Exploring priority and year effects on plant diversity, productivity and vertical root distribution: first insights from a grassland field experiment

1) The order of arrival of plant species during community assembly can affect how species interact with each other. These so-called priority effects can have strong implications for the structure and functioning of plant communities. However, the extent to which the strength, direction, and persistence of priority effects are modulated by weather conditions during plant establishment ("year effects") is not well known. 2) Here we present the first results from a long-term field experiment (POEM: PriOrity Effects Mechanisms) initiated in 2020 in Northern Germany to test how plant functional group (PFG) order of arrival and the year of initiation of an experiment interactively affect the structure and functioning of nutrient-poor dry acidic grasslands, both above and belowground. To do this, we established the same experiment, manipulating the order of arrival of forbs, grasses and legumes on the same site, but in different years. 3) We found that time since establishment was a stronger driver of plant community composition than PFG order of arrival and year of initiation. These three factors interactively affected plant species diversity, with the effect of PFG order of arrival on plant species richness depending on time since establishment. Year of initiation, not PFG order of arrival, was the strongest driver of aboveground community productivity. Although we did not find any effect of PFG order of arrival on root productivity, it had a strong impact on the vertical distribution of roots. Communities where grasses were sown first rooted more shallowly than communities in which forbs or legumes were sown first. 4) Synthesis: Our results demonstrate that plant order of arrival and year effects jointly affect plant diversity and species composition, with time since establishment also playing an important role. While year effects were more important than plant order of arrival in modulating aboveground biomass production in our nutrient-poor grassland, we showed that plant order of arrival can strongly affect the vertical distribution of roots, with communities in which forbs or legumes were sown first rooting deeper than grasses-first communities. These results suggest that a deeper understanding of priority and year effects is needed to better predict restoration outcomes.