Light more than warming impacts understory tree seedling growth in a temperate deciduous forest

Understanding the distinct impacts of temperature and light on seedling growth is crucial for predicting forest regeneration trajectories under future climate change and forest disturbance. This is because temperature and light can change independently or together, influencing the competitive status of tree seedlings and forest herbs. However, most prior studies tend to explore temperature and light effects either separately or in combination, lacking comprehensive full-factorial designs. Here, we utilized two large-scale full-factorial experiments to explore warming and light effects on tree seedlings growing in mesocosms with herbaceous plants. We found that light increased seedling height, diameter, and biomass, while warming alone had no significant effects. Moreover, we observed that there was an interaction effect between light and warming, where warming increased seedling height, diameter, and biomass under illumination. Understory herbaceous plant cover alone did not affect seedling height, diameter, and biomass, but it decreased seedling biomass when mesocosms were warmed or illuminated. This highlights the importance of considering the indirect negative effects induced by the interaction between forest opening and understory herbaceous plants. By disentangling the effects of increased temperature and light availability on understory seedling height, diameter, and biomass, our results contribute valuable knowledge for future forest management. It is imperative to carefully control the size of the gaps artificially created to facilitate the understory regeneration. Small gaps are recommended considering that the herbaceous plants may interact with both warming and light to negatively affect seedlings.