Invasive Species
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Using a wet thermal time model for germination prediction, this study estimated progress toward germination (PTG) of 31 seedlots (10 species) as a function of hourly seedbed temperature (> 0 °C) when soils were above a water potential of −1.5 MPa. Seasonally-summed progress toward germination with a value > 1 indicates that germination will occur for that season. We used near surface (1–3 cm) soil water potential and temperature measurements collected at 24 sites in the Great Basin to determine effects of site, season, and year on PTG. On tree encroached sites, we also determined effects of tree infilling phase at time of tree removal, removal method, and microsite on estimated PTG. Soils were wet and warm enough in early spring, late spring, and fall for PTG > 1 indicating potential germination for most seedlots and species on most sites and years. Prescribed burning increased PTG as much as three times more than either tree cutting or mechanical shredding. Germination prediction could help to screen for plant materials adapted to specific sites or assess effects of seed additives or treatments that time germination to maximize seedling survival.
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In this study, a multivariate dataset was analyzed using principal components analysis to identify “defining factors” that best explained variation among sites. Variation was primarily attributed to an inverse relationship between crested wheatgrass and sagebrush abundance (R2 = 0.69; P < 0.0001) and their affinity for either silty or sandy soil textures, respectively, as well as a negative association between crested wheatgrass abundance and species diversity (R2 = 0.67; P < 0.0001). These results do not support the assumption that crested wheatgrass seedings uniformly remain in vegetation states with low diversity and poor sagebrush reestablishment over the long term (i.e., 43 − 63 yr). We suggest that a broader interpretation of plant community dynamics is needed while avoiding generalizations of how historically seeded Wyoming big sagebrush sites will respond over time.
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In this study, exotic annual grass cover and density were greatly reduced in all treatments where perennial seedlings were planted compared with the control (no seedlings planted). Treatments including crested wheatgrass (Agropyron desertorum) generally limited annual grasses more than other treatments. Most notably, forage kochia (Bassia prostata) reduced exotic annual grasses less than crested wheatgrass and crested wheatgrass planted with forage kochia. This suggests that if forage kochia will be planted, it should be used in conjunction with perennial bunchgrasses in efforts to revegetate exotic annual grass − invaded sagebrush steppe. Established native vegetation also greatly reduced exotic annual grass reinvasion. Though some differences existed among established vegetation treatments, our study highlights that established perennial vegetation prevents redomination by invasives after exotic annual grass control.
View Chapter 12 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
Invasive annual grass research and management in arid and semiarid ecosystems of the Western United States (USA) have historically focused on reducing weed abundance as opposed to ecosystem restoration, which addresses the underlying processes responsible for their persistence. Given the current impact of invasive annual grasses and their continued spread in this region, we identified common characteristics responsible for persistence of the most problematic exotic annual Bromus. For heavily invaded areas, these include transient, yet typically large seed banks, altered soil resource availability and litter production, displacement of native species, and frequent disturbance from fire. To better address these common characteristics for future management, we illustrate how an adaptive management framework can reduce existing uncertainty associated with the restoration of arid and semiarid ecosystems.
View Chapter 11 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
Human land uses are the primary cause of the introduction and spread of exotic annual Bromus species. Initial introductions were likely linked to contaminated seeds used by homesteading farmers in the late 1880s and early 1900s. Transportation routes aided their spread. Unrestricted livestock grazing from the 1800s through the mid-1900s reduced native plant competitors leaving large areas vulnerable to Bromus dominance. Ecosystems with cooler and moister soils tend to have greater potential to recover from disturbances (resilience) and to be more resistant to Bromus invasion and dominance. Warmer and drier ecosystems are less resistant to Bromus and are threatened by altered fire regimes which can lead to Bromus dominance, impacts to wildlife, and alternative stable states.
View Chapter 10 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: Causes, consequences, and management implications.
The factors that determine plant community resistance to exotic annual Bromus species are diverse and context specific. They are influenced by the environmental characteristics and attributes of the community, the traits of Bromus species, and the direct and indirect interactions of Bromus with the plant community. Environmental factors, in particular ambient and soil temperatures, have significant effects on the ability of Bromus to establish and spread.
View Chapter 9 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
A prominent goal of invasive plant management is to prevent or reduce the spread of invasive species into uninvaded landscapes and regions. Monitoring and control efforts often rely on scientific knowledge of suitable habitat for the invasive species. However, rising temperatures and altered precipitation projected with climate change are likely to shift the geographic range of that suitable habitat. Here, we review experimental and modeling studies of climatic limits to exotic annual Bromus distribution in the Intermountain West in the context of projections of future climate change.
View Chapter 8 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
Abiotic factors have a strong influence on where annual Bromus species are found. At the large regional scale, temperature and precipitation extremes determine the boundaries of Bromus occurrence. At the more local scale, soil characteristics and climate influence distribution, cover, and performance. In hot, dry, summer rainfall-dominated deserts (Sonoran, Chihuahuan), little or no Bromus is found, likely due to timing or amount of soil moisture relative to Bromus phenology. In hot, winter-rainfall-dominated deserts (parts of the Mojave Desert), Bromus rubens is widespread and correlated with high phosphorus availability. It also responds positively to additions of nitrogen alone or with phosphorus.
View Chapter 7 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
Bromus tectorum presents a rich resource for soil microorganisms because of its abundant production of biomass, seeds, and surface litter. These organisms interact dynamically with abiotic factors such as interannual variation in weather, with other soil microorganisms, with their hosts, and with each other to create spatially and temporally varying patterns of endemic or epidemic disease. Five principal soil borne pathogens, Ustilago bullata, Tilletia bromi, Pyrenophora semeniperda, Fusarium, and a new species in the Rutstroemiaceae (bleach blonde syndrome pathogen), are known to have sometimes major impacts on B. tectorum seed bank dynamics, seedling emergence, and seed production. Naturally occurring fungal pathogens that can have a strong negative impact on B. tectorum success have also been considered as candidate organisms for B. tectorum biocontrol using an augmentative mycoherbicidal strategy.
View Chapter 3 of the book, Exotic brome-grasses in arid and semiarid ecosystems of the western US: causes, consequences, and management implications.
An understanding of the impacts of exotic plant species on ecosystems is necessary to justify and guide efforts to limit their spread, restore natives, and plan for conservation. Invasive annual grasses such as Bromus tectorum, B. rubens, B. hordeaceus, and B. diandrus (hereafter collectively referred to as Bromus) transform the structure and function of ecosystems they dominate. Experiments that prove cause-and-effect impacts of Bromus are rare, yet inferences can be gleaned from the combination of Bromus-ecosystem associations, ecosystem condition before/after invasion, and an understanding of underlying mechanisms.