Article featured on the website of the Native Plant Society of Texas
Posted on July 10, 2010 by Bill Ward, geologist and writer from Boerne
There are a couple of myths about “mountain cedar” or Ashe juniper (Juniperus ashei) that seem to be part of the conventional wisdom. One is that mountain cedar doesn’t really belong here because it only recently invaded the Hill Country. The other is that merely clearing away mountain cedar will bring spring flow.
For several reasons, both of these persistent ideas might seem plausible, but are there any data to verify they are true? As I used to try to convince my students, plausible assumption is not scientific fact.
Many people I’ve met around the Hill Country believe that most of the mountain cedar moved up from Mexico during the early Twentieth Century, and some people are not certain it should be considered a native tree. Certainly, during the Twentieth Century, mountain cedar did become more widespread over the Hill Country, because prairie fires were inhibited and grasslands were overgrazed. Now millions of acres of the Edwards Plateau are overgrown with dense cedar brakes, but is this the result of a relatively recent invasion of the species to this area?
In 1845, Ferdinand Lindheimer, Father of Texas Botany, wrote, “The cedars form wide strips of forest along the river banks” of the Comal River. In the same letter he described the hilly terrain north of the Comal as having bare hill tops, streams with a few kinds of trees, mostly elms, and “otherwise only cedars on the slopes of the hills and in low-lying places” (from “A Life Among Texas Flora” by M. A. Goyne).
The following year, referring to a geranium he had collected, Lindheimer wrote, “It grows in the hills, you see, on the plateau, which is here 200 feet high, full of ravines that are densely covered with cedars and underbrush,
and to which one has few ways of access along the slopes.” I know those sorts of “ravines” too well, having been over-heated and scratched-up trying to move through their cedar brakes. They are typical of the Hill Country today.
But what about before Lindheimer’s time? How long has mountain cedar been in the Hill Country? Robert P. Adams of Baylor University has done 30 years of research that involves that very subject. Dr. Adams is an authority on the phylogenetic (life tree) relationships of all living juniper species based on DNA sequencing, and he has studied both extinct and living varieties of mountain cedar (Juniperus ashei).
Adams has concluded that based on pollen profiles, J. ashei grew mixed with deciduous trees in Central Texas during the late Pleistocene (about 125,000 to 13,000 years ago). However, by the end of the last major glacial period 10,000-13,000 years ago, populations of mountain cedar in Central Texas probably were extinct because the area was too wet and cool. Most likely that variety of mountain cedar had been pushed into West Texas and Mexico, where in remains today.
After the Ice Age ended, a different variety of the mountain cedar species emerged from a remnant population and colonized Central Texas limestone outcrops. This variety was competitive in invading grasslands. The Hill Country has had the current variety of Ashe juniper for thousands of years.
There are several reasons people might hold mountain cedar in great disdain, especially landowners wanting to maintain their grassland and people who suffer cedar allergies. But many think the most important reason for wanting to rid the Hill Country of cedar is that they are water guzzlers. Does scientific research validate that plausible assumption? There are some interesting conclusions coming from recent studies. That will be the topic for another post.
Mountain Cedar – Water Guzzler or Not?
By Bill Ward
August 15, 2010
August 15, 2010
For years I’ve heard many people say, “David Bamberger cleared the cedar off his land, and his springs started flowing.”
Keep in mind, I don’t mean David Bamberger said that; it’s what other people keep saying. If all he did was clear the mountain cedar (Ashe juniper), the Bamberger Ranch in Blanco County would not be that award-winning showcase for ecology-friendly land management famous around the world.
Judicious cedar clearing is just a part of Bamberger’s land management. He also works hard to restore grasslands and change slope drainage to increase infiltration and decrease runoff, thereby enhancing the local recharge of groundwater. To see what David Bamberger really says about cedar and water availability, check out the interesting April 25, 2010 issue of Bamberger Ranch Journal.
For many people, the implication in “clear the cedar and start the springs” is that cedars are sucking up water that would otherwise filter down to recharge the aquifer and increase spring flow. Why don’t other common trees have the same effect as cedar? It always has puzzled me that the small needles of a seemingly xeric plant like Ashe juniper are transpiring more water into the atmosphere than are the broader leaves of larger trees. Apparently, that is a much too simplistic way to consider the matter.
According to research done by Keith Owens (A&M Experimental Station at Uvalde), there are two different issues to be considered with Ashe juniper: (1) loss of water by interception and evaporation and (2) loss of water by transpiration.
The scale-like juniper needles presumably capture rainfall and hold moisture in the needles. Supposedly juniper canopies can intercept 45% percent of rainfall, most of which can be lost through evaporation. Because so much of the Edwards Plateau is covered by Ashe juniper, this has important implications for hydrologists trying to estimate recharge based on rainfall. A smaller amount of water falling on junipers is funneled by stem flow to the root system. The contribution of the stem-flow water to groundwater recharge was not determined.
Dr. Owens, now at Oklahoma State, joined Dr. J.L. Heilman of Texas A&M and others in research focused on how water availability and aquifer recharge are affected by live oak and Ashe juniper encroachment into Edwards Plateau grasslands and savannas. This study that was published last year in the Journal of Hydrology.
The research was conducted on the Freedman Ranch near San Marcos during 2005 and 2006. Their conclusion was that the oak-juniper woodland relied
heavily on water from recent rains, rather than water from deeper zones of root penetration. This casts doubt on the assumption “that woody species on the Plateau withdraw substantial amounts of water stored deep within the fractured bedrock which otherwise would find its way into the aquifer.” They also found live oak used more water than juniper in that study area. Dr. Heilman has been quoted as saying the idea of brush removal to save water is a case of where policy gets ahead of science.
The year before that report by Heilman and his associates, C.A. Jones and L. Gregory published a Texas Water Resources Technical Report in which they considered the effects of brush management on water resources. One of their conclusions is that removal of brush such as juniper and live oak from upland areas some distance from streams may increase stream flow and/or recharge aquifers, especially where the brush is dense and reduces the amount of rainfall reaching the soil surface and where seeps and springs feed the area streams. However, where soils and rocks “are not conducive to increased runoff and/or subsurface flows to streams or to aquifers,” brush control is unlikely to significantly increase water yields.
Is the jury still out? It sounds to me as if one model is not going to fit all areas of the Hill Country. A lot depends on the local geology. Of course, more work needs to be done, but at least scientific studies are being conducted. One day we’ll know the truth about Ashe juniper’s thirst.