Long- and short-term impacts of a defoliating moth plus mistletoe on tree growth, wood anatomy and water-use efficiency

Climate warming and biotic stressors are expected to reduce tree radial growth and performance at short and long time scales. However, the impacts of different biotic stressors on performance throughout a tree's life are largely understudied. Here we assessed the effects of a past nun moth (Lymantria dispar) outbreak and related defoliation on Scots pine (Pinus sylvestris) trees, which were later severely infested by the hemiparasite mistletoe (Viscum album subsp. austriacum). We compared the responses of trees severely infested or not infested by mistletoe in a wet vs. a dry site to quantify the relative importance of biotic stressors under different climate conditions. We used dendrochronology to quantify: long- and short-term changes in radial growth (resilience), differences in wood anatomy during the outbreak, and recent changes in intrinsic water-use efficiency (WUEi). The outbreak caused a sharp growth reduction in 1953 (50% decrease in basal area increment –BAI) and the formation of tracheids of small transversal lumen diameter (33% decrease in diameter). Recent mistletoe infestation caused a difference in growth between infested and non-infested trees lasting 34 and 21 years in the wet and dry sites, respectively. Growth (BAI) decreased more steeply in severely infested than in non-infested trees, the post-drought resilience decreased in severely infested trees, and the WUEi increased, particularly in the dry site. The BAI of severely infested trees was more negatively impacted by warm and dry conditions during the growing season than in non-infested trees, particularly in the dry site. Tree rings recorded historical effects of biotic stressors (L. monacha outbreak), which may constrain responses to recent stressors (mistletoe).