Topography-based statistical modelling reveals high spatial variability and seasonal emission patches in forest floor methane flux

Abstract

Abstract. Boreal forest soils are globally an important sink for methane (CH4), while these soils are also capable of emitting 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 by ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a random forest model using digital-elevation-model-derived topographic indices, based on which we upscaled the forest floor CH4 flux. The modelling was performed for two seasons: May–July and August–October. Additionally, we evaluated the number of flux measurement points needed to get an accurate estimate of the flux at the whole study site merely by averaging. Our results demonstrate high spatial heterogeneity in the forest floor CH4 flux resulting from the soil moisture variability as well as from the related ground vegetation. The mean measured CH4 flux at the sample points was −5.07 µmol m−2 h−1 in May–July and −8.67 µmol m−2 h−1 in August–October, while the modelled flux for the whole area was −7.42 and −9.91 µmol m−2 h−1 for the two seasons, respectively. The spatial variability in the soil moisture and consequently in the CH4 flux was higher in the early summer (modelled range from −12.3 to 6.19 µmol m−2 h−1) compared to the autumn period (range from −14.6 to −2.12 µmol m−2 h−1), 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, changing their dynamics to CH4 uptake. The mean values of the measured and modelled CH4 fluxes for the sample point locations were similar, indicating that the model was able to reproduce the results. For the whole site, upscaling predicted stronger CH4 uptake compared to simply averaging over the sample points. 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. To predict the CH4 fluxes over large areas more reliably, the locations of the sample points should be selected based on the spatial variability of the driving parameters, in addition to linking the measured fluxes with the parameters.

High-resolution spatial patterns and drivers of terrestrial ecosystem carbon dioxide, methane, and nitrous oxide fluxes in the tundra

“…Our study creates new understanding about high-resolution upscaling of GHG fluxes by incorporating more chamber measurements, predictors, models, and environmental gradients compared to earlier efforts (see e.g., Fox et al, 2008;Dinsmore et al, 2017;Räsänen et al, 2021;Juutinen et al, 2022;Vainio et al, 2021). For example, we included chamber measurements from 101 plots whereas earlier local-scale upscaling studies have usually had circa 30 plots.…”

“…However, the most important environmental variables (i.e., soil moisture, temperature, biomass) had a high predictive performance. Nevertheless, future studies could explore the performance and information derived by upscaling GHG fluxes using high-resolution satellite or drone-derived remotely sensed indices directly (Siewert and Olofsson, 2020;Vainio et al, 2021;Berner et al, 2018).…”

Very High Spatial Resolution Soil Moisture Observation of Heterogeneous Subarctic Catchment Using Nonlocal Averaging and Multitemporal SAR Data

“…It is known that, in wet enough conditions, upland soils can act as CH 4 sources, but the interesting question is what kind of spatial and temporal variation in CH 4 exchange can be expected. Within a single catchment, there can be surfaces or patches with high CH 4 emission and those with an order of magnitude lower CH 4 sink, controlled mainly by soil moisture [33]. Hence, normal spatial averaging or Lee filtering [8] to reduce speckle is not applicable.…”

Excess soil moisture and fresh carbon input are prerequisites for methane production in podzolic soil

“…In upland soils, highaffinity methanotrophs can consume CH 4 at atmospheric concentrations (Knief et al, 2003;Kolb, 2009). Despite the abundance of oxygen in the boreal upland forest soil, there are some indications of smaller-scale CH 4 -producing areas, such as wet depressions (Christiansen et al, 2012;Megonigal and Guenther, 2008;Vainio et al, 2021). In addition, some studies have found that upland forest soils may become CH 4 sources of varying significance after long periods of heavy precipitation (Lohila et al, 2016;Savage and Moore, 1997).…”