Limits to photosynthesis: seasonal shifts in supply and demand for CO2 in Scots pine

Summary Boreal forests undergo a strong seasonal photosynthetic cycle; however, the underlying processes remain incompletely characterized. Here, we present a novel analysis of the seasonal diffusional and biochemical limits to photosynthesis (Anet) relative to temperature and light limitations in high‐latitude mature Pinus sylvestris, including a high‐resolution analysis of the seasonality of mesophyll conductance (gm) and its effect on the estimation of carboxylation capacity ( ). We used a custom‐built gas‐exchange system coupled to a carbon isotope analyser to obtain continuous measurements for the estimation of the relevant shoot gas‐exchange parameters and quantified the biochemical and diffusional controls alongside the environmental controls over Anet. The seasonality of Anet was strongly dependent on and the diffusional limitations. Stomatal limitation was low in spring and autumn but increased to 31% in June. By contrast, mesophyll limitation was nearly constant (19%). We found that limited Anet in the spring, whereas daily temperatures and the gradual reduction of light availability limited Anet in the autumn, despite relatively high . We describe for the first time the role of mesophyll conductance in connection with seasonal trends in net photosynthesis of P. sylvestris, revealing a strong coordination between gm and Anet, but not between gm and stomatal conductance.