COS 201-2 - Decoupling of photosynthesis and stomatal conductance under extreme heat – unavoidable process, adaptive response, or total myth?

Abstract: The Earth has already warmed by over 1 °C, and warming may reach 3 °C or more by the second half of the century. High air temperatures increase atmospheric vapor pressure deficit (VPD) and the severity of drought, threatening forests by reducing gross primary productivity and increasing evapotranspiration. Plants regulate stomata to maximize carbon gain and minimize water loss, resulting in a close coupling between net photosynthesis (A_net) and stomatal conductance (g_s). Evidence for the decoupling of A_net from g_s has been found under extreme heat, where photosynthesis declines to near zero but transpiration continues, but it is unclear if or how decoupling varies among species or plant functional types. We measured leaf gas exchange on hot days (maximum air temperature >40 °C) in 26 different tree and shrub species to investigate if there is widespread evidence for photosynthetic decoupling. Species were growing either as potted plants in the glasshouse or in situ at an urban site in Sydney, Australia.

We hypothesized that on hot days plants with ample water access would continue to transpire, despite reductions in photosynthesis (i.e., photosynthetic decoupling), whereas plants with limited water access would rely on other mechanisms to avoid excessively high leaf temperatures. Instead, there was little evidence for decoupling in well-watered plants in the glasshouse, except for one temperate rainforest species. Most plants with water access had high g_s, up to double model predictions, which effectively cooled leaves below air temperature and allowed photosynthesis to continue. Stronger evidence of decoupling was found for hotter, droughted plant leaves with near-zero A_net and g_s up to three times model predictions. Most tree species growing in situ at the urban field site had evidence of decoupling, with the strongest evidence for four broadleaf evergreen or deciduous species, some evidence for another five broadleaf species, and no evidence for three deciduous species. Contrary to our hypothesis, some urban trees showing decoupling were at wilting point with limited water access. Our observations seem more consistent with the interpretation that photosynthetic decoupling was an adaptive response to high temperature. This extreme-temperature response of sustained transpiration is not yet included in any land surface model, suggesting that model predictions may become inaccurate under conditions of high temperature and high VPD.