Photosynthetic Characteristics of Alpine Meadow Plants along Different Successional Gradients on the Qinghai-Tibetan Plateau

The photosynthetic characteristics of a plant reflect its adaptively to a given environment. To understand the dynamics of restoration and succession in the enclosure of alpine meadows and to investigate the changing patterns of plant photosynthetic characteristics and their influencing factors in the process of enclosure restoration, the present study was conducted to investigate the sample plots of five typical communities (1, 3, 5, 15, and 30 years) in the experimental site of subalpine meadows in the northeastern part of the Tibetan Plateau in China that had been enclosed for more than 30 years, and to determine photosynthetic indexes and leaf functional traits in the dominant species of the various communities as well as the parameters of the three common species. We measured the photosynthetic indexes and leaf functional parameters of each community’s dominant species and the three common species. We determined the soil’s water and total nitrogen content (0–20 cm) in the community. The results showed that a succession sequence of herbaceous plants → semi-shrubs → shrubs was formed in this enclosure, and the soil water content and total nitrogen content of the community gradually increased with the succession sequence. With the succession time (1–30 years), the net photosynthetic rate (Pn), photosynthetic water use efficiency (WUE), and relative chlorophyll content (SPAD) of the dominant species in the community showed a decreasing trend. There was no strict and consistent pattern in the changes of their leaf nitrogen content (Nmass), photosynthetic nitrogen use efficiency (PNUE), and specific leaf area (SLA) with the succession, which was more of a difference between different plant functional groups; Different dominant species indicate different successional stages, and the dominant species at different successional stages show a gradual decrease in Pn and SPAD values from the pre-successional stage to the post-successional stage, while Nmass and SLA show an overall trend of increasing, then decreasing and then increasing. The above results showed that the fenced area transitioned from a community with strong resource acquisition ability to a community with strong resource retention ability in restoration succession, and the soil water content and total nitrogen content were the main factors driving this transition. Knowing the photosynthetic physiological dynamics of the enclosure community in the succession process has specific theoretical significance for restoring alpine degraded meadows.