Soil mutualists modify priority effects on plant productivity, diversity, and composition

Question The importance of priority effects on community assembly is well recognized, as is, increasingly, the role of environmental context in shaping those priority effects. Distinguishing the effects of soil nutrients and soil mutualists may be critical in the context of restoration, but remains poorly understood. Additionally, we asked whether pioneer species and soil conditions influenced trade-offs or complementarities between vegetation production and species diversity. Location Plant Growth Center, Montana State University, Bozeman, MT, US. Methods Using experimental seed addition in mesocosms, we investigated how the priority effects of different plant species influenced productivity, diversity and composition of the resulting plant community. We also experimentally manipulated soil nutrients and mycorrhizae to determine how they modified those priority effects. Results We found that species-specific priority effects of pioneers strongly governed the productivity, diversity and species composition of the resulting community. Pioneer grasses, one native and one non-native species, both preempted light and space, prevented the colonization and growth of the subsequent community and restricted community dispersion (i.e. differences in species composition among mesocosms). By contrast, the pioneer forb species had little influence on the recruitment and establishment of the later colonizing species, and supported the assembly of divergent communities. We found evidence to suggest that amendments of soil mutualists, but not soil nutrients, modified the influence of one pioneer species on characteristics of the resulting community by enhancing community productivity and diversity in mesocosms with non-native grass pioneers. Across all treatments, species diversity declined as vegetation production increased, indicating an overall trade-off between productivity and diversity. However, relationships between productivity and diversity varied among pioneer treatments and mycorrhizal amendments. Conclusions Our results emphasize important interactions between priority effects and the use of soil mycorrhizal amendments in restoration, as well as highlight how pioneer species and mycorrhizae influence potential trade-offs between restoration objectives aimed at enhancing productivity vs species diversity.