Pine processionary moth outbreaks and droughts have different tree ring signatures in Mediterranean pines

Defoliating insects' outbreaks play a critical role in trees' carbon cycle. The pine processionary moth (PPM; Thaumetopoea pityocampa) is the major defoliating insect of Mediterranean coniferous forests. The frequency and intensity of PPM outbreaks is projected to increase as winter temperatures become milder due to climate warming. An accurate evaluation of this projection requires a wide spatial baseline of the historical PPM incidence. PPM outbreaks affect tree secondary growth leading to narrow rings, providing a tree level signal. However, PPM defoliation rings can be confounded with drought rings, the most frequent cause of narrow rings in Mediterranean environments. Thus, an accurate identification of PPM rings demands the consideration of additional tree ring traits. Here, we introduce a multiproxy approach to identify and distinguish PPM and drought events. We sampled four Pinus nigra (3) and P. sylvestris (1) stands in Spain. We identified and verified years of PPM defoliation using remote sensing analysis and field observations of Regional Forest Service. We identified drought events through the combination of climatic data with radial growth reductions. We considered climate growth residuals, among-trees growth variability, latewood percentage (%LW), intrinsic water-use efficiency (iWUE) and minimum blue intensity (BI) to discern between droughts and PPM outbreaks. In comparison with drought rings, PPM rings showed 1) more negative residuals in climate growth models, 2) higher secondary growth variance, 3) higher percentage of latewood and 4) lower iWUE. Minimum BI did not differ between drought and PPM rings, but was lower than in the rest of the rings. The combination of these traits provides a signature to identify PPM rings, opening the opportunity to reconstruct PPM incidence on a broad spatio-temporal scale.