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 degrees C day/night) or elevated (30/24 degrees 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.