Plant-soil feedback mechanisms in medusahead invasion in Californian grasslands
The occurrence of alternative states and thresholds has become a central issue in Ecology and alternate states represent major shifts in ecosystem function due to changes in abundance and composition of dominant species. This particularly occurs in species invasion with thesholds, which lead to invasion when the invasive species reaches a certain density. While seed production, seed dispersal and competitive interactions have been shown to influence these thresholds, belowground processes remain poorly studied. To redress this, we test how soil processes change depending on the density of the invasive plant species Taniatherum-caput medusahead (Medushead) in highly invaded Californian grasslands. Specifically, we hypothesize that Medushead induce positive plant-soil feedbacks which favour its own growth, through change in microbial community and litter input. Using a seed addition experiment, we created a gradient of Medusahead density ranging from 0 to 1800 individuals in 35 plots of 1m2. After one year of establishment, we measured several soil processes along the Medusahead density gradient and during the growing season, such as soil respiration, rate of litter decomposition, soil available nitrogen, microbial nitrogen and soil communities, specifically fungal to bacterial ratio, through PLFA.
Along the density gradient of Medusahead, soil respiration began to increase from 20 individuals, and continued to increase linearly until the patch reaches 1000 individuals to finally stabilize up to 1800 individuals. Results showed that, while dependent of soil humidity during the growing season, soil respiration was significantly higher in plots highly invaded by Medusahead compared to the control with two times more CO2 efflux. These results demonstrate that Medusahead can accelerate soil processes depending on its density and induces major shift in ecosystem functioning. Soil nitrogen also increased along the gradient of Medusahead, showing that the invasive species leads plant-soil feedbacks, which improve growth conditions. Because Medusahead also produces high litter quantity, which constraints growth of the other species, the improved soil conditions are likely to favour its own growth. On-going analysis of microbial communities will confirm if Medusahead induces a shift in soil fungal to bacterial ratio, which is known to determine the rate of soil processes. Together, findings of this experiment will highlight the importance of soil processes in species invasion and more particularly in thresholds which lead to species expansion.