The elevational ascent and spread of exotic annual grasslands in the Great Basin, USA

In the Great Basin of the U.S., sagebrush (Artemisia spp.) and salt desert shrublands are rapidly transitioning to exotic annual grasslands, a novel and often self-reinforcing state that threatens the economic sustainability and conservation value of western grazing lands. Climate change is predicted to directly and indirectly favor annual grasses, potentially pushing annual grassland transitions into higher elevations. We used recently developed remote sensing-based rangeland vegetation data to retrospectively quantify expansion and elevational range shift of annual grassland transitions in the Great Basin from 1986–2019. During this period, we document an alarming six-fold increase in annual grassland area (to >75,000 km2) occurring at a rate of 1,950 km2 yr-1. Annual grasslands now occupy one fifth of Great Basin rangelands. This rapid expansion has been in part facilitated by a broadening of elevational range limits, with the leading edge of annual grassland transitions moving upslope at 60–110 m decade-1. Accelerated intervention is critically needed to conserve the fragile band of rangelands being compressed between annual grassland transitions at lower elevations and woodland expansion at higher elevations.SignificanceExotic annual grasses became widespread throughout the western U.S. Great Basin in the last century and now rank among the most vexing challenges facing western rangelands. Once established, these invaders can transform native sagebrush (Artemisia spp.) and salt desert shrublands into virtual monocultures of highly flammable exotic annual grasses with severely diminished biological and economic value. Capitalizing on a recently developed remote sensing vegetation product providing continuous spatial and annual temporal coverage of western US rangelands, we map the expansion of exotic annual grasslands over the past three decades. Our analysis reveals the alarming pace at which native shrublands are transitioning to annual grasslands, and confirms the movement of these transitions into ever higher elevations as the climate of the western U.S. warms.

A Sense of Scale

The native vegetation communities in the sagebrush steppe, a semi-arid ecosystem type, are under threat from exotic annual grasses. Exotic annual grasses increase fire severity and frequency, decrease biodiversity, and reduce soil carbon storage amongst other ecosystem services. The invasion of exotic annual grasses is causing detrimental impacts to land use by eliminating forage for livestock and creating a huge economic cost from fire control and post-fire restoration. To combat invasion, land managers need to know what exotic annual grasses are present, where they are invading, and estimates of their biomass. Mapping exotic annual grasses is challenging because many areas in the sagebrush steppe are difficult to access; yet field measurements are the main method to identify and quantify their existence. In this study, we address this challenge by exploring the use of both landscape-scale and plot-scale observations with remote sensing. First, we use satellite imagery to map where exotic annual grasses are invading and identify the native species which are being encroached upon. Second, we investigate the use of fine-scale imagery for non-destructive measurements of biomass of exotic annual grasses. Understanding the location of exotic annual grasses is important for restoration efforts, e.g. large swath (~100m) herbicide spraying. Restoration efforts are expensive and often ineffective in areas already dominated by exotic annual grasses. Early detection of exotic annual grasses in sagebrush and native grasses communities will increase the chances of effective ecosystem restoration. We used Sentinel-2 satellite imagery in Google Earth Engine, a cloud computing platform, to train a random forest (RF) machine learning algorithm to map vegetation in ~150,000 acres in the sagebrush steppe in southeast Idaho. The result is a classification map of vegetation (overall accuracy of 72%) and a map of percent cover of annual grass (R2 = 0.58). The combination of these two maps will allow land managers to target areas of restoration and make informed decisions about where to allow grazing. In addition to knowing what exotic annual grasses exist and their percent cover, detailed information about their biomass is important for understanding fuel loads and forage quality. Structure from Motion (SfM) is a photogrammetry technique that uses digital images to develop 3-dimensional point clouds that can be transformed into volumetric measurements of biomass. The SfM technique has the potential to quantify biomass estimates across multiple plots while minimizing field work. We developed allometric equations relating SfM-derived volume (m3) to biomass (g/m2) for a study area in southeast Oregon. The resulting equation showed a positive relationship (R2 = 0.51) between the log transformed SfM-derived volume and log transformed biomass when litter was removed. This relationship shows promise in being upscaled to larger surveys using aerial platforms. This method can reduce the need for destructively harvesting biomass, and thus allow field work to cover a greater spatial extent. Ultimately, increasing spatial coverage for biomass will improve accuracy in quantifying fuel loads and carbon storage, providing insights to how these exotic plants are altering ecosystem services.

Timing Aminopyralid to Prevent Seed Production Controls Medusahead (Taeniatherum caput-medusae) and Increases Forage Grasses

Exotic annual grasses such as medusahead [Taeniatherum caput-medusae(L.) Nevski] and downy brome (Bromus tectorumL.) dominate millions of hectares of grasslands in the western United States. Applying picloram, aminopyralid, and other growth regulator herbicides at late growth stages reduces seed production of most exotic annual grasses. In this study, we applied aminopyralid toT. caput-medusaeto determine how reducing seed production in the current growing season influenced cover in the subsequent growing season. At eight annual grassland sites, we applied aminopyralid at 55, 123, and 245 g ae ha−1in spring just beforeT. caput-medusaeheading. The two higher rates were also applied pre-emergence (PRE) in fall to allow comparisons with this previously tested timing. When applied in spring during the roughly 10-d period between the flag leaf and inflorescence first becoming visible, just 55 g ae ha−1of aminopyralid greatly limited seed production and subsequently reducedT. caput-medusaecover to nearly zero. Fall aminopyralid applications were less effective againstT. caput-medusae, even at a rate of 245 g ae ha−1. The growing season of application, fall treatments, but not spring treatments, sometimes reduced cover of desirable winter annual forage grasses. The growing season after application, both spring and fall treatments tended to increase forage grasses, though spring treatments generally caused larger increases. Compared with other herbicide treatment options, preheading aminopyralid treatments are a relatively inexpensive, effective approach for controllingT. caput-medusaeand increasing forage production.