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Fire in the

Cross Timbers

An Ecoregion of Oak Savannas, Woodlands, & Forests

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The Cross Timbers

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Most Notable Cross Timbers Species

Post Oak

Quercus stellata

If you have samples of post oak leaves, etc., have students pass them around.

The post oak is included in any description of Cross Timbers flora as the most prevalent tree species. The post oak is a member of the “white oak” group of trees. Click Oaks in this group have leaves that are “lobed” Click without “bristle-tips.” Post oak leaves are sometimes described as having a “cross-like” shape. Some attribute the “Cross Timbers” name to this. Click Others think the name comes from the typical manner in which post oak branches grow in right angles from the trunk Click Click often giving the tree itself a cross-like appearance. In some areas, post oaks are referred to as cross oaks. Since no one knows for sure how the “Cross Timbers” name originated, there are quite a few other explanations as to how the name came about! Make a quick sketch of a post oak leaf in the box labelled “Post Oak” on your worksheet. Give students time to draw their leaves. Click

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Most Notable Cross Timbers Species

Blackjack Oak

Quercus marilandica

If you have samples of blackjack oak leaves, etc., have students pass them around.

Blackjack oaks are usually the second most abundant species of tree in the Cross Timbers. Click The blackjack oak is a “red oak.” Red oaks have bristle-tipped leaves. Click This is obvious in the strangely shaped blackjack oak leaves which lack the typical lobes found on most oaks. The origin of the “blackjack” name is as mysterious as “post oak.” Perhaps a source of the name is the tree’s deeply fissured bark Click with rectangular or squarish blocks that is very dark, nearly black. Many early explorers that passed through the Cross Timbers even thought the blackness of the bark indicated that blackjack oaks had survived fires. Make a quick sketch of a blackjack oak leaf in the box labelled “Blackjack Oak” on your worksheet. Give students time to draw their leaves. Click

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Where are the Cross Timbers found?

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Where are the Cross Timbers found?

Let’s try to narrow down this ecosystem’s distribution. Using this “satellite view,” draw a boundary around where you think the Cross Timbers are found. Keep in mind how ecosystems/biomes change across these three states. The satellite image shows biome variations and transitions that might be helpful. Since trees are an important component of the Cross Timbers, knowing where trees can be found could be helpful. Feel free to use what you already know about biomes! Be sure to respond to the prompts beneath the map concerning the evidence and reasoning you used. After students have a minute to come up with their range maps, have them compare with their colleagues’. Now, take a moment to compare your map with your neighbors’. How are your maps matching up? Now let’s see how well your maps compare with those of experts. Click

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Where are the Cross Timbers found?

This map is a portion of the “Level III Ecoregions of the Continental United States” map produced by the US Environmental Protection Agency. Click Ecoregion “29” is the Cross Timbers. Click This next map is a product of research conducted by The Ancient Cross Timbers Consortium. If you take a moment, you can see the similarities in the boundaries of both maps. How do these maps compare with yours? The Cross Timbers extend from southeastern Kansas, through eastern Oklahoma, and on to north central Texas. They are a transition from the Eastern Temperate Forests to the Great Plains. If adjustments need to be made to your map, use a differently colored pen/pencil to draw the “corrected” Cross Timbers boundary. Make a key for your map that describes your initial and corrected ranges. Give students time to correct their maps and to make an appropriate key. Now we’re going to take a look at maps showing two abiotic factors that influence the distribution of biomes, temperature and precipitation. Click

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Factors Influencing Distribution

Average Annual Temperature

As we look at the maps, refer to your Cross Timbers distribution map to see if there are any apparent relationships between abiotic factors and where the Cross Timbers are located. Click Here’s the first map. With what you know about the biomes in Kansas, Oklahoma, and Texas, is this map showing the trend for temperature or the trend for precipitation? Temperature. And how do the colors used support your answer? “Cool” and “warm” colors. Click This map shows average annual temperature. In the small map at the bottom of page 1, reproduce this map showing the trend (or gradient) for temperature. Give students time to draw their maps. Click This second map is a reminder of where the Cross Timbers is found. Next to the map you just drew, describe the “trend” for temperature in this region and how this trend could affect the distribution of the Cross Timbers. Give students time to complete the table. Would anyone like to share their trend description for temperature and impacts on the Cross Timbers? Average annual temperature increases when going from north to south or decreases when going from south to north. Warmer temperatures in the south and cooler temperatures in the north could be restricting the Cross Timbers from southern Kansas to central Texas. There doesn’t appear to be a similar east-west relationship. Let’s move on to precipitation. Click

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Factors Influencing Distribution

Average Annual Precipitation

Before the precipitation map is revealed, would anyone like to share what they think the trend will be and what colors might be used to represent the range of precipitation values? Give students an opportunity to share some of their ideas. Click Click Which color represents drier conditions and which one represents wetter conditions. Brown/tan colors show drier areas than those represented by shades of green. In the map at the top of page 2, reproduce this precipitation map showing the trend (or gradient). Once again, after “Abiotic Factor” to the right of your map, title your map by writing “Precipitation.” Give students time to draw their maps. Click In the table next to your precipitation map, describe the “trend” for precipitation in this region and how this trend could affect the distribution of the Cross Timbers. Give students time to complete the table. Would anyone like to share their responses? Average annual precipitation increases when going from west to east or decreases when going from east to west. The Cross Timbers appear to occur in the “whitish” transition zone between the drier west and wetter east. However, the Cross Timbers do not continue along the “whitish” zone deeper into Kansas or Texas. Click

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Factors Influencing Distribution

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Cross Timbers - Forest

Here’s an area of forest in the Cross Timbers. But what’s the difference between this and a woodland? Ecologists have ways of distinguishing forests from woodlands. One approach is to determine the canopy cover. A forest ecosystem has a canopy cover of 80-100%. To estimate percent canopy cover, ecologists have developed a variety of tools. Click A classic tool is the densiometer which uses a spherical mirror to quickly estimate percent canopy cover. Click A more modern approach is a digital camera equipped with a fisheye lens combined with Click computer software which can help reduce human error and/or bias. Click You can imagine the results from either to look something like this. So, what do you think the term “canopy cover” means? Give students a moment to attempt to define the term. Canopy cover can be defined as the proportion of the forest floor covered by the vertical projection of the tree crowns. So looking at the spherical image, can you visualize that the canopy would cover 80-100% of the ground when projected vertically downward? Reproduce this spherical image on your student worksheet in the “Variation in the Cross Timbers” section. Include “Forest” as the name, “80-100%” as the % Canopy, and, for the Description, include a few of this forest’s characteristics as seen in the images. Don’t worry about “Fire Frequency” in the box. This will be covered later. Click Here’s an example to help with your first entries. Give students time to complete the first box in the “Variation in the Cross Timbers” on their worksheet. Now let’s move on to a woodland. Click

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Cross Timbers - Woodland

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Cross Timbers - Savanna

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Cross Timbers - Prairie

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Disturbance in the Cross Timbers

Woodland

Forest

Woodland or

Savanna

In this aerial view, distinct Cross Timbers systems are separated by roads. What type of system (forest, woodland, savanna, prairie) is found in area “1”? Click The canopy cover in area “1” appears to be very dense, so this would be categorized as a forest. How about areas “2” and “3”? Click Depending upon where the observation is made, area “2” could probably be considered a woodland or savanna, and Click area “3” would probably be called a woodland. Since these three areas are found so close together, they receive the same precipitation and temperature variations. Therefore, other factors must be causing their differences. We’re now going to take a look at a factor that contributes to such variation within the Cross Timbers. This factor is a naturally occurring disturbance. Click

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Fire in the Cross Timbers

Fire. The presence of this naturally occurring disturbance insures a variety of plant species which results in a greater overall biodiversity in the Cross Timbers. Click This is an example of a fire passing through a Cross Timber woodland during the dormant or non-growing season. Click Here’s the aftermath of a growing season fire. Notice that not all of the ground vegetation is burned and that the majority of larger trees are not damaged. The disturbance or impact resulting from fires such as these maintain open areas within the Cross Timbers where grasses and forbs can thrive. This broadens the types of habitats available. For this very reason, the fires pictured were actually set on purpose. Click Prescribed fires have specific management goals and are only conducted under “prescribed” weather conditions with adequate personnel and equipment to reduce the chance of impacts outside of the burn unit. Click

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Fire in the Cross Timbers

Every

Year

Every

2 Years

Every

3 Years

Never

Burned

Fire frequency can result in significant differences in dominant vegetation types. Click If an area has fire pass over it every year, Click woody vegetation may be reduced to such an extent that it’s unnoticeable. Most native vegetation in the Cross Timbers, such as the post oak, Click is fire tolerant. Fire tolerant species are able to resprout, or grow back, from undamaged belowground biomass. Click Fire every two years Click usually allows some woody vegetation to survive and, perhaps, grow into larger trees. Click When fire occurs every three years, Click woodlands are the dominant ecosystem. Click In most parts of the Cross Timbers, when fire has a frequency greater than 3 years or does not occur at all, Click forests will take over as trees begin to exclude other vegetation. Return to the “Variation in the Cross Timbers” section of the worksheet. In each of the systems’ descriptions, include the fire frequency usually associated with each. Give students time to include the fire frequency for each ecosystem category. Click

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Fire in the Cross Timbers

If you have samples of eastern redcedar “leaves”, berries, branches, bark, etc., have students pass them around.

Lack of fire or suppression of fire could result in the transition from an oak system to one dominated by eastern redcedar. This is a fire intolerant native species in the Cross Timbers. Even though it is a native species, eastern redcedar is often considered to be invasive. An invasive species causes, or has the potential to cause, significant changes in ecosystem composition, structure, or function. Fire suppression has resulted in the expansion of eastern redcedar in the Cross Timbers. Where eastern redcedar dominates, species diversity is commonly low. Eastern redcedar does not survive on sites subject to frequent fire because it reproduces solely by seed; there is no “resprouting” like most other native trees. Eastern redcedar is susceptible to fire kill because of its thin bark, shallow roots, inability to resprout, and highly combustible evergreen foliage. Click Lack of frequent fire also permits fuels to accumulate which could generate more severe wildfires. Click

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Fire in the Cross Timbers

1. What type of Cross Timbers system(s) would you expect to encounter if an area were burned every 1-2 years? Why?

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Fire in the Cross Timbers

2. What type of Cross Timbers system(s) would you expect to encounter if an area were burned every 3+ years? Why?

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Fire in the Cross Timbers

3. Why is fire such an important disturbance in the Cross Timbers?