Impact of defoliation on tree growth for sessile oak and Scots pine following heat-drought events

European forests have experienced several droughts and heat waves in recent years leading to an increasing level of tree dieback. Quantifying the effects of dieback on tree growth is a crucial issue for forest managers. We monitored tree crown defoliation and tree growth for sessile oak and Scots pine from 2020 to 2023 in the OPTMix experimental network located in central France. We quantified the relationship between tree growth and tree crown defoliation under varying neighborhood conditions. We hypothesized that crown defoliation, tree size and neighborhood competition (local density and composition) would all affect tree growth. We carried out a magnitude analysis to quantify the importance of these different effects on tree growth. We showed that crown defoliation had a strong negative effect for sessile oak (-19.5 % of growth from 42 % to 57 % of defoliation) and a very strong negative effect for Scots pine (-29.5 % of growth from 39 % to 51 % of defoliation). Tree size and neighborhood density had the strongest effects on tree growth for both species, with, respectively, a positive and a negative effect. The proportion of heterospecific species in the neighborhood had the weakest effect, and was positive for sessile oak and negative for Scots pine. We found no interaction between crown defoliation and other predictors, thus indicating that they do not modify the relationship between crown defoliation and radial growth for either species. Forest managers can use defoliation as a rapid indicator of tree health and to help them assess the impact of defoliation on tree growth in various oak-pine forests. • Defoliation strongly reduced 2020–2023 tree growth for sessile oak and Scots pine. • The reduction in growth was more important for Scots pine than for sessile oak. • Tree size and neighborhood density had the strongest effects on tree growth. • Neighborhood composition affected growth but to a lesser extent than did defoliation. • Tree size and local competition did not modify the growth-defoliation relationship.