## 1 Introduction  
In the global context of terrestrial ecosystems, forests play a crucial role in the carbon (C) cycle, acting as a major C sink and storing about 90% of the total terrestrial C. Moreover, as significant regulators of the global climate, forests have the ability to influence and shape climate change dynamics [1; 2]. Latvia's forest area ranks fourth among European Union countries, comprising approximately 50% to 52% of its total land area [3; 4]. According to the data of the National Forest Inventory of Latvia in 2018, forests with wet organic soil account for 10.3% and forests with drained organic soil account for 12.6% of the total forest area. Forests of both of these two types combined cover 739.87 kha [3]. In Latvia, organic soils are considered the largest source of greenhouse gas (GHG) emissions, emitting more than 6.1 million tons of carbon dioxide (CO\({}_{2}\)) yr\({}^{-1}\)[5]. CO\({}_{2}\) and methane (CH\({}_{4}\)) are the most abundant GHGs responsible for anthropogenic climate change. Nitrous oxide (N\({}_{2}\)O) is another important GHG that is about 300 times as potent as CO\({}_{2}\) and depletes the ozone layer [6]. To gain a better understanding of C cycling in forests, other sinks and sources of GHGs as well as soils need to be studied in depth. Until now, trees have been neglected and only recently has their potential for regulating GHG fluxes been recognized. Considering the number of trees worldwide, even small emissions from individual stems can upscale to large global fluxes [7]. Two pathways can be distinguished for GHG emissions from tree stems. First, GHGs may be produced by the physiological processes of a tree or the microorganisms inhabiting it [8; 9; 10]. Second, GHGs could be produced and accumulated in deep soil layers, and transported through the roots to the whole tree and further to the atmosphere [9; 11; 12]. The GHG fluxes from tree stems are determined by tree species and their characteristic growth rates, distribution area, dimensions, age, root system type and size and whether the tree is living or dead. Faster-growing trees emit more on average than slower-growing ones [1; 12; 13]. GHG emissions are also significantly affected by climatic and edaphic factors. The results of several studies show that greater amounts of GHG emissions through the surfaces of tree stems are released in flooded or overmoistened areas [14; 15; 16]. Tree respiration is a significant component of the C budget in forests at plant, stand and ecosystem levels and CO\({}_{2}\) is produced as a byproduct. Tree stem respiration also plays a crucial role due to the assimilates consumed and the increase in wood as the stand develops [17]. Woody tissue respiration (mainly determined by stem respiration) accounts for about 50% of the total above-ground autotrophic respiration in temperate deciduous forests; however, respiration is not necessarily synonymous with CO\({}_{2}\) efflux [18]. Some studies show that CO\({}_{2}\) efflux is reduced by bark photosynthesis when CO\({}_{2}\) re-fixation occurs [19; 20]. It has been known since the 1970s that trees are capable of storing large amounts of CH\({}_{4}\) in their stems, but the idea that trees are capable of emitting this GHG through the surface of their stems has only recently gained acceptance [21; 22]. The anaerobic environment of a tree stem can promote the activity of methanogenic microorganisms involved in CH\({}_{4}\) production [12]. In a study carried out in eutrophic peatlands, CH\({}_{4}\) from tree stems accounted for up to 30% of net ecosystem CH\({}_{4}\) emissions. The CH\({}_{4}\) fluxes from stems were higher during the wet season when the water table was high and temperatures were lower [13]. Evidence suggests that in wetlands, mangroves and well-drained forests, tree stems most likely emit N\({}_{2}\)O produced in the soil by microbial nitrification and denitrification processes [23; 24; 25].