Changing desert shrublands, past and present

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The unexpected science behind managing our natural resources
Publication date: 
Wed, 02/06/2013
Understanding how plants have responded to changes in climate in the past can help predict how they may respond to changes in the future.

By Karen Bagne and Deborah Finch

Blackbrush - the name may sound unfamiliar, but you would probably recognize this plant as part of iconic western landscapes in places like the Grand Canyon and Joshua Tree National Park, perhaps misnaming it as sagebrush. It may not grab your attention, but blackbrush or Coleogyne ramosissima is an important player in desert ecosystems, dominating the transition from warm to cold deserts of the western U.S. and providing forage and cover for bighorn sheep as well as seeds for a variety of birds and small mammals. It is also a key research subject, revealing how climate shapes the world around us.

National Forest and Grassland managers are tasked with protecting a diverse set of resources on public lands that have a wide range of responses to their environment. This task is becoming increasingly challenging as climate changes lead to stronger and more variable weather patterns. Understanding how valued species such as blackbrush have changed in the past and how they may change in the future can help guide managers as they make decisions. Two Rocky Mountain Research Station (RMRS) scientists in Provo, Utah, Bryce Richardson and Susan Meyer, examined blackbrush DNA and combined these findings with previous studies to create a picture of how blackbrush populations responded to warming following the last glacial period. Richardson notes, "the fundamental interaction between organisms, or organisms and their environment, is through their genes." Previous to this study, blackbrush had been thought by many to be a relict species, a species with little genetic diversity better adapted to past conditions, implying little adaptability to change. Instead, genetic analysis revealed high genetic diversity representing two distinct population groups, suggesting that blackbrush persisted through past climate change events during the Early Holocene and Late Pleistocene with large populations and a widespread geographic distribution.

Blackbrush shrublands occupy the transition area between the cold and warm-desert plant communities of the Great Basin and southwestern United States. Credit: R. Pendleton

Blackbrush was found to have two genetically distinct populations: one in the warmer Mojave Desert and the other in the cooler Colorado Plateau (approximate distribution shown in red). Credit: B. Richardson and S. Meyer, 2012.

A higher elevation blackbrush community showing mass flowering in Browse, Utah; evidence shows that blackbrush colonized these more northern areas after the last Ice Age. Credit: B. Pendleton.

At Los Lunas, New Mexico. Blackbrush from different regions are grown together to reveal differences that may help some plants cope with novel climate conditions. Credit: R. Pendleton.

In addition to the pattern of genetic variation, evidence for past distributions comes from ancient packrat middens where packrats have deposited debris that preserve local plant and animal communities. These middens contain a wealth of information and under the right conditions may preserve a record of the local environment going back more than 50,000 years. When middens are examined across a landscape, they reveal patterns of past distributions of plants and animals. Following the last ice age, blackbrush appears to have shifted northward and upward in elevation. Combined with the genetic evidence, the scientists concluded that the two blackbrush population groups likely diverged during this shift because the high elevation plateaus in southern Utah pose a barrier to gene flow and colonization.

Researchers Rosemary and Burt Pendleton of Albuquerque, New Mexico and Stan Kitchen of Provo, Utah, are also contributing to our understanding of ecological differences by using a "common garden" approach to examine blackbrush adaptations to local conditions such as winter temperatures and monsoonal rains. Common garden experiments typically take plants from different native environments and rear them in a common area, in this case providing information on how different populations of blackbrush might survive and grow under novel climatic conditions. Rosemary says, "Understanding how plants have responded to changes in climate in the past can help predict how they may respond to changes in the future". Their results showed a great deal of variation in blackbrush responses to weather conditions.

Altogether, much of this research shows that blackbrush’s genetic diversity may help this species adapt to future climate conditions. But as in the past, there are physical barriers that will limit dispersal. In addition, blackbrush establishes poorly following fire. Therefore, although blackbrush has successfully migrated in response to climate in the past, the expansion of exotic annual grasses that have led to an increase in wildfires will likely limit natural migration, and more active management may be required. Recently Rose, Burt, Susan Meyer and others evaluated how blackbrush could be successfully restored given problems with major fires and invasive plant species. They concluded that restoration focused at higher, and cooler, elevations would be advantageous as global temperatures warm.

Understanding the past helps scientists and managers predict where blackbrush will shift in the future and where natural migration is unlikely to occur, which will influence not only blackbrush, but animal species that depend on it as well. Knowledge of local adaptations and response to disturbance in the present can direct managers towards successful management actions such as establishing protected areas, collecting seed for restoration, and reducing fire frequency. Interactions among temperature, precipitation, genetic adaptations, topography, and disturbance illustrate the complexity of predicting species response to climate change, but also the value of science to successful management of natural resources.

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