Community biomass and biodiversity effects in Phase II
Community biomass significantly increased with plant species richness in Phase II (Table 1, Fig. 3b). However, plant richness effects on community biomass significantly varied with soil moisture treatments, resulting in a significant plant species richness × soil moisture in Phase I × soil moisture in Phase II interaction (Table 1).
Over all treatments, the BE increased with plant species richness (Table 2, Fig. 4d). However, the CE and SE showed different directions of responses to plant species richness. Positive CE values were greater in higher plant species richness levels, while negative SE increased with higher plant species richness (Table 2, Fig. 4e and f). Soil moisture treatments in Phase I significantly influenced the BE, CE and SE in Phase II (Table 2). Both the BE and CE were greater in mesocosms with drought treatments than those with ambient treatments from Phase I (Fig. 4d and e). However, drought treatments from Phase I caused more negative SE in Phase II (Fig. 4f). Drought in Phase II generally decreased the magnitude of positive BE and CE values, and it had no general influence on SE (Table 2, Fig. 4).
Plant species richness and soil moisture treatments of Phase I and II interacted to influence both the CE and SE (Table 2). In mesocosms with two species, the CE did not vary among soil moisture treatment combinations, while at higher richness levels, the CE was greater in the Drought-Ambient treatment and lower in the Ambient-Ambient treatment compared to the other treatment combinations (Fig. 4e). There were no significant differences in SE among soil moisture treatment combinations for mesocosms with two or four species. However, in treatments with six or twelve species, the SE was positive in the Ambient-Ambient treatment, and the values were negative for the other treatment combinations. The negative SE was greatest in the Drought-Ambient treatment and with twelve plant species (Fig. 4f). Relationships between species richness and the BE did not vary among treatment combinations of soil moisture in Phase I and II (Fig. 4d), and thus there was no plant species richness × soil moisture in Phase I × soil moisture in Phase II interaction (Table 2).