Analysis of US Grazingland Area Response to Management and Environmental Forcings through Vegetative Phenology and Evapotranspiration

Regenerative management strategies are being adopted in a wide range of agricultural systems in an effort to improve soil and agroecosystem health. In grazing lands, adaptive grazing practices are being evaluated, where stock movement and grazing schedules are re-actively refined through the season based on metrics of paddock health and recovery rather than prescribed at the beginning of the season. In some research fields, adaptively and prescriptively managed paddocks are adjacent placed to study the time evolution in pasture health post-management change. Evaluation of the impact of such management practices is enhanced in having a more comprehensive understanding of site dynamics prior to the start of the experiment, regarding phenological patterns in water use and plant growth and their response to climate drivers (e.g. rainfall, drought, temperature). This study investigates a 24-year (2001-2024) time history of vegetation dynamics and climate forcings at grazing land experiment sites in Wyoming, Oklahoma, and Michigan, capturing a gradient in precipitation and cover conditions across US grazing lands. Seasonal timing of vegetative greenup, leaf area index (LAI, and evapotranspiration (ET) are analyzed in correspondence to variations in temperature, precipitation, and land cover classification in the experiment fields and surrounding areas.

Greenup data is obtained from the MODIS MCD12Q2V61 greenup product, and contains the day of year of 15% greenness at a 500 m spatial resolution. MODIS leaf area index (LAI) is also analyzed. When compared to anomalies temperature and precipitation data from NASA's DAYMET reprojection and the US drought monitor, we can identify correspondence between greenup and weather for various land cover types – primarily forest, grassland and cropland over these sites. The time history of the Evaporative Stress Index (ESI), representing standardized anomalies in the ratio of remotely sensed actual-to-reference ET, is also evaluated at these sites as an indicator of vegetative water stress. ESI is available across the continental United States (CONUS) at a 4 km spatial resolution and has shown good response to agricultural drought events in the U.S. and globally.

At the 4-km scale, we found very different degrees of sensitivity in ESI and LAI anomalies to drought events across the climate gradient of our three research sites, due in part to variations in landcover composition and frequency and severity of drought. 30-m daily ET and LAI products for 2022-2023, developed through a fusion Harmonized Landsat-Sentinel data facilitate diagnosis the sub-pixel variability in these coarser-resolution/longer timeseries indices. We use these assess the differential response of different landcover types to climate forcings.

These results set a baseline by which future ecosystem response resulting from management practice can be evaluated, for example possible increase in resilience to drought due to improved soil health. Small scale spatial variability in environmental conditions is better understood in context with the long term, coarse scale climatologies. Going forward, we will continue to utilize remote sensing to observe ESI, greenup, and other land surface variables at these grazing land sites for the duration of this experiment and beyond. Analysis of within - field variability of vegetative and soil health may shed light on the benefits of adaptive management.