Variation in Photosynthetic Capacity of <i>Sorbus rehderiana</i> and <i>Quercus aquifolioides</i> Along Elevational Gradients on the Eastern Edge of the Qinghai-Tibetan Plateau, China

Elevation provides a critical environmental context for studying the photosynthetic adaptation mechanisms of plants and is of great significance for understanding plant responses and acclimation to environmental change. Although elevational variation strongly influences photosynthetic capacity, the underlying regulatory mechanisms remain insufficiently understood. This study addresses this gap by systematically examining the photosynthetic and anatomical adaptations of two dominant tree species with contrasting functional types, Sorbus rehderiana (deciduous broad-leaved tree) and Quercus aquifolioides (evergreen broad-leaved tree), distributed widely along the eastern edge of the Qinghai-Tibetan Plateau, China. Our findings reveal distinct adaptive strategies. S. rehderiana enhances CO2 assimilation efficiency at higher elevations (in the 2600-3400 m a.s.l) by increasing the surface area of mesophyll cells and chloroplasts exposed to intercellular airspace (Sm and Sc), reducing cell wall thickness (Tcw), and optimizing nitrogen allocation within the photosynthetic apparatus. These modifications maximize carbon gain during a short growing season. In contrast, Q. aquifolioides adopts a conservative strategy by reducing photosynthetic capacity at higher elevations, allocating more nitrogen to structural tissues, increasing Tcw, and reinforcing mechanical defenses, thereby prolonging leaf lifespan in high-elevation environments. This study reveals the intrinsic mechanisms underlying the elevational variation in photosynthetic capacity between deciduous and evergreen trees, and provides a scientific basis for improving predictions of the plant' response to future climate change.