Recruitment and growth decline in Pinus halepensis populations after recurrent wildfires in Catalonia (NE Iberian Peninsula)

Despite many Mediterranean species show efficient mechanisms to withstand fires, they are currently submitted to fire regimes characterized by anthropogenically increased levels of fire recurrence, which may overpass their recovery capacity. We studied fire recurrence effects on Pinus halepensis populations at a regional level in Catalonia (NE Iberian Peninsula), by comparing post-fire populations in paired areas once-burnt (1994) and twice-burnt (once from 1978 to 1991, and again in 1994). Survey was conducted 10 years after fire in 14 study sites: density, height, and diameter were recorded, and dendrochronological analyses were conducted to deal with time of recruitment and radial growth. In both types of areas, post-fire recruitment took place mainly just after the summer fire, in 1994 (87% of individuals), and also in 1995 (12%). Density was significantly lower in twice-burnt areas than in once-burnt ones, which is attributed to fire intervals shorter than 15 years, the minimum time-length required by P. halepensis populations to recover pre-fire density levels. Height and diameter records evidenced a growth decline in twice-burnt areas. Dendrochronological analyses revealed that radial pine growth was lower in twice-burnt areas along the first post-fire years (from 1994 to 1999), thus resulting in lower total radial growth than in once-burnt areas. Such radial growth differences are attributed to differences in soil nutrient availability along the first post-fire years (post-fire nutrient boom), resulting from differences in pre-fire fuel loads between once- and twice-burnt areas. According to our results, drastic changes in P. halepensis forests in the western Mediterranean Basin are likely to be expected under fire regimes characterized by high levels of fire recurrence.