Priority Effects Created by Manipulating Plant Functional Group Order of Arrival Affect Root Productivity in Grassland Ecosystems

Different timing of arrival of plant functional groups (PFG) can create priority effects that influence plant community assembly and ecosystem functioning (e.g., aboveground primary productivity). The effect of such priority effects on belowground productivity, however, remains unexplored. In order to investigate how PFG order of arrival affects belowground productivity in mesic grasslands, we performed a field experiment from 2012 to 2015 on two different soil types. In this experiment, we manipulated PFG order of arrival and measured root length density and root productivity in the topsoil in 2012 and 2014. In both years, our results showed that sowing legumes first created strong and consistent priority effects belowground (lower root length density), despite the fact that species composition markedly changed from 2012 to 2014. Root productivity was only affected by soil type. In a second experiment started in 2016, we were interested to know if the belowground priority effects described earlier could also be observed in dry acidic grasslands characterized by nutrient-poor soils. To answer this question, we set up a mesocosm experiment combining biodiversity (sown diversity) and assembly (PFG order of arrival) approaches. Preliminary results strongly suggest that root productivity in dry acidic grasslands follows a similar pattern to the one observed for mesic grasslands (plots where forbs/legumes were sown first were less productive than the others). Interestingly, root productivity did not seem to be affected by sown diversity. Since the mechanisms behind priority effects are still poorly understood, we recently set up controlled experiments using rhizoboxes to see how vertical root distribution and root system architecture drive these effects. The analysis of the vertical distribution of roots is currently being performed using root image analysis software tools. Altogether, our results show that PFG order of arrival can strongly affect root productivity in both mesic and dry acidic grasslands.