Growth projections reveal local vulnerability of Mediterranean oaks with rising temperatures

Growth projections using ecological models fitted to data collected along climatic gradients can help to understand how forests will respond to climate change. Stem growth of two Mediterranean oaks was predicted using nonlinear multiplicative models as a function of precipitation and minimum temperature of the hydrological year fitted to dendrochronological data. The growth of both species increased nonlinearly with accumulated precipitation before reaching an asymptote, but the species with a warmer niche (Q. ilex, an evergreen species) required lower levels of precipitation to achieve high relative growth. The species-specific relationship between growth and minimum temperature exhibited an optimum for the two species. Trees were negatively affected by high minimum temperatures whereas they responded negatively (Q. ilex) or neutrally (Q. pyrenaica, a deciduous species) to low temperatures along the climatic gradient analyzed. Growth would decrease rapidly when minimum temperatures rose above approximately 7 °C for Q. pyrenaica and 9 °C for Q. ilex. Most growth projections suggest a likely future decrease in productivity along the species range for Q. pyrenaica and particularly at species-specific warm, dry locations pointed to a future drastic reduction in productivity as a result of the increase in temperatures without a paired increase in precipitation forecasted by the different climate scenarios considered. In agreement with results from studies modeling future distribution of species this suggests that Q. pyrenaica could be threatened by climate change at the species local dry edge where, in addition, stands often present a lack of seed regeneration. More drought tolerant Q. ilex might profit from warming temperatures at cold northern locations but would also reduce productivity at warm, dry locations. Stem growth was successfully modeled using biologically meaningful species-specific responses to climate which provided key ecological information to understand the functional response of the two species. The models used have much potential to be applied with dendroecological data to study the response of forests to climate change.