Leaf Silicon and Phytolith Dynamics of Castanopsis fargesii Along an Elevational Gradient in Subtropical Forests

Silicon (Si) plays a crucial role in forest ecosystem functioning by influencing plant nutrient cycling, stress resistance, and biogeochemical processes; however, how Si and phytolith characteristics respond to environmental gradients in subtropical forests remains poorly understood. This study investigated the variations in silicon and phytolith characteristics in the leaves of Castanopsis fargesii across different elevations in the Guoyan Mountain Nature Reserve (Nanping City, China). It aimed to identify the dominant phytolith morphologies and their proportions, and to examine the effects of elevation, leaf nutrient elements, and soil physicochemical properties on these parameters. Plots were established at 100 m elevation intervals within the natural distribution range of Castanopsis fargesii forests in the Guoyan Mountain Nature Reserve, covering elevations from 600 to 900 m. Fresh leaves, leaf litter, and soil samples (0–20 cm depth) were collected to analyze fresh leaf silicon content and phytolith contents, leaf nutrient elements, and soil physicochemical properties. From 600 to 900 m elevation, silicon content in fresh leaves, leaf litter, SCDI, and phytolith content all exhibited consistent patterns, with significantly higher values at 900 m than at 600 m. The dominant phytolith morphologies in leaves were elongate, flabellate, acute, trapeziform psilate, and blocky. At higher elevations (900 m), the proportions of elongate, acute, and trapeziform psilate phytoliths were relatively higher, while flabellate and blocky forms were more abundant at lower elevations (600 m). However, elevation had minimal influence on the overall proportions of dominant phytolith morphologies. Leaf calcium (Ca) and soil properties—including available calcium, organic matter, and available silicon—were identified as major factors affecting fresh leaf silicon content and phytoliths. Elevation, leaf nutrient elements, and soil physicochemical properties significantly influenced silicon and phytoliths in Castanopsis fargesii leaves. Certain phytolith morphologies in the leaves may be indicative of environmental changes.