Photo: Madrone trees near Guadalupe Mountains National Park. © 2017 Delena Norris-Tull
Impacts of Herbicides on Native Plants
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
Rinella, et al., 2009, posed the important question, “Which is worse for native biota, invaders or things done to control invaders.”
There is plenty of evidence that invasive species have a negative impact on native plant diversity. Nevertheless, there are many instances wherein invasive species have lived among native plant communities for decades, without having a huge impact on that community. Rinella, et al, 2009, ask us to consider the following reality, “That invasive species so often imperil natives without outright extirpating them begs a precautionary approach to ecosystem management. Specifically, once invaders render natives rare, avoiding practices that might further rarify them can very clearly seem the prudent course of action. However, the prudent course of action can become less clear with management practices used explicitly for suppressing invaders. On the one hand, these practices often do have negative side effects on native species [both plant and animal species]…. On the other hand, these negative effects may be outweighed by positive effects on native species because natives often rebound when management suppresses invaders.” Thus, “it is sometimes difficult to know a priori whether management practices will make matters better or worse.”
Rinella, et al., 2009, specifically examined the impact of herbicides on native species. “Some herbicides used in natural areas, such as glyphosate, are phytotoxic to the vast majority of species, both native and nonnative. Others, such as 2,4-D and picloram, are highly toxic only to dicotyledonous species, but these chemicals are widely used where native and exotic dicots co-occur…. Natural-area spraying is not tracked consistently, making it difficult to reasonably estimate global hectares sprayed…. The U.S. Bureau of Land Management, Forest Service, and Park Service spray over 120,000 ha of natural area each year.”
In a cattle ranch in Montana, Rinella, et al., 2009, “applied dicot-specific herbicide treatments… to a grassland with relatively abundant monocots (i.e., grasses) and exotic dicots (forbs), and relatively rare native dicots (forbs, a subshrub, and a shrub). In factorial combination with the herbicide treatments, [they] applied grazing treatments (i.e., cattle grazing, no cattle grazing).” They collected data intermittently for 16 years. Leafy spurge was the most abundant invasive plant.
Prior to their experimental treatments, in 1982, 800 ha of the area had been treated with picloram, in an unsuccessful attempt to remove invasive forbs and increase forage and restore native plant communities. Rinella, et al., 2009, used the center of this previously treated area for their experiments, which “consisted of twelve 12X12 plots each of which received one of the following treatments: (1) cattle grazing, (2) herbicide), (3) both cattle grazing and herbicide,” or (4) no treatment. On this working cattle ranch, grazing consisted of the “take half leave half” principle that is commonly used in Western rangelands. Rinella, et al., 2009, “visually estimated percentage plant cover of each species independently, except for grasses, which we estimated as a group.” They collected cover data “one month and one, two, and four years after herbicide application. Then, after realizing the grazing enclosures were still intact, [they] gathered data again 16 years after application.” In addition to cover, they estimated leafy spurge and grass biomass production, at each time interval.
In data analysis, Rinella, et al., 2009, found that they could exclude data prior to year four, “because most dicot species were completely absent from herbicide-treated plots during” years one to three, probably due to persistence of herbicide at phytotoxic concentrations. “Some natives were absent from most cover-sampling frames.” They concluded that in any sampling-frames in which a native species survived, “that frame was not appreciably affected by herbicide.”
Rinella, et al., 2009, found that, “Except for Euphorbia esula (leafy spruge), herbicide initially greatly depleted all exotic forbs and none of them recolonized appreciably by one year after application… However, all but one exotic forb recolonized by four years after application, and by 16 years after application exotic-forb frequencies did not differ statistically between sprayed and not-sprayed plots…. [Similarly] herbicide continued to suppress three natives… one year after application…, and these plants recovered by the study’s end.” One native forb was apparently unaffected by herbicide. “Two native forbs that were initially greatly suppressed by herbicide never fully recovered from herbicide use regardless of grazing… Three native forbs… and a subshrub… were significantly rarer in sprayed plots 16 years after spraying but only when grazing was excluded.”
In the biomass data, Rinella, et al., 2009, found that herbicide reduced production of leafy spurge for several years. And “herbicide increased grass production” for the first and likely the second year. But there was a “0.76 probability herbicide increased [leafy spurge] production” by year 16. This is likely to have been caused by reduced production by native dicot species.
Rinella, et al., 2009, concluded that, after an early increase in grass forage, “herbicide provided little benefit to the livestock producer or the ecosystem we studied…. In addition to E. esula, all other exotic species also eventually recovered from spraying. In contrast to the exotics, herbicide caused long-term suppression of several native forbs.”
They reference a similar study by Rice et al., 1997, in which Centaurea maculosa was treated with picloram. In that study, native forb species were only mildly affected and recovered over time. That study showed that different native forbs may respond differently to herbicide toxicity, pointing to the need for more research to demonstrate under what conditions, and with which herbicides, different native species can survive without damage. However, Rice at al., 1997, also found that the invasive species recovered over time. Thus, both studies call into question the benefits of using herbicides to increase forage production.
Elodea removal in Alaska
Elodea was first recorded in Alaska in 1982 and has rapidly become invasive. The frequent use of floatplanes in remote areas of Alaska has facilitated its spread. Both mechanical removal, highly expensive, and biocontrol with grass carp have been ruled out as viable forms of control. Grass carp are too likely to result in unintended spread, resulting in the consumption of native aquatic plants. Sethi, et al., 2017, examined the impacts of the herbicides diquat and fluridone, used to treat Elodea infestations. They studied four lakes in the Kenai Peninsula of Alaska, examining herbicide impacts on zooplankton and native macrophytes. They compared two treated and infested lakes with two untreated, uninfested, subartic lakes.
While a study in Washington found that fluridone reduced native aquatic plants, few studies have been conducted in the extreme climatic and photo period conditions of Alaska. Elodea has been shown to be particularly sensitive to fluridone, and thus the herbicide can be used at low concentrations. Fluridone has been shown to have low toxicity to aquatic invertebrates and fish. In the study by Sethi, et al., 2017, on Beck Lake, a whole lake treatment with fluridone was carried out in June and September 2014. The larger Daniels Lake received a partial lake treatment, in June and September 2014, and July and October 2015. A single targeted application of diquat was applied, at half the label recommended rate, in Daniels Lake on June 2014. Diquat has higher toxicity than fluridone.
Sethi, et al., 2017, results:
Water quality: While each lake had distinct results for most water quality indicators (DO, conductivity, and pH), “as a group, treated lakes tended to be of lower clarity and higher Chlorophyll a load.” But it is likely that the water quality indicator differences were largely unrelated to treatments.
Zooplankton: Much of the variation in zooplankton was likely unrelated to treatment. “Treated lakes as a group tended to have higher zooplankton density and biomass than untreated lakes at the onset of the sampling season, however lakes converged to consistent low density and biomass levels as the season progressed.”
Macrophytes: Elodea declined in 2014, corresponding to treatment in June. It was absent from both treated lakes through 2015 and through the single spring 2016 sampling event. “No systematic impacts to the abundance of native macrophyte associated with herbicide treatment were identified…Treatment lakes…tended to exhibit higher rates of lily pad senescence earlier in the summer season as compared to untreated lakes…Total taxonomic richness as detected by rake sampling increased in treated lakes after a year of treatment, and both Beck and Daniels [Lakes] had higher richness than the untreated replicate lakes after Elodea was eradicated by end of 2015.”
Woodland caribou study
Woodland caribou is a threatened species in Alberta, Canada. It is thought that historically, the caribou evaded predators, particularly wolves, by browsing in areas less favored by other ungulates. High elevation tundra provides such a habitat. Mihajlovich & Blake, 2004, tested the use of glyphosate in reducing forage favorable to other ungulates (such as moose), with the goal of facilitating a larger browse area suitable for caribou. Mihajlovich & Blake, 2004, treated a recently harvested and reforested area with glyphosate. They found that the herbicide did not have a negative impact on lichen, a browse needed for caribou, but it did reduce the density of blueberries.
Mihajlovich & Blake, 2004, concluded: “Results of this experiment suggest broadcast application of glyphosate herbicide for browse reduction would not negatively impact the supply of caribou lichen (and hence caribou forage) on treated areas. However, herbicide treatment would result in some reduction of blueberries thus reducing forage for species other than woodland caribou.”
References:
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Impacts of Herbicides on Native Plants
Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, October 2020.
Rinella, et al., 2009, posed the important question, “Which is worse for native biota, invaders or things done to control invaders.”
There is plenty of evidence that invasive species have a negative impact on native plant diversity. Nevertheless, there are many instances wherein invasive species have lived among native plant communities for decades, without having a huge impact on that community. Rinella, et al, 2009, ask us to consider the following reality, “That invasive species so often imperil natives without outright extirpating them begs a precautionary approach to ecosystem management. Specifically, once invaders render natives rare, avoiding practices that might further rarify them can very clearly seem the prudent course of action. However, the prudent course of action can become less clear with management practices used explicitly for suppressing invaders. On the one hand, these practices often do have negative side effects on native species [both plant and animal species]…. On the other hand, these negative effects may be outweighed by positive effects on native species because natives often rebound when management suppresses invaders.” Thus, “it is sometimes difficult to know a priori whether management practices will make matters better or worse.”
Rinella, et al., 2009, specifically examined the impact of herbicides on native species. “Some herbicides used in natural areas, such as glyphosate, are phytotoxic to the vast majority of species, both native and nonnative. Others, such as 2,4-D and picloram, are highly toxic only to dicotyledonous species, but these chemicals are widely used where native and exotic dicots co-occur…. Natural-area spraying is not tracked consistently, making it difficult to reasonably estimate global hectares sprayed…. The U.S. Bureau of Land Management, Forest Service, and Park Service spray over 120,000 ha of natural area each year.”
In a cattle ranch in Montana, Rinella, et al., 2009, “applied dicot-specific herbicide treatments… to a grassland with relatively abundant monocots (i.e., grasses) and exotic dicots (forbs), and relatively rare native dicots (forbs, a subshrub, and a shrub). In factorial combination with the herbicide treatments, [they] applied grazing treatments (i.e., cattle grazing, no cattle grazing).” They collected data intermittently for 16 years. Leafy spurge was the most abundant invasive plant.
Prior to their experimental treatments, in 1982, 800 ha of the area had been treated with picloram, in an unsuccessful attempt to remove invasive forbs and increase forage and restore native plant communities. Rinella, et al., 2009, used the center of this previously treated area for their experiments, which “consisted of twelve 12X12 plots each of which received one of the following treatments: (1) cattle grazing, (2) herbicide), (3) both cattle grazing and herbicide,” or (4) no treatment. On this working cattle ranch, grazing consisted of the “take half leave half” principle that is commonly used in Western rangelands. Rinella, et al., 2009, “visually estimated percentage plant cover of each species independently, except for grasses, which we estimated as a group.” They collected cover data “one month and one, two, and four years after herbicide application. Then, after realizing the grazing enclosures were still intact, [they] gathered data again 16 years after application.” In addition to cover, they estimated leafy spurge and grass biomass production, at each time interval.
In data analysis, Rinella, et al., 2009, found that they could exclude data prior to year four, “because most dicot species were completely absent from herbicide-treated plots during” years one to three, probably due to persistence of herbicide at phytotoxic concentrations. “Some natives were absent from most cover-sampling frames.” They concluded that in any sampling-frames in which a native species survived, “that frame was not appreciably affected by herbicide.”
Rinella, et al., 2009, found that, “Except for Euphorbia esula (leafy spruge), herbicide initially greatly depleted all exotic forbs and none of them recolonized appreciably by one year after application… However, all but one exotic forb recolonized by four years after application, and by 16 years after application exotic-forb frequencies did not differ statistically between sprayed and not-sprayed plots…. [Similarly] herbicide continued to suppress three natives… one year after application…, and these plants recovered by the study’s end.” One native forb was apparently unaffected by herbicide. “Two native forbs that were initially greatly suppressed by herbicide never fully recovered from herbicide use regardless of grazing… Three native forbs… and a subshrub… were significantly rarer in sprayed plots 16 years after spraying but only when grazing was excluded.”
In the biomass data, Rinella, et al., 2009, found that herbicide reduced production of leafy spurge for several years. And “herbicide increased grass production” for the first and likely the second year. But there was a “0.76 probability herbicide increased [leafy spurge] production” by year 16. This is likely to have been caused by reduced production by native dicot species.
Rinella, et al., 2009, concluded that, after an early increase in grass forage, “herbicide provided little benefit to the livestock producer or the ecosystem we studied…. In addition to E. esula, all other exotic species also eventually recovered from spraying. In contrast to the exotics, herbicide caused long-term suppression of several native forbs.”
They reference a similar study by Rice et al., 1997, in which Centaurea maculosa was treated with picloram. In that study, native forb species were only mildly affected and recovered over time. That study showed that different native forbs may respond differently to herbicide toxicity, pointing to the need for more research to demonstrate under what conditions, and with which herbicides, different native species can survive without damage. However, Rice at al., 1997, also found that the invasive species recovered over time. Thus, both studies call into question the benefits of using herbicides to increase forage production.
Elodea removal in Alaska
Elodea was first recorded in Alaska in 1982 and has rapidly become invasive. The frequent use of floatplanes in remote areas of Alaska has facilitated its spread. Both mechanical removal, highly expensive, and biocontrol with grass carp have been ruled out as viable forms of control. Grass carp are too likely to result in unintended spread, resulting in the consumption of native aquatic plants. Sethi, et al., 2017, examined the impacts of the herbicides diquat and fluridone, used to treat Elodea infestations. They studied four lakes in the Kenai Peninsula of Alaska, examining herbicide impacts on zooplankton and native macrophytes. They compared two treated and infested lakes with two untreated, uninfested, subartic lakes.
While a study in Washington found that fluridone reduced native aquatic plants, few studies have been conducted in the extreme climatic and photo period conditions of Alaska. Elodea has been shown to be particularly sensitive to fluridone, and thus the herbicide can be used at low concentrations. Fluridone has been shown to have low toxicity to aquatic invertebrates and fish. In the study by Sethi, et al., 2017, on Beck Lake, a whole lake treatment with fluridone was carried out in June and September 2014. The larger Daniels Lake received a partial lake treatment, in June and September 2014, and July and October 2015. A single targeted application of diquat was applied, at half the label recommended rate, in Daniels Lake on June 2014. Diquat has higher toxicity than fluridone.
Sethi, et al., 2017, results:
Water quality: While each lake had distinct results for most water quality indicators (DO, conductivity, and pH), “as a group, treated lakes tended to be of lower clarity and higher Chlorophyll a load.” But it is likely that the water quality indicator differences were largely unrelated to treatments.
Zooplankton: Much of the variation in zooplankton was likely unrelated to treatment. “Treated lakes as a group tended to have higher zooplankton density and biomass than untreated lakes at the onset of the sampling season, however lakes converged to consistent low density and biomass levels as the season progressed.”
Macrophytes: Elodea declined in 2014, corresponding to treatment in June. It was absent from both treated lakes through 2015 and through the single spring 2016 sampling event. “No systematic impacts to the abundance of native macrophyte associated with herbicide treatment were identified…Treatment lakes…tended to exhibit higher rates of lily pad senescence earlier in the summer season as compared to untreated lakes…Total taxonomic richness as detected by rake sampling increased in treated lakes after a year of treatment, and both Beck and Daniels [Lakes] had higher richness than the untreated replicate lakes after Elodea was eradicated by end of 2015.”
Woodland caribou study
Woodland caribou is a threatened species in Alberta, Canada. It is thought that historically, the caribou evaded predators, particularly wolves, by browsing in areas less favored by other ungulates. High elevation tundra provides such a habitat. Mihajlovich & Blake, 2004, tested the use of glyphosate in reducing forage favorable to other ungulates (such as moose), with the goal of facilitating a larger browse area suitable for caribou. Mihajlovich & Blake, 2004, treated a recently harvested and reforested area with glyphosate. They found that the herbicide did not have a negative impact on lichen, a browse needed for caribou, but it did reduce the density of blueberries.
Mihajlovich & Blake, 2004, concluded: “Results of this experiment suggest broadcast application of glyphosate herbicide for browse reduction would not negatively impact the supply of caribou lichen (and hence caribou forage) on treated areas. However, herbicide treatment would result in some reduction of blueberries thus reducing forage for species other than woodland caribou.”
References:
- Mihajlovich, M., & Blake, P. (2004). An evaluation of the potential of glyphosate herbicide for woodland caribou habitat management. Alces, 40:7-11.
- Rinella, M.J., Maxwell, B.D, Fay, P.K., Weaver, T., & Sheley, R.L. (Jan., 2009). Control effort exacerbates invasive-species problem. Ecological Applications, 19 (1): 155-162.
- Sethi, S.A., Carey, M., Morton, J.M., Guerron-Orejuela, E., Decino, R., Willette, M., Boersma, J., Jablonski, J., & Anderson, C. (Aug., 2017). Rapid response for invasive waterweeds at the arctic invasion front: Assessment of impacts from herbicide treatment. Biological Conservation, Part A, 212: 300-309.
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