Abstract: Background, aim, and scope Recent studies have indicated that annual growth rings are widespread in the roots of herbaceous species and formed annually. However, little is known about the presence and characteristics of annual growth rings in alpine meadow of northwestern Yunnan, China, the age of herbaceous species is also largely unknown. Materials and methods We cut off the main roots approximately 5—10 cm beneath the ground surface from each species (3—5 individuals for per species) that were growing well. The root samples were put in sealing bags in the field and stored in 75% alcohol after washing the samples clean. In the laboratory, we cut cross-section (10—15 μm thickness) from the proximal end of the root samples to avoid missing annual rings with pulley section cutter, and stained the paraffin sections with 1% safranine water solution and 0.5% fast green ethanol solution. Finally, we measured the ring width and vessel parameters with WinCELL automatically. Results Eleven species showed annual growth ring structures and 60% of species showed clear or relatively clear demarcated annual ring boundaries. The period spanned 2012—2017 with an average age of 3.8 years (2—6 years old). Meanwhile, the growth rates and hydraulic conductivity indicated increasing tendency with age. Whereas, the vessel size indicated decreasing tendency over the years. Discussion The annual growth ring structure were significantly lower than others, which might be due to a relatively low number of species sampled in our study area. Meanwhile, perennial herbaceous species were relatively young, resource stress above alpine treeline might contribute to shorter life history strategies. Finally, we analyzed the changing trend of ring width and the structure of vessels. The main reason for the shortened life history of species in this area may be caused by environmental stress above the alpine timberline. In the early stage of herbaceous plant growth, it is primarily the stage of vegetative growth. There is a close relationship between the radial growth of herbaceous plants and climates, and the impact of climates change on alpine meadow vegetation directly affects the physiological and ecological processes of herbaceous plants. High altitude areas own plenty of light and plants have strong photosynthesis, so that the growth rate and water transfer efficiency of perennial herbs increase with age, which may also be a positive feedback mechanism for global warming. With the increase of age, herbaceous plants will adopt a series of self-protection measures to increase the probability of survival and reproduction success. They will avoid this injury by transforming their life history strategies, reducing the number of vessels and reducing the size of the vessels.Conclusions We found that the true annual growth rings occurred in the secondary root xylem of the perennial herbaceous species in northwestern Yunnan, and the growth ring showed fluctuations between species. This is a trade-off performance for drought stress conditions caused by climate warming, while increasing the probability of reproduction to reduce the risk. Our results provide further evidence for the sensitive and rapid response of radial increment growth in roots of perennial herbaceous species in northwestern Yunnan. Recommendations and perspectives The authors predict that there was similar result elsewhere in an alpine meadow region and set out to do further statistical analysis combining with meteorological factors.
Keywords: perennial herbaceous species; anatomical structure and pattern; ring width; vessel; age distribution