Recent declines in radial growth and wood density characterize dieback in European beech and pedunculate oak

In the last decades, climate change has boosted the occurrence of severe dry and warm episodes that impair the functioning of forests. Elevated evaporative demand, i.e., high vapour pressure deficit, during the growing season limits the ability of trees to fix carbon and growth thus increasing the likelihood of tree dieback and mortality. While climate change impacts on tree growth are widely documented, we still lack a clear understanding on how wood density responds to temperature increases, particularly for hardwood tree species near their warm distribution edges. We evaluated the annual growth and wood density variability of European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) in two forests suffering tree mortality in northern Spain. We used dendrochronology to quantify tree-ring-width and basal area increment (BAI) and densitometry analyses to obtain annual wood density profiles of non-declining (ND) and declining (D) trees. In addition, resistography was employed to estimate resistance drilling density. We found a higher BAI of ND as compared with D trees in the two species, since the severe 2012 drought for beech and after the 1960s for oak. By contrast, differences between vigour classes in wood density were only evident in oak during the last decade, when D trees showed a decreasing trend. Beech growth was enhanced by wet-cool summer conditions, whereas density increased in response to dry-warm spring conditions and cool temperatures during the previous autumn. Oak growth was enhanced by dry-warm conditions in the prior winter and in the summer of the year of growth. High minimum temperatures during the previous autumn, spring and summer increased wood density in ND oak trees, while high precipitation during spring reduced wood density in D trees. We found that resistance drilling profiles did not differ between vigour classes in beech, but D oak trees showed lower resistance to penetration in the sapwood than ND trees, consistent with densitometry profiles. Our results indicate that radial growth data have better capacity to differentiate declining and healthy trees than density and resistograph profiles in European beech and pedunculate oak.