Assessing physiological responses of plants to experimental CO2 enrichment – an opportunity to improve predictions of future carbon uptake

Temperate forests are a major sink for carbon released by humans from burning fossil fuels and as a result of deforestation. As such, they modulate the impact that human CO₂ emissions have on atmospheric CO₂ concentrations and subsequently the global climate. Atmospheric CO₂ fertilisation directly affects photosynthesis and thus plant growth, impacting the productivity of the forest and the services it provides. However, the land carbon feedback to increasing CO₂ is very uncertain, making it difficult to predict the magnitude of the observed effects. The Birmingham Institute of Forest Research (BIFoR) sets out to tackle some of these open questions and aims at quantifying the effect of climate change on forests with its free-air CO₂ enrichment (FACE) facility.

A major source of uncertainty in the response of photosynthesis to increasing CO₂ concentrations is the effect on the CO₂ concentration inside the chloroplast, determined by the mesophyll conductance to CO₂ diffusion (g_m). Just as stomata restrict CO₂ diffusion from the ambient air into the intercellular air space, mesophyll components restrict the onward diffusion to the site of carboxylation. However, unlike stomatal conductance, the magnitude and nature of g_m has been highly debated over the years. Increasing evidence suggests that g_m is not a constant, but variable in the short-term in response to changes in environmental parameters, such as CO₂ concentration, light intensity, and temperature. However, to date, no conclusive agreement has been reached as to what are the major contributors to this variability. Here, we investigate the physiological mechanisms that influence how g_m responds to short-term environmental changes. We formulate a mechanistic model that seamlessly integrates into the Farquhar-von Caemmerer-Berry photosynthesis model and that well-describes the short-term variability of g_m. Our results are discussed in the context of improving estimates of the global carbon cycle and the role of BIFoR-FACE.