A comparison of water use strategies between pure and mixed forests on the Chinese Loess Plateau

To elucidate how pure and mixed forests adjust their water use strategies under interannual precipitation variability, we conducted a systematic investigation of root architecture, water dynamics, and physiological regulation in pure Pinus tabuliformis, pure Robinia pseudoacacia, and their mixed forests on the Chinese Loess Plateau during a wet year (2022) and a normal year (2023). By analyzing fine root vertical distribution, root length density, soil moisture, stand transpiration, stable isotope-based water sources, and stomatal behavior, we found that mixed forests accumulated higher fine root biomass than either pure forest type, while exhibiting intermediate root length density. Seasonal fluctuations in soil water content and stand transpiration were more pronounced in mixed forests than in pure P. tabuliformis stands, though less variable than in pure R. pseudoacacia stands. Transpiration dynamics closely tracked root length density patterns and responded strongly to environmental drivers such as air temperature, net radiation, and vapor pressure deficit. Importantly, as conditions shifted from the wet to the normal year, all forests reduced surface water use, but mixed forests showed a markedly greater shift toward deep soil water extraction compared to pure stands. Stomatal conductance analysis further indicated that mixed forests adopted more stringent stomatal regulation, reflecting a more conservative water use approach relative to pure forests. These results demonstrate that mixed forests develop a more efficient and conservative water-use strategy by optimizing root distribution for deep-water access and enhancing stomatal control. Our findings offer key physiological and ecological insights for predicting the stability and adaptability of plantation ecosystems under future climate variability on the Loess Plateau.