The differential responses of gas exchange and evapotranspiration to experimental drought for seedlings of two typical tree species on the Loess Plateau

Seedling survival is vital for ecological restoration and carbon neutrality on the Loess Plateau, yet large-scale afforestation in North China has reduced soil water availability in arid and semi-arid areas, complicating efforts to sustain young trees. The water consumption strategies of commonly planted trees and their ecohydrological responses to varying water conditions remain unclear. Two common tree seedlings, Robinia pseudoacacia and Pinus tabuliformis, were cultivated using soil column experiments to simulate the initial stage of afforestation. Six irrigation treatments (in 100, 200, 300, 400, 500 and 600 mm) were applied to investigate the effect of rainfall recharge on tree physiological traits and water consumptions from June to August in 2021-2022. Results indicated that the both species experienced reduced leaf gas exchange under drought (<200 mm) and flood (>500 mm), and it was decreased when the vapour pressure deficit >2 kPa. R. pseudoacacia exhibited lower stomatal conductance and transpiration but higher leaf water use efficiency than P. tabuliformis under drought, indicating better drought resilience. The daily evapotranspiration (ET) of the two species was both limited by soil moisture when irrigation <300 mm, while it was promoted by solar radiation when irrigation > 300 mm. A precipitation threshold of 300 mm was identified as critical for seedling growth. Moreover, the mean ET of R. pseudoacacia and P. tabuliformis was 190.43-648.40 and 196.30-620.40 mm, respectively, in the 100-600 mm irrigation treatments during June to August in 2021-2022, and the mean water use efficiency of P. tabuliformis was 55.1 % higher than that of R. pseudoacacia at the plant level. This study will provide critical insights into water consumption strategies to water stress on early afforestation, offering guidance for species selection and ecological restoration on the Loess Plateau.