Indirect effects mediate direct effects of climate warming on insect disturbance regimes of temperate broadleaf forests in the central U.S.
Climate warming directly affects insect disturbance regimes by altering temperature dependent population development. Indirect effects of climate change on insect disturbance may mediate or accelerate direct effects via vegetation feedback (e.g., host-tree demographic dynamics). However, such indirect effects have rarely been incorporated in predicting insect disturbance regimes. We included both direct and indirect effects in a forest landscape modeling framework to simulate red oak borer (ROB, Enaphalodes rufulus [Coleoptera: Cerambycidae]) disturbance regimes under a warming climate from 2000 to 2150 in the Central Hardwood Forests, U.S. We quantified effect sizes and relative importance of direct effects and indirect effects of warming climate on the ROB disturbance using a factorial experimental design and two-way ANOVA. Both direct and indirect effects acted positively in the short and medium term (e.g., 0-100 years), while the effect size of indirect effects changed over time from positive (+30.9 kha) to negative (-17.3 kha) and mediated the positive direct effects in the long term (e.g., 100-150 years). Direct effects had greater influence than indirect effects on ROB disturbed areas in the short term (e.g., <50 years), whereas indirect effects were more important (w2= 0.38 vs. 0.26) in the long term. This was because the host-tree abundance significantly declined over time under warming climates, which decreased the forest susceptibility to ROB and thus overrode the outbreak-promoting effects of warming climates in the long term. Synthesis and applications. This study reveals indirect effects of warming climates mediate direct effects on insect disturbance regimes by altering primary host-tree demographic dynamics. We highlight that indirect effects are important in understanding insect disturbance regimes under warming climates as they may mediate or even reverse the expectation of increased insect disturbance. Long-term predictions of insect disturbance without considering indirect effects may overestimate its impacts under warming climates. Our findings also indicate that different management interventions are required at different time scales to maintain oak forests' health and sustainability in the U.S. central temperate broadleaf forests.