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From before the Europeans came to America to now, the amount of sagebrush-steppe has almost been cut in half in America. This can be attributed to livestock grazing, fragmentation, cultivation of land, and a higher frequency of fires. Fire is a natural occurrence and can be either good for the ecosystem or devastating. Historically fire return intervals were much greater in sagebrush-steppe of the Great Basin. However, since the arrival of Europeans to America and invasion of exotic plants such as cheatgrass (Bromus techtorum), fire return internals have been shortened.
This has led to significant losses of sagebrush-steppe. This influences which species of plants can thrive in this new and changing environment. Sagebrush is an important resource for sagebrush obligate species such as greater sage grouse and Columbia River basin pygmy rabbits for all stages of life.
Sagebrush-steppe is one of the largest ecosystems of the Western United States. It provides food and habitat for over three hundred different species of plants and animals.
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A lot of this land is regulated by the federal government for multi-use. It is also one of the biggest places for livestock production on public land. People use the sagebrush-steppe to hunt, recreate, and camp.
There has been a loss of nearly half the sagebrush communities. Before European settlement, there was an estimated twenty-five million hectares of sagebrush (Artemisia tridentate); now there is an estimated thirteen million hectares. Loss of sagebrush steppe has been primarily due to land management approaches resulting in overutilization of forage species, changes in fire frequency, slow recovery time, and invasion by annual grasses (Mata et al, 2018, Shinneman and McIlroy, 2016).
A potential threat to sagebrush steppe is overgrazing by domestic livestock. Sagebrush ecosystems evolved with little pressure until the introduction of cattle. Overgrazing the annual and perennial grasses that inhabit the gaps between sagebrush shrubs changes the community (Baker, 2006). Cattle herbivory was found to be associated with reduced native bunchgrass abundance, shifts in bunchgrass composition to only the most grazing-tolerant species, and aggregated bunchgrasses beneath protective sagebrush canopies. These collective cattle-induced changes appear to ripple through the community by decreasing the size and connectivity of gaps between perennial vegetation. As gaps get smaller and less connected, both living and dead herbaceous soil cover (litter) decreases and the amount of bare soil increases (Reisner et al, 20013).
It’s hard to determine how frequent fires were in the past on sagebrush communities due to lack of vegetation living after the fire. Fire frequency is typically aged by years between burns in the trunk of a tree. However, when fire passes through sage brush it kills it. Some sagebrush communities have large trees nearby that can be used to determine fire frequency. Estimates of fire frequency range from thirty years to two hundred years (Davies et al, 2012, Davies et al, 2007).
Analysis of sagebrush communities showed that there are significantly more fires now than before European settlement. Between the late 1980s and late 1990s, the frequency of fires doubled, and average fire size increased by 400% in the Great Basin (Eiswerth et al, 2009). Potential reasons for more frequent fires are climate change, increased ignitions by people, invasive-plant fire cycles, and other land-use effects (Baker, 2013).
The increased size of fires has altered the dynamics of the sagebrush communities. Fires are now burning larger patches and reducing the amount of species present. Virtually all the species present on the most frequently burnt sites are introduced annuals. The frequency of fires has accelerated soil erosion and decreased the resilience and resistance to exotic and invasive species (Swanson et. al, 2018, Whisenant, 1992).
The biggest problem with exotic plants are they fill the gaps were bunchgrasses use to be. Bunchgrasses created gaps which restricted fuel connectivity. Exotic plants fill the holes and gaps and create a continuous fuel source for fire. Invasion by exotic annual grasses can create fire return intervals that are too short for sagebrush to reestablish (Davies et al, 2007). If invasive species were present before a fire, they, like all plants, will be reduced by fire. This leaves no patches of unburnt sagebrush to reseed. The invasive species that fill post-fire niches tend to tolerate and fuel subsequent fires. The non-native species can successfully compete with native species for soil water after fire, as well as nutrient uptake facilitated by adequate soil moisture, thus negatively affecting native plant growth and productivity (Shinneman and McIlroy, 2018, Davies et al, 2012).
One of the most notable exotic plant species to move in is cheatgrass (Bromus techtorum). Cheatgrass was introduced by livestock and is now one of the biggest threats to loss of sagebrush communities (Reisner et al, 2013). Ecologists and land managers have long been concerned that invasion by cheatgrass and increased fire risk from human activity can interact to generate a wildfire-invasion feedback cycle that results in fundamental changes to sagebrush steppe ecosystem structure and function. The fires are more frequent and extensive burns reduce or eliminate sagebrush and other woody perennials (Davies et al, 2012).
There are two primary reasons of concern for cheatgrass. One, it outcompetes native annual vegetation and increases the frequency of fires. When spring comes it is one of the first plants to come up and starts to take advantage of the resources. With disturbances like fire and overgrazing, cheatgrass begins to dominate (Whisenant, 1992). This chokes out and prevents other more desirable plants from reestablishing. Sagebrush communities of sagebrush in Northern Nevada and Southern Idaho are rapidly being converted to monocultures of cheatgrass. Cheatgrass is well adapted for invading arid dry environments (Eiswerth et al, 2009).
The other primary concern with cheatgrass is the increase in frequency of fires. Cheatgrass grows in the early spring and by the high part of the summer it has already seeded out and senescenced (Shinneman and McIlroy, 2016). The cheatgrass is dried out and is much easier to catch on fire by lightning strike, accidental fires from humans, or by other means. Cheatgrass fires are characterized by being large, high-intensity fires that leave fewer and smaller unburnt patches of sagebrush, that are especially vulnerable to further cheatgrass invasion (Baker, 2006). Fire frequency and cheatgrass relative frequency have a correlation value of 0.65 (Figure 1). Showing fires are more frequent with more cheatgrass present (Whisenant, 1992).
Recovery time for sagebrush after a fire is about thirty-five years to hundreds of years (Shinneman and McIlroy, 2016). When flames reach sagebrush mortality is nearly 100%. Fires do not thin sagebrush communities or lower their density by killing a certain fraction of sagebrush. Sagebrush doesn’t sprout from the crown or roots following the fire like many other plant species (Baker, 2006 & Moffet et al, 2015). It is relatively unimportant how fast the fire moves, how hot the fire is, or what the fire intensity is. If a fire front passes through an area the sagebrush will be killed. Thus, sagebrush is temporarily eliminated from the plant community (Young and Evans, 1978).
It may only take thirty years for sagebrush to become the dominant species on the range and choke out perennial and annual grasses. In order to restore the natural balance between grasses and shrubs, prescribed burns were used (Young and Evans, 1978). Fire has been documented since the 1930s as a management tool (Moffet et al, 2015). Potential benefits of fire are reduction of woody plants, decreased risk of high-intensity fire, and increased production of herbaceous vegetation for some wildlife species (Swanson et al, 2018). Livestock managers used fire to shift dominance from sagebrush to desirable herbaceous plants. This increased the forage availability and palatability for livestock, but has now been devastating with cheatgrass introduction (Davies et al, 2007, Moffet et al, 2015).
The other reason sagebrush stand recovery so slowly is their poor seed dispersibility. Sagebrush are established entirely by seeds. 90% of sagebrush seeds fall within two meters of the mother plant. The seed viability is generally a year (Eiswerth et al, 2009). Sagebrush can reestablish from seeds following a fire; however, the seeds are short lived and if a second fire occurs before the new plants produce seed (four-six years), the species may undergo local extinction (Whisenant, 1992). Thus, sagebrush is dependent on patches of unburnt areas to reseed and establish the burnt area. However, increased fire size and frequency has led to complete burns of sagebrush stands leaving no sagebrush seeds to reestablish on the burnt area (Mata et al, 2015, Whisenant, 1992).
Sagebrush is an important component for wildlife species. There are over 350 plant and animal species that use sagebrush for food, shelter, or a place for reproduction activities (Moffet, 2015, Shinneman & McIlroy, 2016). Some of these species include rare or threatened species like the greater sage grouse (Centrocercus urophasianus) and endangered species Columbia Basin pygmy rabbit (Brachylagus idahoensis). These species are sagebrush obligates and can only survive in the conditions sagebrush presents. Greater sage grouse need sagebrush to create nests and raise their young. Sagebrush also provides habitat for bugs and insects the sage grouse prey on (Davies, 2012).
Though we may not know the complete history of fires on sagebrush-steppe, we can see a change in recent history that should concern us. Increased frequency of fires and greater intensity of those fires can destroy sagebrush and set it in a fire cycle that is too quick for recovery. We need to concentrate our efforts on protecting as much sagebrush-steppe as we can. Without it we would loss a great diversity of animals and plants.
Dooming of Sagebrush Steppe. (2022, Apr 25). Retrieved from https://studymoose.com/dooming-of-sagebrush-steppe-essay
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