Peak photosynthesis at summer midday in Acacia trees growing in a hyper-arid habitat

Key message: Desert Acacia trees photosynthesize during the hot dry summer, and use stored carbon for summer growth. Trees that grow in hyper-arid environments can provide important insight into the role of carbon use and carbon storage for tree survival and growth in extreme conditions. Acacia trees, in particular, experience some of the most arid conditions in which trees can grow on the planet, enduring high temperatures, high radiation and drought. Here we measured for two years photosynthesis along the day, stem circumference growth, and seasonal changes in non-structural carbohydrates in adult Acacia trees in their natural hyper-arid habitat (Arava desert, southern Israel). The peak of net CO₂ assimilation was at midday in all seasons, even during summer when vapor pressure deficit was at maximum of 6 kPa and light intensities were at high levels (1800 µmol photons m⁻² s⁻¹). Tree growth started in the spring and increased in the hot summer season and during the dry fall season (autumn). Starch concentrations in the branches were highest in the winter and spring (17% in dry matter) decreasing in the summer and fall (7% in dry matter). Our observations indicated that carbon assimilated during the winter was stored in the branches as starch reserves, which were later used for tree growth in summer. Still, most of the growth was subsidized by concurrent assimilation during the dry season. These findings show that Acacia trees are able to photosynthesize in conditions that other trees cannot, indicating a strong potential to contribute to ecosystem carbon sequestration in warming and drying climates.