Abstract:

Objective: To analyze changes in 1?3 order root traits and its the effects on height and collar diameter of seedlings grwon in soils with different drought stresses so as to reveal response mechanisms of 1?3 order root on seedling growth of tree species with large divergence in biological characteristics. Method: Taking one-year-old seedlings including Acer truncatum, Prunus davidiana, Quercus variabilis, and Koelreuteria paniculata as research objects, whose biological and ecological characteristics are quite different, a field experiment with different gradient of soil moisture was implemented in half control environment. The mean root diameter (RD), specific root length (SRL), specific root surface area (SRA), and root tissue density (RTD) of roots in order 1?5 were measured. The relative interaction index (RII) was introduced to evaluate the relative impact of drought intensity on seedling growth and root traits. Effects of soil drought on relative growth of seedling height (H) and collar diameter (CD) were analyzed, and mechanism effects of RD, SRL, SRA and RTD of lower order root on H and CD were studied. Result: H and CD of four tree species were significantly affected by soil drought (P<0.001), and the effects were remarkably shaped by tree species (P<0.001). The inter species differences are much greater than the inter drought gradient differences. In extreme drought treatment, H and CD of A. truncatum and K. paniculata were decreased, and P. davidianadeveloped thin and tall stem, while H and CD of Q. variabilis was increased. This growth pattern was obviously constrained by changes in RD, SRL, SRA, and RTD of lower order root. The correlation between root traits at the 4th and 5th order levels is low. The relative growth of seedling height and ground diameter are significantly positively correlated with RD and SRA of lower order roots (P<0.05), but significantly negatively correlated with SRL and RTD of lower order roots (P<0.05), Namely, seedling growth was determined by soil resources acquisition strategies to some extent. The higher H and CD of A. truncatum and K. paniculata was accompanied with higher RD and SRA of lower order root so that A. truncatum and K. paniculata adapted drought soil environment using soil acquirement strategy. While the lower H and CD of P. davidiana and Q. variabilis was accompanied with higher SLR and RTD of lower order root so that P. davidiana and Q. variabilis adapted soil drought environment using conservation strategy. Conclusion: The effects of soil drought on aboveground seedling height and collar diameter of trees is closely related to changes in root traits of lower order root, and this relationship is mediated by tree species. In drought soil environment, A. truncatum and K. paniculata acquired soil resources by large amount of fine root, hence the root development of A. truncatum and K. paniculata is fast investment, while the root diameter of P. davidiana and Q. variabilis is larger, therefore, the root development of P. davidiana and Q. variabilis is slow investment. In general, absorptive root development of the four tree species is drought avoidance.

Key words: lower order root, root functional traits, soil drought, relative growth of seedlings, interspecific variation