The bare karst terrain in South China is one of the largest and most densely-populated tropical karsts in the world. It is distinguished for its harsh natural conditions. Most karst areas have a characteristic "double layer" geological structure; the soil layers and fissures formed after prolonged intensive karstification cause a loss of surface water and subsequent desiccation. Seasonal drought is the main feature of karst ecosystems and is a major constraint upon agricultural production and vegetation restoration. In the South China karst region, particularly the bare-rock karst areas, plants have evolved adaptations to the rocky and xeric conditions of karst soils; the dominant plants commonly have robust root systems. One such plant, Cyclobalanopsis glauca, is an evergreen broadleaf species and a dominant species in the Guilin karst forest. These plants survive well in higher mountain elevations, even in very thin and rocky soils, and karst forests can grow under seasonal drought conditions.
We set up a water supply device in combination with a simulated karst structure comprising soil, limestone, and epikarst layers. The setup had a diameter of 100 cm. Soil, limestone, and epikarst layers had heights of 30, 50, and 20 cm, respectively. Epikarst layer had two treatments-watered and unwatered-and the water content of the soil layer was controlled within 30%-40%, 50%-60%, 70%-80%, or 90%-100%. Each treatment was replicated three times. We investigated the effects of this karst structure on the growth, leaf area (LA), specific leaf area (SLA), biomass, osmotic solutes, SPAD value, root-shoot ratio, leaf relative water content (RWC), and ratio of dry weight to fresh weight (RDF) of 2-year-old seedlings of C. glauca that were transplanted from a nursery into the simulated karst substrate. The controlled soil water treatments lasted for 4 months, after which the plant physiological parameters were tested. Results showed that plant roots could extend into the epikarst layer, and seedling growth was not affected by topsoil drought stress when water was added to the epikarst layer. When the epikarst layer had no water, the physiology of the seedlings was greatly influenced by the soil water content (SWC). When the topsoil underwent drought, LA, RWC, and SPAD decreased, while RDF and root-shoot ratio increased. Malondialdehyde (MDA), soluble sugars, and proline are important osmotic solutes; they are products of lipid peroxidation and the most prominent indicators of oxidative stress in plants. Compared with well-watered treatments and treatments watered in the epikarst layer, MDA, soluble sugars, and proline content increased significantly (analysis of variance, Duncan's test; P<0.01) in the leaves of unwatered C. glauca seedlings, suggesting that they suffered drought stress. The results also showed that using water in the epikarst layer was an important adaptive strategy for C. glauca seedlings in drought-stricken karst soils. In this study, we also found that the seedling roots could penetrate through the rock layer to reach the epikarst layer when the latter was watered. However, no roots penetrated this layer when it was not watered. The results suggested that ground water in the epikarst layer plays an important role in the growth and survival of C. glauca seedlings and in their water use.