The role of vegetation burn severity in daily soil respiration patterns during the first post-fire growth season in a Mediterranean pine stand

Mediterranean pine forests will become increasingly vulnerable to stand-replacing wildfires, as fire regimes intensify and, in particular, fire recurrence intervals become too short for the formation of seedbanks that can endure sustainable pine recruitment rates and/or if crown fires produce elevated mortality of the aerial seedbank.

Tree crown fires can produce greater indirect impacts than ground fires. Namely, a protective ground cover of scorched needle cast is not only highly effective to reduce post-fire soil (fertility) losses but may also improve conditions for vegetation and soil recovery, by increasing soil wetting and reducing temperature and moisture fluctuations at the soil surface and its upper depths. While tree crown consumption has shown some potential as indicator of post-fire pine recruitment, its role on soil recovery processes has received little research attention.

To this end, the present study assessed if daily patterns in soil respiration differed for pine trees of a single stand with partial vs. complete crown consumption. Daily soil respiration patterns were measured at hourly intervals for a total of six collars, three per treatment, using the Li-8100A automated gas flux analyzer. This was done at repeated occasions with contrasting weather conditions, i.e. at the start, middle and end of the first post-fire spring as well as by early summer as the end-of-spring campaign had to be interrupted due to a rainfall event. The focus on the spring period was justified by eddy-covariance measurements in the same burnt area, indicating that carbon emissions during the preceding fall and winter periods were very reduced.

In a nutshell, the obtained results revealed that crown consumption did not play an important role in post-fire soil respiration during any of the four occasions. At the same time, they showed noticeable differences in soil respiration rates between the four occasions, suggesting that soil temperature was the main control of overall respiration rates while soil wetting was responsible for pulses in carbon emissions.