North American Oak Savannas

Although the present web site deals primarily with Midwestern oak savannas, for comparison the west coast (California/Oregon) and Southwestern U.S. oak savannas are discussed briefly here.

Southwestern U.S. oak savannas

Oak savannas are found in vast areas of southeastern Arizona, southwestern New Mexico, and adjacent land in Mexico. These are predominantly grasslands with scattered oaks, mainly at altitudes between 4000 and 6000 feet.

	Open-grown Emory oaks (Que-rcus emoryi) on a hillside in southeastern Arizona. Vast acreages of oak savannas can be found at the appropriate altitudes in this part of Arizona.
	Typical open-grown Emory oak in southeastern Arizona. This species is evergreen, but generally loses some or all leaves during the early summer drought. These habitats are primarily prairie grasslands with scattered oaks. Fire is a constant threat, either human- or lightning-induced, and under appropriate conditions great acreages burn. The large oaks are fire-resistant.

,

A good overview of California oak savannas can be found at this link.

There are vast oak savanna areas in interior California, primarily at altitudes of between 200 and 2000 feet in the Coast Ranges and in a ring around the Central Valley in the foothills of the Sierra Nevada and Coast Ranges. (See map above) They are situated between annual grasslands at lower altitudes and mixed conifer or ponderosa pine forests at higher elevations.

The dominant species of oaks in California oak savannas are blue oak (Quercus douglassii), valley oak (Q. wislizenii), coast live oak (Q. agrifolia), and Engelmann oak (Q. engelmannii). Much of the California oak savannas are in private hands and are used primarily for cattle grazing.

The oak savanna would not exist without fire. Although fire resistant, oaks are shade intolerant and are unable to maintain populations underneath the shade of maples and other shade-tolerant trees. Thus, in the absence of fire maples and other shade-tolerant trees are eventually able to take over the forest.

Why are oaks fire resistant?

• The thick bark of oaks is very fire resistant and has low thermal conductivity so that heat does not penetrate well to the living tissues underneath.
• Oaks form dormant buds at the base of the trunk. These buds are protected from fire by the root collars. These dormant buds are below the soil surface where they are protected from the lethal effects of fire. If the stem is killed by fire, one or more of these dormant buds grow.
• Oak saplings become resistant to fire when they are smaller than those of other tree species.
• Oak saplings subject to periodic (annual) fires produce extensive root systems even if top-killed. These large root systems permit better growth of shoots in fire-free years.
• Tree competitors are destroyed by fire, thus permitting selective oak reproduction.
• The seeds of oaks (acorns) serve as food for rodents. Rodents bury acorns under ground, thus bringing them into an environment where they are protected from fire.

The table below compares the shade tolerance and fire sensitivity of various hardwood trees to oaks. As seen, the oaks are the most fire resistant of the hardwoods.

Shade tolerance and fire sensitivity of deciduous treespecies in upland Midwest forests.

Trees which are intolerant or very intolerant to shade, such as aspen and birch, are pioneer colonizers of open land, but are unable to reproduce in their own shade. Also, tree species which are very tolerant to shade, such as maple and beech, are able to form dense forests with essentially no tree reproduction except for their own species. However, all of these species are very sensitive to fire.

The oaks are intolerant to shade and hence are unable to reproduce under their own canopy. Oaks, however, are resistant to fire (in the case of bur oak, very resistant) and hence are able to reproduce in a fire-prone ecosystem. Although the table is an oversimplification, it can be used to predict the type of forest that will eventually evolve under different sorts of regimes. This table clearly explains why the oaks are the savanna trees par excellence.

The most fire-resistant of the oaks is the bur oak, Quercus macrocarpa, which is a preeminent tree of the oak savanna. The bark of the bur oak is very thick and unusually fire resistant.

Measuring the oak canopy with fish-eye lens photography A fish-eye lens takes a photograph that encompasses a complete 180 degrees, from the horizon to the azimuth. Such photographs can be used to measure the oak canopy at different locations on a site. Simple measurements on these photos with a grid system can be used to calculate the percent canopy, or more complex measurements with computer software can be used to obtain more precise readings. For oak savanna restoration, the complex measurements are not needed.

Typical fish-eye lens photograph of a bur oak savanna, with a canopy cover of about 50%. Although measurements of canopy cover with fish-eye lens photography are not necessary to assess the restoration value of a site, they do permit an objective assessment of the canopy.

Impact of Forestry and Agriculture

The early surveyors called oak savannas “second” or “third rate timber”, indicating that there was little wood suitable for commercial cutting. The oaks did provide wood for many uses by early settlers, however, such as material for log homes and barns, furniture, fence posts, and fire wood. However, without power equipment, cutting trees is a laborious occupation, so that much of the oaks, especially the larger trees, were probably left alone.
The focus of early settlers was on producing food for their own needs, so that the land that could be cultivated without clearing was favored. Oak savannas, with their open-grown character, were, however, suitable for pasturage, so that almost all of the savannas in settled areas were grazed. The widely spaced oaks provided no difficulty for grazing animals, which generally benefitted from the shade they provided.

Savanna areas for grazing were burned every spring, in order to “green up” the pasturage. In many parts of the Midwest, burning of savannas by farmers continued well into the 20th century. The open character of these savannas is strikingly revealed by the air photography that was carried out by the U.S. Soil Conservation Service in the late 1930s. Open-grown trees (almost certainly oaks) stand out on all these air photos. Many of the oaks that can be seen on these old air photos still exist on the landscape today, especially in hill country where row-crop agriculture was not possible.

Oak savannas and forestry

Professional foresters, interested primarily in woodlands for commercial production, often do not look favorably on oak savannas, because silvicultural characters such as “stocking rate” and “reproduction rate” are low. From the forester’s point of view, the ideal oak forest is one sufficiently “stocked” so that it is able to “produce” at maximum rate. The term “basal area per acre” is often used to express the “quality” of a forest.

Landowners with potentially high quality oak savannas may be discouraged by professional foresters from managing their woodlots as savannas. It must be emphasized that oak savanna management and commercial logging are incompatible activities. This is unfortunate for the future of oak savanna restoration. Since there is no reasonable way that an oak savanna can be managed for a profit, many landowners may be discouraged from savanna restoration.

However, the aesthetic value of an oak savanna is vastly superior to that of a logged forest.

Nuzzo, V.A. 1986. Extent and status of Midwest oak savanna: presettlement and 1985. Natural Areas Journal 6: 6-36.

McPherson, Guy R. 1997. Ecology and Management of North American Savannas. University of Arizona Press, Tucson.

Jeanine M. Rhemtulla, David J. Mladenoff and Murray K. Clayton (2007). Regional land-cover conversion in the U.S. upper Midwest: magnitude of change and limited recovery (1850–1935–1993). Landscape Ecology Volume 22:

Anderson, Roger A., Fralish, James S. and Baskin, Jerry M. editors.1999. Savannas, Barrens, and Rock Outcrop Plant Communities of North America. Cambridge University Press.

McClaran, Mitchel P. and McPherson, Guy R. Oak savanna in the American Southwest. pp. 275-287 in Anderson et al. 1999.

Allen-Diaz, Barbara, Bartolome, James W., and McClaran, Mitchel P. California oak savannas. pp. 322-339 in Anderson et al. 1999..