Old oaks show increasing growth synchrony after pollarding cessation as climate becomes more arid

Abstract

Pollarded oak woodlands have been historically managed by people to produce firewood and timber. Pruning cessation and climate warming could contribute to their decline, especially in southern Europe under ongoing aridification. Widespread pollarding abandonment could make oaks more responsive to drought stress and increase between-site growth synchrony. To fill this research gap, we compared the basal area increment (BAI) trends and the responses of growth indices to climate variables and a drought index of formerly pollarded oak stands (five stands of Quercus faginea, one stand of Q. humilis) located in northern and eastern Spain. Radial growth indices were correlated with monthly climate variables (mean maximum and minimum temperatures, precipitation) and a multiscalar drought index. BAI was also projected based on recent trends to forecast stands viability in the 21st century. Major growth suppressions (MGS), which were attributed to pollarding, were reconstructed using dendrochronology. Wet-cool conditions from prior winter to current early summer improved oak growth. Year-to-year growth synchrony has increased since 1850 as summer conditions became more arid, but BAI did not significantly decline in four out of the six study sites. Synchrony among sites regarding MGS was low indicating a high spatial variability in oak pollarding. The strongest BAI decline was observed in La Guarguera site, where growth cessation was forecasted in the mid-21st century. In contrast, BAI data suggested strong growth improvement in sites Campillo de Dueñas and Valsalobre, which were pollarded in the 1960–1970s. These findings confirm that pollarding is a suitable management tool to keep vigorous old oak stands under more arid climate conditions.