## 3 Results and Discussion
### Impact of Burning on Vegetation Phenology  
Pasture 14 was burned in 2014. As a result, the EVI showed earlier green-up and higher peak values (peak EVI >0.7) than the other pastures (peak EVI <0.6, Figure 4). Pastures 16, 18, and 20 were burned in 2015. As a result, the EVI showed earlier green-up and higher peak values at these pastures than the remaining pastures. In 2018, burning effects on early green-up and higher forage production did not appear due to dry conditions during spring. Total precipitation during January to May 2018  
Figure 3: Seasonal dynamics of the enhanced vegetation index (EVI) in three selected years with different distribution patterns of precipitation at the uppermost landscape position (Pasture 14). The pasture was burned in 2014.  
was only 147 mm (Table 3), the lowest for that period over the previous two decades. Consequently, vegetation phenology in all pastures was similar in 2018, regardless of burning treatment. We compared the dynamics of EVI for P15 in dry (2012) and wet (2017) burned years vs. dry (2011) and wet (2015) unburned years to further examine the impacts of burning on vegetation phenology (Figure 5). Similarly, the dynamics of EVI were compared for P16 in dry (2011) and wet (2015) burned years vs. dry (2012) and wet (2017) unburned years. In general, vegetation greened-up and peaked earlier in burned years. Although 2012 was a dry year, the spring was warm and wet causing early green-up in 2012 in both pastures, though P16 was not burned in 2012. However, vegetation greened-up slightly earlier in the burned P15 than unburned P16. In comparison, both 2015 and 2017 were wet years and burning showed a clear impact on early green-up during both years. Vegetation greened-up and peaked early in 2017 (burned year) in P15 and 2015 (burned year) in P16. In P16, vegetation greened-up late and the EVI values were lowest during the drought-affected year (2011) despite application of a prescribed burn. The EVI values were greater in the unburned P15 than the burned P16 during the 2011 drought, illustrating negative impacts of the combination of a prescribed burn and drought. Figure 4: Seasonal dynamics of the enhanced vegetation index (EVI) at six tallgrass prairie pastures for the three selected years. In general, burning induces earlier green-up of vegetation due to greater soil heating and increased solar radiation at the soil surface by removing the litter layer [3]. We also observed early green-up of vegetation in burned years under normal climatic (i.e., no drought) conditions (Figure 4). However, the burning had minimal or no impact on the timing of green-up in burned and unburned pastures in 2012 (Figure 6), when spring was warm (average T\({}_{\text{a}}\) was \(\sim\)18 \({}^{\circ}\)C for March-May period as compared to long-term, 1981-2010, mean of \(\sim\)15 \({}^{\circ}\)C) with well-distributed precipitation. These results indicate that greater temperature and availability of solar radiation at the soil surface are the main reasons for early green-up of vegetation following prescribed spring burns. Burning also enhances below- and above-ground plant productivity in tallgrass prairie because of more plant-available nitrogen (N) due to increased mineralization from soil organic matter (SOM) as a consequence of higher soil temperature and additional impacts on biophysical properties and plant physiological responses [4; 30; 31]. We also observed relatively higher peak values of EVI during burned years under normal precipitation but not during dry years (Figure 4) due to reduced soil water availability. Burning can reduce availability of soil water in numerous ways: by increasing evaporation from bare soil immediately after burns, and by increasing plant transpiration due to earlier green-up and enhanced vegetative growth. Burning substantially reduced SWC at all depths up to 1.5 m in bluestem prairie in the Flint Hills, Kansas [32].