How do Plant-Associated Fungi Mediate Vegetation and Process Shifts in Response to Interactive Global Change Factors in Phosphorus-Limited Dry Forest-Grassland Systems?

Jason Hoeksema, University of Mississippi (Principal Investigator)
Nicole Hynson, University of Hawaii at Manoa (Co-Investigator)
Jennifer Bhatnagar, Boston University (Co-Investigator)
Edward Brzostek, West Virginia University (Co-Investigator)

The compounding effects of multiple global change factors such as elevated atmospheric CO₂ concentrations (eCO₂), drought, temperature, and land use change are causing fundamental changes in biogeochemical cycles on a global scale. To date, free-air CO₂ enrichment (FACE) experiments have provided profound insights into how predicted future climate change scenarios may continue to impact ecosystems services upon which all life relies. However, FACE experiments have largely focused on fertile temperate ecosystems, which only represent a quarter of the planet's terrestrial biomes. Therefore, it remains unclear (and unlikely) that findings from temperate zone FACE sites can be applied to the remaining three quarters of the globe. For instance in dryland, infertile subtropical forests, CO₂ enrichment has not led to an increase in ecosystem storage of carbon (C); instead, eCO₂ has caused increases in soil respiration and fundamental shifts in the understory plant community composition from mixed shrubs and grasses to specific (C3) grasses dominating. C3 grass invasion and its negative ramifications for important ecosystem functions such as water and nutrient cycling are becoming commonplace in much of the tropics and subtropics, and are a pressing area of concern. Sites such as EucFACE in southeastern Australia provide a foreboding forecast of how future predicted increases in atmospheric CO₂, warming, and drought will further enhance these invasions and their negative impacts. However, the mechanisms by which understory plant communities shift under changing climate conditions are unknown, thus limiting the ability to predict which ecosystems may be most at risk.

The overarching hypothesis is that a shift to increasing relative dominance of C3 grasses under eCO₂ is accompanied by shifts in the composition and traits of critical root symbiotic fungi, resulting in reduced C storage in soil. Objective 1: Assess the consequences of eCO₂ for microbial communities and their traits and resulting shifts in dry forest understories and carbon (C) cycling. Objective 2: Model and experimentally test how the interaction between eCO₂ concentrations and increased predominance of C3 grasses impacts plant-soil-microbe interactions, microbial traits, and C cycling in dry subtropical forest systems. These objectives will be accomplished through a combination of three approaches: (a) Field sample analysis at EucFACE, in which microbial, and plant biogeochemical cycling (C, N, and P pools and fluxes) will be measured, as well as plant and fungal gene expression in roots and soils, (b) Controlled-environment Experimentation, using growth-chamber experimentation with soil from EucFACE, in which the interactive effects of eCO₂ and understory vegetation on morphological and molecular traits of mycorrhizal fungi, and association of those traits with the performance of key C3 and C4 grasses will be tested; and (c) Ecosystem Modeling, in which observational and experimental data will be leveraged to improve the ability of a plant-microbial interactions model (FUN-CORPSE) to predict the impacts of eCO₂ on understory vegetation shifts, and shifts in fungal traits on coupled C-N-P cycling in forests. Outcomes and benefits of the project will include an improved understanding of the mechanisms and ecosystem consequences of vegetation shifts in the understory of dry forests in response to eCO₂, and substantial improvements in the ability of ecosystem models to capture those dynamics and project their impacts on important ecosystem services such as nutrient cycling and carbon storage.