Photo: Cheatgrass, Southwest Montana. © 2020 Delena Norris-Tull
Effects of lower soil nitrogen in sagebrush ecosystems
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
September 2020.
Below, I summarize a study that was conducted in the late 1980s, that would not be carried out today, as it would be considered unethical to purposely disturb such a large area of sagebrush steppe. However, I am including the details here because this experimental study provides insight into the successional sequence of a group of invasive plants. And it uncovers an interesting method that could be used to try to reverse some of the damage of disturbances that occur in that environment, modification of phosphorus levels in soils.
It is important to note that the studies reported in this section were studies carried out over multiple years. As pointed out by Lars Baker, grants to fund this kind of research often only provide 2-3 years of funding (typical for graduate students pursuing master’s degrees), which is not enough time to adequately assess the effectiveness of most methods of treatment.
McLendon and Redente, 1991, studied the effects of adding nitrogen and phosphorus fertilizers to a disturbed site in a high altitude (2020 m) semi-arid sagebrush steppe community in northwestern Colorado. They removed vegetation and 5cm of topsoil, and mixed the next 35 cm of soil with a grader. They disturbed enough area to create four treatment plots, each 500 m2. Their intention was to simulate the type of disturbances caused by energy development throughout the Colorado Plateau. This disturbance removed 90% of the seed bank. They studied plant successional responses to the addition of P and N over five years after the soil disturbance. After each year, they counted number of species present, and aboveground relative biomass.
McLendon and Redente, 1991, counted 46 plant species over the five years, 30 of which were so infrequent (<1% relative biomass) that they were removed from analysis.
They compared the success of early successional annuals and late-seral perennial grasses. “Control plots were dominated by annuals for 3 years following disturbance. Annuals declined in importance the 4th year, at which time herbaceous perennials became major components. Perennial grasses and shrubs dominated the plots the 5th year as annuals continued to decline….
“Two annuals, Salsola iberica [Russian thistle, a non-native invasive annual] and Chenopodium berlandieri [netseed lambsquarters, an invasive native annual that used to be an important part of prehistoric cultivation by Native Americans], dominated the control plots the first year, with large amounts of two other annuals, Bromus tectorum [cheatgrass, a non-native annual forage grass that has become widely invasive] and Sisymbrium altissimum [tumble mustard, a non-native invasive annual]. Salsola iberica increased in dominance the 2nd year. The 3rd year, Bromus tectorum replaced Salsola iberica as the dominant species. Bromus tectorum declined the fourth year but remained a major component along with the biennial forb Melilotus officinalis [yellow sweetclover, a non-native invasive annual or biennial]. Perennial grasses, as a group, were a third major component…. [In the 5th year] the early-seral shrub Chrysothamnus nauseosus [Douglas rabbitbrush, a native perennial forb, sometimes treated as an invasive] became a co-dominant, and four perennial grass species (Agropyron smithii [western wheatgrass], Agropyron dasystachyum [thickspike wheatgrass], Oryzopsis hymenoides [Indian ricegrass], and Stipa comata [needle-and-thread grass]) [all native perennial grasses that provide good forage for livestock and wildlife] became major components. Together these five perennial species comprised 47% of the aboveground biomass on these [control] plots, compared to 25% the year before and 11% the third year. Annuals decreased to <40%, and most of this was Bromus tectorum (19%).”
Dr. Norris-Tull's comments: This data shows that native perennial grasses and shrubs are somewhat successful in returning with no treatment. However, the fact that cheatgrass was gaining such a hold indicates that, over time, it might dominate the habitat. In addition, big sagebrush (Artemisia tridentata), a critical shrubby species for this habitat, barely made a comeback by year 5, producing only 0.1% biomass in the nitrogen treatment, and 0.3% biomass in the control. At the time of this study, cheatgrass was not considered a weed. It is still used as a forage in Western States today, but has more and more become viewed as a weed that has caused a serious threat to sagebrush habitat.
In the nitrogen treatment plots, McLendon and Redente, 1991, found “greater and longer dominance by [invasive] annuals… Annuals, primarily Salsola iberica, dominated all plots (80% relative aboveground biomass)…. Annuals continued to dominate… in the 2nd and 3rd years… However, significant compositional change occurred during the 3rd year. There was a dramatic decline in Salsola and a continued increase in Kochia scoparia [a non-native annual invasive forb] and Bromus tectorum. When compared to control plots, nitrogen plots had higher levels of annuals (95% and 82%, respectively) and Kochia scoparia was present in greater amounts and Bromus tectorum in lesser amounts…. [By the 4th and 5th years,] shrubs and perennial grasses were becoming dominant on the control plots,… whereas annuals, primarily Kochia scoparia, continued to dominate the nitrogen plots…. The nitrogen treatment did not result in a significantly lower species diversity, as compared to the control, until the 3rd year, after which the gap widened.”
McLendon and Redente, 1991, also found that, “Phosphorus did not affect species distribution, species richness, or aboveground biomass production.”
For the results in the control plots, McLendon and Redente, 1991, “hypothesize that the high growth rate of Salsola iberica allowed it to assimilate limited resources during the 2nd year, thereby dominating the plots…. Competition appears to have become the major factor the 3rd year as Bromus tectorum largely replaced Salsola iberica…. A major ecological advantage possessed by Bromus tectorum is its ability to begin growth earlier than most associated species.”
McLendon and Redente, 1991, found that, “Phosphorus was not… significant but nitrogen did significantly affect succession for all years except the first.”
“The addition of nitrogen to this system altered succession by slowing the rate of succession and allowing [invasive] annuals to dominate through the 5th year…. Dominance of disturbed nutrient-rich sites by annuals may be a function of two major factors: seed availability and rapid potential growth rate. The relatively high levels of available resources often characteristic of initial conditions in secondary succession allow the potentially high growth rate of early-successional annuals to be expressed.
“The higher potential growth rates of early-successional annuals might allow them to outperform the slower-growing perennials as long as resources are relatively abundant. However, as resources [i.e., nitrogen] become more limiting, [perennial] species with lower nutrient requirements (per unit biomass) should have the advantage.”
Spotted knapweed, Centaurea maculosa, is an invasive rangeland biennial or short-lived perennial species that infests over 7 million hectares in the northwestern US and Canada. It can outcompete native perennial grassland species when soil nitrogen is relatively high. Sheley, et al, 1996, building on the data from on McLendon and Redente, 1991, on the impact of nitrogen on secondary successional sequences, proposed that planting species that sequester nitrogen, such as rye, may assist in the growth of desired native perennials and enable them to outcompete knapweed.
Herron, Sheley, Maxwell, and Jacobsen, 2001, took the information discovered by McLendon and Redente, 1991, and attempted to apply it to attempts to alter the successional sequence to favor the desired late-seral perennials over the invasive annuals and biennials.
Herron, et al., 2001, tested the hypothesis that “succession from a weedy plant community toward a desired late-seral [native] plant community could be accelerated by altering nutrient availability.” They studied the effects of adding and removing nitrogen and phosphorus from the soil, and the effects of growing annual rye (a crop plant) and bottlebrush squirreltail (Elymus elimoides, a native wild rye grass), on the growth of spotted knapweed and bluebunch wheatgrass (Pseudoroegneria spicatum, a native grass).
They conducted an experiment by growing knapweed and wheatgrass seedlings together in pots, in various densities, with five separate treatments: addition of nitrogen, addition of phosphorus, an annual rye cover crop, bottlebrush squirreltail; and a no nutrient control.
Herron, et al, 2001, results: Annual rye (Secale cereale) and bottlebrush squirreltail both were verified to remove nitrogen uptake in bluebunch wheatgrass. Annual rye reduced N uptake within bluebunch wheatgrass five times more than did squirreltail. N uptake by wheatgrass was not affected by the addition of N or P. “Phosphorus application dramatically increased uptake” within wheatgrass. “Annual rye decreased P uptake” by wheatgrass.
Nitrogen or Phosphorus “addition did not affect N uptake by knapweed… Bottlebrush squirreltail increased N uptake by spotted knapweed, while annual rye decreased” N uptake by knapweed by the greatest amount. Both “annual rye and bottlebrush squirreltail dramatically decreased P uptake by knapweed. Nitrogen addition did not alter P uptake by spotted knapweed, while P addition [dramatically] increased P uptake.”
When comparing the biomass of knapweed and wheatgrass in the five treatment pots, the results were:
Nitrogen addition had no effect on the competitive relationship between knapweed and bluebunch wheatgrass. The addition of Phophorus or the presence of bottlebrush squirreltail reduced the ability of knapweed to compete with bluebunch wheatgrass. “Annual rye shifted the competitive advantage from spotted knapweed to bluebunch wheatgrass.” Thus, both annual rye and bottlebrush squirreltail reduced the soil nitrogen levels enough to give a competitive advantage to [native] bluebunch wheatgrass.
Herron, et al., 2001, believe that “bluebunch wheatgrass has a lower N requirement than spotted knapweed, and, therefore, has the ability to out-compete spotted knapweed at lower N levels…. Our study provides initial evidence supporting the theory that nutrient levels can be altered to accelerate successional change from a weedy plant community toward a desired plant community. Ephemeral cover crops or mid-seral species could be used in restoration projects to lower N availability. Lower N availability could accelerate the establishment and domination of late-seral species over earlier successional weedy species.”
References:
Links to additional Rangeland Restoration practices:
Links to additional Innovative Solutions:
Effects of lower soil nitrogen in sagebrush ecosystems
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western,
September 2020.
Below, I summarize a study that was conducted in the late 1980s, that would not be carried out today, as it would be considered unethical to purposely disturb such a large area of sagebrush steppe. However, I am including the details here because this experimental study provides insight into the successional sequence of a group of invasive plants. And it uncovers an interesting method that could be used to try to reverse some of the damage of disturbances that occur in that environment, modification of phosphorus levels in soils.
It is important to note that the studies reported in this section were studies carried out over multiple years. As pointed out by Lars Baker, grants to fund this kind of research often only provide 2-3 years of funding (typical for graduate students pursuing master’s degrees), which is not enough time to adequately assess the effectiveness of most methods of treatment.
McLendon and Redente, 1991, studied the effects of adding nitrogen and phosphorus fertilizers to a disturbed site in a high altitude (2020 m) semi-arid sagebrush steppe community in northwestern Colorado. They removed vegetation and 5cm of topsoil, and mixed the next 35 cm of soil with a grader. They disturbed enough area to create four treatment plots, each 500 m2. Their intention was to simulate the type of disturbances caused by energy development throughout the Colorado Plateau. This disturbance removed 90% of the seed bank. They studied plant successional responses to the addition of P and N over five years after the soil disturbance. After each year, they counted number of species present, and aboveground relative biomass.
McLendon and Redente, 1991, counted 46 plant species over the five years, 30 of which were so infrequent (<1% relative biomass) that they were removed from analysis.
They compared the success of early successional annuals and late-seral perennial grasses. “Control plots were dominated by annuals for 3 years following disturbance. Annuals declined in importance the 4th year, at which time herbaceous perennials became major components. Perennial grasses and shrubs dominated the plots the 5th year as annuals continued to decline….
“Two annuals, Salsola iberica [Russian thistle, a non-native invasive annual] and Chenopodium berlandieri [netseed lambsquarters, an invasive native annual that used to be an important part of prehistoric cultivation by Native Americans], dominated the control plots the first year, with large amounts of two other annuals, Bromus tectorum [cheatgrass, a non-native annual forage grass that has become widely invasive] and Sisymbrium altissimum [tumble mustard, a non-native invasive annual]. Salsola iberica increased in dominance the 2nd year. The 3rd year, Bromus tectorum replaced Salsola iberica as the dominant species. Bromus tectorum declined the fourth year but remained a major component along with the biennial forb Melilotus officinalis [yellow sweetclover, a non-native invasive annual or biennial]. Perennial grasses, as a group, were a third major component…. [In the 5th year] the early-seral shrub Chrysothamnus nauseosus [Douglas rabbitbrush, a native perennial forb, sometimes treated as an invasive] became a co-dominant, and four perennial grass species (Agropyron smithii [western wheatgrass], Agropyron dasystachyum [thickspike wheatgrass], Oryzopsis hymenoides [Indian ricegrass], and Stipa comata [needle-and-thread grass]) [all native perennial grasses that provide good forage for livestock and wildlife] became major components. Together these five perennial species comprised 47% of the aboveground biomass on these [control] plots, compared to 25% the year before and 11% the third year. Annuals decreased to <40%, and most of this was Bromus tectorum (19%).”
Dr. Norris-Tull's comments: This data shows that native perennial grasses and shrubs are somewhat successful in returning with no treatment. However, the fact that cheatgrass was gaining such a hold indicates that, over time, it might dominate the habitat. In addition, big sagebrush (Artemisia tridentata), a critical shrubby species for this habitat, barely made a comeback by year 5, producing only 0.1% biomass in the nitrogen treatment, and 0.3% biomass in the control. At the time of this study, cheatgrass was not considered a weed. It is still used as a forage in Western States today, but has more and more become viewed as a weed that has caused a serious threat to sagebrush habitat.
In the nitrogen treatment plots, McLendon and Redente, 1991, found “greater and longer dominance by [invasive] annuals… Annuals, primarily Salsola iberica, dominated all plots (80% relative aboveground biomass)…. Annuals continued to dominate… in the 2nd and 3rd years… However, significant compositional change occurred during the 3rd year. There was a dramatic decline in Salsola and a continued increase in Kochia scoparia [a non-native annual invasive forb] and Bromus tectorum. When compared to control plots, nitrogen plots had higher levels of annuals (95% and 82%, respectively) and Kochia scoparia was present in greater amounts and Bromus tectorum in lesser amounts…. [By the 4th and 5th years,] shrubs and perennial grasses were becoming dominant on the control plots,… whereas annuals, primarily Kochia scoparia, continued to dominate the nitrogen plots…. The nitrogen treatment did not result in a significantly lower species diversity, as compared to the control, until the 3rd year, after which the gap widened.”
McLendon and Redente, 1991, also found that, “Phosphorus did not affect species distribution, species richness, or aboveground biomass production.”
For the results in the control plots, McLendon and Redente, 1991, “hypothesize that the high growth rate of Salsola iberica allowed it to assimilate limited resources during the 2nd year, thereby dominating the plots…. Competition appears to have become the major factor the 3rd year as Bromus tectorum largely replaced Salsola iberica…. A major ecological advantage possessed by Bromus tectorum is its ability to begin growth earlier than most associated species.”
McLendon and Redente, 1991, found that, “Phosphorus was not… significant but nitrogen did significantly affect succession for all years except the first.”
“The addition of nitrogen to this system altered succession by slowing the rate of succession and allowing [invasive] annuals to dominate through the 5th year…. Dominance of disturbed nutrient-rich sites by annuals may be a function of two major factors: seed availability and rapid potential growth rate. The relatively high levels of available resources often characteristic of initial conditions in secondary succession allow the potentially high growth rate of early-successional annuals to be expressed.
“The higher potential growth rates of early-successional annuals might allow them to outperform the slower-growing perennials as long as resources are relatively abundant. However, as resources [i.e., nitrogen] become more limiting, [perennial] species with lower nutrient requirements (per unit biomass) should have the advantage.”
Spotted knapweed, Centaurea maculosa, is an invasive rangeland biennial or short-lived perennial species that infests over 7 million hectares in the northwestern US and Canada. It can outcompete native perennial grassland species when soil nitrogen is relatively high. Sheley, et al, 1996, building on the data from on McLendon and Redente, 1991, on the impact of nitrogen on secondary successional sequences, proposed that planting species that sequester nitrogen, such as rye, may assist in the growth of desired native perennials and enable them to outcompete knapweed.
Herron, Sheley, Maxwell, and Jacobsen, 2001, took the information discovered by McLendon and Redente, 1991, and attempted to apply it to attempts to alter the successional sequence to favor the desired late-seral perennials over the invasive annuals and biennials.
Herron, et al., 2001, tested the hypothesis that “succession from a weedy plant community toward a desired late-seral [native] plant community could be accelerated by altering nutrient availability.” They studied the effects of adding and removing nitrogen and phosphorus from the soil, and the effects of growing annual rye (a crop plant) and bottlebrush squirreltail (Elymus elimoides, a native wild rye grass), on the growth of spotted knapweed and bluebunch wheatgrass (Pseudoroegneria spicatum, a native grass).
They conducted an experiment by growing knapweed and wheatgrass seedlings together in pots, in various densities, with five separate treatments: addition of nitrogen, addition of phosphorus, an annual rye cover crop, bottlebrush squirreltail; and a no nutrient control.
Herron, et al, 2001, results: Annual rye (Secale cereale) and bottlebrush squirreltail both were verified to remove nitrogen uptake in bluebunch wheatgrass. Annual rye reduced N uptake within bluebunch wheatgrass five times more than did squirreltail. N uptake by wheatgrass was not affected by the addition of N or P. “Phosphorus application dramatically increased uptake” within wheatgrass. “Annual rye decreased P uptake” by wheatgrass.
Nitrogen or Phosphorus “addition did not affect N uptake by knapweed… Bottlebrush squirreltail increased N uptake by spotted knapweed, while annual rye decreased” N uptake by knapweed by the greatest amount. Both “annual rye and bottlebrush squirreltail dramatically decreased P uptake by knapweed. Nitrogen addition did not alter P uptake by spotted knapweed, while P addition [dramatically] increased P uptake.”
When comparing the biomass of knapweed and wheatgrass in the five treatment pots, the results were:
Nitrogen addition had no effect on the competitive relationship between knapweed and bluebunch wheatgrass. The addition of Phophorus or the presence of bottlebrush squirreltail reduced the ability of knapweed to compete with bluebunch wheatgrass. “Annual rye shifted the competitive advantage from spotted knapweed to bluebunch wheatgrass.” Thus, both annual rye and bottlebrush squirreltail reduced the soil nitrogen levels enough to give a competitive advantage to [native] bluebunch wheatgrass.
Herron, et al., 2001, believe that “bluebunch wheatgrass has a lower N requirement than spotted knapweed, and, therefore, has the ability to out-compete spotted knapweed at lower N levels…. Our study provides initial evidence supporting the theory that nutrient levels can be altered to accelerate successional change from a weedy plant community toward a desired plant community. Ephemeral cover crops or mid-seral species could be used in restoration projects to lower N availability. Lower N availability could accelerate the establishment and domination of late-seral species over earlier successional weedy species.”
References:
- Herron, G.J., Sheley, R.L., Maxwell, B.D., & Jacobsen, J.S. (Sept, 2001). Influence of nutrient availability on the interaction between spotted knapweed and bluebunch wheatgrass. Restoration Ecology, 9 (3): 326-331.
- McLendon, T., & Redente, E.F. (1991). Nitrogen and phosphorus effects on secondary succession dynamics on a semi-arid sagebrush steppe. Ecology, 72: 2016-2024.
Links to additional Rangeland Restoration practices:
- Prairie Restoration
- Sagebrush Steppe Restoration
- Revegetation with Native Plants
- Dogs as detectors of noxious weeds
Links to additional Innovative Solutions: