Forests play a crucial role in mitigating climate change by absorbing carbon dioxide (CO₂) from the atmosphere. However, recent research has revealed that forests also emit significant amounts of other greenhouse gases (GHGs), such as methane (CH₄) and nitrous oxide (N₂O), from both tree and soil sources. These GHGs have a much greater warming potential than CO₂, making it essential to accurately assess their emission levels to fully understand the role of forests in climate change mitigation.

Despite the critical role of forests in mitigating climate change, our understanding of the magnitude, variability, and drivers of all GHG fluxes from forests of different regions is still incomplete. Before initiating this Research Topic in 2021, we carried out literature research and found that only a very small fraction of the forest studies measured CH₄ fluxes from trees (i.e., leaves and stems, 11 out of 340 studies) or at the ecosystem scale (13 studies) (Figure 1). These studies are particularly lacking in boreal and tropical regions due to various factors, including limitations in site accessibility, availabilities of instrumentation and skilled labor as well as regional/national policies. Since only a limited number of studies were conducted at the ecosystem level, the role of forests in CH₄ exchange is largely uncertain, varying from a weak CH₄ sink to a strong source, calling for more observations in different forest ecosystems. Studies on ecosystem scale N₂O fluxes in forests are even less (Eugster et al., 2007; Mammarella et al., 2010).

This Research Topic aims to cover the current state of research regarding processes associated with GHG fluxes in forest ecosystems and their biotic or abiotic drivers. Studies in this Research Topic cover GHG fluxes at soil, tree and ecosystem scales and include a wide range of ecosystems (from a boreal forest in Europe to a temperate forest in Oceania and tropical forests/plantations in Asia and south America), providing new knowledge on how environmental changes, especially land managements, affect the GHG potentials of forests.

Nitrogen (N) fertilization has been a forest management practice to enhance the tree growth in Scandinavia countries for many decades (Hedwall et al., 2014). Besides the increased CO₂ sequestration, the N addition via fertilization may also act as substrates that boost N₂O emissions from forest soils. Rütting et al. investigated the consequences of long-term fertilization on N₂O emissions in a boreal forest in Northern Sweden using a combination of manual/automatic chambers and gas probes. Although the overall N₂O emissions were low, the fertilization generally doubled the emissions. Surprisingly, N₂O emissions were higher in the winter than in the growing seasons. Winter has conventionally been assumed as a dormant period for most biological processes and the observed high winter N₂O emissions suggest that the winter GHG fluxes must be considered in annual budgets. At the same time, the large variance in winter N₂O emissions also calls for more studies to investigate the underlying processes under various winter environments (e.g., microbial communities for denitrification).

In Southeast Asia, N fertilizations are heavily applied to the widespread oil palm plantations in the region. Between the plantations and nearby rivers, an area of natural riparian forests is often kept as buffer zones for local ecosystems. The added N in the plantation could substantially contribute to soil N₂O emissions in the plantation as well as the adjacent riparian forests via leaching. So far, data on GHG emissions from oil palm plantations are limited, particularly for N₂O (Skiba et al., 2020). The study of Drewer et al. has confirmed the soil in oil palm plantation as a strong source for both N₂O and CH₄. Importantly, they found that N₂O emissions from the riparian forests were even higher than in the oil palm plantation themselves, highlighting the unexpected effects of land management on ecosystems around the managed areas. Long-term observations are still needed to further evaluate how the GHG potentials will evolve over time from both oil palm plantations and the surrounding buffer zones.

Soil decomposition is an important process that releases CO₂ into the atmosphere, where tree roots are a direct source of carbon substrates for decomposition (Dijkstra et al., 2021). Carrillo et al. found that root morphological traits (e.g., root diameter) play a critical role in determining the decomposition rates. Changes of root traits have significant importance, especially when plants are under environmental stresses (e.g., drought), because they can lead to increased CO₂ emissions through decomposition. Besides the contribution to carbon emissions via roots, tree stems are also known as an important emitter of GHG (Covey and Megonigal, 2019). Soosaar et al. focused on the aguaje palms (Mauritia flexuosa) and boarwoods (Symphonia globulifera) in a Peruvian Amazon and found that the CH₄ emission rates from tree stems can reach up to 44% of those from soils. However, the large variances in stem CH₄ fluxes among different locations, stem heights and species highlight the necessity to further study the mechanisms and drivers of these fluxes.

So far, our knowledge regarding the forest GHG fluxes and their underlying processes is still very limited and the conventional notion of mitigating climate change by simply planting trees has faced growing criticism. To ensure that forests remain an effective tool for mitigating climate change, we need to improve our understanding of the dynamics of GHG emissions from forests and their controls. Further research is needed to better understand the mechanisms of GHG fluxes, particularly CH₄ and N₂O fluxes from trees. Moreover, long-term observations at the ecosystem level can help reveal the potential changes in forest GHG emissions in response to climate change and land use. Filling these knowledge gaps are essential for developing mechanism models, estimating GHG balance at regional and global scale. Ultimately, the knowledge will enable the development of effective strategies to manage forest ecosystems to maximize their potential as carbon sinks and minimize the release of potent GHGs such as CH₄ and N₂O.