Enhanced erosion of pyrolyzed soil surfaces drives rapid recovery of post-fire landscape hydrologic functions

The prevailing paradigm is that recovery of post-fire soil-hydrologic properties is dominated by pedogenic processes that drive soil structure formation, a critical process for regaining soil hydrologic functioning. The primary drivers for soil structure formation are climate and vegetation required for soil biological activity. Evidence shows that following perturbations of agricultural soils (e.g., compaction) or abrupt land use change soil structure recovery may take years to decades. In contrast, measurements from post-fire soils show that recovery of critical soil hydrologic properties (notably soil saturated hydraulic conductivity) is rapid (generally within 1 to 3 yrs) and occur at rates faster than expected from soil structure regeneration. To reconcile the rapid post-fire recovery rates, we propose a new conceptual framework for recovery of post-fire soil-hydrologic properties driven primarily by accelerated erosion of the unstable and structureless pyrolyzed surface soil layer. In this framework, initial recovery occurs not by redeveloping new structure in the pyrolyzed surface soil layer, but rather by removing it, thus exposing minimally-affected sublayers as new soil surfaces. Based on wildfire characteristics, a typical depth of pyrolyzed soil layer is estimated to be a few centimeters (<5 cm) depending on fuel load, burning times and heat transport. A tentative peak temperature of 300 C (torrefaction limit) defines the extent of loss of binding organic carbon thus creating a fragile and easily transported layer by wind or water erosion. Examples of the proposed mechanism in several Western US post-fire landscapes will be presented with discussion of various landscape geomorphic controls (topography, post-fire rainfall, ash transport and more).