Effects of fire recurrence on plant growth in three Mediterranean perennial woody species

Recurrent fires at short fire intervals tend to simplify vegetation structure and to reduce phytomass production in Mediterranean plant communities. Here we compared growth of three Mediterranean perennial woody species between areas affected by one fire versus areas affected by two fires during their recent fire history (20 years). Design consisted of 13 study sites distributed over Barcelona and Tarragona provinces in Catalonia (NE Iberian Peninsula). Each study site corresponded to Pinus halepensis-dominated communities, and was conformed by two areas: an area burned only once (in 1994) and an area burned twice (once between 1975-1989, and again in 1994). Selected species were the seeder subshrub Rosmarinus officinalis, the resprouter shrub Pistacia lentiscus and the resprouter tree Quercus ilex, which are common and widespread in the Mediterranean Basin. We measured branch elongation and basal area increment during three growing seasons, between 10 and 11 years after the last fire, which was in 1994 in all areas. Significant growth differences were found for R. officinalis, whose basal area increment was higher in twice-burned areas relative to once-burned ones, as reburning likely resulted in a release from competition. Contrastingly, Q. ilex and P. lentiscus showed similar basal area increments. No significant differences in branch elongation were found for any species, probably because light was not a limiting resource in these environments. These results suggest that re-burning at short fire intervals resulted in a release from competition that allowed enhanced growth of the seeder species but did not affect growth of the resprouter species, which may rely mostly on resources stored in underground organs.