Abstract Understanding the drivers of net ecosystem exchange of carbon (NEE) between forests and the atmosphere is crucial for the prediction of future global carbon dynamics. We therefore analyzed the long-term (1999–2014) ecosystem carbon fluxes of a mixed coniferous/deciduous forest (Brasschaat forest) in the Campine ecoregion of Belgium. The carbon uptake of this forest increased over the 16-year study period. By consecutively performing time series decomposition and the statistical technique of random forests, the correlative strength between multiple meteorological drivers, tropospheric pollutants and biotic indices with NEE was quantified at different time scales: i.e., long-term, seasonal and weekly, and separately for day- and nighttime NEE fluxes. The drivers that were most correlated with the trend in carbon sink capacity were the increasing atmospheric CO2 level and soil recovery from acidification. The radiation-saturated carbon uptake increased remarkably and explained much of the long-term variability of daytime NEE. When the long-term and seasonal variation were extracted the remaining weekly variation in daytime NEE was most strongly correlated with variation in the incoming radiation and cloudiness, and to a lesser extent by variation in vapor pressure deficit. In contrast to daytime NEE, nighttime NEE did not show a steady trend over time, but fluctuated, peaking in 1999 and in 2011. The long-term variability in nighttime NEE was most strongly correlated with the groundwater table depth. Air temperature was highly correlated to the seasonal as well as to the remaining weekly variation, i.e., after removal of the long-term and seasonal variability, in nighttime NEE. Biotic drivers (e.g., quantum yield and radiation-saturated carbon uptake) explained less of the variation in NEE on a seasonal and short-term scale, but were more important at the long term.

中文翻译：

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气候、污染和生物因素驱动净森林 - 温带混交林不同时间尺度的 CO2 大气交换

摘要 了解森林和大气之间的净生态系统碳交换 (NEE) 的驱动因素对于预测未来的全球碳动态至关重要。因此，我们分析了比利时 Campine 生态区的针叶/落叶混交林（Brasschaat 森林）的长期（1999-2014）生态系统碳通量。在 16 年的研究期间，这片森林的碳吸收量有所增加。通过连续进行时间序列分解和随机森林统计技术，在不同的时间尺度上量化了多个气象驱动因素、对流层污染物和生物指数与NEE的相关强度：即长期、季节性和每周，分别为天- 和夜间 NEE 通量。与碳汇容量趋势最相关的驱动因素是大气 CO2 水平的增加和酸化后土壤的恢复。辐射饱和的碳吸收显着增加，并解释了白天 NEE 的大部分长期变化。当提取长期和季节性变化时，白天 NEE 剩余的每周变化与传入辐射和云量的变化最密切相关，与蒸汽压差的变化相关性较小。与白天 NEE 相比，夜间 NEE 并未随时间呈现稳定趋势，而是波动，在 1999 年和 2011 年达到峰值。夜间 NEE 的长期变化与地下水位深度相关性最强。在夜间 NEE 中，气温与季节性以及剩余的每周变化高度相关，即在去除长期和季节性变化后。生物驱动因素（例如，量子产率和辐射饱和碳吸收）在季节性和短期范围内对 NEE 变化的解释较少，但从长期来看更为重要。