Disentangling the effects of climate and defoliation on forest growth: The case of an outbreak of a Thaumetopoea pityocampa population with a shifted phenology in a Pinus pinaster monoculture

Forests are widely affected by disturbances, both abiotic, climate related, and biotic, like insect outbreaks. However, the impact of the interaction of these disturbances on tree growth is still not well understood. On the present work the impact of climatic and long-term defoliation by Thaumetopoea pityocampa (Dennis and Schiff.) during summer on Pinus pinaster growth was evaluated. At the National Pine Forest of Leiria, in Portugal, we sampled defoliated and non-defoliated forest plots covered by monocultures of young P. pinaster trees. Growth was assessed using dendrochronological techniques, and its relationship with climatic variability, including severe droughts, and defoliation, was analysed using hierarchical partitioning models. Results suggest that long term defoliation result in trees not being able to take advantage of favourable climatic conditions, leading to a reduced radial and height growth. Droughts were the major climatic disturbance, causing growth reduction as well, although the impacts of both disturbances did not act synergistically. Additionally, models showed that previous year climatic conditions have a significant effect on annual growth, namely, previous year annual precipitation and temperature. Yet, the best model obtained include a positive effect of the previous year annual precipitation and a negative effect of the interaction between this climatic parameter and defoliation. This work illustrates the potential of defoliation and climate to reduce forest productivity and the importance of water availability for tree development in Mediterranean climates. In addition, our results also emphasise the need to simultaneously evaluate the effects of different disturbances, particularly in a time of global change when an increase in forests disturbances is expected. This approach will certainly contribute to a more informed management of production forests.