Carbon sink strength and its environmental controls in ecosystems along an elevational gradient in dryland mountains

Abstract

Ecosystems in dryland mountains occur along pronounced elevational gradients and exhibit high environmental heterogeneity, playing key roles in the global carbon cycle. However, their CO2 flux dynamics and environmental controls remain insufficiently understood. In this study, we established three eddy covariance flux sites along an elevational gradient in a dryland mountain region of Northwest China, representing a Larix principis-rupprechtii plantation, a Rhododendron lapponicum shrubland, and an alpine meadow. Continuous observations from 2020 to 2022 quantified seasonal and cross-ecosystem variations in net ecosystem productivity (NEP), gross primary productivity (GPP), and ecosystem respiration (Re), and examined their responses to environmental and physiological variables. Annual GPP and Re decreased significantly with increasing elevation. The plantation showed the strongest net CO2 uptake, the shrubland maintained variable net CO2 uptake, and the alpine meadow remained close to CO2 neutrality, shifting between weak net CO2 uptake and weak net CO2 release. Monthly anomaly analysis showed that NEP anomalies were most pronounced during the peak growing season and were mainly driven by GPP anomalies, while Re anomalies either amplified or offset GPP effects on NEP. Random forest and structural equation modeling analyses revealed that dominant controls shifted along the elevational gradient, with lower-elevation ecosystems being primarily regulated by vegetation activity, photosynthetic capacity, and atmospheric water demand, whereas higher-elevation ecosystems showed stronger temperature and water limitations. These results highlight that magnitude of net CO2 uptake was jointly influenced by vegetation activity, thermal conditions, and water availability through their contrasting effects on photosynthetic CO2 uptake and ecosystem respiration.