Altitudinal patterns and drivers of moss plant traits in the genera Didymodon and Tortula in eastern Pamir Plateau, China

Mosses are widely distributed in alpine deserts; however, the altitudinal variation patterns of their functional traits remain poorly understood. In this study, two pleurocarpous moss genera, Tortula and Didymodon, from the Eastern Pamir Plateau (China) were investigated to examine the variation of their morphological characteristics and chlorophyll fluorescence parameters along five altitude gradients (2022.99–2793.07 m), and to identify the key environmental factors driving these patterns, aiming to reveal the ecological adaptation mechanisms of different moss genera in alpine desert environments. The results showed that the two moss genera exhibited distinct morphological differences: Tortula had taller plants but smaller leaf length and width compared with Didymodon. With increasing altitude, Tortula exhibited a significant increase in leaf width and midrib width (P < 0.05), enhancing its light-harvesting capacity; whereas in Didymodon, plant height and stem diameter increased significantly (P < 0.05), while leaf width and midrib width decreased significantly (P < 0.05), indicating an adaptive strategy of strengthening structural rigidity and forming compact leaves to cope with high-altitude environments. Regarding chlorophyll fluorescence parameters, Tortula showed significantly higher Fo, Fm, Fv, ABS/CSo, TRo/CSo, and DIo/CSo with increasing altitude (P < 0.05), suggesting that enhanced energy absorption and dissipation help maintain photosystem stability. In contrast, most fluorescence parameters in Didymodon did not vary significantly with altitude, but its higher PIInst indicated stronger photosynthetic stability. Trait variation in the two moss genera was driven by different environmental factors (explained variance: 22.98–67.24%). Overall, the functional trait matrix of Tortula was mainly influenced by soil moisture and nutrient availability, while that of Didymodon was primarily regulated by soil nutrient levels. The two moss genera exhibited distinct functional differentiation through divergent regulation of morphological traits and chlorophyll fluorescence parameters under alpine desert conditions. Didymodon tends to adopt a conservative, efficiency-based strategy, whereas Tortula follows a more acquisitive strategy. These findings provide new physiological and ecological evidence for understanding the adaptive mechanisms and ecological functions of non-vascular plants in mountain desert environments.