The future of forests: thermal acclimation in the JULES land surface model

A key driver of the terrestrial carbon sink is photosynthesis. Accurate representation of this process in Earth System models is important to help understand and quantify the resilience of the global carbon sink to future climate change. In the JULES land surface model (the land surface component of the UK Earth System model - UKESM), we implement thermal adaptation and acclimation of photosynthesis using the latest scheme from Kumarathunge et al., (2019), which is based on data from 141 C₃ species covering a diverse range of biomes from tropical rainforest to arctic tundra. Additionally, we explore the sensitivity of photosynthetic acclimation to rising atmospheric CO₂ concentrations. In model simulations using forcing based on RCP8.5 to explore the model response to increasing temperatures, we show that thermal adaptation and acclimation has a positive effect on GPP that persists to 2050, but the size of the response diminishes over time. Broken down by biome, this effect is most notable in the tropics. Additionally, opposite effects of temperature adaptation and acclimation are seen in tropical (adaptation effect decreases GPP over time, whereas acclimation increases GPP) versus temperate and boreal regions (adaptation effect is constant, whereas acclimation decreases GPP over time). The attenuation of the adaptation effect in the tropics is because high temperatures in this region cause a shift in the J_max:V_cmax ratio such that photosynthesis becomes light-limited earlier (in contrast to simulations where thermal adaptation and acclimation is not activated). The light-limited rate of photosynthesis is less sensitive to increasing atmospheric CO₂ concentrations, therefore photosynthetic rates are reduced. This effect is not seen in the temperate/boreal regions because of the cooler temperatures here. Thermal acclimation results in seasonal shifts in the optimum temperature for photosynthesis. In the tropics, the optimum temperature for photosynthesis increases compared to control simulations without acclimation allowing for higher photosynthetic rates at leaf temperatures around the optimum. This increases the resilience of tropical vegetation to higher temperatures and heat extremes. In the temperate and boreal region, thermal acclimation lowers the optimum temperature for photosynthesis to adjust photosynthetic capacity to cooler spring temperatures. Acclimation of the J_max:V_cmax ratio to increasing atmospheric CO₂ concentration results in large decreases in GPP as CO₂ concentrations rise across all biomes. Enabling thermal adaptation and acclimation in the JULES land surface model therefore leads to a lower CO₂ fertilisation response of tropical vegetation as photosynthesis transitions from CO₂-limited to light-limited earlier, however vegetation productivity benefits from adjustment of its thermal sensitivity of photosynthesis to local temperatures.