Impact of black cherry on pedunculate oak vitality in mixed forests: Balancing benefits and concerns

The vitality of European forests continues to decline due to new pests and diseases, climate-change related disturbances and high loads of atmospheric nitrogen deposition. Deteriorating soil health is a major factor underpinning the low vitality of West-European forests. Selecting tree species with soil ameliorative traits is proposed as an avenue to counteract soil acidification and improve overall forest vitality. Here we evaluate the impact of black cherry (Prunus serotina Ehrh.), a known rich litter species, on the vitality of neighboring pedunculate oak (Quercus robur L.) in ten mixed forests on sand in Germany, Belgium and the Netherlands. We found that black cherry admixture increases foliar N and P to a surplus whereas it causes deficiencies in foliar Mg, thereby resulting in an overall negative effect on oak foliar nutrient concentrations. Contrary, defoliation of oak leaves by herbivory decreases with the proximity of black cherry. Using structural equation modelling (SEM), we tested the hypothesized 'improved soil health' pathway. Our analyses showed that black cherry admixture leads to lower accumulation in the humus layer, resulting in higher soil base saturation which has a positive effect on foliar Ca yet a negative effect on total chlorophyll. Moreover, the SEM illustrated that herbivory of oak leaves decreases when black cherry is admixed, both via dilution and improved soil health. Indirect effects of black cherry on oak vitality via "improved soil health" in our SEM are however small in comparison to direct relations. Hence, our study showed that the combined positive and negative impacts of black cherry on oak vitality are limited, which tempers the potential benefits of using the rich litter species to counteract oak decline via improved soil health - yet, the concern of black cherry as an invasive alien species negatively affecting the vitality of mature pedunculate oak trees may also be exaggerated.