Grassification in forest understories: Increasing dominance of graminoids affects tree seedling growth and survival

Temperate forests in Europe are experiencing increasingly hotter and drier summers. Because of increasing forest disturbances, canopies are also opening up, creating brighter, but also drier conditions at the forest floor. This can lead to a dominance of drought-resistant species, such as graminoids, potentially leading to a grassification of the understorey in the long term. It remains unclear how graminoid dominance is impacting tree seedling recruitment in forests because of a lack of experiments that allow disentangling the effects of different interacting drivers. Here we introduce a novel experimental platform and report the early-stage results on seedling growth and survival. We test two hypotheses: (1) light and recent forest soil promote graminoid growth, while drought has little effect at this very early stage; and (2) the presence of graminoid negatively affects tree seedling growth. We planted 12 tree seedlings within a matrix of monocultures of graminoids together in trays filled with forest soils and subjected to three two-level factorial treatments of light, rainfall, and land-use legacies. Graminoid shoot length significantly increased after exposure to light, which suppressed tree seedling height growth. Drought had no effect, but light and land use significantly influenced seedling growth through the mediation of graminoid shoot length. Light increased the height of Acer pseudoplatanus when growing alongside shorter Deschampsia cespitosa but decreased Acer pseudoplatanus and Quercus petraea height within mesocosm dominated by taller graminoid species, such as Calamagrostis epigejos. Recent forest soil only increased Acer pseudoplatanus height when grown in combination with Brachypodium sylvaticum, the shortest graminoid species within the studied species pool. This study presents a novel experimental platform where we demonstrate that light and post-agricultural soil promote graminoid growth more than tree seedlings, while their interaction with graminoids significantly affects seedling growth.