Published: Nov. 8, 2016 By

From my campsite on Smoky Mountain I could look over the Burning Hills to Fifty Mile Mountain and Navajo Mountain to the east, and I could see Lake Powell to the south.

It was a stunning vista that held my attention. But as the time passed, my attention turned to blackbrush, the most common shrub between my camp and the southern Utah's Burning Hills.

Systematists say that blackbrush, Coleogyne ramosissima, is in the rose family. This came as a surprise to me, for I admire the wide range of blooms born by roses.

Blackbrush drops its leaves to minimize water loss to desiccating desert winds. Photo by Jeff Mitton.

Blackbrush drops its leaves to minimize water loss to desiccating desert winds. Photo by Jeff Mitton.

In contrast, although a nearly monospecific stand of blackbrush is distinctive, I see not delicate beauty, but spare, tough, austere plants that create a stark and prickly landscape. Its flowers have no petals, but the small, yellow flowers are made of colorful sepals. Blackbrush owes its common name to its dark bark that turns black after rain.

Wearing shorts to bear the heat, I soon learned not to brush past them as I walked. Surely similar experiences taught cowboys to wear chaps while riding the range. Botanists describe them as spinescent, meaning that the shrubs have rigid branches and spines. A dense stand of blackbrush would be impenetrable.

Blackbrush can form nearly monospecific stands, or it might be a minor component of communities dominated by big sagebrush. It occupies the ecotones between warm and cold deserts in the Colorado River drainage, the Great Basin and southeastern California.

Arid environments place some specific stresses on plants, and foremost among these are the moisture stress of the long, hot, dry summer. Blackbrush minimizes its loss of water by dropping its leaves in summer, and biologists who have studied this conclude that dropping leaves is a response more to aridity than heat. Leaves appear in early spring, flowers are seen in April and May, and fruits mature in July. The photo was taken Aug. 2, when blackbrush were devoid of leaves.

Western deserts are riddled with holes, many of them occupied by little mammals looking for something to eat each day. Blackbrush is wind-pollinated, and it is obligately outcrossing.

Blackbrush in the Granite Mountains in California reach ages exceeding 1,250 years.

To insure pollination and to satiate seed predators, blackbrush has the strategy of masting. During mast years, it produces an abundance of flowers, pollen and fruits. Abundant pollen assures high success of wind pollination and the abundance of fruits overwhelms the appetites of the mammals — some seeds escape to germinate in the following spring.

During years between masts, pollen and fruits are quite rare. Masting requires reserves of energy and some development of flower buds; so mass flowering is triggered by opportune weather in previous years, but is not at all correlated with current-year weather. Some years, masting works satisfactorily, but a sudden drought can cause reproductive failure.

Species living in environments with stochastic fluctuations in weather frequently respond by developing exceptional longevity. Bristlecone and limber pine high in the mountains and Mojave yucca and creosote bushes in the Mojave Desert, all with longevities exceeding 1,000 years, come to mind.

Blackbrush in the Granite Mountains in California reach ages exceeding 1,250 years.

Blackbrush deter herbivores such as mule deer and desert bighorn with thorns and prickly branches, but it also has chemical defenses, and foremost among these are tannins.

A masting species living in the desert must be frugal but sensible with its chemical defenses. Leaves have moderate levels of tannins, but they only last a few months before being jettisoned. So it makes sense that the twigs have higher tannin concentrations, for they must last much longer and therefore deserve greater protection. Furthermore, twigs at the periphery of the shrub have more tannins than do interior twigs, which may be out of the reach of herbivores.

Deposition of tannins has clearly been shaped by natural selection driven by long-term pressures from persistent herbivores.