3.1 Height and LWC
The height of O. ochrocephala decreased significantly under drought stress compared with that of the CK (40.93% and 60.76% under the D1 and D2 treatments, respectively) (P <0.05). The heights of K. humilis, P. annua, and S. pulchra increased by 51.72%, 132.92%, and 137.35% respectively, under the H2 treatment (P <0.05), while that of O. ochrocephaladecreased significantly (45.15%) (P <0.05). The heights of K. humilis and S. pulchra increased significantly under the D2H2 treatment (75.42% and 81.93%, respectively) (P <0.05) (Figure 1).
The LWC of K. humilis, P. annua, and S. pulchra decreased significantly (39.75%, 28.12%, and 37.48%, respectively) under the D2 treatment. The LWC of K. humilis and P. annua increased significantly (P <0.05) under the high temperature and the combined stress owing to the interaction of drought and heat. The LWC of S. pulchra increased significantly only under the H2 and D2H2 treatments (P <0.05). The LWC change of O. ochrocephala was non-significant in all treatments (P >0.05). Drought inhibited the LWC of K. humilis, P. annua , and S. pulchra , while high temperature and the compound stress of drought and heat promoted an increase in the moisture content of the leaves (Table 2).
3.2 Aboveground and belowground biomass
The AGB of K. humilis decreased significantly under the H2 treatment (72.66%) (P <0.05). Its BGB increased significantly under the D2H2 treatment (98.13%) and decreased significantly under the H2 treatment (35.68%) (P <0.05). The AGB of P. annua decreased significantly under the D1H1 treatment (79.84%) (P <0.05) and increased slightly, but not significantly (P >0.05), under the D2 treatment (14.48%). The BGB change of P. annua was not significant for all treatments.
The AGB of O. ochrocephala and S. pulchra decreased with the high temperature and drought stress increase, as well as under the compound heat and drought stress. The AGB of O. ochrocephaladecreased significantly under the H2, D1H1, and D2H2 treatments (86.95%, 73.65%, and 87.95%, respectively) (P <0.05). The AGB of S. pulchra decreased the most under the D2H2 treatment (95.50%). The BGB of O. ochrocephala and S. pulchraincreased gradually with the increase of high temperature stress, and decreased with the increase of the compound heat and drought stress; the BGB of both species decreased significantly under the D2H2 treatment (55.47% and 34.42%, respectively) (P <0.05) (Figure 2).