Shrub Steppe of the Columbia Basin: Our Natural Heritage

I was recently invited to put together a presentation about the biodiversity of the Columbia Basin for a regional conference about shrub steppe. Unfortunately, the conference was canceled. Rather than let the presentation sit idle on my computer, I thought I’d post it here as it seemed to be a relevant sequel to my previous post about shrub steppe. So, here it is…

The Columbia Basin’s shrub steppe contributes important and unique diversity to our regional natural heritage. In this presentation, I hope to inspire you with the subtle beauty and diversity of this landscape and highlight some of the conservation issues associated with preserving it for future generations.

I will first define what our ‘natural heritage’ is, discuss the concept of shrub steppe as this term can be used in a very broad or very specific manner, and then will provide a quick tour of the variety of ecological systems found in the Columbia Basin. Finally, I will conclude with a few slides about some of conservation issues surrounding shrub steppe.

Our Natural heritage consists of the diversity of natural features occurring in Washington State. The Washington Natural Heritage Program focuses on Washington’s biodiversity and this element of our natural heritage is what I will focus on in this presentation.

Biodiversity can be described as the genetic, species, and ecosystem diversity that results from the unique combinations of climate, geology, soils, and biological interactions that occur on the landscape.

The Washington Natural Heritage Program takes a fine filter/coarse filter approach to conserving biodiversity. The assumption is that if we focus on protecting rare species (fine filter) and rare and high-quality examples of common ecosystems (coarse filter) we’ll be able to effectively conserve Washington’s full suite of biodiversity. To achieve this goal, our primary objectives are to inventory the locations of these conservation targets, maintain a database of those locations, and provide and utilize those data to affect conservation actions.

Photo Credit: Rex Crawford, Washington Natural Heritage Program

What is shrub steppe? Depending on your perspective, shrub steppe can be defined numerous ways. Some use shrub steppe to describe a landscape. For example, the Columbia Plateau Ecoregion and Columbia Basin (a physiographic region within the Columbia Plateau Ecoregion) are often referred to as shrub steppe. From a rancher’s perspective, shrub steppe may be just another name for a rangeland--a place where livestock can thrive. Ecologists, always in search of technical terms, might choose to use standardized classification names such as “Inter-Mountain Basins Big Sagebrush Steppe Ecological System” when referencing shrub steppe. Generally speaking, shrub steppe is a grassland with shrubs.

When we talk about shrub steppe as a specific ecosystem, we are referring to those sites where shrubs (typically sagebrush) occur over a layer of bunchgrasses such as bluebunch wheatgrass and Idaho fescue. Historically, it is thought that, on average, shrub cover was probably around 10% as periodic fires kept sagebrush from becoming more dense. However, fire suppression and the introduction of grazing is though to have increased shrub cover in many areas.

We can also refer to shrub steppe as a landscape unit…in our case, the Columbia Plateau Ecoregion. A dry climate in this ecoregion has resulted in the dominance of non-forested vegetation. Most of this vegetation is comprised of the specific shrub steppe ecosystem I just described; however, there is a great deal of variation associated with shrub steppe as there are a host of other small habitats embedded within this matrix of sagebrush. 

In addition, shrub steppe is not limited to the Columbia Plateau and is actually a major vegetation type throughout the Intermountain West. But, here we’ll focus on the Columbia Plateau or, when possible, even more specifically on the Columbia Basin which is mostly limited to Washington and small portions of Idaho and Oregon.

For the moment, let us focus on shrub steppe as a landscape…the Columbia Plateau Ecoregion. Despite what may appear to some as a monotonous landscape, there is much diversity in the Ecoregion. There at least 239 plant and animal species in this area which are considered to be globally vulnerable—in other words, they are threatened with extinction. There are also 450 plant associations found throughout the Ecoregion. These plant associations reflect a myriad of habitats distributed across the region. 

The shrub steppe of the Columbia Basin can be categorized into different vegetation types, based on the dominant plants found in each area. These include:

• Shrub steppe, shown here in the green, which is mostly dominated by sagebrush and bunchgrasses such as bluebunch wheatgrass (Pseudoroegneria spicata) and Idaho fescue (Festuca idahoensis)
• Palouse prairie, shown here in the blue, which is dominated by Idaho fescue and bluebunch wheatgrass.
• Canyon grasslands, shown here in pink, which are dominated by bluebunch wheatgrass and Sandberg’s bluegrass (Poa secunda); and
• Klickitat meadow steppe, shown here in brown, which is similar to Palouse Prairie, but has slightly different species composition and environmental setting.
  
  

This slide shows an elevation model of the Basin, with the various colors showing variation in elevation. Starting from near the center of the Basin near Richland, you can see that there is a gradual rise in elevation as you move the north and east. This rise in elevation, and distance away from the rain shadow cast by the Cascade Mountains, results in a gradual increase in moisture. Thus, although the Basin appears to be a monotonous swath of sagebrush (and orchards and wheat fields) there are very discrete changes relative to this moisture gradient. We can take the map I just showed in the previous slide and further divide the Basin’s vegetation types based on this moisture gradient.

Rexford Daubenmire, in his classic study of shrub steppe in the Columbia Basin, categorized the shrub steppe landscape into different vegetation zones, based on the dominant plants found in each area. These zones essentially represent the moisture gradient I just discussed.

So, what I previously described as shrub steppe (shown here in the green, orange, and pink) can be subdivided into even finer vegetation zones, each representing slightly different ecological conditions. For example, in the center of the basin, which is the hottest and driest portion, Basin Big Sagebrush (Artemisia tridentata subsp. tridentata), Wyoming big sagebrush (Artemisia tridentata subsp. wyomingensis), and bluebunch wheatgrass are the dominant species. As one moves further north and east, Idaho fescue increases in dominance resulting in a Big Sagebrush/fescue zone (orange) and in even moister environments, a three-tip sagebrush (Artemisia tripartita)/fescue zone. 

The Palouse Prairie can be split up in a similar manner. The western portion of the Palouse is slightly drier and consists of an open, bluebunch wheatgrass-fescue grassland (or steppe). As you move east, the grassland becomes closed (very dense) and is dominated by fescue, rose, snowberry, and a high diversity of forbs. 

In summary, the shrub steppe, as a landscape, is much more diverse than might first come to mind. In fact, the diversity of ecological types is much greater than shown here as even this map is a broad explanation of the diversity of habitats found in the Basin.

Photo Credit: Rex Crawford (bottom three), Washington Natural Heritage Program

Ecologist can focus ecological classification at a variety of spatial scales. For example, the previous few slides classified the shrub steppe landscape at a very coarse scale (e.g. from a basin-wide perspective). We could also zoom in much finer and divide the landscape into much smaller, yet very discrete units. This slide shows different spatial scales (vertical gradient) at which the Washington Natural Heritage Program classifies ecological communities. Although we mostly use a vegetation-based classification system, in this presentation I will only focus on what we call an Ecological System classification, which was developed by NatureServe. Ecological Systems are essentially repeatable patterns of species which occur in similar environments. In other words, this classification scheme considers both vegetation and environmental factors such as soils, climate, geology, and disturbance regimes. This slide shows the different scales at which we can split up the Shrub Steppe landscape into various ecological types or habitats.

The vegetation zones I showed previously are similar to what the Ecological System classification refers to as a Matrix Ecological System. So, the shrub steppe shown in the light green in the previous slide is what we would call the Inter-mountain Basins Big Sagebrush Steppe. Other Ecological Systems occur at smaller spatial scales such as the Columbia Plateau scablands, riparian areas or vernal pools. 

This is a list of all the Ecological Systems types which occur in the Columbia Basin. As you can see, the Basin isn’t just “sagebrush”; it supports a large variety of ecological systems. For the next few slides, I am going to give you a quick photographic tour of many of these systems (as well as some rare species which they support) in order to provide a sense of the diversity of our natural heritage in the Columbia Basin.

We’ll start with the ecological systems which characterize what we think of “shrub steppe” habitat. In other words, these reflect “shrub steppe” as a specific ecosystem.

The Inter-Mountain Basins Big Sagebrush Steppe Ecological System is one of the matrix types in the Basin. It covers a vast area of the Columbia Plateau. This system is characterized by Basin and Wyoming Big Sagebrush with an understory dominated by bluebunch wheatgrass. This is an example from Washington Department of Natural Resource’s Two Steppe Natural Area Preserve.

This is the Columbia Plateau Low Sagebrush Steppe Ecological System. It is typically found on mountain ridges and flanks on shallow soils. Low sagebrush (Artemisia arbuscula) is dominant along with Idaho fescue, bluebunch wheatgrass, and Sandberg’s bluegrass. Within the Columbia Basin, this type is mostly limited to the southwest portion in the Yakima Folds area.

The Columbia Plateau Scabland Shrubland Ecological System is locally referred to as scabland or lithosol communities. These sites have very shallow soils with exposed rock and gravel being very common. Common species include stiff sage (Artemisia rigida), along with numerous dwarf-shrub buckwheats (Eriogonum ssp.), balsamroot (Balsamorhiza ssp.), and Sandberg’s bluegrass. These small patch sites are very common in the Basin. 

At higher elevations, within the forested zones, mountain sagebrush (Artemisia tridentata subsp. vaseyana) can dominate open flats, ridges, and slopes. Higher moisture at this elevation can result in a species rich understory of both grasses and forbs. This system is mostly found outside the Columbia Plateau Ecoregion, but represents a high-elevation expression of shrub steppe.

The following couple of slides show a few of the rare plants found in these shrub steppe habitats. As a matter of fact, there are nearly 50 rare plants which occur in these shrub steppe and grassland habitats.

This particular plant is Piper’s daisy and is considered to be globally vulnerable. It is listed by Washington Natural Heritage Program as a State Sensitive Species, is endemic to the Columbia Basin, and is mostly found in the Inter-Mountain Basins Big Sagebrush Steppe Ecological System.

Photo Credit: John Gamon, Washington Natural Heritage Program

The Columbia milk-vetch is found in a variety of shrub steppe habitats (shrub steppe, lithosols, etc.) but is restricted to a 5 X 25 mile area along the western side of the Columbia River in Yakima, Kittitas, and Benton counties. It is considered to be a State Threatened species, is on the U.S. Fish & Wildlife Service’s Species of Concern list and occurs in a variety of shrub steppe habitats. 

Photo Credits: Washington Department of Fish and Wildlife

These are a few of the sagebrush-obligate vertebrate species that are struggling due to the loss of shrub steppe habitat. There are many other species that have been impacted by this habitat loss but I’m just going to mention a few things about these critters. 

The Sage Grouse has experienced a large decline due to conversion of shrub steppe to agriculture and degradation of remaining habitat and fragmentation. The Washington Department of Fish and Wildlife estimates that there are less than 1,000 grouse remaining in Washington, occupying only 8-10% of historic habitat. The Sage Grouse is considered a State Threatened species. 

The Sage Sparrow commonly nests within or beneath sagebrush and is closely tied to shrub steppe habitats. It is considered a Candidate for State listing as it is also sensitive to many of the same threats experienced by the Sage Grouse.

The Pygmy rabbit is a Federal and State Endangered species and as of 2003, there was less than 30 of these critters left in Washington. They typically are found in dense stands of big sagebrush growing in deep, loose soils. The deep soils are critical to them being able to burrow for cover and possibly nesting. 

The next group of ecological systems we’ll explore is the Palouse Prairie and related grasslands found in the Basin.

As its name suggests, the Columbia Basin Palouse Prairie occurs in the Palouse region of southeast Washington and adjacent areas in Idaho. Historically, it was characterized by rolling topography composed of loess hills and plains over basalt bedrock. The Palouse prairie is located along the cooler and moister eastern rim of the Basin, getting anywhere from 15-30 inches of rain/year. Species diversity is relatively high due to having more moisture and slightly cooler climate than other parts of the Basin. The Palouse prairie was once an extensive grassland that has been almost completely lost from the landscape. Most remnants of this grassland are located on steep and rocky sites or are small and isolated sites that do not function at the same scale this grassland once did. 

Photo Credit: Washington Natural Heritage Program

This is the Columbia Basin Foothill and Canyon Dry Grassland Ecological System. It is mostly found in the canyons and steep slopes of the Snake River drainage in southeast Washington. Bluebunch wheatgrass and Sandberg’s bluegrass are common, especially on south-facing slopes while Bluebunch wheatgrass and Idaho fescue are also common, especially on the north slopes. These grasslands differ from others in that they are located on different landforms, have shallow soils, and have lower moisture and thus lower species diversity than grasslands such as the Palouse Prairie. 

Outside of the Palouse region, yet within cooler and moisture portions of the Basin, the Northern Rocky Mountain Lower Montane, Foothill, and Valley Grassland can be found. This particular example is a site in the northern portion of the Basin where three-tip sagebrush, Idaho fescue, and a rich diversity of herbaceous species dominate. 

Photo Credit: Chris Chappell, Washington Natural Heritage Program

Scattered mostly throughout the central part of the Columbia Basin, are Columbia Plateau Steppe and Grasslands. These extensive grasslands occur on sites which experience frequent fire or a fire of sufficient size which precludes sagebrush from gaining a foothold. In other words, these grasslands are maintained by fire. This grassland type is probably most abundant near the Hanford area.

Photo Credit: John Gamon, Washington Natural Heritage Program

The following couple of slides show a few of the rare plants found in these grassland habitats. As mentioned previously, there are nearly 50 rare plants which occur in these shrub steppe and grassland habitats. Thus, this is just a small taste of the rare plants which occur in these habitats.

The first species is the Palouse Goldenweed. The Washington Natural Heritage Program considers is to be State Threatened and U.S. Fish and Wildlife Service consider it to be a Species of Concern. This plant is endemic to the Palouse prairie. 

Photo Credit: John Gamon & Reid Schuller, Washington Natural Heritage Program

Jessica’s aster is a State Endangered species and considered a Species of Concern by U.S. Fish and Wildlife Service. It is also endemic to the Palouse region and occurs in Palouse prairie and forest/grassland transition areas. 

One of the more interesting critters unique to the Palouse prairie is the Giant Palouse earthworm. Little is known about the Giant Palouse earthworm. But, it is believed to grow up to 1 m (~3 ft) in length. However, contemporary specimens have only been observed up to about half that length. As with many of the other species in the Basin, it suffers from habitat destruction. Since the original sighting of this species in 1897, the worm has rarely been seen, with the most recent observation being in 2005 when a Washington State University graduate student found one near Palouse, Washington.

Tucked inside the matrix of Columbia Basin shrub steppe and grasslands are numerous smaller habitats such as wetlands, riparian areas, vernal pools, cliffs, and sand dunes. Although small in size, these habitats are an important part of the Basin’s natural heritage. The following couple of slides introduce just a few of these types. 

Photo Credit: Rex Crawford, Washington Natural Heritage Program

The Columbia Basin Foothill Riparian Woodland and Shrubland is found below lower treeline and mostly along the Columbia River and along some of its major tributaries. Cottonwood (Populus trichocarpa), hawthorn (Craetaegus ssp.), alder (Alnus ssp.), and willows (Salix ssp.) are common dominate species. Water management (diversions, irrigation, dams, etc.) and heavy grazing have both reduced the extent of this type and degraded remaining examples. The structural diversity of the vegetation provides important habitat for a variety of creatures. 

Scattered through the Basin, but concentrated in the northern and eastern section, are small, seasonally wet depressions called Columbia Plateau Vernal Pools. Rain and snowmelt fill the pools in winter and spring, and impervious underlying basalt bedrock or clay keeps the pools wet until mid-summer when they completely dry out, as shown here. This unique hydrological regime results in a specialized environment where species specifically adapted to the ephemeral nature of these wetlands thrive. Because of this harsh and unpredictable environment, 63% of the plants found in these wetlands are annual species. In addition, the unique nature of these pools supports about 12 rare plants.

These pools are very similar to those found in California which have received far more conservation attention than those in Washington. Research funded by The Nature Conservancy increased our understanding of the types of plants found in these wetlands but there is still much more work needed to be done in order to understand the biological importance of these pools.

Pothole Ponds Photo Credit: Rex Crawford, Washington Natural Heritage Program

Numerous other wetland types occur in the Basin and all provide important habitat for wildlife. Wetland Ecological Systems such as the North American Arid Freshwater Marsh, Northern Columbia Plateau Basalt Pothole Ponds, and Inter-Mountain Basins Playa & Alkaline Closed Depressions can be found in the Basin.

Photo Credit: Washington Natural Heritage Program

The Inter-Mountain Basins Active and Stabilized Dunes Ecological System is found throughout the Basin although it has been reduced about 75% from its historical extent. The Washington Natural Heritage Program recently conducted a conservation assessment of sand dunes in the Basin and found that these sites support a unique array of rare species and plant communities and have become an important conservation target.

Another small but important habitat in the Basin are the Inter-Mountain Basins Cliffs and Canyons. This Ecological System can have sparse vegetation occurring on steep cliff faces and unstable scree and talus slopes, as shown here. Serviceberry (Amelanchier alnifolia), netleaf hackberry (Celtis reticulata), and sumac (Rhus glabra) are common shrubs found in these areas. Cliffs and canyons provide habitat for reptiles, birds, and other small critters.

Now that we have had an opportunity to enjoy a few photos and hopefully gain an appreciation of shrub steppe biodiversity, I thought I’d turn to the somber conservation issues facing the Columbian Basin’s shrub steppe. 

The contemporary distribution of shrub steppe vegetation types has changed drastically from its historical extent. Originally, over 10 million acres were covered by a sea of sagebrush and bunchgrasses but the onset of development and agriculture has eliminated over 50% of the original coverage. Not only has the absolute loss been great, but you can see in this slide that the remaining shrub steppe is highly fragmented. And, as of 1995, only 1% of the original extent was protected as some form of Natural Area.

Photo Credit: from Washington Natural Heritage Program 2007 Plan

These are the ‘Natural Area’ sites which comprise the ~1% of shrub steppe with protection status. These sites are managed specifically for their natural heritage values. Designation and ownership of these sites ranges from State Natural Area Preserves, Washington State University Biological Study Areas, Federal Research Natural Areas and Areas of Critical Environmental Concern, and private preserves (for example, The Nature Conservancy).

There are many other areas which contribute some form of protection of shrub steppe. For example, the Yakima Training Center, Hanford Reach National Monument, State and Federal Wildlife Areas, etc. are all important areas for shrub steppe conservation. However, because these areas manage for multiple land uses, the Washington Natural Heritage Program does not consider these sites as ‘protected’ due to the fact that some of these land uses can have detrimental effects on biodiversity. Nonetheless, some of these sites support the largest and most intact shrub steppe landscapes in the Basin. 

To highlight these land use differences, I wanted to quickly review this generalized, conceptual model of what happens to the ecological integrity of shrub steppe with a given land use. Sites managed for biodiversity conservation will focus management activities to maintain shrub steppe condition within the bounds of its historic range of variability. Historic range of variability refers to the range of climatic, disturbances, and compositional changes that these ecosystems evolved with. In some cases, the historic range of variability may have included human-induced disturbances such as deliberate fire management. However, most contemporary human-induced disturbances occur at are novel experiences for many ecosystems. The types, intensity, and duration of land use of the contemporary landscape is mostly outside the bounds of an ecosystem’s historic range of variability. Thus, these land uses can often have detrimental effects on ecological integrity. 

Managing a site for occasional, light, or selective grazing may result in the ecological system being in some form of an ‘alternative steady state’. These sites maintain many of the species you’d expect in an ecosystem functioning within its historic range of variability, but they may be missing some species sensitive to anthropogenic disturbances or suffer from slight changes in ecological processes. Severe or prolonged grazing could greatly simplify the system. Typically, only native weedy species or nonnative and invasive species occur at these sites. In addition, many of the key ecological processes have been altered or even eliminated. For example, overgrazing may destroy the cryptogamic crust found in many shrub steppe ecological systems. These soil crusts (a combination of algae, lichens, and mosses) are important for many ecosystem functions such as encouraging infiltration of precipitation, preventing soil erosion, and nutrient cycling. These crusts can also serve as a barrier to many aggressive nonnative species such as cheatgrass (Bromus tectorum). When these crusts are trampled and broken a part, bare soil is exposed and cheatgrass and other invasive annual species are able to establish. Even further degradation could force the ecosystem to a tipping point where it becomes something entirely different. An example would be a former shrub steppe site that is now completely dominated by nonnative annual grasslands, such as cheat grass. Of course, land conversion results in the direct loss of the ecological system.

Photo Credit: Rex Crawford (top right), Washington Natural Heritage Program

Here is a photographic interpretation of the previous slide. In the top left corner, is an example of an intact shrub steppe site where vegetation is within the historic range of variability (HRV). Sagebrush and bunchgrasses are dominant along with a diversity of forbs and intact cryptogamic crust on the soil surface.

The photo in the upper right is an example of a site that has experienced some light grazing. Most of the native bunchgrasses are not very tolerant of grazing, thus they begin to decline with continued grazing. As you can see in this photo, there is much more space between bunchgrass than in the site functioning within HRV. The cryptogamic crust has been impacted but is still functioning. However, overall species composition is pretty similar to the HRV site. Restoration of these sites is often just a matter of removing or suspending current land use.

In the lower right is a simplified site that has been degraded due to overgrazing. Bunchgrasses have declined dramatically, but shrubs are still present. The cryptogamic crust has mostly been eliminated and much of the ground cover consists of cheatgrass. These changes have altered any ecological processes. Restoring such a site requires much more effort and the likelihood of getting back to the HRV is increasingly low.

The photo in the lower left shows a site that has crossed a threshold into a new type--in this case an annual grassland. Shrubs and bunchgrasses are pretty much gone and species composition consists almost entirely of nonnative or weedy native species. Restoration of these is extremely difficult and costly.

This is, of course, a simplistic model and the relationship between each of these states does not always follow in this linear order. The intensity and duration of each type of land use have a different impact on shrub steppe. 

Despite all those photos of nice shrub steppe I showed earlier, most of the shrub steppe in the Columbia Basin is functioning outside the HRV state. Thus, not only have we lost a over half of the historical extent of shrub steppe, but the remaining shrub steppe is not always in the ecological condition we’d prefer, at least from a conservation point of view. 

For example, In the early 1980’s, the Washington Rangeland Committee and Washington Conservation Commission conducted an assessment of the ecological condition of Washington’s rangelands. In this study, condition was rated as the similarity of vegetation composition to “natural, undisturbed plant communities”. The results were disturbing. The study found that, of the remaining shrub steppe on the landscape, only about 11% was comparable to historical conditions while nearly 40% was in poor condition. In summary, as of 1984, nearly 70% of remaining shrub steppe is in fair to poor condition. The ecological condition of shrub steppe has likely not improved since then.

Due to direct loss and degradation of remaining shrub steppe stemming from a variety of land uses, sagebrush habitats are among the most imperiled ecosystems in North America. The impact on biodiversity is not evenly distributed across the different habitat types found in the shrub steppe landscape. For example, of those sites in Washington which have been converted to agriculture, 75% occur on loamy soil sites whereas <15% of shallow soil sites have been converted. These sites support different types of shrub steppe and associated critters. In addition, the remaining shrub steppe now occurs in a matrix of agriculture and is highly fragmented. Such fragmentation disrupts many of the natural ecological processes such as fire and species movement between habitats. Fragmentation has had a detrimental impact on many species, especially birds. Many researchers now think that shrubland and grassland birds are declining faster than any other group of species in North America.

Because of the dramatic loss and continual degradation of shrub steppe in eastern Washington, the Washington Department of Natural Resource’s Natural Heritage Program has initiated an inventory and assessment of the conservation value of shrub steppe remaining on State Trust lands. For the past 3 years, the Washington Natural Heritage Program has been conducting field work to determine:

• what kinds of ecological systems occur on Trust lands
• their current ecological condition
• whether or not they support any rare species
• whether they serve as critical wildlife habitat, and
• whether they serve as an important ecological corridor.

To date, we have collected data from about 75 % of the DNR Trust Lands we are targeting in eastern WA. Depending on funding, we hope to finish the assessment in the next few years.

There are many other shrub steppe conservation efforts occurring in the Basin. To more effectively and efficiently formulate and implement conservation strategies for shrub steppe, these efforts need to be coordinated. Such a broad-scale partnership should consider the following objectives in order to effectively protect the Columbia Basin’s natural heritage:

• rare & high quality examples of ecological systems
• rare species
• ecological corridors
• wildlife habitat
• ecological services

I’ll conclude with a quote from one of our eminent grassland ecologists, John Weaver, and one of the first to study Washington’s shrub steppe. I really like this quote because it reminds us that our natural heritage is not easily replaced. If we are to maintain the natural heritage of the Columbia Basin’s shrub steppe landscape, we need to conserve, whether through protection or compatible land uses, as much of the remnant high-quality shrub steppe while we still can and restore those sites that are currently in fair or poor condition.

Note: Unless otherwise noted, photos are my own.

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