Geophysical Investigation of Boreal Forest Hydrogeology and Implications for Greenhouse Gas Fluxes

Abstract

<p>Boreal forest ecosystems constitute the second largest forest biome after tropical rainforests yet have been estimated to contribute to 18-20% of the global terrestrial CO<sub>2</sub> sink (~0.68±0.88 Pg C y<sup>-1</sup> 2006-2018) making our understanding of them critical during this period of changing climate. In Fennoscandia, ~90% of the boreal forest is managed. Increasing the amount of managed forests worldwide has been suggested as a way to improve C mitigation potential. However, due to the complex environmental effects caused by, for example, rotation forestry, further investigations are needed to understand all interactions that may occur. These potential ecosystem changes are not confined to the above ground environment, but also affect the subsurface, a part of the forest ecosystem that is often lesser understood spatially. One crucial consideration is alterations to the subsurface hydrogeology, which may lead to shifts in the local soil moisture regime, water table depth and depths to capillary zones. Such changes could have strong effects on localized greenhouse gas (GHG) fluxes. </p><p>During the period of 2017-2020, we have conducted extensive hydrogeophysical investigations at the ICOS Norunda forest research site in Sweden (30 km north of Uppsala). The Norunda boreal forest research station is part of the European Integrated Carbon Observation System (ICOS) and offers one of the longest (>20 y) timeseries of local micrometeorological and GHG flux measurements. Here we have integrated electrical resistivity tomography (2-D profiling and monitoring), self-potential (2-D profiling and 3-D monitoring) and ground penetrating radar survey results to increase our knowledge of the local subsurface. Combining our geophysical observations with <em>a priori</em> knowledge of GHG fluxes in different zones of the forest (e.g. mature, thinned, marshy) allows us to investigate the relationships between hydrogeology and surface GHG fluxes. Here we present the highlight results from our study that include (i) spatial characterization of differing hydrogeological zones within Norunda from ERT, SP and GPR; and (ii) an evaluation of the importance of subsurface hydrology for soil GHG fluxes. </p>