Grazing Treatment Influences Recovery of Mesic Grassland from Seasonal Drought: An Assessment Using Unmanned Aerial Vehicle−Enabled Remote Sensing

Drought is frequent on grasslands, often creating patches with reduced coverage of green vegetation that influence the trajectory of vegetation recovery following drought. Limiting the extent and sizes of low-cover patches thus should be a target of adaptive management on grazed grasslands, but quantifying patch sizes requires measurements at small spatial resolution. We used airborne (unmanned aerial vehicle) estimates of green plant cover at 8-cm spatial resolution to quantify the aerial extent and patch sizes of mesic grassland in central Texas, United States in which green plant cover was eliminated during a growing-season (seasonal) drought in each of two grazing treatments (rotational, continuous). Remote measurements of the normalized difference vegetation index (NDVI), indicative of plant biomass, were then used to compare postdrought recovery of vegetation in patches that lacked versus retained green plant cover during drought (not green vs. green). Below-average precipitation from April through July 2018 (41% of the mean) eliminated green plant cover from 5%−12% of grassland area. Fractional coverage of the not-green area was greater by a factor of 2 under continuous grazing. Formerly not-green patches exhibited full recovery in seasonal mean values of NDVI during the year following drought in both grazing treatments. NDVI recovery was facilitated by a marked increase in abundance of early-season annual species in pasture dominated by summer-maturing perennial grasses. Spring NDVI of formerly not-green patches was greatest under continuous grazing during 2 post-drought yr, implying that annual species limited colonization of perennial vegetation more under continuous than rotational grazing. Our results imply that managers can facilitate drought recovery of perennial vegetation in mesic grassland by 1) adaptively managing grazing to reduce patch sizes of grassland that suffer complete loss of green plant cover and 2) employing rotational grazing to eliminate grazing pressure for several consecutive months annually following drought.