Methane cycling in temperate forests.

Temperate forest soils are considered significant methane (CH) sinks, but other methane sources and sinks within these forests, such as trees, litter, deadwood, and the production of volatile organic compounds are not well understood. Improved understanding of all CH fluxes in temperate forests could help mitigate CH emissions from other sources and improve the accuracy of global greenhouse gas budgets. This review highlights the characteristics of temperate forests that influence CH flux and assesses the current understanding of the CH cycle in temperate forests, with a focus on those managed for specific purposes. Methane fluxes from trees, litter, deadwood, and soil, as well as the interaction of canopy-released volatile organic compounds on atmospheric methane chemistry are quantified, the processes involved and factors (biological, climatic, management) affecting the magnitude and variance of these fluxes are discussed. Temperate forests are unique in that they are extremely variable due to strong seasonality and significant human intervention. These features control CH flux and need to be considered in CH budgets. The literature confirmed that temperate planted forest soils are a significant CH sink, but tree stems are a small CH source. CH fluxes from foliage and deadwood vary, and litter fluxes are negligible. The production of volatile organic compounds could increase CH's lifetime in the atmosphere, but current in-forest measurements are insufficient to determine the magnitude of any effect. For all sources and sinks more research is required into the mechanisms and microbial community driving CH fluxes. The variability in CH fluxes within each component of the forest, is also not well understood and has led to overestimation of CH fluxes when scaling up measurements to a forest or global scale. A roadmap for sampling and scaling is required to ensure that all CH sinks and sources within temperate forests are accurately accounted for and able to be included in CH budgets and models to ensure accurate estimates of the contribution of temperate planted forests to the global CH cycle.