Abstract:

Aims The variations in population structure and the quantitative dynamics of plants are a reflection of the local ecology and environment. Hippophae tibetana is a dwarf shrub, and a pioneer species of vegetation succession which is peculiar to the alpine region of the Qingzang Plateau. It has excellent water and soil conservation effects and good ecological adaptability to high-altitude environments. However, little research has been made into the changes in the population structure and dynamics at different altitudes. Such an understanding is important to understanding the ecological strategy of H. tibetana adaptation to alpine habitats. The Qilian Mountains in the northeastern margin of the Qingzang Plateau contain degraded alpine grassland and are ecologically fragile. It is thought that the irregularly distributed native species, H. tibetana, plays an important role in water conservation at 2 700-3 300 m in this region.

Methods The structure, dynamics, life span and morphological characteristics of H. tibetana populations distributed at three altitudes (2 868, 3 012, and 3 244 m) in Tianzhu Zangzu Autonomous County, Qilian Mountains were studied and quantified, establishing static life tables and population survival curves. This quantitative analysis of the population dynamics was used to try to determine future population development trends.

Important findings We found: 1) The plant height, basal diameter and crown width of H. tibetana decreased with elevation. The age structures of all populations at the three altitudes were roughly spindle-shaped, with abundant mature individuals but few seedlings and few old plants. Populations were found to be stable. 2) The survival curves of all populations tended to approach the Deevey-II type, and the survivability was greatest at low altitude, medium at middle altitude, and lowest at high-altitude. The mortality and disappearance rates were both high, ranking from highest to lowest with high-altitude > low-altitude > middle-altitude. Seedlings were proportionally more abundant at the middle and high altitudes. Life expectancy was greatest at middle-altitude > low-altitude > high-altitude. 3) The dynamic index (V′_pi) of each altitude population was close to 0, indicating that all populations were stable, and the maximum of probability under random disturbance (P_max) of the middle altitude population was the smallest, indicating that the middle-altitude populations were the most resiliant to random interference. The middle altitude was the most suitable for the growth of H. tibetana. 4) The proportion of seedlings in all populations is likely to decrease, while the proportion of mature and old plants will increase over the next 2, 4 and 6 age classes of time. Any decline in the total populations at the three altitudes is likely due to the lack of young individuals, interspecific and intraspecific competition, and environmental stress. These findings will enable us to predict future growth and death of H. tibetana populations and provide a reliable theoretical foundation for the protection of natural forests in this region. This will be important to the future management of these alpine environments as global climate warming makes its impact.

Key words: Qingzang Plateau, Hippophae tibetana, age structure, population dynamics, altitude gradient