Aims Linking spatial patterns to ecological processes is a central topic in ecology. Inter-specific relationships in arid plant communities have been extensively studied by investigating aboveground patterns, but research on underground distribution patterns is inadequate. Our objective was to explore the inter-specific relationships between two dominant species, Achnatherum splendens (grass) and Sophora alopecuroides (forb), in a typical grass-forb community of arid/semi-arid regions of China.
Methods We selected a 30 m×30 m plot in a representative A. splendens-S. alopecuroides community in Sand Lake, Ningxia Hui Autonomous Region, Northwest China. The two dominant species usually spatially assemble as clumps aboveground. We partitioned the spatial gradient from the base of the assemblages to the non-vegetated open spaces between assemblages into under-canopy, transitional and open subareas to characterize the horizontal differences of microhabitat. Five assemblages of the two species were randomly chosen, and a 150 cm deep soil profile with coverage of 100 cm×100 cm was excavated in each subarea. Root biomass in terms of fine roots (diameter ＜2 mm) and coarse roots (diameter ≥2 mm) of the two species was investigated at soil depths of 0-10, 10-30, 30-60, 60-100 and 100-150 cm.
Important findings Horizontally, biomass density of the total roots of A. splendens decreased along the gradient from under-canopy to open subareas. Also, ANOVA showed that biomass density of the total roots significantly differed among the three subareas for A. splendens, but no significant differences were detected for S. alopecuroides. Vertically, the soil layers with the highest biomass density of the total and fine roots of A. splendens increased in depth from under-canopy to open subareas, while that of the total roots of S. alopecuroides remained at 10-30 cm depth and that of the fine roots of S. alopecuroides tended to decrease in depth along the gradient. Also, the fine roots of the two species were different in the highest biomass density layer in the under-canopy subareas where root systems were densely distributed, but showed similar vertical patterns in the transition and open subareas. Niche separation in the under-canopy subareas and discrepancy in horizontal patterns of the two dominant species could represent an important adaptive strategy of avoiding direct competition and facilitating coexistence in arid communities.