Do fire severity effects on soil change in space and time in the short-term? What ash tells us

Abstract

n the absence of data, the impact of fire, especially wildfires, is measured analysing the fire severity. This post-fire assessment is very useful because allow to identify the degree of destruction imposed by the fire. Among the techniques used to determine fire severity, ash colour is often used, that permit identify the degree of organic matter consumption (darker ash uncompleted combustion, lighter ash completed combustion). The objective of this paper was observed if fire severity changes in space and time, according to ash colour analysis, applying an index. The ash colour analysis was carried out one and fifteen days after the fire. In this area we identified ash with four different colours, black (B) dark grey (DG), light gray (LG) and white colour (W) and some uncovered areas classified as bared soil (BS). Black and DG represent medium fire severity, LG and W, higher severity. The results showed that in the studied fire, the severity was high and a great part of the plot was uncovered by ash (BS). Fifteen days after BS increased as the fire severity index, from 6.05 to 6.45, showing that during this period the ash redistribution in a short period after the fire can influence the fire severity assessment. We did not identified significant differences between measurements and the coefficient of variation (CV%) remained the same. However significant differences were identified with the spatial correlation analysis with Global Moran's I and the spatial structure of fire severity index. This is evidence that ash color changed in this period in the space and the traditional statistical methods did not detected, only with spatial analysis. The analysis of fire severity using ash color some days after the fire can induce important errors, because wind can (re)mix ash and a particle produced in one area can be easily exported to other