Atmospheric feedbacks reverse the sensitivity of modeled photosynthesis to stomatal function

Stomata mediate fluxes of carbon and water between terrestrial plants and the atmosphere. These fluxes are governed by stomatal function and can be modulated in many Earth system models by an empirical parameter within the calculation of stomatal conductance, the stomatal slope (g1M). Intuitively, g1M represents the marginal water cost of carbon, relating it to the emergent plant property of water use efficiency. Observations show that g1M can range widely across and within plant types in varying environments, and this distribution of g1M is not captured within Earth system models which represent each plant type with a single g1M value. Here we examine how g1M influences photosynthesis using coupled Earth system model simulations by perturbing g1M to observed 5th and 95th percentiles for each plant type. We find that high g1M reduces photosynthesis nearly everywhere, while low g1M has regionally dependent responses. Under fixed atmospheric conditions, low g1M increases photosynthesis in the Amazon and central North America but decreases photosynthesis in boreal Canada. These responses reverse when the atmosphere responds interactively due to spatially differing sensitivity to increases in temperature and vapor pressure deficit. Choice of g1M also influences photosynthetic response to changes in atmospheric carbon dioxide (CO2), with lower and higher g1M modifying total global response to elevated 2x preindustrial CO2 by 6.4% and -9.6%, respectively. Our work demonstrates that atmospheric feedbacks are critical for determining the photosynthetic response to g1M assumptions and some regions are particularly sensitive to choice of g1M.

Methods
This dataset contains model output from simulations using the Community Earth System Model version 2 (CESM2; Danabasoglu et al., 2020), which is comprised of the Community Land Model 5 (CLM5; Lawrence et al., 2019), the Community Atmosphere Model 6 (CAM6; Bogenschutz et al., 2018), and a slab ocean based on output from the CESM2 Coupled Model Intercomparison Project Phase 6 (CMIP6; Danabasoglu & Gent, 2009) preindustrial control run.