Short-term effects of different fire severities on soil properties and Pinus halepensis regeneration

Considering that diverse fire severities can affect soil properties differently, the aim of this study was to examine to what extent changes in soil properties caused by fire could condition seedling establishment. This new approach is for identifying a new fire cause-effect chain to qualify the impacts of fire on soils with the purpose of using fire as a tool in forest management to favour Pinus halepensis Mill. regeneration. The study area was a reforested P. halepensis area which had been crossed by fire for 78.8 ha, causing various degrees of damage. The forest was subdivided into three large areas according to the gravity of crown scorch, [low (LS), medium (MS) and high (HS) severity], on the basis of needle yellowing which usually occurs after exposure to direct flames. Results showed significant differences in soil properties with respect to fire severity. In the HS area, total nitrogen and carbon were considerably reduced while ash and phosphorus contents significantly increased. The changes in soil properties, in particular to nutrient levels, affected P. halepensis regeneration, mainly the first year after the fire. Greater regeneration occurred in areas affected by moderate fire severity in which the temperatures reached increased the mineralization of soil organic matter with the consequent release of nutrients available for seedling growth. Additionally, moderate fire severity suppressed the regeneration of grasses, reducing the interspecific competition. Heights of seedlings were inversely proportional to the density of grasses. Where the number was abundant (LS), the height was modest; conversely, where the number was low (HS), the greater hypsometric differentiation of pine seedlings was observed. These results suggest that moderate fire severity represents an environmental stress (hormesis) altering microscale conditions to increase pine germination and establishment. The exposure of P. halpensis to a moderate environmental factor that is damaging at higher intensities, induces an adaptive beneficial effect on seedling regeneration. This data can re-evaluate the assertion that coniferous burned areas, if left unmanaged, would remain unproductive for an indefinite period.