Intra‐annual growth dynamics of forest understorey herbaceous plant species in response to global change

Abstract Interspecific variation in intra‐annual growth dynamics is an important determinant of species coexistence in plant communities. Species are expected to have contrasting growth responses to global changes, especially in the forest understorey where diverse phenological strategies have evolved as a result of tree canopy shading. Accordingly, impacts of climate warming, canopy disturbances, atmospheric nitrogen deposition and legacy effects from former land‐use will likely have implications for competitive interactions and may in turn alter community composition and functioning. Yet, intra‐annual growth dynamics in forest understorey species, which are crucial for forest biodiversity and functioning, have seldom been studied, certainly in response to global change. Our study provides the first comprehensive characterization of growth curves in temperate forest understorey species in response global changes. We performed a mesocosm experiment and carried out high‐resolution plant height and cover measurements to assess species‐specific growth curves and study growth phenology responses to warming, illumination (simulating canopy opening), nitrogen deposition and soil land‐use history (comparing ancient and post‐agricultural forest soil). Furthermore, we investigated how plant height and cover responses to global change treatments varied throughout the year. We identified four distinct growth patterns among the nine studied species, demonstrating the diversity of growth strategies in the understorey. Interspecific variation in growth phenology responses to global change was very large, but all species exhibited advances in response to experimental warming. Light and nitrogen addition delayed growth phenophases in most cases, but only had an effect on late‐spring and summer‐growing species and evergreens. Soils with an agricultural land‐use legacy produced advances for some species and delays for others. Furthermore, our findings revealed that traditional snapshot data collections can only uncover a limited number of effects from global change treatments. Synthesis . This study provides new insights into the growth dynamics of forest understorey species and their responses to global change drivers. Our findings suggest that global changes, such as climate warming, canopy disturbances, atmospheric nitrogen deposition and land‐use legacy effects will continue to shift growth phenologies, therewith disrupting competitive balances in plant communities. These insights can help to support adaptive forest management in a changing world.