Understanding the mechanisms underlying net ecosystem CO2 exchange (NEE) in mountain grasslands is important to quantify their relevance in the global carbon budget. However, complex interactions between environmental variables and vegetation on NEE remain unclear; and there is a lack of empirical data, especially from the high elevations and the Mediterranean region. A chamber-based survey of CO2 exchange measurements was carried out in two climatically contrasted grasslands (montane v. subalpine) of the Pyrenees; assessing the relative contribution of phenology and environmental variables on CO2 exchange at the seasonal scale, and the influence of plant functional type dominance (grasses, forbs and legumes) on the NEE light response. Results show that phenology plays a crucial role as a CO2 exchange driver, suggesting a differential behaviour of the vegetation community depending on the environment. The subalpine grassland had a more delayed phenology compared to the montane, being more temperature than water constrained. However, temperature increased net CO2 uptake at a higher rate in the subalpine than in the montane grassland. During the peak biomass, productivity (+74%) and net CO2 uptake (NEE +48%) were higher in the subalpine grassland than in the montane grassland. The delayed phenology at the subalpine grassland reduced vegetation's sensitivity to summer dryness, and CO2 exchange fluxes were less constrained by low soil water content. The NEE light response suggested that legume dominated plots had higher net CO2 uptake per unit of biomass than grasses. Detailed information on phenology and vegetation composition is essential to understand elevation and climatic differences in CO2 exchange.

中文翻译：

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物候学和植物功能类型优势驱动比利牛斯山脉半天然草原的二氧化碳交换

了解网络生态系统 CO 背后的机制2山地草原的交换（NEE）对于量化它们在全球碳预算中的相关性很重要。然而，NEE上环境变量和植被之间的复杂相互作用仍不清楚。并且缺乏经验数据，尤其是来自高海拔地区和地中海地区的经验数据。基于室内的 CO 调查2交换测量在两个气候对比的草原（山地v. 亚高山）的比利牛斯山脉；评估物候和环境变量对 CO 的相对贡献2季节性尺度的交换，以及植物功能类型优势（草、杂草和豆类）对 NEE 光响应的影响。结果表明，物候作为 CO 起着至关重要的作用2交换驱动程序，表明植被群落的不同行为取决于环境。与山地相比，亚高山草原的物候更延迟，温度高于受水限制。然而，温度增加了净 CO2亚高山的吸收率高于山地草原。在生物量峰值期间，生产力 (+74%) 和净二氧化碳2亚高山草原的吸收（NEE + 48％）高于山地草原。亚高山草原的物候延迟降低了植被对夏季干旱和 CO2交换通量受土壤含水量低的限制较小。NEE 光响应表明以豆科植物为主的地块具有较高的净 CO2单位生物量的吸收量高于草类。有关物候学和植被组成的详细信息对于了解 CO 的海拔和气候差异至关重要2交换。