Tropical dry forests (TDF) are one of the most widely distributed tropical land‐cover types in Mexico, but their regional‐to‐global contribution to the carbon and water cycles is still highly uncertain. We measured CO₂ and water vapor fluxes between the ecosystem and atmosphere using the eddy covariance technique from 2016 to 2018 in an old‐growth TDF in northwestern Mexico. First, we investigated the magnitude and seasonality of evapotranspiration (ET), net ecosystem production (NEP) and its contributing fluxes, gross ecosystem production (GEP), and ecosystem respiration (Reco). Second, we explored the main environmental factors controlling carbon and water fluxes as well as tested if this ecosystem acted as a net carbon source or sink. During the study period, all precipitation entering the ecosystem went back to the atmosphere through ET (738.9 ±58.26 mm y⁻¹). Ecosystem respiration (2203.16 ±244.2 g C m⁻² y⁻¹) was consistently larger than GEP (1975.32 ±295.52 g C m⁻² y⁻¹), determining an annual NEP (−227.6 ±59.4 g C m⁻² y⁻¹) that resulted in net annual carbon losses. This forest maintained its water use efficiency (WUE; GEP/ET) across years (2.53–3.24), but water availability constrained light use and maximum carbon assimilation rates. Our results render relevance to the feedback between soil water content and net radiation as the main environmental variables controlling ecosystem fluxes in this old‐growth TDF.

中文翻译：

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古老的热带干旱森林中碳和水通量的环境控制

热带干旱森林（TDF）是墨西哥分布最广泛的热带土地覆盖类型之一，但是它们对碳和水循环的区域乃至全球贡献仍然不确定。我们测量了CO ₂在2016年至2018年期间，使用涡度协方差技术在墨西哥西北部的一个老龄TDF中研究了生态系统与大气之间的水汽通量。首先，我们调查了蒸散量（ET），净生态系统产量（NEP）及其贡献通量，生态系统总产量（GEP）和生态系统呼吸（Reco）的大小和季节。其次，我们探索了控制碳和水通量的主要环境因素，并测试了该生态系统是否充当净碳源或汇。在研究期间，进入生态系统的所有降水都通过ET（738.9±58.26 mm y ^-1）返回大气。生态系统呼吸（2203.16±244.2 g C m ^-2 y ^-1）始终大于GEP（1975.32±295.52 g C m⁻² y ^-1），确定年度NEP（−227.6±59.4 g C m ^-2 y ^-1），导致净年度碳损失。多年来（2.53-3.24），该森林保持了其用水效率（WUE； GEP / ET），但是水资源的可用性限制了光的使用和最大的碳同化率。我们的研究结果与土壤水分和净辐射之间的反馈相关，这是控制该老龄TDF中主要生态变量的环境变量。