Abstract. Boreal forest soils are globally an important sink for methane (CH₄), while these soils are also capable to emit CH₄ under favourable conditions. Soil wetness is a well-known driver of CH₄ flux, and the wetness can be estimated with several terrain indices developed for the purpose. The aim of this study was to quantify the spatial variability of the forest floor CH₄ flux with a topography-based upscaling method connecting the flux with its driving factors. We conducted spatially extensive forest floor CH₄ flux and soil moisture measurements, complemented with ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a Random Forest model using topography, based on which we upscaled the forest floor CH₄ flux – this was performed for two seasons: May–July and August–October. Our results demonstrate high spatial heterogeneity in the forest floor CH₄ flux, resulting from the soil moisture variability, as well as on the related ground vegetation. The spatial variability in the soil moisture and consequently in the CH₄ flux was higher in the early summer compared to the autumn period, and overall the CH₄ uptake rate was higher in autumn compared to early summer. In the early summer there were patches emitting high amounts of CH₄, however, these wet patches got drier and smaller in size towards the autumn, which was enough for changing their dynamics to CH₄ uptake. The results highlight the small-scale spatial variability of the boreal forest floor CH₄ flux, and the importance of soil chamber placement in order to obtain spatially representative CH₄ flux results. We recommend that a site of similar size and topographical variation would require 15–20 sample points in order to achieve accurate forest floor CH₄ flux.

中文翻译：

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基于地形的建模揭示了林地甲烷通量的高空间变异性和季节性排放斑块

摘要。北方森林土壤是全球重要的甲烷（CH ₄）汇，而这些土壤也能够在有利的条件下排放CH ₄。土壤湿度是众所周知的CH ₄通量的驱动力，可以通过为此目的开发的几种地形指数来估算湿度。这项研究的目的是通过将通量与其驱动因素联系起来的基于地形的放大方法来量化林地CH ₄通量的空间变异性。我们进行了空间广阔的林地CH ₄北方松树林中的通量和土壤湿度测量值，以及地面植被分类的补充。然后，我们使用地形随机森林模型对土壤湿度进行了建模，在此基础上我们放大了林地的CH ₄通量-进行了两个季节：5月至7月和8月至10月。我们的结果表明，由于土壤水分的变化以及相关地面植被，林地CH ₄通量具有很高的空间异质性。与秋季相比，初夏时土壤水分和CH ₄通量的空间变异性更高，总体上CH ₄秋季的吸收率比初夏高。在初夏，有一些小块散发出大量的CH ₄，但是，这些湿小块在秋天到来之前变得更干燥，尺寸更小，足以将其动态变化为吸收CH ₄。结果突出了北方森林地表CH ₄通量的小范围空间变异性，以及为了获得具有空间代表性的CH ₄通量结果而放置土室的重要性。我们建议，具有相似大小和地形变化的站点需要15–20个采样点，才能获得准确的林地CH ₄通量。