How well do stomatal conductance models perform on closing plant carbon budgets? A test using seedlings grown under current and elevated air temperatures

Future carbon and water fluxes within terrestrial ecosystems will be determined by how stomatal conductance (g_s) responds to rising atmospheric CO₂ and air temperatures. While both short-and long-term CO₂ effects on g_s have been repeatedly studied, there are few studies on how g_s acclimates to higher air temperatures. Six g_s models were parameterized using leaf gas exchange data from black spruce (Picea mariana) seedlings grown from seed at ambient (22/16°C day/night) or elevated (30/24°C) air temperatures. Model performance was independently assessed by how well carbon gain from each model reproduced estimated carbon costs to close the seedlings' seasonal carbon budgets, a long-term indicator of success. A model holding a constant intercellular to ambient CO₂ ratio and the Ball-Berry model (based on stomatal responses to relative humidity) could not close the carbon balance for either treatment, while the Jarvis-Oren model (based on stomatal responses to vapor pressure deficit, D) and a model assuming a constant g_s each closed the carbon balance for one treatment. Two models, both based on g_s responses to D, performed best overall, estimating carbon uptake within 10% of carbon costs for both treatments: the Leuning model and a linear optimization model that maximizes carbon gain per unit water loss. Since g_s responses in the optimization model are not a priori assumed, this approach can be used in modeling land-atmosphere exchange of CO₂ and water in future climates.