Boston University Boston, Massachusetts, United States
Invasive species can disrupt the biogeochemical cycles of ecosystems by changing composition, processes, and ecosystem structure, but there is uncertainty about how climate change may exacerbate these impacts.
Cogongrass (Imperata cylindrica) is invading the Southeastern United States. Prior work established that cogongrass reduces biodiversity and pine seedling survival. However, we have no estimates of how cogongrass mediates carbon and water cycles in adult pine stands. In addition, elevated CO2 may accelerate the growth of C3 species like pine, while elevated temperature may favor C4 grasses like cogongrass. We examine two questions:
Does cogongrass alter the carbon and water fluxes of the pine stands it invades?
Will competitive relationships change as atmospheric CO2 increases?
We test these questions by modeling cogongrass invasions using the Ecosystem Demography 2 model (ED2). ED2 was first calibrated through a Bayesian meta-analysis and two targeted field campaigns. Cogongrass invasions were simulated for four managed pine forests across a temperature and precipitation gradient under future climate scenarios until 2100. At each site we explored a range of scenarios involving combinations of different climate futures, different levels of CO2, and three different vegetation competition scenarios: only pine forests, only cogongrass, and competing levels of cogongrass and pine forests.
This work has three major conclusions. First, we were able to demonstrate the utility of targeted field work, which reduced uncertainty in in cogongrass LAI estimate by ~ 2.5. Second, we found that cogongrass invaded quickest in elevated temperatures. Finally, we found that invasions alter carbon processes via increase in overall biomass, and mediate water processes by increasing competition for water. These findings disagree with field studies that have found temporary facilitation between cogongrass and pines saplings during droughts – suggesting different competitive dynamics between saplings and adult pines. They also suggest that C4 photosynthesis confers a benefit under climate change.
While C4 photosynthesis alone cannot explain cogongrass' spread over the landscape (native grasses also employ a C4 strategy), it explores one likely advantage of many and how that will change through time. This is also an example of how dynamic vegetation models can test previously untestable questions using modeling experiments.