Impacts of pine processionary moth defoliation on tree water use patterns

Stomatal conductance is the principal mechanism of plants to regulate transpiration rates in response to environmental conditions. However, disturbances directly affecting leaves, such as outbreaks of defoliating insects, can impact the ability of trees to control canopy water loss, leading to significant shifts in plant water relations and forest water budget dynamics. One such example is the pine processionary moth (Thaumetopoea pityocampa; PPM), the main defoliating insect of pines and cedars in the Mediterranean Basin, that could have a significant effect on forest ecohydrology and tree vulnerability to drought. However, despite its potential relevance, PPM effects on tree water use patterns remain largely unexplored. Our study aimed to assess the effects of PPM defoliation on tree hydraulic patterns over time, and on tree stomatal sensitivity to soil water limitations during and after defoliation periods. We conducted a 12-month study of two stands in a Pinus nigra forest affected by PPM in Spain by combining measurements of stem sap flux, soil water content, and micrometeorology. The selection of the study site was based on the high presence of PPM nests during October and November, marking the onset of the defoliation period. In each stand, we installed 15 sap flux sensors on trees with contrasting abundance of PPM nests. These devices recorded sap flux density (Js), air temperature and air relative humidity at an hourly resolution. Ten TMS-4 dataloggers were also placed in each stand to measure soil temperature and soil water content (SWC) to a depth of 14 cm. The percentage of defoliation was visually assessed both at the beginning and at the end of the defoliation period, in November and next season May. Sensitivity in tree water use to changes in soil moisture was determined using stepwise regression models to estimate the breakpoint at the tree level between SWC and Js, representing the point at which constraints on water use shifts from SWC to atmospheric vapor pressure deficit (VPD). We hypothesized that defoliation would initially increase tree water use in relation to VPD due to damage to the leaf cuticle and/or reductions in stomatal sensitivity to aridity as the leaves are consumed by PPM. Results indicate a transitory increase in sap flux density with defoliation, with differences between the trees that tend to be reduced over time. By offering novel insights into the importance of defoliation in water use patterns and its regulation in relation to environmental conditions, our study contributes to enhanced decision-making for water conservation efforts.