El Niño-Southern Oscillation modulates insect outbreaks in humid subtropical China: Evidences from tree rings and carbon isotopes

Insect outbreaks are key disturbances triggering decline episodes in humid subtropical evergreen forests in eastern China. However, their long-term variability and linkages to climate are still unclear largely due to the shortness of records reporting forest damage. A retrospective approach would improve our understanding of long-term outbreak variability and its dependence on climate and relevant atmospheric circulation patterns such as the El Niño-Southern Oscillation (ENSO). This study provides a dendrochronological reconstruction of Dendrolimus punctatus outbreaks affecting Pinus massoniana in humid subtropical eastern China. We used tree-ring width and carbon isotope data from sites where outbreaks were or not were reported for the past fifty years, here considered as host and non-host sites, respectively. The results showed that inter-annual changes in carbon isotope discrimination are more sensitive to outbreaks than changes in tree-ring width. Based on carbon isotopes, we inferred that photosynthetic rates increased as a compensatory mechanism in response to insect defoliation. We reconstructed eleven insect outbreaks since 1915, which corresponded to ENSO positive phases and below-average sea surface temperatures in the central and eastern tropical Pacific Ocean. These conditions correspond to “La Niña” episodes which induce hot and dry climate conditions across the study region and these may facilitate the occurrence of D. punctatus outbreaks.