Grasslands and Climate Change

Preparers

Karen Bagne, Paulette Ford, and Matt Reeves; Rocky Mountain Research Station

Issues

Grasslands cover a broad expanse of the U.S. and encompass a diverse set of environmental conditions and ecological communities. Grasslands are major contributors to U.S. food production and provide many other services valuable to humans including aquifer recharge, pollination, and recreational opportunities. While typically defined as lands on which the existing plant cover is dominated by grasses, undeveloped grasslands consist of more than just grass. They are highly diverse communities of grasses, forbs, and non-vascular and woody plants, punctuated by wetlands that provide critical wildlife habitat. Although climate is an important driver of grassland ecosystems, disturbances such as fire and grazing also play a key role in sustaining grasslands. Some systems, such as temperate savannas commonly grade into grasslands, and are maintained as grasslands principally by human-caused disturbance [1].

The largest U.S. grassland region is the Great Plains, a vast area of prairie, agriculture, and rangelands extending from the Dakotas through Iowa, Nebraska, Kansas, Oklahoma and parts of Texas, and including the eastern parts of Montana, Wyoming, Colorado, and New Mexico (see grasslands map below).

Grasslands are also an important component in other regions including the Central Valley of California, the western coast of the Gulf of Mexico, parts of the Great Basin, and arid regions of the Southwest. Temperate savannas (used frequently to describe temperate grasslands with scattered trees) include pine savannas of the southeastern Gulf Coastal Plain, California oak savannas, piñon/juniper savannas of the Southwest, and aspen parklands of Canada. We do not discuss marshes, alpine meadows, or tundra for this topic page.

The grassland ecosystems we see today have an extensive history of human activity including burning, hunting, crop production, livestock grazing, and urban development. In response to these alterations, many native grasslands and grassland species are in decline and climate change is expected to add to or exacerbate existing stressors that threaten these ecosystems. As climate conditions shift geographically so will the distributions of many plants and animals. The relatively flat terrain of grasslands increases vulnerability to climate change impacts, because habitats and species must migrate long distances to compensate for temperature shifts. This contrasts sharply with mountainous terrain, where conditions change over short distances due to steep increases in elevation [2]. Warmer temperatures bring greater evaporation and alter rainfall patterns, which will further deplete aquifers and threaten water-dependent habitats. Of particular concern are the small isolated wetlands that facilitate groundwater recharge, support a critical component of diversity in many grassland ecosystems (e.g., vernal pools, playa lakes, prairie potholes) and are already threatened by current land use practices [3].

History and human values influence not only how grassland ecosystems will respond to climate change but also what management options will be available. The extensive fragmentation that already characterizes grassland ecosystems will limit opportunities for species to disperse in concert with climate. Global market trends affect local human activities in grassland ecosystems, often with important consequences. For example, changing commodity prices can alter crop choice and subsequent aquifer withdrawals [4]. In some grasslands, ecosystem engineers (e.g., bison and prairie dogs), are missing from much of their former range [5], and fragmentation and agricultural practices have reduced pollinator species. Because these species play an active role in ecosystem dynamics, this will further limit ecosystem resilience to climate change impacts.

Likely Changes

On a large scale grassland-suitable habitat in the U.S. is expected to increase, but projections vary by grassland type. Model projections for grasslands are at a broad scale (1km) and give a sense of likely changes summarized below [6]:

• Climate suitable for Great Plains grasslands is expected to remain relatively stable with some expansion to the north in Canada and retraction on the eastern and southern boundaries.
• Climate suitable for semi desert grasslands in the southwestern U.S. is expected to expand while contracting in Mexico.
• Climate suitable for Great Basin shrub-grasslands is expected to decline with contraction primarily along the eastern boundary in Colorado, Montana, and eastern Idaho.
• Climate suitable for California valley grasslands is likely to have significant declines, shifting towards oak woodlands and desert scrub, along with a high proportion of no analog climates (i.e., projected climates do not match any contemporary biomes).
• Climate suitable for Gulf Coastal grasslands is expected to contract towards southeastern Texas and have a high proportion of no analog climates.

Other expected changes not captured by the distribution models include effects of CO₂ and nitrogen availability.

When not limited by other factors, increasing CO₂ increases plant growth, as well as water use efficiency, which is especially important in drier regions. However, the benefits of elevated CO₂ will be limited by factors including water availability [7, 8] and available nutrients, particularly nitrogen [9]. Thus effects of elevated CO₂ on plant growth will vary with local climate patterns,species adaptations to water limitations, and nitrogen availability. Studies indicate that nutrient depletion may happen faster in drier regions, and with factors such as plant community composition and grazing [8]. Nitrogen deposition from air pollutants and increased mineralization from higher temperatures can increase plant productivity, but increases are often accompanied by a reduction in biodiversity as faster growing plants outcompete others [10]. A study of a California grassland found that global change may speed reductions in diversity and forb species are most vulnerable to this process [11].

Grassland community composition will likely be affected by climate change, but complex interactions make predictions problematic. For example, some experiments have shown that under certain conditions elevated CO₂ favors woody plants, herbaceous forbs, and legumes over many grasses, because of their different photosynthetic pathways, (C4, warm-season versus C3, cool-season) [12] or possibly differences in root depth and groundwater uptake [13]. Shrub encroachment of montane meadows under warmer temperatures has also been supported [14]. But other evidence suggests that C4 grasses may be favored in arid areas with reduced precipitation because of their greater ability to resist desiccation, high temperatures, and low nitrogen levels [15]. Although a number of expanding exotic grass species are C4, any competitive disadvantage from elevated CO₂ so far is not universally apparent. Changes in species composition will likely vary by region and by year and will depend on depth and timing of available soil water as well as disturbance factors such as grazing, fire, and disease, which can have strong influence on plant communities.

Extreme weather conditions can induce a rapid and severe response that alters both ecosystems and human communities [16]. Increasingly severe and frequent droughts, floods, fires, and hurricanes are likely to affect U.S. grassland ecosystems. Drought exacerbates soil erosion and aquifer depletion. Greater variability in precipitation will favor more frequent fires, which can reduce encroachment of woody plants into grasslands[17]. Fires are a natural element in grassland ecosystems, but extreme conditions, along with land use variables, can encourage fires that impact very large areas in a short period of time, such as the Texas fires of 2011. Similarly, floods occur regularly, recharge aquifers, transport nutrients, and provide habitat for some wildlife species, but greater intensity of run-off events will also decrease retention of organic matter and flush out aquatic organisms in wetlands [18]. Other widespread disturbance events, such as insect outbreaks, can speed the conversion of forests and woodlands to grasslands [19].

The extent and duration of open water in wetlands is expected to decrease even in regions where precipitation is expected to be higher, because of the greater evaporation expected with warmer temperatures [20]. Greater evaporation also increases salinity. Sea level rise will inundate coastal grasslands with salt water and increase erosion. These changes will impact wetland plants as well as migratory bird populations that breed, winter, or migrate through grassland habitats, which are significant components along the Pacific, Central, and Mississippi Flyways. Although the distribution and timing of available water in aquatic habitats will be sensitive to climate change, regional effects, such as geomorphology, water demand, and soils, have not been well explored [21].

Options for Management

Management options to sustain grassland ecosystems under global change are many, but are mostly untested in their ability to maintain or enhance resource values into the future. Tenants of managing climate change response, also called adaptation, include evaluating the success of current management programs, implementing anticipatory actions, and maintaining the flexibility to modify strategies. Local climate alterations may also affect management decisions such as when prescribed fires can be applied. Low risk and "no regrets" options include actions that are expected to have benefits on valued resources now. Examples include management to reduce current stressors that threaten the ecosystem services from grasslands and actions that increase ecosystem resilience such as altering grazing patterns to increase biodiversity. More drastic measures include assisted migration, where species are moved to new regions expected to have favorable climates, and assisted transition, where managers encourage conversion of an area to a projected state [22]. Managers need to consider the expected impacts to key resources at a relevant temporal and spatial scale as well as uncertainty surrounding those expectations. For example, wetlands suitable for breeding waterfowl in the northern Great Plains are projected to shift in location to wetter regions in the northeast where few wetlands are protected, thus conservation efforts for these species needs to shift focus [23].

Connectivity

Landscape connectivity and species’ dispersal ability are important concepts in climate change adaptation because of the expected spatial shifts in suitable habitat for plants and animals. The widespread fragmentation of grasslands makes this an especially critical consideration for anticipating species response. Dispersal can be manipulated through establishment of corridors and translocations. Shifts of geographic distributions should anticipate not just emigration but also immigration of new species to a defined location.

Livestock

Productivity and changes to growth periods, both in their duration and timing, will need to be taken into consideration for livestock management. Livestock management can include altering of stocking rates, livestock breeds, and livestock species to better match new conditions, along with monitoring of indicators that practices are compatible with sustaining healthy grasslands.

Restoration

Restoration of degraded grasslands can increase resilience to climate change along with providing protection from soil erosion, carbon loss, and other negative impacts. In regions where climate is expected to no longer support current communities, restoration can focus on a function such as aquifer recharge or on species expected to be more tolerant of new conditions. Conservation of native grasslands often focuses on localized areas at a small spatial scale, but a focus on the surrounding matrix becomes increasingly important as climate and natural communities shift, especially for grasslands because of their intensive human use and fragmentation. Genetic diversity incurs greater resilience to changing and uncertain conditions and should be considered in restoration or translocation of species. Greater biodiversity and redundancy of species functional roles creates greater stability of ecosystems and is associated with greater resilience in the face of changing conditions [24, 22]. Intermediate levels of disturbance, including soil disturbance, fire, and grazing, can encourage greater biodiversity. Disturbance, however, is also conducive to invasion by exotic plants [25]. Control of invasive species can reduce stress on the system making it better able to resist climate change.

Extreme events

Extreme events should be anticipated and steps taken to mitigate their impacts. This may take the form of building infrastructure or reducing fuel for fires, but could also be addressed through contingency plans that can be put into action quickly when the need arises. Providing for redundancy in habitats across a large spatial scale can also increase resilience to extreme events. Extreme weather events can also have positive effects. Hurricanes, in part, prevent forest establishment and maintain grasslands in coastal areas [26,27].

Management tools

Many decision support tools are available for managers. Scenario planning, where impacts are assessed across a range of climate futures, can identify actions that are robust under multiple projections. Where and how to direct management resources can be guided by vulnerability assessments, because they identify priorities and areas of vulnerability where effective actions can be directed. Risk and decision analysis can help evaluate the potential outcomes and uncertainty surrounding possible management actions. Find more tools in the "tools" tab.

Opportunities

Although there are many reasons for concern, some climate change impacts may be positive. Increases in plant productivity and longer growing seasons in some regions may support more livestock, increase wildlife species, and increase economic benefits. Increases in fires may reduce encroachment by woody species and further encourage conversion to grasslands. Native grassland restoration efforts may get a boost from the expected increase in carbon sequestration these sites provide. In some cases, climate change may incur negative impacts to problematic invasive species in favor of natives [28]. The variability in climate expected can be an opportunity for effective management, such as invasive control and revegetation, as actions can be timed to the most effective conditions [29]. For example, native plants can be more tolerant of drought conditions than non-natives allowing for control measures to be applied when populations are low [30]. Finally, climate change is a good motivator for developing a broader, more responsive, and collaborative management paradigm.