Quantifying the dynamic of cereals and broad leaves plants in semi-arid ranging grasslands using PlanetScope® satellite imaging

The vegetation composition in grasslands is temporarily changed during its lifespan, due to the different physiological development of broadleaves and cereals species. Determination of the temporal vegetation composition and its dynamic, which is additionally termed: 'phenological state', is needed for maximal pasture utilization and sustainability. The widely used low-spatial and temporal resolution satellites are primarily suited to flat landscapes and low frequent imaging. Therefore, they are less suitable for the patchy landscape of arid regions and their plant development, characterized by rapid physiological processes. However, determining these processes using high-resolution satellite imagery needs to be studied better. Therefore, the following objectives were defined: (i) Determining the dynamic of cereals and broad-leaves vegetation in an arid grassland using timelines of High-resolution satellite imagery and (ii) Determining the influence of different grazing intensities on grassland vegetation phenology. The study site is in Migda experimental farm, northern Negev, Israel. Fifty-three grassland plots, sized 10X10m, were located. Seventeen plots were grazed, six of them heavily (90 min. per plot), five moderately (60 min.), and six lightly (30 min.). The grazing was carried out by a small ruminant herd, composed of 120- 130 animals, mainly sheep, at the cereals fallow stage (Between April end and May beginning) during 2019 and 2023. As a reference, thirty-six fenced and ungrazed plots were located; half of them are covered by cereals (Conserved), and half are covered by broad- leaves, due to spraying selective anti-cereals herbicide in January of each year (Sprayed). Imagery data from PlanetScope® was acquired (Spatial resolution: 3m pixel -1, Temporal: Once in three days, Spectral: RGB and NIR). Normalizing the different treatments NDVI, which was found to be the most suitable vegetation index by the Conserved values, indicates a phenology of a three-phase scheme. The first and the third phases (absent in part of the treatments) resemble a parabolic increase of Fresh vegetation reflectance (FVR) from the treatment, and the second resembles a parabolic increase in FVR from the Conserved. In most treatments, the duration and maximal NDVI of the first phase were positively correlated to the rainfall amounts, while a less dominant trend was found in the second phase. A rehabilitation trend over the years was calculated for the first and third phases of the lightly and moderately grazed treatments, correlated to increased broadleaf biomass, and a degradation trend was calculated for the heavily grazed. The finding indicates a parabolic change of vegetation in arid grassland. This novel methodology may be suited to other ecosystems.