Impact of secondary pests on carbon allocation in declining beech trees after the 2018-2020 drought episode

Abstract During the extreme drought period of 2018-2020, secondary pest attacks were observed in some declining beech trees ( Fagus sylvatica ). The main species identified are Agrilus viridis and Taphrorychus bicolor , which are known to attack beech trees more frequently after periods of intense drought. The aim of this study is to investigate if and how these two cambium-feeding insects in interaction with drought contribute to beech decline. We assessed the modification of the carbon reserves stock and carbon allocation to growth and fruiting in declining beech trees. Past radial growth, non-structural carbohydrates (NSC) concentrations in several organs and the quantity of beechnuts were measured in 38 beech trees with contrasting levels of biotic attack in 2020. Total amount of carbon reserves was calculated on the basis of NSC concentrations, and the amount of carbon allocated to growth and fruiting at the tree level was assessed using allometric relationships. Beech trees most severely attacked by secondary pests showed lower radial growth for last ten years than trees that have not been attacked. NSC concentrations and the amount of carbon reserve in the coarse roots and stems of trees were also lower in beech trees with intense biotic symptoms compared to unaffected trees. However, the amount of carbon allocated to growth and fruiting during the 2020 growing season did not differ significantly among biotic attack classes. Synthesis: Regardless of biotic attack intensity, declining beech trees exhibited abnormally low carbon reserves. Trees most severely affected by insects in 2020 appeared to be the most vulnerable, as evidenced by reduced radial growth over the past decade. Future experiments are needed to explore further whether insects impacted directly carbon reserves, or whether low carbon reserves in trees is a vulnerability factor promoting insects attacks. Finally, declining beech trees allocated a similar amount of carbon to fruiting as reported in healthy stands, suggesting that fruiting is a prioritised carbon sink, even under severe drought and biotic attack.