Changes in Soil Structure and Hydraulic Properties in a Wooded‐Shrubland Ecosystem following a Prescribed Fire

Abstract

Pre‐ and post‐fire measurements were made for a low‐intensity prescribed fire in a semiarid, shrub‐woodland transition zone, and objectives were to: (i) determine changes in near‐saturated hydraulic conductivity (Kf measured with a tension infiltrometer), air permeability (ka measured with an air permeameter), and soil physical properties at shrub undercanopy and interspace microsites immediately before and after a fall burn and for a 13‐mo period; and (ii) quantify the importance and effect of post‐fire soil structure on hydraulic properties using pre‐ and post‐fire measurements. At undercanopy microsites, structure deteriorated from a moderate to a weak subangular blocky structure after the fire that broke down to a structureless soil 10 mo later. At interspace microsites, post‐fire soil structure deteriorated from a moderate‐strong subangular blocky structure with hard dry consistency to a weak subangular blocky structure with soft dry consistency. After 10 mo, the intercanopy maintained a weak‐moderate soil structure that became structureless‐weak after 13 mo. Immediately after the fire, at both microsites, there was incomplete organic combustion, a decrease in bulk density, and an increase in ka; however, at undercanopy microsites, there was no significant change in Kf even though there was a slight to moderate hydrophobicity, whereas at interspace microsites where no water repellency existed, Kf increased. These changes may be a result of expansion of vaporized water through soil pores that broke up aggregates, deteriorating soil structure. Thus, mechanisms that contributed to changes immediately and after the first year post‐fire were different for low‐intensity burns than for higher intensity burns.