Comparative analysis of annual rings of perennial forbs in the Loess Plateau, China

Although recent studies have demonstrated that annual growth rings are present among perennial forbs species at high northern latitudes, little is known about whether there are demarcated growth rings of perennial forbs in the Loess Plateau of China where plant growth is strongly limited by dry climate conditions and severe soil erosion. In this study, we collected the main roots of 11 perennial forbs species along the precipitation gradient in the Loess Plateau, and analyzed the growth rings in the secondary root xylem. We found that ten species showed distinct annual growth rings, and the anatomical patterns, including vessel size and density, varied considerably among different families. Our results suggest, for forbs species in the Loess Plateau, that vessel diameter in the root xylem was strongly correlated with growth rate of the forb's roots. Ring widths of the forbs showed a significant declining trend, reflecting the deteriorating signal of growth condition with age. In comparison to other families, forb species of Fabaceae usually have the evidently larger vessels that link directly to higher hydraulic capacity and growth rate. In terms of annual ring width patterns, this study provides an applicable approach to detecting effects of limited climatic conditions and life history strategies on herbaceous vegetation in the Loess Plateau.