Growth Responses to Climate and Drought in Relict Cork Oak Populations as a Benchmark of the Species Tolerance

We still lack information on the long-term growth responses to climate of relict tree populations, which often persist in topoclimatic refugia. To fill that research gap, we studied three relict cork oak (Quercus suber) populations located in northern Spain using dendrochronology. The sites were subjected to humid (Zarautz), continental (Bozoo) and xeric (Sestrica) climate conditions. Cool-wet conditions during the current spring enhanced growth in Bozoo and Sestrica, whereas wet conditions in the previous October enhanced growth in Zarautz. In this site, growth also increased in response to dry conditions in the prior winter linked to high North Atlantic Oscillation indices. Correlations between the precipitation summed from the previous September to the current May peaked at the driest site (Sestrica). The strongest growth responses to drought severity were also found at this site, where growth negatively responded to 9-month early-summer droughts, followed by the continental Bozoo site, where growth was constrained by 1-month July droughts. Growth declined in response to 6-month January droughts in the wettest site (Zarautz), where cork oak was vulnerable to previous late-summer to autumn drought stress. Despite warmer and drier spring conditions that would negatively impact cork oak at the Bozoo and Sestrica sites, trees from these populations could tolerate further aridity.