Combined heat and drought events reduce first-year survival of tree seedlings, especially at southern range margins

In forests, first-year tree seedlings experience high mortality rates, yet we know relatively little about how extreme climate events contribute. In this study, we exposed over 700 containerized seedlings of eight tree species to three heat and three drought treatments, and their combinations, that represent extreme conditions currently beyond the climate typically experienced by species at the temperate-boreal ecotone. We measured the height and survival of all seedlings and compared these responses to species traits. We found that, overall, combined heat and drought reduced survival more than either stressor alone. Reductions in survival were strongest for two northern conifer species with relatively low root-to-shoot biomass ratios. In contrast, two more southerly distributed species with the largest root systems had higher survival. Treatments did not impact growth or biomass of surviving seedlings. These results establish that higher temperatures or shifting precipitation regimes may not pose a major threat to first-year seedlings separately, but combined warming and drought have the potential to cause widespread mortality, particularly at southern range margins. These findings demonstrate the importance of first-year seedling survival under combined stressors and clarify how these rare but extreme events can cause regeneration failure. Although difficult to predict, these events should be considered in adaptive management planning and forest modeling due to their potential for profound impacts.