Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO, USA.
Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, USA.
Glob Chang Biol. 2021 Oct;27(20):5186-5197. doi: 10.1111/gcb.15775. Epub 2021 Jul 14.
Satellite-derived sun-induced chlorophyll fluorescence (SIF) has been increasingly used for estimating gross primary production (GPP). However, the relationship between SIF and GPP has not been well defined, impeding the translation of satellite observed SIF to GPP. Previous studies have generally assumed a linear relationship between SIF and GPP at daily and longer time scales, but support for this assumption is lacking. Here, we used the GPP/SIF ratio to investigate seasonal variations in the relationship between SIF and GPP over the Northern Hemisphere (NH). Based on multiple SIF products and MODIS and FLUXCOM GPP data, we found strong seasonal hump-shaped patterns for the GPP/SIF ratio over northern latitudes, with higher values in the summer than in the spring or autumn. This hump-shaped GPP/SIF seasonal variation was confirmed by examining different SIF products and was evident for most vegetation types except evergreen broadleaf forests. The seasonal amplitude of the GPP/SIF ratio decreased from the boreal/arctic region to drylands and the tropics. For most of the NH, the lowest GPP/SIF values occurred in October or September, while the maximum GPP/SIF values were evident in June and July. The most pronounced seasonal amplitude of GPP/SIF occurred in intermediate temperature and precipitation ranges. GPP/SIF was positively related to temperature in the early and late parts of the growing season, but not during the peak growing months. These shifting relationships between temperature and GPP/SIF across different months appeared to play a key role in the seasonal dynamics of GPP/SIF. Several mechanisms may explain the patterns we observed, and future research encompassing a broad range of climate and vegetation settings is needed to improve our understanding of the spatial and temporal relationships between SIF and GPP. Nonetheless, the strong seasonal variation in GPP/SIF we identified highlights the importance of incorporating this behavior into SIF-based GPP estimations.
卫星衍生的太阳诱导叶绿素荧光（SIF）已越来越多地用于估算总初级生产力（GPP）。然而，SIF 与 GPP 之间的关系尚未得到很好的定义，这阻碍了卫星观测到的 SIF 向 GPP 的转化。先前的研究通常假设 SIF 与 GPP 之间存在线性关系，无论是在日尺度还是更长时间尺度上，但缺乏对此假设的支持。在这里，我们使用 GPP/SIF 比来研究北半球（NH）SIF 与 GPP 之间关系的季节性变化。基于多个 SIF 产品和 MODIS 和 FLUXCOM GPP 数据，我们发现，在高纬度地区，GPP/SIF 比具有很强的季节性驼峰模式，夏季的值高于春季或秋季。通过检查不同的 SIF 产品，我们证实了这种驼峰状的 GPP/SIF 季节性变化，除了常绿阔叶林外，这种变化在大多数植被类型中都很明显。GPP/SIF 比的季节性幅度从北方/北极地区到干旱地区和热带地区逐渐减小。在 NH 的大部分地区，GPP/SIF 的最低值出现在 10 月或 9 月，而最大值则出现在 6 月和 7 月。GPP/SIF 的季节性幅度在中等温度和降水范围内最为明显。GPP/SIF 与生长季节早期和晚期的温度呈正相关，但在生长高峰期的几个月中则没有。在不同月份之间，温度与 GPP/SIF 之间的这种变化关系似乎在 GPP/SIF 的季节性动态中起着关键作用。有几种机制可以解释我们观察到的模式，需要涵盖广泛气候和植被条件的未来研究来提高我们对 SIF 与 GPP 之间空间和时间关系的理解。尽管如此，我们确定的 GPP/SIF 强烈的季节性变化突出表明，在基于 SIF 的 GPP 估算中，必须考虑这种行为。
