth of the United States. Thousands of 200- to 400-year old post oak survive in the Cross Timbers, and red cedar (Juniperus virginiana) trees over 500-years old have also been found on fire-protected blufflines. Our failure to understand the ancient Cross Timbers is contributing to the ongoing destruction and fragmentation which is a major threat to the viability of this ecosystem and to the biodiversity, water quality, and recreational values it supports.
    The Ancient Cross Timbers Consortium has been established to promote research, education, and conservation in this interesting and authentic American woodland. These eight figures provide a brief overview of the ecosystem and the conservation challenges we confront.

Figure 1. The potential natural distribution of the Cross Timbers (red) and Post Oak Savanna (green) forest types along the eastern margin of the southern Great Plains. This map was digitized from Kuchler (A.W. Kuchler, 1964 "Potential Natural Vegetation of the Coterminous United States" American Geographical Society, Special Publication 36) and includes Kuchler's type 75 ("Cross Timbers: Quercus-Andropogon), which covers an estimated 30,526 square miles (7,909,700 hectares). The Cross Timbers on level soils have been largely cleared for cultivation or grazing. However, extensive field surveys indicate that hundreds of square miles of ancient Cross Timbers survive in a fragmentary pattern on steep terrain from southeast Kansas, across Oklahoma, and into central Texas. Old growth woodlands also survive locally in the Post Oak Savanna of southern Texas (which Kuchler mapped as type 91, oak-hickory).

Figure 2. This map illustrates the location of ancient post oak forests actually documented with core samples by the University of Arkansas Tree-Ring Laboratory. This map certainly does not represent all of the ancient forests that survive in the Cross Timbers and Post Oak Savanna, but is sufficient to document their presence throughout the ecosystem.

Figure 3. This is the oldest post oak tree ever found. It is only some 20 feet tall, but careful tree-ring analysis indicates that it is over 400 years old. It is near the Keystone Ancient Forest Preserve, recently established in Sand Springs, Oklahoma, and managed by The Nature Conservancy.

Figure 4. A typical view of the ancient Cross Timbers vegetation type. The post oak trees shown here average only 15" in diameter and 30' tall, but are in the 200- to 300-year age class (proven non-destructively with increment cores). Due to the small stature and slow growth rate of the dominant post oak trees, most of the Cross Timbers are not suited for commercial sawlog production. Consequently, the Cross Timbers remain one of the least disturbed forest ecosystems in the eastern United States. However, land clearing, suburban development, and the increased production of wood chip mills threaten even these marginal, slow-growing woodlands.

Figure 5. An ancient eastern red cedar growing on a sandstone bluffline in the Cross Timbers, at the Keystone Ancient Forest Preserve. This tree is over 500 years old. Note the strip bark growth form, which is often indicative of old-growth conifers. The Cross Timbers, contain the largest and least disturbed stands of eastern red cedar known to still exist in the eastern United States.

Figure 6. Undisturbed savannas have become extremely rare in the eastern United States, but are still relatively common within the mosaic of closed canopy forest, open savanna, and glade that constitutes the ancient Cross Timbers. The open grown savanna tree shown here is over180 years old. 

Figure 7. One of the many small glades dominated by prairie grasses that punctuate the vegetation mosaic of the Cross Timbers. These grassy openings are so common that we think it is justified to suggest that the largest amount of unplowed, ungrazed, and largely undisturbed glade grassland that still exists in the eastern United States may be found within the ancient Cross Timbers. The rugged terrain typical of many ancient Cross Timber remnants creates microenvironments that promote a surprising degree of diversity in these authentic and endangered ecosystems.  

Figure 8. Green development? Most of the ancient Cross Timbers are privately owned, and the integrity of these ecosystem remnants will depend largely on the environmental ethics and civic-mindedness of the landowners. But we see both economic and environmental opportunities in the future of the ancient Cross Timbers and this photograph helps illustrate one example. This is a photograph of a new subdivision under development on private property in the hills above the U.S. Army Corps of Engineers reservoir of Skiatook Lake north of Tulsa, Oklahoma. Ancient Cross Timbers still grace many of the hills in the viewshed of the lake. This particular subdivision simply bulldozed and burned the ancient post oak that were present and will replace them with new homes, bermuda grass lawns, and hybrid pear trees.  We doubt that the developer realized that he destroyed an ancient forest with 200- to 400-year old post oak trees and many other interesting plants and animals. As an alternative to this conventional development, future builders in the ancient Cross Timbers might consider a more "green design," with a minimum footprint cleared for road, home, and yard, and a two-story structure with elevated deck to take advantage of the magnificent view over the top of a canopy of virgin 20' to 30' tall post oak and beyond to the lovely lake and stunning Oklahoma skies. In fact, many buyers might well prefer a piece of the low-maintenance well-adapted ancient forest, rather than the thirsty and exotic bermuda grass lawn. People might even pay more for the privilege of an ecologically friendly place to live.