Short-term recovery of soil functional parameters and edaphic macro-arthropod community after a forest fire

Forest fires can result in severe economic and environmental consequences, and little is known about the ecological patterns and processes that may lead to the recovery of burnt areas. In the last decades, Portugal has been the Southern European country with the highest number of fire events and with the highest burnt area per hectare. With this work, we proposed to study the effect of a forest fire on the terrestrial ecosystem. More specifically, this work intended to evaluate the short-term recovery of several soil chemical, biochemical (microbial enzymatic activities) and biological (edaphic macro-arthropod community) variables in a burnt pine tree forest area. Soil and macro-arthropod sampling was carried out in a burnt area (transects BI, BII and BIII) and in a neighbouring unburnt area (U) 3 and 8 months after the fire, coinciding with autumn and spring. Soil was collected for the determination of physical (pH and conductivity) and chemical parameters (moisture and organic matter) and soil enzymes (cellulase, acid phosphatase and nitrogen mineralisation rate). Edaphic macro-fauna was captured using pitfall traps. Univariate and multivariate statistics revealed, overall, that burnt sites displayed lower acid phosphatase and cellulase activities and higher conductivity and pH values than the unburnt area. There was a recovery in the measured soil parameters between autumn and spring in the most interior parts of the burnt areas (BII and BIII), but the outer transect (BI, close to a road) still displayed considerable differences to the remaining burnt transects as well as to the unburnt area. A total of 47 macro-arthropod taxa were captured in both seasons, with Linyphiidae spiders (20.2%) and insect families Formicidae (13.4%) and Staphylinidae (11.9%) being the most abundant. Dominance by some taxa was overall stronger in the burnt than in the unburnt area, although dominant taxa varied between seasons. In autumn, the burnt area was dominated by ants and had also a high abundance of scavengers, carrion feeders and some ground active hunters. In spring, there was a general increase in taxa diversity, richness, and total catches; in the burnt area, there was a re-colonisation by several organisms sensitive to litter quality, such as isopods and pseudoscorpions, particularly in the outer transect (closest to the unburnt area). Differences in soil parameters between burnt and unburnt areas were most likely due to the deposition of nutrient-rich alkaline ashes. However, low cellulase activity in the outer part of the burnt area (BI) indicated compromised microbial activity in both sampling seasons. Recovery of soil functional parameters was delayed in the outer zone of the burnt area because of (i) fire intensity in that area or (ii) proximity to the road (enhancing erosion and exposure to contaminants). The pattern of arthropod re-colonisation of the burnt area followed the inverse recovery pattern (from the outer zone to the inner zone), stressing the primary role of the adjacent unburnt area as a source of potentially colonizing organisms. Direct and indirect effects of fire on soil parameters (soil alkalinisation and nutrient enrichment) and edaphic fauna had a short-term persistence in the burnt area, and signs of recovery were evident 8 months after the fire (spring). The adjacent unburnt area seemed to act as an important source of arthropod colonisers. More prolonged studies on the recovery of soil functional parameters and arthropod community structure are required to understand long-term re-colonisation patterns. Researchers and authorities should also endeavour in the implementation of measures that favour and protect survivors and new indigenous colonisers (microbes, plants and animals) after a forest fire.