Invasive Annual Cheatgrass Research Articles

Invasive annual cheatgrass enhances the abundance of native microbial and microinvertebrate eukaryotes but reduces invasive earthworms

Invasive annual cheatgrass enhances the abundance of native microbial and microinvertebrate eukaryotes but reduces invasive earthworms

Pollinator-friendly flora in rangelands following control of cheatgrass (Bromus tectorum): a case study

AbstractInvasive winter annual grasses, such as cheatgrass (Bromus tectorumL.) are considered serious threats to regional biodiversity. Pollinator populations that depend on the native flora are likely to be negatively impacted as these native species may be displaced by the invasive grass species. Colonization by cheatgrass is also predicted to increase risk of wildfires, as dead plant parts provide fuel in the already dry and arid regions of the western United States. Biocontrol, grazing, prescribed burning, or use of broad-spectrum nonselective herbicides have been suggested as possible means to controlB. tectorum. Efficient control may facilitate regrowth of native flora that could in turn support other ecosystem functions. Reporting our findings as a case study, we describe here the results of the application of a preemergent herbicide, indaziflam, that limits germination ofB. tectorumseeds. Herbicide was applied to the study locations during the months of December 2016, January 2017, and February 2017. The data reported here on the diversity of flowering plants were collected between May through September 2018. Herbicide-treated plots showed an increase in diversity and abundance of flowering plants compared to the untreated control within two seasons after cheatgrass control was implemented, suggesting that effective reduction of the population of the invasive annual cheatgrass may help facilitate the growth of native forbs. Further studies are necessary to understand mechanisms that facilitate reestablishment of native flowering species, the long-term consequences of reducing invasive annual grasses and to document any residual effects of the herbicide on ground-nesting pollinators.

Weather and distance to fire refugia limit landscape‐level occurrence of fungal disease in an exotic annual grass

AbstractThe enemy release hypothesis proposes that invasion by exotic plant species is driven by their release from natural enemies (i.e. herbivores and pathogens) in their introduced ranges. However, in many cases, natural enemies, which may be introduced or managed to regulate invasive species, may fail to impact target host populations. Landscape heterogeneity, which can affect both the population dynamics of the pathogen and the susceptibility and the density of hosts, may contribute to why pathogens fail to control hosts despite established negative disease impacts.We explored patterns of post‐fire infection of the fungal head‐smut pathogenUstilago bullataon the invasive annual cheatgrassBromus tectorum, which has caused the notorious grass‐fire cycle and ecosystem degradation across Western North America. We asked whether infection level was a driver of host density or vice‐versa, and how weather affected infection and how spatial patterns of infection varied with time since fire, using a combination of structural equation modelling (SEM), proportional odds modelling and entropy‐based local indicator of spatial association (ELSA) on data from >700 plots spanning >100,000 ha remeasured annually for 4 years.Observed infection levels increased with greater prior‐year cheatgrass cover, and disease severity did not suppress cheatgrass populations. Warm, humid fall/winters and proximity to fire refugia (unburned patches) were associated with more infections. Infection clustering was most evident 2–3 years following fire with warm‐wet fall–winter conditions and decreased after two drier, colder winters.Synthesis. Severity of fungal disease did not result in measurable reductions of populations of a non‐native, invasive host species, cheatgrass, which suggests that natural enemies may not strongly regulate cheatgrass in its introduced range. Landscape heterogeneity associated with disturbance and weather limited population‐level infection of hosts by the fungal pathogen. Disturbance (specifically wildfire) and variable weather are key components of this and similar invasion systems, and likely need to be considered when evaluating disease dynamics and potential for natural enemies to influence invasion potential.

Potential for post‐fire recovery of Greater Sage‐grouse habitat

AbstractIn the western United States, fire has become a significant concern in the management of big sagebrush (Artemisia tridentata Nutt.) ecosystems. This is due to large‐scale increases in cover of the fire‐prone invasive annual cheatgrass (Bromus tectorum L.) and, concurrently, concerns about declining quantity and quality of habitat for Greater Sage‐grouse (Centrocercus urophasianus). The prevailing paradigm is that fire results in a loss of sage‐grouse habitat on timescales relevant to conservation planning (i.e., 1–20 yr), since sagebrush cover can take many more years to recover post‐fire. However, fire can have effects that improve sage‐grouse habitat, including stimulating perennial grass and forb production. The conditions under which fire results in the permanent loss or enhancement of sage‐grouse habitat are not well understood. We used long‐term data from the Utah Division of Wildlife Resources Range Trend Project to assess short‐term (1–4 yr post‐treatment) and long‐term (6–10 yr post‐treatment) effects of fire on vegetation cover at 16 sites relative to sage‐grouse habitat vegetation guidelines. Sagebrush cover remained low post‐fire at sites considered historically unsuitable for sage‐grouse (<10% initial sagebrush cover). In contrast, at sites that had higher (>10%) pre‐fire sagebrush cover, sagebrush cover decreased to <10% in the short‐term post‐fire, but by 6–10 yr after fire, most of these sites exhibited a recovering trajectory and two sites had recovered to >10% cover. Post‐fire sagebrush cover was positively related to elevation. Across all sites, perennial grasses and forbs increased in cover to approximately meet the habitat vegetation guidelines for sage‐grouse. Cheatgrass cover did not change in response to fire, and increased perennial grass cover appears to have played an important role in suppressing cheatgrass. Our results indicate that, while fire poses a potential risk for sage‐grouse habitat loss and degradation, burned sites do not necessarily need to be considered permanently altered, especially if they are located at higher elevation, have high sagebrush cover pre‐fire, and are reseeded with perennial grasses and forbs post‐fire. However, our results confirm that fire at more degraded sites, for example, those with <10% sagebrush cover, can result in cheatgrass‐dominated landscapes and sagebrush loss at these sites should be avoided.

Effects of Sagebrush Restoration and Conifer Encroachment on Small Mammal Diversity in Sagebrush Ecosystem

Conifer encroachment in sagebrush ecosystems reduces habitat heterogeneity, niche space, and resource availability, all of which negatively affect many wildlife populations. Sagebrush restoration is recommended as a management action to mitigate conifer encroachment and restore wildlife across millions of hectares in the Great Basin. Despite this recommendation, the effects of conifer encroachment and sagebrush restoration are unknown for most wildlife species. Small nonvolant mammal communities include keystone species, consumers and prey; facilitate energy flow and ecological function; and provide important ecological goods and services. We assessed causal relationships between conifer encroachment and sagebrush restoration (conifer removal and seeding native plants) on small mammal communities over 11 yr using a Before-After-Control–Impact design. Sagebrush habitat supported an additional small mammal species, twice the biomass, and nearly three times higher densities than conifer-encroached habitat. Sagebrush restoration increased shrub cover, decreased tree cover, and density but failed to increase native herbaceous plant density. Restoration caused a large increase in the non-native, invasive annual cheatgrass (Bromus tectorum L.). Counter to prediction, small mammal diversity did not increase in response to sagebrush restoration, but restoration maintained small mammal density in the face of ongoing conifer encroachment. Piñon mice (Peromyscus truei), woodland specialists with highest densities in conifer-encroached habitat, were negatively affected by sagebrush restoration. Increasing cheatgrass due to sagebrush restoration may not negatively impact small mammal diversity, provided cheatgrass density and cover do not progress to a monoculture and native vegetation is maintained. The consequences of conifer encroachment, a long-term, slow-acting impact, far outweigh the impacts of sagebrush restoration, a short-term, high-intensity impact, on small mammal diversity. Given the ecological importance of small mammals, maintenance of small mammal density is a desirable outcome for sagebrush restoration.

Cascading effects of fire retardant on plant-microbe interactions, community composition, and invasion.

Climate change, historical fire suppression, and a rise in human movements in urban-forest boundaries have resulted in an increased use of long-term fire retardant (LTFR). While LTFR is an effective fire-fighting tool, it contains high concentrations of nitrogen and phosphorus, and little is known about how this nutrient pulse affects terrestrial ecosystems. We used field surveys and greenhouse experiments to quantify effects of LTFR on plant productivity, community composition, and plant interactions with the ubiquitous root symbiont arbuscular mycorrhizal fungi (AMF). In the field, LTFR applications were associated with persistent shifts in plant communities toward exotic annuals with little or no dependency of AMF. Plants exposed to LTFR were less colonized by AMF, both in field surveys and in the greenhouse, and this was most likely due to the substantial and persistent increase in soil available phosphorus. All plants grew bigger with LTFR in the greenhouse, but the invasive annual cheatgrass (Bromus tectorum) benefitted most. While LTFR can control fires, it may cause long-term changes in soil nutrient availabilities, disrupt plant interactions with beneficial soil microbes, and exasperate invasion by some exotic plants.

The invasive annual cheatgrass releases more nitrogen than crested wheatgrass through root exudation and senescence.

Plant-soil feedbacks are an important aspect of invasive species success. One type of feedback is alteration of soil nutrient cycling. Cheatgrass invasion in the western USA is associated with increases in plant-available nitrogen (N), but the mechanism for this has not been elucidated. We labeled cheatgrass and crested wheatgrass, a common perennial grass in western rangelands, with (15)N-urea to determine if differences in root exudates and turnover could be a mechanism for increases in soil N. Mesocosms containing plants were either kept moist, or dried out during the final 10days to determine the role of senescence in root N release. Soil N transformation rates were determined using (15)N pool dilution. After 75 days of growth, cheatgrass accumulated 30% more total soil N and organic carbon than crested wheatgrass. Cheatgrass roots released twice as much N as crested wheatgrass roots (0.11 vs. 0.05mgN kg(-1) soil day(-1)) in both soil moisture treatments. This occurred despite lower root abundance (7.0 vs. 17.3g dry root kg(-1) soil) and N concentration (6.0 vs. 7.6gN kg(-1) root) in cheatgrass vs. crested wheatgrass. We propose that increases in soil N pool sizes and transformation rates under cheatgrass are caused by higher rates of root exudation or release of organic matter containing relatively large amounts of labile N. Our results provide the first evidence for the underlying mechanism by which the invasive annual cheatgrass increases N availability and establishes positive plant-soil feedbacks that promote its success in western rangelands.

The invasive annual cheatgrass increases nitrogen availability in 24-year-old replicated field plots.

Previous studies comparing invaded and non-invaded sites suggest that cheatgrass (Bromus tectorum L.) causes soil N cycling to increase. Unfortunately, these correlative studies fail to distinguish whether cheatgrass caused the differences in N cycling, or if cheatgrass simply invaded sites where N availability was greater. We measured soil C and N concentrations and net and gross N-cycling rates on 24-year-old replicated field plots in a sagebrush-steppe ecosystem that had been plowed, fumigated, and seeded to different plant communities in 1984. Laboratory assays of soil collected throughout the soil profiles (0-60 cm) showed that soil NO3 (-), organic C and N, and net N mineralization, net nitrification, and soil respiration rates were all greater beneath cheatgrass than in sagebrush-perennial grass plots. In surface soils (0-10 cm), field and lab assays on five sampling dates during 2 years showed gross N mineralization, net N mineralization, and net nitrification rates were all faster beneath cheatgrass than in sagebrush-perennial grass plots. Modeling analyses based on soil respiration and gross N-cycling rates suggest that cheatgrass provides soil microbes with lower C:N substrates and that this could explain the faster N-cycling rates beneath cheatgrass. This is the first long-term replicated field study to conclusively show that cheatgrass created greater soil organic N pool sizes and stimulated N-cycling rates compared to similar-aged stands of sagebrush and native perennial grasses. Increased N-cycling rates may represent a positive plant-soil feedback that promotes continued dominance by cheatgrass, even in the absence of soil disturbance or fire.

Effects of perennial semi-arid bunchgrass spatial patterns on performance of the invasive annual cheatgrass (Bromus tectorum L.)

Plant spatial patterns critically influence community dynamics, including plant interactions, resource distribution, and community invasibility. Research suggests that resistance of western US plant communities to further invasion by the exotic annual grass Bromus tectorum may be linked to the positions of, and spacing between, perennial plants. In particular, gaps between aggregated clusters of perennial plants may facilitate B. tectorum invasion by providing safe sites for seed germination and establishment. We tested the effects of random, regular, and aggregated bunchgrass patterns, manipulated at both community (plot) and neighborhood scales, on B. tectorum biomass and spikelet production after experimental seed addition. We found strong evidence of treatment effects on both biomass and spikelets, which varied between treatments by approximately 2.5-fold. Mean biomass and spikelet counts were lowest in plots in which bunchgrasses were aggregated at both community and neighborhood scales, likely due to the increased competition. Although not statistically distinguishable from most other treatments, B. tectorum biomass and spikelet counts were highest in plots with bunchgrass patterns that were random at the community scale and aggregated at the neighborhood scale. These plots were characterized by relatively large gaps between bunchgrass clusters, suggesting that B. tectorum may exploit gaps between aggregated perennial plants. Our results support the emerging hypothesis that community resistance to B. tectorum invasion could be increased through manipulation of perennial vegetation to reduce basal gap size and connectivity.

Comparison of Postfire Soil Water Repellency Amelioration Strategies on Bluebunch Wheatgrass and Cheatgrass Survival

Soil water repellency can limit postfire reseeding efforts and thus increase the susceptibility of a site to weed invasion. We evaluated the effectiveness of wetting agents and simulated anchor chaining for improving seedling growth and survival in water-repellent soil, for the native perennial bluebunch wheatgrass (Pseudoroegneria spicata) and invasive annual cheatgrass (Bromus tectorum). Research was performed in a glasshouse, on 20-cm-diameter soil cores that were excavated from underneath burned Utah juniper (Juniperus osteosperma) trees. The experiment was arranged as a randomized split-plot design, with the two grass species sown separately under four soil treatments: 1) no treatment (control), 2) simulated anchor chaining (hereafter referred to as “till”), 3) wetting agent, and 4) till plus wetting agent. Soil water content was highest in the wetting agent treatment, lower for till, and lowest in the control. Overall, the response of bluebunch wheatgrass and cheatgrass was similar among treatments. At the conclusion of the study, wetting agent cores had twice as many seedlings as the control, while the till and control were similar. Despite a lower number of seedlings, tilling in general resulted in the same level of biomass as the wetting agent treatment. Overall, biomass in the till and wetting agent treatments was at least twofold higher than the control. No benefit was found in applying both till and wetting agent treatments together in comparison to just applying wetting agent. Because of a lack of correlation between glasshouse and field settings the results of this study need to be interpreted with caution. Our data may indicate that if cheatgrass is not already present on the site, anchor chaining or treating the soil with wetting agent can increase establishment of seeded species.

Nitrogen Uptake by Perennial and Invasive Annual Grass Seedlings: Nitrogen Form Effects

Invasive annual cheatgrass (Bromus tectorum L.) and medusahead wildrye [Taeniatherum caput‐medusae (L.) Nevski ssp. asperum (Simonk.) Melderis] have decreased livestock productivity and biological diversity and increased the frequency of wildfire on rangelands in the western United States. On disturbed sites, squirreltail (Elymus sp.), a short‐lived North American perennial, appears to compete against invasive exotic annuals when available soil N and nitrification are reduced. We tested the hypothesis that differences in N uptake activity could account for this phenomenon. North American seedlings of perennial bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Á. Löve], four populations of squirreltail, and two invasive exotic annuals were cultured in the laboratory on NH4+, NO3−, or NH4NO3 nutrient solutions, and N uptake activity (mol kg−1 root dry wt. h−1) was measured. The overall biomass means of 4‐wk‐old seedlings cultured with NO3−, NH4NO3, and NH4+ were 72, 67, and 42 mg seedling−1, respectively. Regardless of N form, cheatgrass biomass was as much as 4.2‐fold greater than any of the other grasses including medusahead, which exceeded the biomass of all perennials except one. Cheatgrass had 1.5‐ to 2.2‐fold greater NO3− uptake activity than the perennials, but the NO3− uptake activity of medusahead exceeded only two of the squirreltail populations. Ammonium uptake activities of perennials were not consistently more favorable than those of the annuals. The vigorous seedling growth of the invasive annuals coupled with the greater NO3− uptake activity of cheatgrass appear to be primary traits of the invasive annuals, driving their superior N capture and competition compared with these North American perennial grasses.

Distribution of ecosystem C and N within contrasting vegetation types in a semiarid rangeland in the Great Basin, USA

Semiarid sagebrush ecosystems are being transformed by wildfire, rangeland improvement techniques, and exotic plant invasions, but the effects on ecosystem C and N dynamics are poorly understood. We compared ecosystem C and N pools to 1 m depth among historically grazed Wyoming big sagebrush, introduced perennial crested wheatgrass, and invasive annual cheatgrass communities, to examine whether the quantity and quality of plant inputs to soil differs among vegetation types. Natural abundance δ15N isotope ratios were used to examine differences in ecosystem N balance. Sagebrush-dominated sites had greater C and N storage in plant biomass compared to perennial or annual grass systems, but this was predominantly due to woody biomass accumulation. Plant C and N inputs to soil were greatest for cheatgrass compared to sagebrush and crested wheatgrass systems, largely because of slower root turnover in perennial plants. The organic matter quality of roots and leaf litter (as C:N ratios) was similar among vegetation types, but lignin:N ratios were greater for sagebrush than grasses. While cheatgrass invasion has been predicted to result in net C loss and ecosystem degradation, we observed that surface soil organic C and N pools were greater in cheatgrass and crested wheatgrass than sagebrush-dominated sites. Greater biomass turnover in cheatgrass and crested wheatgrass versus sagebrush stands may result in faster rates of soil C and N cycling, with redistribution of actively cycled N towards the soil surface. Plant biomass and surface soil δ15N ratios were enriched in cheatgrass and crested wheatgrass relative to sagebrush-dominated sites. Source pools of plant available N could become 15N enriched if faster soil N cycling rates lead to greater N trace gas losses. In the absence of wildfire, if cheatgrass invasion does lead to degradation of ecosystem function, this may be due to faster nutrient cycling and greater nutrient losses, rather than reduced organic matter inputs.