Age Structure and Self-Thinning of Four Poisonous Forbs in Alpine Meadows on the Qinghai-Tibetan Plateau

Age structure and self-thinning indicate the development of plant species and their adaptability to the environment, and the proportion of poisonous plants could reflect the status of grasslands degradation. Little is known about age structure and self-thinning of poisonous dicotyledonous forbs in the alpine meadows on the Tibetan Plateau. We selected four poisonous dicotyledonous species, namely Anaphalis lacteal, Stellera chamaejasme, Pedicularis kansuensis, and Gentiana straminea, and determined their age structures by using number of branches, crown diameters, and plant heights and their self-thinning by using dry weights and different components (branches, leaves, and inflorescences). The populations of the four species were pyramid shaped with a rich seedling and juvenile base, which demonstrated a strong ability to increase in number rapidly and adapt to the environment. The Godron index indicated that the stability of age structures of the four species were A. lacteal > S. chamaejasme > G. straminea > P. kansuensis. Significant self-thinning was exhibited by branches of A. lacteal, branches and leaves of S. chamaejasme, and inflorescences of P. kansuensis. The self-thinning exponents from the allometric relationships between biomass and density differed at the level of branches, leaves, and inflorescences in the four poisonous forbs. We concluded that the populations of the four poisonous species were stable and able to increase in number rapidly on the Qinghai-Tibetan Plateau.