Anderson, Jay, and Richard Inouye. 2001. Landscape-scale changes in plant species abundance and biodiversity of a sagebrush steppe over 45 years. Ecological Monographs 71(4):531-556.
RELEVANT TO: GRASSLANDS/SHRUBLANDS
LIVESTOCK
INVASIVE SPECIES
FIRE
DESCRIPTION
The Idaho National Engineering and Environmental Laboratory, established in 1950, exists as the largest reserve of sagebrush steppe in North America. This study analyzes cover, density, and frequency data for plants sampled nine times during 45 years on permanent plots. Forty-seven of the 79 permanent plots have been free of livestock since 1950, and these are the focus of this article. Only two of the plots have burned since 1950. Pronghorn, mule deer, elk, black-tailed jackrabbits, cottontail and pygmy rabbits and locally abundant small mammals are present.
When the reserve was established in 1950, it had apparently been "severely overgrazed," and a drought had existed since 1933, continuing until 1957. The area had "very low cover of perennial grasses [0.5%], low density and richness of perennial forbs, dominance of plots by two subspp. of Artemisia tridentata [big sagebrush], and relative vegetative homogeneity of plots..." (p. 544) The vegetation total cover was 18%.
Hypotheses considered were that (1) the vegetation would be quite static, given the longevity of the sagebrush steppe woody species, usurping of resources by mature woody species, and lack of propagules (e.g., seeds) of herbaceous species; (2) annual or longer term variation in precipitation would be a major driver of vegetative change; (3) plant cover and stability of cover are positively associated with species richness; and (4) vulnerability to invasion by exotics is inversely related to native plant cover and richness.
MAJOR FINDINGS
By 1995, vegetation cover was 38%, with shrubs contributing 52% of the cover, grasses 15%, and perennial forbs 7.5%. Annuals and biennials accounted for 24% of the total, of which nearly half consisted of exotics, primarily cheatgrass. Basal cover underestimates the importance of perennial grasses relative to shrubs and forbs in these communities. "Furthermore, the substantial increase in cover of perennial grasses through 1975 was not at the expense of the shrub overstory" (p. 545).
Cheatgrass has only rarely displaced native vegetation, and cover of cheatgrass is "strongly inhibited by cover of native species..."(p. 553). "Thus, the bulk of the evidence available suggests that where native plant populations in sagebrush steppe are thriving, B. tectorum [cheatgrass] does not pose the threat that it does where populations of native perennials have been depleted" (p. 553).
Density of big sagebrush has decreased in recent decades; perennial grasses increased sharply in the 1970s, and then have leveled off; and perennial forbs have increased significantly over the past two decades. Species richness of all three have increased in the 45 years. Cover of shrub and perennial grass species have been positively correlated with species richness in most years. "[T]he data clearly support the prediction that cover in sagebrush steppe is generally higher in areas having greater richness of vascular plants..." (p. 551).
The number of plots on which nonnative species were recorded increased, but the average density of nonnative species didn't. In 1995, the greater the cover of native species, the lower the cover of nonnative species, including cheatgrass. "Thus, good cover of perennial species in sagebrush steppe appears to increase resistance to invasion." (p. 552)....[C]over of B. tectorum [cheatgrass] is strongly inhibited by cover of native species." (p. 553).
"Despite the depauperate and homogenous conditions in 1950, vegetation on the permanent [livestock-free] plots... has been anything but static over the past 45 [years], clearly refuting the prediction of long-term stability under shrub dominance. Mean richness per plot of all growth forms increased steadily..." (p. 545).
"The 45-yr period has witnessed a substantial increase in heterogeneity in vegetative structure among plots" (p. 549).
Classic range "succession" has not taken place; instead species increases and decreases have fluctuated over the 45 years. (pp. 546-7). Likewise, prediction of static dominance by big sagebrush has not been shown. Likely this is the result of having "a large number of plots that better represent landscape-scale dynamics" (p. 548).
"Indeed a few other studies have shown that arid and semiarid communities can respond to reduced stocking rates and favorable precipitation with increased production of native perennial grasses and forbs if residual populations of such natives are present [citations]" (p. 548).
"Our data do not support Laycock's [citation] opinion that 'if grazing were completely removed, diversity of both plants and animals on the landscape level probably would decrease on most areas of public lands' in the United States....Laycock's conclusion is probably correct for grassland ecosystems having a long evolutionary history of intense grazing pressure, but it may not be applicable to semiarid ecosystems where unpalatable shrubs are dominants (e.g., sagebrush steppe) or may quickly become dominants with heavy livestock grazing or drought.....[L]ivestock grazing in such shrub-dominated systems intensifies shrub dominance, reduces or eliminates populations of palatable species, and tends to homogenize vegetation at the landscape scale." (p. 550).
Regarding the variable relations individual species had in relation to precipitation: "Our data indicate that individual species within the guild of perennial grasses responded individualistically to temporal environmental variation." (p. 550).
The authors conclude that:
- The increases in species diversity and heterogeneity 1950-1975 are largely recovery from drought and grazing;
- species richness is an important factor driving the functioning of semiarid sagebrush-steppe ecosystems; and
- "...[A]dequate cover of native species appears to render these semiarid communities more resistant to invasion." (p. 553)
"Grime [citation] speculated that high biodiversity might be 'vitally important' in semiarid and other ecosystems that 'experience drastic fluctuations on a seasonal or longer time scale.' Our results support this view by providing evidence for links between plant wpecies diversity and function in sagebrush steppe" (p. 551).
QUESTIONS RAISED FOR THE THREE FORESTS
- Do current Forest policies acknowledge native sagebrush understory as a key feature in maintaining natural fire cycles, resisting invasive species, and supporting cover and native biodiversity?
- Do current Forest policies take into account the negative impacts of livestock grazing in conjunction with drought in sagebrush steppe?
RELEVANCE TO FOREST MANAGEMENT
- Livestock grazing permits must acknowledge synergism with drought and indicate evidence for the grazing to support natural ecosystems in light of drought.
"[Under drought conditions,] [t]he most palatable species -- perennial grasses and forbs -- are likely to be the least drought tolerant. Thus, drought and grazing have similar, synergistic effects [citation]." (p. 545).
- Large blocks of sagebrush habitat must be livestock-free to protect and restore native biodiversity, reduce invasive species spread, and provide long-term comparison with sagebrush habitat that continues to be grazed by domestic livestock.
- Capability and suitability of vegetation for livestock grazing must be based on actual, not potential vegetation, including understory vegetation.