Summer climate variability over the last 250 years differently affected tree species radial growth in a mesic Fagus-Abies-Picea old-growth forest

Sustainable forest management has to consider the long-term effects of climate change on species growth
to develop adaptation measures. In this perspective, dendrochronology provides valuable information on
climate-growth relationships over long time periods. Tree-ring analyses in mixed stands can elucidate
how different species respond to climate change within the same environmental conditions. However,
few studies have investigated such stands, especially in South-Eastern Europe. In the forest reserve of
Lom, in Bosnia and Herzegovina, we had the opportunity to study three co-occurring species (silver fir,
Norway spruce, European beech) in an old-growth forest characterized by reduced human and natural
disturbances, and a climate favourable to the tree species. We evaluated tree growth response to climate
on inter-annual and decadal time scales over about 250 years. Response to inter-annual climate variability
changed over the study period. Climate signal in beech was generally low, with a negative correlation
to April temperature in the last decades. In fir and spruce, the positive effect of current year’s spring temperature
decreased, while the negative effect of the previous summer temperature considerably
increased over the last century. At the decadal scale, different responses have been detected among
species: spruce was mostly negatively affected by summer temperature whereas summer precipitation
benefited fir growth, probably balancing high evapotranspiration. Beech showed a peculiar delayed
response, and protracted drought periods led to severe growth reductions. Despite a temperature
increase over the last three decades, fir did not experience any growth reduction, while a strong decline
was evident in spruce and beech productivity. Mesic sites are commonly underrepresented in tree-ring
research. In our analysis, covering about 250 years, we observed that climate also affects species-specific
growth patterns in these areas. Within a global change perspective, specific divergent responses are likely
to occur even where current environmental conditions appear to be not limiting for tree species. Future
management strategies should consider these outcomes.