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.