Populus pruinosa decline in a riparian tugai forest on the Zarafshon River, central Uzbekistan: edaphic conditions as predisposing factors and drought as the triggering factor

Abstract Ecological and anthropogenic changes have reduced the area of Central Asian riparian forests ( tugai ), involving dieback of Populus pruinosa Schrenk, one of the tugai ’s principal tree species. In a tugai forest on the Zarafshon River, Central Uzbekistan, we investigated the role of environmental factors in P. pruinosa dieback by comparing one healthy and one proximate declining stand. We measured the widths of tree rings of the past 25 years (1999–2023), analyzed their carbon isotope ratios (δ 13 C; 2004–2023), determined physical and chemical soil variables, and retrieved data on groundwater depths and SPEI (Standardised Precipitation Evapotranspiration Index). Over the 25-year period, radial growth did not differ between healthy and declining trees, but tree growth of the declining stand decreased, and in the last 6 years (2018–2023), during and after 2 exceptionally dry years (2018 and 2019), radial increment was significantly smaller. Correlations between radial growth, δ 13 C and SPEI, indicative of drought stress, were only found in the declining stand’s trees. Soil of the declining stand had a higher clay content in the subsoil (30–60 cm), higher salt concentrations in the uppermost layer (10 cm) and in the subsoil, and a lower field capacity across the entire soil profile. There was no groundwater decline during the study period. For the first time, evidence is provided that a drought spell in combination with predisposing unfavorable soil conditions can cause tree dieback in Central-Asian tugai forests at a relatively short distance from the water table. Our study may also contribute to initiate further research for analyzing interrelationships between hydrological, edaphic, ecophysiological and meteorological factors in dieback processes of Central-Asian riparian forests, especially in regions that are strongly underrepresented in ecological research.