Growth response of two annual herbs to karst soil depth and water supply

The morphology, biomass accumulation, and distribution characteristics of Xanthium sibiricum Patrin ex Widder and Bidens pilosa L. were studied under simulated karst habitats with different soil depths and water availability. Results showed that decreased resources (i.e., water supply, soil depth, and water supply + soil depth) inhibited plant growth. The biomass accumulation and stem investment ratio of both plants decreased significantly. The decrease in water supply had no significant effect on the root-shoot ratio, root length, or specific leaf area of the species, whereas the root-shoot ratio, root length, and specific leaf area of both species increased significantly with the decrease in soil depth and water supply + soil depth. Results also showed that the inhibition effect of reduced soil depth on plant growth was greater than that of reduced water supply. When resources were slightly reduced, the root and leaf investment ratios were stable at the expense of stem investment to ensure water absorption and photosynthesis. When resources were severely reduced, plants countered the increased pressure of the underground environment by investing in underground systems, e.g., increasing specific root length.