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The post-fire growth responses and changes in wood C and N isotope composition depend on site water availability and fire severity in Mediterranean Aleppo pine forests.

Mediterranean forests are subjected to recurrent wildfires and summer droughts. Under warmer and drier conditions, it is required to determine how Mediterranean pines recover after wildfires, and how this translates into changes in tree radial growth and function (e.g. intrinsic water-use efficiency-iWUE). We analysed four Aleppo pine areas located in SE Spain affected by 1994 wildfires and subjected to different water availability, ranging from mesic to semi-arid conditions. We combined dendrochronological analyses with delta C-13 and delta N-15 wood isotopes to quantify the changes in radial growth (expressed as Basal Area Increment-BAI) and functional responses (iWUE and N cycling) to three fire severities (unburned sites, low and medium severities). We expected that the post-fire release in nutrients and a reduction in competition for water would enhance radial growth. We found that fire did not significantly alter growth patterns at the driest sites, but increased BAI at the wettest sites. delta C-13 was significantly (P a parts per thousand currency sign 0.01) more negative only in burned stands located at the wettest site indicating a decreased iWUE and thus improved water availability. However, the delta N-15 was higher in severely burned than in unburned plots from all sites but the wettest site, indicating a potential fertilization effect of fire in sites subjected to mild drought severity. Site water availability determined how fire affected subsequent modifications in growth and tree functioning of Aleppo pine forests, that is, changes in iWUE and N cycling. Therefore, site dryness should be explicitly considered to forecast the growth and functioning responses of Mediterranean pine forests to the predicted increasing recurrence of fire events due to global warming.