Effects of soil thickness on component properties, biomass structure and expanding ability of Euphorbia thymifolia

Aiming to address the lack of suitable turfgrass species that can be used in stepping stone gaps in subtropical areas, four soil layers (2-, 4-, 8-, and 12-cm-thick) were set in outdoor floor tiles gaps, in which the native grass Euphorbia thymifolia was planted as the experimental material. Plant survival rate, component properties, biomass, and expansion ability were measured after summer. The effect of soil depth on the survival, growth, and reproduction strategies were analyzed. The results showed that: 1) Soil depth significantly affected the survival rate in summer (P < 0.05). Survival rate increased with the increase in soil depth; even in 2-cm-thick soil, with 16.90% soil water content and 76.80 ℃ surface temperature, the survival rate of E. thymifolia reached 85.67%. 2) The order of influence of soil depth on leaf characteristics was number > area > thickness > length > width, on stem characteristics was secondary branch number > stem length > plant height > primary branch number, on root characteristics was indefinite root number > main root diameter > lateral root number > main root length, and on flower characteristics was flower number per plant > inflorescence number per plant > flower number per inflorescence > seed number per flower > 1 000 seed weight. E. thymifolia exhibited extreme morphological plasticity, and could significantly adjust component properties according to soil depth (P < 0.05); therefore, E. thymifolia could complete its life cycle with plump seeds at all four soil depths. 3) Soil depth significantly affected biomass structure of E. thymifolia (P < 0.05). The biomass in 2-cm-thick soil was preferentially distributed to the root and adventitious root; however, in 4-cm-thick soil it was to the stem, whereas in soil depth of ≥ 8 cm it was to the flower and fruit. Therefore, the biomass could be reasonably distributed among components, which showed a growth strategy with reasonable biomass allocation. 4) Soil depth significantly affected branching and expansion ability of E. thymifolia (P < 0.05), with the order of influence being branching rate > expansion space > expansion area > extension density. For soil depths of 2–12 cm, the branching rate of stems 1 cm in length was 1.72～8.31 branch, the coverage area of each plant was 841～2 656 cm², and the expansion density was 2.65～4.81 branch·cm⁻³. To summarize, E. thymifolia exhibited strong heat and drought resistance, and could adjust its growth strategy according to soil depth; therefore, it is suitable for promoting as a turfgrass for planting in stepping stone gaps.