The following content is from a publication from the Forest Service Office of Sustainability and Climate, the full content can be viewed/downloaded in PDF
Droughts can result in reduced growth rates, defoliation, and increased stress on vegetation, with accompanying ecological, economic, and social effects in rangeland areas. Droughts may be caused by a reduction in precipitation or an increase in temperature. These “hot droughts” reduce the supply of water through increased evaporation and faster melting of snow and ice. During these droughts, plants increase their demand for water through increased evapotranspiration and longer growing seasons. (Udall & Overpeck, 2017).
Droughts have been increasing in frequency and severity over the last 50 years in much of the United States (Figure 1) (Peters, Iverson, & Matthews, 2014). This trend is expected to continue in the future, particularly in the Central Plains and Southwest rangelands (Cook, Ault, & Smerdon, 2015).
Figure 1 - Cumulative Drought Severity Index (meteorological drought) compared over two time periods (1960–1986 and 1987–2013) (Peters et al., 2014). Click here to learn more about this dataset and zoom in to your area of interest. For an annual comparison of relative moisture surplus and deficit from 2000–2016 (in 3-year increments), view a time series webmap here.
Drought and Rangeland Soils
In the 1930s Dust Bowl, portions of the Midwest were transformed into a “moonscape, empty and hideous,” with no plants or wildlife to be seen (Egan, 2006). This is an extreme example of the effects of drought and poor soil management, but even moderate droughts can have profound ecological effects.
Some of the most profound effects of drought are on soils. As plant cover and water availability declines, soil temperatures increase and soil chemistry changes (Sanaullah, Rumpel, Charrier, & Chabbi, 2012). Soil acidification and salinization can occur, microbial activity can decline, organic matter in the soil can decrease, and soil runoff and erosion can increase. These changes can affect carbon and water storage: for any given soil type, the amount of water may double or triple, depending on how much carbon is in the soil (Saxton & Rawls, 2006).
These variables create feedback loops: as plant cover and water availability decline, this leads to changes in soil quality, which further reduce plant cover and water storage in the soil. These feedback loops worsen the effects of drought. In addition, drought may also lead to increased fire activity in some regions, which has further ecological consequences on the system (Westerling, Gershunov, & Cayan, 2003).
Drought and Rangeland Vegetation
Precipitation is a strong driver of ecosystem function and productivity. Research from the United States’ Great Plains, African Serengeti, and Mongolian Plateau have all found that primary productivity increases steadily with increases in rainfall (Sala, Gherardi, Reichmann, Jobbagy, & Peters, 2012). Grasslands are highly dynamic, with frequent droughts (Xiaomao, Harrington, Ciampitti, & Knapp, 2016) and large annual differences in productivity (M. Reeves, 2017; Xiao, Liu, & Stoy, 2016). Drought can reduce vegetation productivity, but vegetation in turn can also affect droughts. For example, deep-rooted perennials store more soil organic carbon than annual grasses (such as the invasive cheatgrass, Bromus tectorum) (Rau et al., 2011). As cheatgrass spreads and reduces the storage of organic carbon by the soil, this reduces the soil’s water-holding capacity, thus reducing the resilience of the system to drought.
Moreover, because cheatgrass spreads fire more readily than perennial grasses (Klemmedson & Smith, 1964) and fire can promote the spread of invasive species, this can act as a positive feedback loop for the spread of fire and invasive species (Abatzoglou & Kolden, 2011; Finch et al., 2016). By drying out fuels and increasing the stress on native vegetation, drought can drive this loop to further spread fire and invasive plants (Abatzoglou & Kolden, 2011). Researchers find differential sensitivity to drought in different species, among both native and invasive plant species (Alpert, Bone, & Holzapfel, 2000; Cavaleri & Sack, 2010; Heberling & Fridley, 2013). While some plant species steadily decline in productivity as drought increases, others show rapid declines past some threshold (Reeves et al., in preparation). Some warm-climate plants are better adapted to drought and can respond quickly with rapid bursts of productivity when precipitation increases (Reeves et al., in preparation).
There is also variation by climatic region. Research by Smith (2017) found that arid rangelands were more sensitive to droughts than mesic rangelands, both in their productivity and in vegetation community responses to drought (see also Tielbörger et al. (2014). Additionally, sensitivity to drought varies over time. Arid regions are affected in the first year of a drought while plant communities in mesic regions are not strongly affected until the third year, with differences in recovery times as well (Smith, 2011).
Overall, drought indices only explained around 40 percent of the difference in productivity in some rangeland systems; the rest of the variation may be due to differences in soils, vegetation, topography, and other factors (Reeves et al., in preparation). While programs such as the United States Drought Monitor provide valuable information on drought intensity, they do not account for all the differences in local vegetation productivity. The greater sensitivity of arid rangelands to drought is also relevant in the face of continued climate change. As mesic systems become more arid, they may also become less resilient to year-to-year fluctuations in rainfall.
Drought and Rangeland Livestock and Wildlife
Droughts affect not just rangeland plants and soils, but the entire ecological and social systems that depend on them. Lower plant productivity means fewer arthropods, which means fewer sage grouse (Gregg & Crawford, 2009; Guttery et al., 2013). This also means less forage and water available for ungulates and other herbivores, which can reduce their survival (Bender, Lomas, & Browning, 2007; Frank & McNaughton, 1992).
These effects extend to livestock production and to the local economies that depend on these animals. In 2013, following a major drought, the U.S. calf crop was the lowest since 1951, according to the National Agricultural Statistical Service (NASS) (2017), and major feedlots were forced to close. Several years later, cattle businesses were still recovering.
Drought is complex. Managers should use an integrated and coordinated approach to manage rangeland vegetation, water, and soils, in order to maintain healthy rangeland communities before and after droughts. The timeframes for improving resilience to the effects of drought fall into three categories: pre-drought, during drought, and postdrought.
- Reduce and prevent incursions of invasive plants including cheatgrass, which can reduce the ability of native vegetation to resist the effects of drought and to recover after drought (Stewart & Hull, 1949).
- Maintain cover, vigor, and diversity of native perennial grasses, forbs, shrubs, and other desirable plants. Increased production increases litter, which aids soils moisture retention, adds to carbon accumulation, and decreases soil temperature (Bot & Benites, 2005). Increased density of perennials improves site resistance to infestation by weedy annuals. Plants that are more vigorous are likely to recover more quickly after drought and have better reproductive capacity to replace plants lost due to drought.
- Maintain adequate biological soil crust and plant litter to maintain cover and reduce erosion potential, especially wind erosion, during a drought (Bot & Benites, 2005; Munson, Belnap, & Okin, 2011).
- Follow appropriate grazing practices to maintain plant vigor and productivity. Vigorous plants have deeper root systems than plants that have been weakened by inappropriate management, and are better able to survive drought and recover more quickly after a drought (Ekanayake, O’Toole, Garrity, & Masajo, 1985). Appropriate livestock management supports herbage production and ensures litter cover is present in adequate quantities to maintain soil moisture, mediate soil temperatures, and support the nutrient cycle (Amiri, Ariapour, & Fadai, 2008; Van Poollen & Lacey, 1979). Good grazing management also limits soil compaction, maintaining moisture infiltration into the soil (Amiri et al., 2008). Improved grazing practices include conservative stocking rates, appropriate grazing seasons and utilization levels, good livestock distribution across management units, and use of the appropriate class of livestock (Krausman et al., 2009).
- Promote appropriate fire regimes. Fuels management and reduction of invasive species such as cheatgrass and buffelgrass (Cenchrus ciliaris) can reduce wildfires and help to maintain the resilience of native plant communities (Finch et al., 2016).
- Use resistance and resilience information based on widely available soils data in advance of drought events to perform risk assessments (Maestas, Campbell, Chambers, Pellant, & Miller, 2016) and develop contingency plans for when droughts occur. Resistance and resilience concepts can be used as a framework for planning land management treatments and actions, and for responding to threats at multiple scales (Chambers et al., 2017; Chambers et al., 2016; Chambers et al., 2014).
- Design multi-site, multi-year drought studies to better understand drought recovery dynamics and the interactions between drought and grazing, and to coordinate research between different sites, for example through the Drought-Net program (Smith et al., 2016).
- Regularly assess resource conditions (water availability, vegetation vigor, soils, etc.).
- Coordinate and communicate with stakeholders on adaptation strategies and tactics.
- Adjust livestock use near permanent water sources to minimize impacts to soil, wildlife habitat, and other resources, and to meet wildlife needs.
- Improve distribution of livestock, moving or removing herds as needed and utilizing portable water troughs to improve livestock distribution and reduce the impacts on vegetation, soils, and permanent water supplies.
- Managers can develop expected use maps (Guenther, Guenther, & Redick, 2000) based on slope and distance to water to predict areas where livestock use could be concentrated during a drought and to identify underutilized areas where livestock could be moved if water sources were provided.
- Root growth is an essential part of drought recovery, and may occur at different times than regrowth of above-ground biomass (Hagedorn et al., 2016). Allow sufficient time for full recovery above and below ground when making management decisions on the resumption of rangeland uses.
- Promote recovery of perennial plant cover, litter, and biological crusts after drought to reduce the potential for accelerated erosion and the spread of invasive species. The sooner recovery occurs after a drought, the less likely that increased soil erosion will occur and that invasive plants will spread.
- Monitoring is always important in rangeland management, but it is even more important before, during, and after droughts. Managers need to understand how the plants are responding to these conditions and adjust livestock management to meet long-term needs.
- Recovery after drought may take a long time. It is as important as planning prior to—and management during—drought.
This is an excerpt taken from a synopsis of presentations given in a webinar by the Forest Service, with support from the USDA climate hubs, in June 2017. Presentations highlighted the direct and indirect effects of drought and how they affect the demand for recreation activities. The full synopsis is available as a PDF.
References made on this page can be found in full text pdf.