Abstract: This paper studied the soil physical-properties under Populus davidiana, Quercus liaotungensis, and Populus davidiana-Quercus liaotungensis mixed forest in the Ziwuling forest area of Loess Plateau, and the leaf photosysthetic characteristics of these three types of forests. The results showed that soil moisture content in 0~300 cm layer was the highest under P. davidiana forest, and obviously increased below 200 cm in depth under P.davidiana-Q. liaotungensis mixed forest, which was 10.5%~19.76% higher than that under Q. liaotungensis forest. In 0~60 cm layer, P. davidiana forest showed the highest soil bulk density and the lowest soil porosity, while P.davidiana-Q. liaotungensis mixed forest presented the lowest soil bulk density and the highest soil porosity, and both of these indices surpassed their corresponding values under pure forests, which indicated that the mixed forest could make effective use of water in deep soil, and obviously improved soil physical and chemical properties. P. davidiana and Q.liaotungensis had a higher content of leaf chlorophyll than P. davidiana-Q.liaotungensis mixed forest, and Q. liaotungensis presented the highest leaf chlorophyll content. Q. liaotungensis had the highest photosynthetic rate and stomatal conductance, followed by P. davidiana, and by P. davidiana-Q.liaotungensis mixed forest. The water use efficiency of the forests ranked in the decreasing order of Q. liaotungensis in pure forest, Q. liaotungensis in mixed forest, P. davidiana in mixed forest, and P. davidiana in pure forest. Q. liaotungensis in mixed forest presented the highest F_v/F_m and F_v/F_o, and did not remarkably differ from those in pure forest, but in the mixed forest, the F_v/F_m and F_v/F_o P. davidiana were markedly lower than those of P. davidiana in pure forest. Both the q_p and NPQ of P. davidiana and Q. liaotungensis in pure forests were higher than those in mixed forest, respectively. In Ziwuling forest area, Q. liaotungensis in Q. liaotungensis-dominant climax community tended to be more stable, and grew better than Q. liaotungensis in P.davidiana-Q. liaotungensis mixed forest, and P. davidiana would be gradually replaced. Compared with P. davidiana in pure forest, P. davidiana in its mixed forest was at a disadvantage in its growth.

Key words: Plant granules, Oncomelania hupensis, Poison effect