Ecophysiological responses and vulnerability to other pathologies in European chestnut coppices, heavily infested by the Asian chestnut gall wasp

Dryocosmus kuriphilus Yasumatsu, the Asian chestnut gall wasp (ACGW), is an invasive alien species, which is causing alarm in the chestnut stands of Italy and Europe. It has extensively colonised the Castanea sativa Miller range throughout the country and highly conspicuous, alarming symptoms have appeared on plants. In addition, chestnut trees growing in Mediterranean climates are more frequently subjected to stressful environmental conditions, such as hot-dry periods or extreme weather events, which compromise both yield and productivity. It will be useful, therefore, to gain further insights into the biotic and abiotic disturbances affecting chestnut and their interactions in order to develop management strategies to counteract the loss of productivity. This study was aimed at investigating the effects of ACGW on European chestnut ecophysiology during warm, dry summer conditions, typical of Mediterranean environments. We studied the functional and structural responses of young chestnut sprouts in a coppice stand in the Apennines (Tuscany, Italy), focussing in particular on photosynthetic capacity, leaf morphology, shoot growth, and hydraulic architecture. We also assessed the vulnerability of sprouts to chestnut blight Cryphonectria parasitica (Murrill) Barr. ACGW is a gall-making insect with strong dispersal potential. It spends its larval stages inside the chestnut buds and grows to adult state inside galls, which are located mainly on the main leaf axis or petiole. In this study, we found a reduction of about 30% in CO2 assimilation and stomatal conductance in the blades of galled leaves. PSII efficiency (ΦPO) was not negatively affected by the presence of galls, although lower electron transport to PSI acceptors (ΔVIP) was found in galled leaves, which could have negative consequences for NADP+ production and carboxylation. ACGW strongly affected the photosynthesizing leaf area, which was reduced by about 40% compared with a non-galled leaf. Carbohydrate concentration was higher in leaf blades while galls were richer in starch. Shoot vigour was affected by a massive presence of ACGW, resulting in a smaller leaf area and biomass, and very low capacity for water transport through the wood xylem compared with vigorous shoots. In fact, compared with vigorous shoots, non-vigorous shoots had a higher percentage of impaired xylem conductive area (5.7% vs. 1.3%) and a higher number of obstructed vessels per mm2 (31.2 vs. 7.4). Assessment of shoot vulnerability to chestnut blight revealed a prevalence of hypovirulence in blight infections, probably not directly due to ACGW, but instead related to loss of vigour in the shoot. However, ACGW could play a role in the appearance and spread of Gnomoniopsis sp., which was the most common endophyte on vigorous and non-vigorous shoots and the main coloniser of old galls. ACGW is opening up unpredictable scenarios in European chestnut forests, which are also susceptible to environmental stress factors. Therefore, the simultaneous study of disturbance dynamics and chestnut response, and advanced monitoring of insect spread and climate change would allow timely and effective implementation of adaptive forest management strategies