Texas Naturalist’s Notes By Bill Reiner

As the leaves of most trees and shrubs native to central Texas turn colors and then drop away, two tree species, dominant on the landscape west of the interstate highway, stay green. Plateau Live Oaks are cherished for their gnarled shapes, their longevity, and their shade. Ashe Junipers, also known as “mountain cedars,” well… not so much.

One might think that a tree tough enough to grow on the thin, rocky soils of the Edwards Plateau, cloaking them in green through the gray winter days, would be prized—particularly in this season when evergreen trees are celebrated. Ashe Junipers are the only evergreen conifers common in Austin and its western environs. (To the east of town live their cousins, the Eastern Red-Cedars, and the “lost” Loblolly Pines that remain around Bastrop.) They are the closest thing we have to a native Christmas tree. They also provide food and shelter for many species of wildlife.

But many perceptions of Ashe Junipers—some true, some half-true, some false—conspire to make this tree reviled or at least misunderstood by many.
First, “mountain cedar” is not a true cedar. Like many other plants and animals that Europeans met when they came to the Americas, it was named for its likeness to the cedars of the Eastern Hemisphere. It is, like Eastern Red-Cedar and many cousins in the western United States, a juniper: Juniperus ashei to be precise. But just as the vultures, robins, orioles, and blackbirds of the Americas, who are not related to their namesakes in Europe, junipers will likely continue to be called “cedars” by many people.

Junipers are wind-pollinated, producing pollen in tiny cone-like structures at the tips of twigs of staminate (“male”) trees. The abundance of these pollen cones in mid-winter, when the pollen is dispersing, gives these trees a rusty color. Some of the pollen is blown onto pistillate (“female”) flowers on separate trees; these are the trees that then produce the blue, berry-like seed cones that are favorites of Cedar Waxwings and American Robins. Some people are allergic to the pollen that can produce a haze at times in mid-winter, and that certainly contributes to antipathy toward junipers. However, oaks and grasses also spew copious amounts of allergenic pollen, so that cannot be the sole reason for the scorn some people have for this tree.

Ashe Juniper (Juniperus ashei) cones. Cortaña tract, Balcones Canyonlands Preserve, 29 December 2014

Ripe fruit on a pistillate (female) Ashe Juniper.

One of the most pernicious myths about Ashe Junipers is that they are not native trees. Nothing could be further from the truth, and the myth ignores clear historical facts. Early accounts of the landscape—those that can be pegged to the eastern edge of the Edwards Plateau, from Austin to San Antonio—routinely refer to “cedar-covered” slopes.

The forester William Bray, writing in 1904 the earliest broad description of the timber of the Edwards Plateau, goes into great detail on the different types of forest. “Cedar” he calls “the most abundant species of all.” “The writer knows of no region in which any species of cedar is so uniformly abundant and dominant as is the mountain cedar in the limestone country of Texas.”

The geologist Ferdinand Roemer, visiting New Braunfels and other new German colonies in the 1840s, remarked: “The cedars here are not the stunted shrub-like plants found in the Northern States of the Union, but are stately trees with straight trunks, seldom more than twenty to twenty-five feet in height and one and one-half feet thick.”

Recent studies of Ashe Juniper growth rates found that the trees’ trunks grow very slowly, a rate of approximately 0.6 to 1.0 inch of diameter per decade. At that rate, the trees reported by Roemer ranged from 120 to 300 years old—in the 1840s. Clearly, they were not introduced by Europeans.

One historical account that has been cited to support the idea of vast prairies covering the Hill Country is taken out of context. Frederick Law Olmsted, in his Journey Through Texas, writes “after spending a pleasant week in Austin, we crossed the Colorado, into, distinctively, Western Texas.” Then, in a section entitled “The Western Prairies,” he goes on: “We were, in fact, just entering a vast region, of which live-oak prairies are the characteristic…. The live-oak is almost the only tree away from the river bottoms….”

Only six pages later, however, he writes the following, shortly after his party passed San Marcos on their way to San Antonio: “We pitched our tent at night in a live-oak grove, by the side of a deep pure spring, at the mouth of a wooded ravine closed by rugged hills toward the north. Behind us were the continuous wooded heights, with a thick screen of cedars; before us, very beautiful prairies, rolling off far to the southward….”

Olmsted was following the edge of the Blackland Prairie, a broad swath of grassland that once ran from Dallas to San Antonio. Interstate 35 traces its general course. As he traveled “west”—actually southwest, toward San Antonio—he was following this great sweep of prairie, keeping the “wooded heights,” the Balcones Escarpment, to his right. Such descriptions are common among the first-person narratives of the time—though sometimes mangled in re-tellings and reminiscences.

A major occupation in the first century of Anglo settlement in the Hill Country was that of “cedar chopper.” Entire communities were comprised of people who eked out a living harvesting these trees, which were often used for firewood or building materials. Juniper was a significant source for railroad ties during the great railroad-building era of the late 1800s. Because juniper wood is resistant to decay, smaller trees were used for fence posts—many of which still support fences today.

As a result of this lumbering boom, few “virgin” stands of Ashe Juniper survive. Much of the land was stripped of trees, leaving, in places, a more open, savanna-like landscape that some now believe was the pre-settlement appearance of all of central Texas. This partly accounts for the “invasive” epithet that is often thrown at the tree. In some places on the Edwards Plateau, junipers are encroaching upon what were once prairies or savannas, but in other areas, they are simply reclaiming land from which cedar choppers removed them.

Ashe Juniper is a pioneer (or “invasive”) species, that can grow up in grasslands where overgrazing has thinned the sod and thereby reduced fuel for wildfires. Cattle and other grazers will not eat it, and it is not a favorite food for browsers such as deer. In time, the junipers will shade out the grasses and form a continuous “cedar brake”—the first stage in natural succession that, over many decades, may result in a mature mixed forest. Whether, and how, the cedar brake diversifies depends upon several other conditions, such as water availability and soil depth.

Ashe Junipers are often reviled as “water hogs.” Like all trees, they do take up water. Some studies show junipers taking up more—especially isolated trees—but other studies show them capturing less than other trees, in particular when they grow together to form a closed-canopy forest. One myth claims that they suck water from aquifers, but their roots rarely go deep enough to tap into groundwater. They have also been accused of intercepting a small proportion of rain in their foliage, never allowing it to reach the ground.

This last claim is true, but, rather than a mark against the trees, is more often than not beneficial. Very little of our annual rainfall here in “Flash Flood Alley” comes in small increments. Most arrives in downpours of one, two, five, eight, or even 15 inches at a time. In those events, slowing and holding the water is most important. More water may be lost through unchecked runoff from heavy rains—especially on steep slopes—than from rainfall trapped by juniper foliage and evaporated back to the atmosphere.

An umbrella of juniper foliage may catch the first quarter-inch of rain, but it also slows the velocity of drops striking the ground, reducing their erosive potential. Juniper foliage and branches tend to funnel rain down the main trunk rather than dispersing it to the “drip line” of the outer canopy as in most broad-leaved trees. Water dripping into the interior of the tree’s canopy is then more likely to be absorbed by the spongy mat of fallen needles (or duff) at its base.

The quantity of water may overwhelm the duff’s ability to absorb it, especially under and around small trees that have not accumulated much of a mat. Even then, though, as water starts to run over the surface, the fallen juniper foliage tends to form tiny check dams that slow the overland flow. A small amount of the sediment carried by the flowing water may then settle out. In future rains, that check dam and its previously-caught sediment can retain still more water—and also provide a medium in which seeds can grow. Slowly, over years, a slope of eroded soil—or even bare, broken rock—can be colonized.
Juniper cover and the organic matter in the trees’ litter also enhance and protect the structure of the soil. The clay soils common in the Hill Country, when pounded by direct impact of raindrops, tend to form crusts that impede infiltration of future rains. Juniper duff not only buffers the direct impact of raindrops, it also adds organic matter that helps maintain larger soil pores and prevents crusting.

All these under appreciated benefits of junipers may account for the finding of a recent study that challenges conventional wisdom. Many land managers had assumed that juniper encroachment into former grasslands would cause springs to dry up and streams to stop flowing. But Wilcox and Huang, reviewing streamflows in four Edwards Plateau watersheds that had been degraded through overgrazing, then “invaded” by junipers, found that streamflow actually increased as the trees filled in. The contribution of baseflow—the portion of streamflow contributed by groundwater through springs and seeps—nearly doubled in that time.

William Bray, back in 1904, would not have been surprised. He describes the result of a storm in two small, adjacent watersheds near what is now Wild Basin: one “almost denuded” at its head, the other heavily timbered with “an almost impenetrable growth of cedar and mixed timber….” In the first, “the water at once begins to pour down as from a steep roof, converging into swift streamlets which erode every vestige of organic soil….” In the other, “the downpour… is largely taken up by the porous ground.”

In some locations a healthy prairie may allow for more infiltration with little water lost to runoff. But on the steep, rocky slopes of the Balcones Escarpment, Ashe Junipers may be better water managers than we think they are.

References for this column included The Timber of the Edwards Plateau of Texas; Its Relation to Climate, Water Supply, and Soil (U.S. Dept. of Agriculture, Bureau of Forestry, bulletin no. 49) by William L. Bray, “Energy balance and water use in a subtropical karst woodland on the Edwards Plateau, Texas,” by J. L. Heilman et al, in Journal of Hydrology, and “Woody plant encroachment paradox: Rivers rebound as degraded grasslands convert to woodlands,” in Geophysical Research Letters, by Bradford P. Wilcox and Yun Huang.