Wood anatomy and tree-ring stable isotopes indicate a recent decline in water-use efficiency in the desert tree Moringa peregrina

The ability of desert plants to adapt to future climate changes and maximize their water-use efficiency will determine their survival. This study uses wood anatomy and δ 13 C and δ 18 O isotope analyses to investigate how Moringa peregrina trees in the Egyptian desert have responded to the environment over the last 10 years. Our results show that M. peregrina tree-ring widths (TRWs) have generally declined over the last decade, although individual series are characterized by high variability and low Rbars. Vessel lumen area percentages (VLA%) are low in wet years but increase significantly in dry years, such as the period 2017–2020. Stable δ 13 C isotope values decrease between 2010 (− 23.4‰) and 2020 (− 24.9‰), reflecting an unexpected response to an increase in drought conditions. The mean δ 18 O value (± standard error, SE) for the first ten rings of each tree from bark to pith (2020–2010) is 33.0 ‰ ± 0.85 with a range of 29.2–36.3‰, which indicates a common drought signal. The intrinsic water-use efficiency (iWUE) declines gradually with time, from 130.0 µmol mol −1 in 2010 to 119.4 µmol mol −1 in 2020. The intercellular carbon concentration (C i ) and C i /C a ratio increase over the same period, likely as a result of decreasing iWUE. The results show that M. peregrina trees seem to cool their leaves and the boundary air at the cost of saving water.