Water Use Efficiency of Trees Planted on Abandoned Farmland and Influencing Factors in a Loess Hilly Area

Water use efficiency (WUE) serves as an indicator of the drought resilience strategies employed by plants in water-limited regions and their ability to adapt to their specific growing environments. Nevertheless, a consensus has yet to be reached regarding the primary factors that influence WUE in plantations located in loess hilly areas. This study aimed to quantify the instantaneous water use efficiency (WUE_t) and short-term water use efficiency (WUE_s) in 21-year restored forests consisting of six forestlands and investigated the effects of soil and plant factors on WUE. The results indicated that the WUE_t and WUE_s performance in the monoculture PT forest was the highest, followed by the monoculture PS forest. The highest concentrations of soil organic carbon (SOC), dissolved organic carbon (DOC), total nitrogen (TN), dissolved organic nitrogen (DON), nitrate nitrogen (NO₃^–-N), and ammonium nitrogen (NH₄⁺-N) were found in the RP pure forest. The relative water content of leave (RWC), stomatal conductance (Gs), transpiration rate (Tr), and net photosynthetic rate (An) were highest in the monocultural PS forest. Soil water content (SWC), soil hydraulic conductivity (Ks), and TN exhibited positive correlations with WUE. RWC and An demonstrated positive correlations with WUE. An and Ks were respectively the leaf and soil factors with the highest impact on WUE. Additionally, Ks showed positive correlations with relative water RWC. These results underscore effective water use and soil nutrients vary by forestland types and the intricate interplay of soil and plant factors in relation to leaf WUE.