Tree seedling responses to multiple environmental stresses: Interactive effects of soil warming, nitrogen fertilization, and plant invasion

Temperate deciduous forest ecosystems in northeastern North America are under increasing biotic and abiotic stresses that can have interactive effects on understory vegetation, and thus impact the next generation of forest canopy trees. We examined seedling responses of the common and increasingly dominant species Acer rubrum (red maple) to the combined effects of soil warming (+5 °C), chronic nitrogen fertilization (+50 kg N ha−1 yr−1), and invasion by the phytotoxic plant Alliaria petiolata (garlic mustard), and their interactions. We planted 296 first-year A. rubrum seedlings in a multifactorial field experiment to examine the effects of all combinations of the experimental treatments. Second-year Acer rubrum seedlings demonstrated higher aboveground growth under soil warming conditions. Further, soil warming positively influenced plant-soil feedbacks through higher arbuscular mycorrhizal colonization in fine roots. However, the positive growth responses and mycorrhizal colonization observed under soil warming were moderated by both N fertilization and A. petiolata invasion. Our results highlight the importance of developing management plans that consider how multiple environmental change factors affect tree seedling performance, particularly via the plant-soil interface.