Cohort-based water competition in a land surface model to assess grassland responses to drought

Climate change increases the frequency and intensity of drought events, highlighting the need to accurately predict vegetation responses to water stress to reduce uncertainties in climate projections. Droughts can determine the competitive outcome between plant species, thereby affecting ecosystem carbon, water, and energy fluxes. Capturing species- or cohort-specific responses to drought (where cohorts group species with similar plant traits and water stress strategies) in land surface models requires representing structural and functional diversity, which determines how plants compete for water under stress.

Most land surface models, including QUINCY, represent vegetation using a few fixed plant functional types, defined by shared photosynthetic pathway, phenology, structure, and climatic range, with little or no interaction between co-occurring species. To address this limitation, we introduced vegetation demography into QUINCY, focusing on grasslands. At the site scale, the model now represents multiple cohorts, defined by distinct plant trait combinations that influence water stress responses, which share the same soil resources and allow for explicit water competition between cohorts. By accounting for how plants compete for water at different soil depths, the model links cohort interactions to changes in transpiration driven by soil water availability and root distribution, supporting more process-based simulations of grassland drought responses. To ensure consistent coupling between water and carbon fluxes, physiological water stress is diagnosed based on the reduction in transpiration caused by competition for soil water. This approach maintains coherence between transpiration and carbon uptake under drought conditions.

Site-scale simulations with this cohort-based water competition scheme allow detailed analyses of water use strategies, transpiration partitioning, and drought responses in grassland communities. By incorporating in situ observations of species composition and plant traits to define cohorts, the framework directly connects field data with model simulations, supporting more accurate predictions of grassland responses to drought.

This framework provides a basis for assessing water competition outcomes in grassland communities under future climate scenarios and will be extended to include nutrient competition. Overall, the introduction of cohort-based water competition in QUINCY represents a step toward more realistic simulations of ecosystem responses to environmental stress, offering insights into the role of plant diversity and structure in modulating drought impacts.