Characterizing Land Surface Phenology and Exotic Annual Grasses in Dryland Ecosystems Using Landsat and Sentinel-2 Data in Harmony

Invasive annual grasses, such as cheatgrass (Bromus tectorum L.), have proliferated in dryland ecosystems of the western United States, promoting increased fire activity and reduced biodiversity that can be detrimental to socio-environmental systems. Monitoring exotic annual grass cover and dynamics over large areas requires the use of remote sensing that can support early detection and rapid response initiatives. However, few studies have leveraged remote sensing technologies and computing frameworks capable of providing rangeland managers with maps of exotic annual grass cover at relatively high spatiotemporal resolutions and near real-time latencies. Here, we developed a system for automated mapping of invasive annual grass (%) cover using in situ observations, harmonized Landsat and Sentinel-2 (HLS) data, maps of biophysical variables, and machine learning techniques. A robust and automated cloud, cloud shadow, water, and snow/ice masking procedure (mean overall accuracy >81%) was implemented using time-series outlier detection and data mining techniques prior to spatiotemporal interpolation of HLS data via regression tree models (r = 0.94; mean absolute error (MAE) = 0.02). Weekly, cloud-free normalized difference vegetation index (NDVI) image composites (2016–2018) were used to construct a suite of spectral and phenological metrics (e.g., start and end of season dates), consistent with information derived from Moderate Resolution Image Spectroradiometer (MODIS) data. These metrics were incorporated into a data mining framework that accurately (r = 0.83; MAE = 11) modeled and mapped exotic annual grass (%) cover throughout dryland ecosystems in the western United States at a native, 30-m spatial resolution. Our results show that inclusion of weekly HLS time-series data and derived indicators improves our ability to map exotic annual grass cover, as compared to distribution models that use MODIS products or monthly, seasonal, or annual HLS composites as primary inputs. This research fills a critical gap in our ability to effectively assess, manage, and monitor drylands by providing a framework that allows for an accurate and timely depiction of land surface phenology and exotic annual grass cover at spatial and temporal resolutions that are meaningful to local resource managers.

Vegetative Community Response to Landscape-Scale Post-fire Herbicide (Imazapic) Application

Disturbances such as wildfire create time-sensitive windows of opportunity for invasive plant treatment, and the timing of herbicide application relative to the time course of plant community development following fire can strongly influence herbicide effectiveness. We evaluated the effect of herbicide (imazapic) applied in the first winter or second fall after the 113,000 ha Soda wildfire on the target exotic annual grasses and also key non-target components of the plant community. We measured responses of exotic and native species cover, species diversity, and occurrence frequency of shrubs and forbs seeded before (1 to 2 or 9 to 10 mo) herbicide application. Additionally, we asked whether landscape factors, including topography, species richness, and/or soil characteristics, influenced the effectiveness of imazapic. Cover of exotic annual grass cover, but not of deep-rooted perennial bunchgrass, was less where imazapic had been applied, whereas more variability was evident in the response of Sandberg bluegrass (Poa secunda J. Presl) and seeded shrubs and forbs. Regression-tree analysis of the subset of plots measured both before and after the second fall application revealed greater reductions of exotic annual grass cover in places where their cover was <42% before spraying. Otherwise, imazapic effects did not vary with the landscape factors we analyzed.

Herbicide Control Strategies for Ventenata dubia in the Intermountain Pacific Northwest

Ventenata dubia is an exotic annual grass that has become increasingly invasive in various perennial grass systems throughout the Intermountain Pacific Northwest. Currently, little information is available to landowners about herbicide control options. In our first field study, we evaluated V. dubia control efficacy and perennial grass tolerance of herbicides applied pre-emergence (PRE) at two locations and as an early postemergence (EPOST) application at four different conservation reserve grasslands, with each grassland dominated by different perennial grass species. Treatments included flufenacet plus metribuzin (303 + 76 g ai ha−1 [0.27 + 0.07 lb ai ac−1]), propoxycarbazone-sodium (49 g ai ha−1 [0.04 lb ai ac−1]), rimsulfuron (53 g ai ha−1 [0.05 lb ai ac−1]), sulfosulfuron (53 g ai ha−1 [0.05 lb ai ac−1]), and imazapic (105 g ai ha−1 [0.09 lb ai ac−1]). Rimsulfuron and flufenacet plus metribuzin applied PRE provided > 90% control 10 mo after treatment (MAT). Rimsulfuron and sulfosulfuron applied EPOST provided > 90% control 9 MAT. Herbicide injury to bluebunch and intermediate wheatgrass was negligible across treatments. Imazapic and sulfosulfuron applied EPOST resulted in significant injury to smooth brome and timothy. In our second study, we addressed the following question: Will fall herbicide plus fertilizer treatments improve V. dubia control compared with herbicide treatments alone? We imposed fall herbicide treatments in main plots and fertilizer treatments (fall N, fall P, fall K, fall PK, spring N, NPK) in split plots at three study locations. Herbicide treatments resulted in high levels of V. dubia control. Differences in V. dubia abundance among fertilizer treatments were negligible 9 MAT. Within herbicide control plots, spring N and NPK treatments resulted in significant increases in perennial grass cover and decreases in V. dubia cover (9 MAT). This result indicates that spring N applications timed to the onset of perennial grass growth could be utilized as a component of an integrated management strategy for V. dubia in invaded perennial grass systems.