Effect of fire size and severity on subsequent fires using difference normalized burn ratios on the Osceola National Forest

Background/Question/Methods and Results/Conclusions

Fire is a natural component of ecosystems in Florida. Florida forests naturally experienced frequent low intensity fires, yet fire exclusion polices have altered the forest structure resulting in uncharacteristically severe wildfires. The Osceola National Forest in north Florida has experienced high wildfire occurrence for a number of years. Communities within the Osceola are fire dependent and require regular burning for pine germination and understory maintenance. Forest managers are currently working to reintroduce fire back into this system where it has been absent for decades in hopes of reducing the risk of intense wildfires and restoring ecosystem health. The objective of this study is to use Difference Normalized Burn Ratio (DNBR) to evaluate past fire severity, size, and frequency to determine the appropriate fire frequency for management objectives. A DNBR captures the spectral response in Landsat TM scenes over a time interval, and indicates the degree of change that is due to fire. This analysis has shown that wildfires in unburned areas and wildfires on mesic flatwoods exhibited a higher portion of high severity pixels. Pixels that have not experienced fire in 5 years are indistinguishable from areas that have never burned and the probability of a pixel with high severity reoccurring within 3 years is very low. Using DNBR as a method of analyzing past fire severity is a useful tool for managers to determine the effects of severity level and how to better predict areas of potentially high severity. The analysis has further provided an effective method of determining fire frequencies necessary to maintain the optimum level of wildfire protection.