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.

Monitoring Mediterranean grass phenology from digital terrestrial camera and Sentinel-2 vegetation indices in an oak-grass savanna ecosystem

<p>Annual grasslands are an essential component of Mediterranean oak savannas, the most extensive agroforestry system in Europe, as the primary source of fodder for livestock and wildlife. Monitoring its phenology is key to adequately assess the impacts of global warming on different time scales and identify pre-critical states in the framework of early warning decision making systems. The natural variability of the climatic-hydrological regime in these areas and the usually complex spatial patterns of the vegetation, with sparse distribution and multiple layers, encourage the exploitation of available data from remote sensing sources. This work presents an assessment of vegetation indexes (VI) from Sentinel-2 validated against field data from terrestrial photography in an oak-grass system in southern Spain as a multi-approach method to monitor phenology in grass pastures. The analysis also has provided an insight into the links of the phenology dynamics with hydrological variables under these conditions.</p><p>From December 2017 to May 2019 a quantitative value of grassland greenness was computed using the Green Chromatic Coordinate (GCC) index. The phenological parameters of the start of the season (SOS), the peak of the season (POS) and end of the season (EOS) were extracted using the 50% amplitude method and confirmed using field photography. These values were compared with those provided by eight VI's derived from Sentinel-2 (NDVI, GNDVI, SAVI, EVI, EVI2, MTCI, IRECI and S2REP) and the difference in days between the key phenological dates were estimated. The results showed that for annual grasslands NDVI was the index providing estimations closest to those of ground GCC, with differences below 10 days for all phenological dates and the best correlation with GCC values (r = 0.83, p <0.001). None of the VIs using bands in the red-edge region have improved the NDVI results. Two of them, MTCI and S2REP, followed a different trend that the rest of explored indices, presenting a high temporal variability. The high diversity of species, typical of Mediterranean grasslands, might explain the high variability observed in these values. However, the third index using red-edge bands, IRECI, presented a high correlation with GCC. In this case, the index was designed to focus on the chlorophyll content of the canopy instead of the leaf scale addressed by S2REP. The influence of the vegetation ground coverage and foliage density is then higher and more similar to the broad-band indices. GNDVI also provided good general results. Soil moisture (SM) time-series were also used to estimate phenology and have presented a good agreement with GCC in SOS and EOS estimations, with SM reaching threshold values a few days before greenness ones, as measured by GCC. However, SM was not a good indicator of the POS, presenting significant biases with respect to GCC estimations.</p>