Forest and Population Characteristics of Vulnerable Relict Tsuga forrestii Downie in China

Tsuga forrestii Downie, a vulnerable species endemic to China, is confined to small, fragmented habitats in the northwestern Yunnan and southwestern Sichuan Provinces, China. Understanding the current status of its communities and populations is crucial for protecting existing natural forest resources. We surveyed 33 plots in Yunnan and Sichuan, where T. forrestii is the primary dominant species. We analyzed their community characteristics including vertical stratification, species diversity, and population structure, and classified them into four distinct forest types: (1) Tsuga forrestii evergreen coniferous forest; (2) Tsuga forrestii-Lithocarpus variolosus evergreen coniferous and broad-leaved mixed forest; (3) Tsuga forrestii-Quercus guyayifolia evergreen coniferous and broad-leaved mixed forest; (4) Tsuga forrestii-Abies forrestii evergreen coniferous forest. These forests exhibited a multilayered vertical structure, and T. forrestii as a dominant species appeared in the arborous and shrub layers, and the associated taxa were mainly species of Abies, Lithocarpus, Quercus, Castanopsis, and Rhododendron. No significant differences in species richness or diversity indices or phylogenetic relatedness metrics among the forest types were found. The maximum age of the remaining wild specimens was 344 years, with individuals under 20 years or over 170 years old being rare. The average growth rate of T. forrestii, based on ring width, fluctuated over time, and the range of mean values was 0.92 to 3.31 mm/year. Established seedlings/saplings were rare and mainly found in unstable microhabitats. These findings highlight the poor regeneration and a decline in its populations. Improving its regeneration status is crucial to maintaining its population status.