Population decline, potential habitat shifts, and growth reduction of the endangered tree fern Sphaeropteris lepifera in response to changing canopy density.

Sphaeropteris lepifera, a rare and endangered species belonging to the family Cyatheaceae, is often referred to as a "living fossil" and possesses significant ornamental value. This study investigated the survival status of four wild S. lepifera populations, with a focus on the impact of different canopy densities on the growth of Sphaeropteris lepifera plantlets. Moreover, an ensemble model incorporating ten algorithms was developed to assess the potential suitable distribution ranges of S. lepifera current and future climate conditions. The results indicate the wild population of S. lepifera exhibits a declining trend, which poses a very high risk of extinction. Variations in canopy density have significantly altered the understory light environment, leading to disorganization in the chloroplast grana lamellae structure of S. lepifera under C1 (< 30%) canopy density. This disorganization results in damage to photosystem II, blockage of electron transfer, increased expression of chloroplast genes, and ultimately yellowing of the leaves, which contributes to overall slow plant growth. Predictions from the ensemble model suggest that current suitable habitats of S. lepifera are located along the coastlines of Zhejiang, Fujian, and Guangdong provinces, as well as in Hainan and Taiwan, identified as preferred areas for reintroduction into the wild. The overall suitable distribution of S. lepifera is projected to shift northward, with an increase in total area under future climate conditions, except the extreme climatic conditions (SSP585, 2100). However, this expansion of the suitable habitat does not change the endangered status of S. lepifera, due to the factors such as interspecific competition, man-made interference, and species dispersal capabilities, etc. Therefore, mitigation efforts, including human interventions such as the reintroduction of plantlets, spore propagation, and artificial management of suitable areas, remain essential.