Growth Analysis and Reproductive Allocation of Japanese Forbs and Grasses in Relation to Organ Toughness under Trampling

Abstract: The growth and reproduction of Japanese forbs ( Artemisia princeps and Piantago asiatica ) and grasses ( Digitaria ad-scendens and Eleusine indica ) treated to 25 tramplings (3 g m^-2) per week were investigated in relation to the toughness (tensile strength) of organs. The perennial erect forb, A. princeps , was the most sensitive to trampling in terms of a remarkable depression of plant size and relative growth rate (RGR). RGR and net assimilation rate (NAR) of trampled A. princeps were negative. This was promoted by a loss of organs due to a reduced toughness of organs following trampling. In contrast to this species which did not flower after trampling, the perennial rosette forb, P. asiatica , maintained its plant biomass, NAR, RGR and reproduction under trampling because of tougher organs. However, NAR without trampling was lower in P. asiatica due to a larger leaf dry mass per leaf area (LMA), which could contribute to leaf toughness under trampling. The annual tussock grass, D. adscendens , which has a greater RGR than that of another grass, E. indica , without trampling was intolerant to trampling in terms of decreased biomass and RGR under trampling due to more sensitive organs, although it maintained an ability to reproduce. On the other hand, E. indica showed a marked trampling tolerance, with hardly reduced plant biomass and RGR. This species showed increased toughness of organs when trampled and frequently formed inflorescences in the growing period and produced similar biomass allocation to reproductive organs to untrarnpled plants. Between the grasses, RGR without trampling was slower in E. indica , partly because of its larger LMA. These results suggest that plants face a dilemma between trampling tolerance and efficient assimilative capacity and/or growth rate.