Climate warming and drought modify galling effects on tall goldenrod

Plants must respond to changing climatic conditions while continuing to defend against herbivores. While numerous studies have investigated how one type of stress affects plants, the effects of multiple abiotic and biotic stressors and their interactions are less understood. We used the Rainfall Exclusion eXperiment (REX) at the Kellogg Biological Station Long-Term Ecological Research site (KBS LTER) to quantify the individual and interactive effects of drought and warming treatments (abiotic stressors), and galling by Rhopalomyia solidaginis (biotic stress) on tall goldenrod (Solidago altissima), a common native plant species in Michigan, USA. At the end of the 2021 and 2022 growing seasons, we measured stem height and biomass as a proxy for plant productivity, and seed mass per stem as a proxy for reproductive fitness. We also measured gall biomass, larval chamber number, and larval chamber volume to reflect the effects of drought and warming on the gallmaker. We found that warming mitigated some negative galling effects; galled plants were 7.1 cm shorter than non-galled plants in ambient conditions, but under warming, there was no reduction in height for galled plants. Furthermore, drought exacerbated some galling effects: galled plants experiencing drought conditions had the lowest probability of producing seeds (0.47) compared to plants from all other treatments. Understanding how plants respond to individual abiotic and biotic stressors as well as their interactions will enhance our ability to predict plant fitness and community dynamics under new climate regimes.