## 3 Results and Discussion
### Interannual Variability in Vegetation Phenology during Dornant Seasons  
When EVI\({}_{250\text{m}}\) and NDVI\({}_{250\text{m}}\) were compared for six (from 2015-2016 to 2020-2021) dormant seasons, the EVI (Figure 3a) showed a larger interannual variability than did NDVI (Figure 3b). There were some differences in the patterns of NDVI and EVI among pastures during the dormant seasons as they were influenced by weather conditions (e.g., rainfall) and management practices (e.g., burning, grazing, or hay harvest). The EVI declined immediately after burning (e.g., P13 in 2018, P14 in 2019, P15 in 2017, and P16 in 2021), but this reduction was not evident from the NDVI. The EVI is an improved form of the NDVI, which employs an additional blue band for atmospheric corrections and a background brightness factor (L) for background noise corrections [38]. The failure of NDVI to show reductions in the vegetation index after burning can be attributed to darker soils (e.g., burned soils), resulting in higher values of vegetation index for a given amount of vegetation if the soil background effect is not removed [41]. These results indicate a greater potential of EVI than NDVI to track vegetation phenology in tallgrass prairie during dormant seasons. The results illustrated the interactive effects between prescribed spring burns and rainfall on vegetation phenology. The EVI values were lowest until late March of 2018 for P13 and 2021 for P16 than in other years due to drier conditions after burning (Figure 3a). In comparison, EVI and NDVI increased rapidly in P15 after burning in 2017 due to wetter conditions after burning. During 2016 and 2017, when P18 and P20 were burned, EVI and NDVI values increased rapidly due to warmer and wetter conditions than during other years (Figure 3a,b). These results capture the combined positive impacts of the combination of prescribed burns and non-drought conditions on enhanced growth and early greening of vegetation in March. Greater soil temperature and increased solar radiation at the soil surface following burning and enhanced mineralization of soil nitrogen by burning litter layer induce earlier greening of vegetation in the absence of water stress [7; 9; 42]. However, the results also illustrated the negative impacts of prescribed spring burns on the greening of vegetation during drought conditions due to reduced soil moisture. Soil moisture was substantially reduced by burning at all depths up to 1.5 m in the Bluestem Prairie in Kansas [43]. Figure 2: Monthly average of daily total solar radiation (ATOT) during the non-growing season (November-March) of native prairie at the El Reno Mesonet station. Figure 3: Interannual variability of the Moderate Resolution Imaging Spectroradiometer-derived enhanced vegetation index (EVI) (**a**) and normalized difference vegetation index (NDVI) (**b**) at 250 m spatial resolutions during the non-growing season (November–March) of native prairie. Excluding years when good rainfall (i.e., no occurrence of droughts) occurred in combination with a prescribed burn, there were no observable larger sums of EVI\({}_{250\rm m}\) and NDVI\({}_{250\rm m}\) during warmer and wetter dormant seasons compared to cooler and drier dormant seasons in any of P13, P14, P15, and P16 that were burned in 4-5-year rotations (data not shown). Detritus accumulation limits the early greening of vegetation due to decreased solar radiation and soil temperature at the soil surface [7]. In comparison, annually burned P18 and P20 had larger dormant season sums of EVI\({}_{250\rm m}\) and NDVI\({}_{250\rm m}\) during warmer and wetter years (e.g., 2016 and 2017) compared to other years, due to the positive impact of the combination of prescribed burns and rainfall. Burning of the litter layer increases soil temperature and solar radiation at the soil surface and induces the early greening of vegetation during non-drought conditions [7; 9].