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<p>Among living tree species, <em>Acacia raddiana</em> (Savi) and<em> Acacia tortilis </em>(Forssk), species of the legume family, populate some of the hottest and driest places on Earth. Our research investigates the physiological processes underlying the unique survival of trees in extreme environmental conditions. We measured <em>Acacia</em> trees in their natural habitat together with a controlled experiment under scenarios of drought and low N on a lysimeters system to unravel their water use strategies and growth dynamics. In the field, temperature positively influenced the growth rate of the trees, daily and annual gas-exchange curves showed higher gas exchange during noon and in summer, when temperature and radiation are maximal (44°C, 2000 µmol m<sup>-2</sup> s<sup>-1</sup>), and the air is dry (21% RH). Furthermore, in the controlled experiment, Acacia saplings keep transpiring water (180 g per day), especially at noontime (0.08 g<sub>water</sub>/g<sub>plant</sub>/ min), and therefore continue growing in low soil water content of 5%. These findings suggest a strong potential for acacia trees to contribute to ecosystem carbon sequestration in warming and drying climates.</p>

IntroductionSoil water availability is a key factor in the growth of trees. In arid deserts, tree growth is limited by very dry soil and atmosphere conditions. Acacia tree species are distributed in the most arid deserts of the globe, therefore they are well adapted to heat and long droughts. Understanding why some plants do better than others in some environments is a key question in plant science.MethodsHere we conducted a greenhouse experiment to continuously and simultaneously track the whole-plant water-balance of two desert Acacia species, in order to unravel their physiological responses to low water availability.ResultsWe found that even under volumetric water content (VWC) of 5-9% in the soil, both species maintained 25% of the control plants, with a peak of canopy activity at noon. Moreover, plants exposed to the low water availability treatment continued growing in this period. A. tortilis applied a more opportunistic strategy than A. raddiana, and showed stomatal responses at a lower VWC (9.8% vs. 13.1%, t4= -4.23, p = 0.006), 2.2-fold higher growth, and faster recovery from drought stress.DiscussionAlthough the experiment was done in milder VPD (~3 kPa) compared to the natural conditions in the field (~5 kPa), the different physiological responses to drought between the two species might explain their different topographic distributions. A. tortilis is more abundant in elevated locations with larger fluctuations in water availability while A. raddiana is more abundant in the main channels with higher and less fluctuating water availability. This work shows a unique and non-trivial water-spending strategy in two Acacia species adapted to hyper-arid conditions.