Ecosystem Responses to Fire

Fire determines major vegetation structure and composition of several world biomes, as well as temperature, precipitation, and water balance. The fire-prone vegetation cover constitutes at present 40% of the world's land surface, including especially the Mediterranean regions, except central Chile where fires are less frequent (Table 1). In Mediterranean-type ecosystems, most of the areas occupied by shrublands have the climate potential to be forests and these shrublands are generally fire-maintained. But in contrast with current opinion that often attributes the existence of these ecosystems only to anthropogenic burning, Mediterranean shrublands seem to have naturally expanded in the Late Tertiary, with flammable C4 grassy biomes. In the Mediterranean Basin, natural development of pine forests has also played a determinant role in increasing fire frequency since the Miocene (c. 9 Ma).

Forest fires are generally considered as catastrophic events in the Mediterranean Basin and California where big fires occur, but the evolutionary and ecological influences of fire also represent key parameters for driving landscape diversity, ecosystem heterogeneity, vegetation dynamics, and species differentiation. Fire determines indirect environmental changes with greater fluctuations in temperature and increasing oxygen concentration in soils, increases light and water availability, reduces aboveground competition and determines a proper regeneration niche for fire-adapted plants. In burned ecosystems, a higher abundance and species-richness is quickly observed several months, or even weeks, after fire and important changes in species occurrence and diversity occur. These immediate and profound ecological modifications are considerably attenuated during the second postfire season because Mediterranean ecosystems are particularly resilient and fire-adapted. Nevertheless, these brief structural changes are of particular importance for the regeneration window of fire-adapted species.

Fire represents a major selective force that shapes the evolution of plant reproductive traits in fire-prone Mediterranean environments, and thus ecosystem dynamics. Strong serotiny, fire-stimulated flowering or germination, smoke- or charred wood-induced germination, resprouting ability through lignotubers, stumps or burls, and seed dormancy favor plant persistence and these traits are more common in Mediterranean ecore-gions with a longer fire history or a higher fire frequency. Several lines of evidence corroborated by recent phylo-genetic analysis indicate that there has been stronger selective pressure for fire persistence traits in California than in the Mediterranean Basin. Because of a more drastic fire occurrence in California, plant species have evolved here more frequently toward the association of resprouting capacity and propagule-persistence than Mediterranean Basin plants. Furthermore, another peculiarity is the specialization of Californian annual plants which encompass numerous fire endemics that can persist as dormant seed banks for many decades between fires and that occur only 1 or 2 years after a fire.

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