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Methane is an important greenhouse gas 1 , but the role of trees in the methane budget remains uncertain 2 . Although it has been shown that wetland and some upland trees can emit soil-derived methane at the stem base 3 , 4 , it has also been suggested that upland trees can serve as a net sink for atmospheric methane 5 , 6 . Here we examine in situ woody surface methane exchange of upland tropical, temperate and boreal forest trees. We find that methane uptake on woody surfaces, in particular at and above about 2 m above the forest floor, can dominate the net ecosystem contribution of trees, resulting in a net tree methane sink. Stable carbon isotope measurement of methane in woody surface chamber air and process-level investigations on extracted wood cores are consistent with methanotrophy, suggesting a microbially mediated drawdown of methane on and in tree woody surfaces and tissues. By applying terrestrial laser scanning-derived allometry to quantify global forest tree woody surface area, a preliminary first estimate suggests that trees may contribute 24.6–49.9 Tg of atmospheric methane uptake globally. Our findings indicate that the climate benefits of tropical and temperate forest protection and reforestation may be greater than previously assumed. Studies of in situ woody surface methane exchange in upland tropical, temperate and boreal forest trees find that methane uptake can result in a net tree methane sink that is globally significant and demonstrates an additional climate benefit provided by trees.

There is increasing interest in the measurement of methane (CH4) emissions from tree stems in a wide range of ecosystems so as to determine how they contribute to the total ecosystem flux. To date, tree CH4 fluxes are commonly measured using rigid closed chambers (static or dynamic), which often pose challenges as these are bulky and limit measurement of CH4 fluxes to only a very narrow range of tree stem sizes and shapes. To overcome these challenges we aimed to design and test new semi-rigid stem-flux chambers (or sleeves). We compared the CH4 permeability of the new semi-rigid chambers with that of the traditional rigid chamber approach, in the laboratory and in the field, with continuous flow or syringe injections. We found that the semi-rigid chambers had reduced gas permeability and optimal stem gas exchange surface to total chamber volume ratio (Sc / Vtot) better headspace mixing, especially when connected in a dynamic mode to a continuous flow gas analyser. Semi-rigid sleeves can easily be constructed and transported in multiple sizes, are extremely light, cheap to build and fast to deploy. This makes them ideal for use in remote ecosystems where access logistics is complicated.

Tree stems can act as a conduit for trace greenhouse gases (GHG) produced in the soil. However, the majority of studies describing tree stem fluxes of methane (CH ) and nitrous oxide (N O) have ₄) and nitrous oxide (N₂O) have ₂O) have focused on wetland ecosystems. Tree stem fluxes of GHGs on free-draining soils are understudied, but they are assumed to be a source of CH and a weak source of N O. The work presented in this ₄) and nitrous oxide (N₂O) have ₂O) have thesis aimed to determine how climatic variables, soil abiotic conditions, and tree species influence CH and N O fluxes in forests on free-draining soil. Soil and stem CH and N O fluxes were measured in ₄) and nitrous oxide (N₂O) have ₂O) have lowland tropical rainforest in Panama, Central America and temperate woodland in the UK, using chambers installed on the forest floor or strapped to individual stems of two common tree species. Air samples were collected every two to four weeks during 5 months in 2014 and during November 2015 at the tropical site, and between February 2015 and January 2016 at the temperate site.Tree stem CH fluxes differed significantly between species at both sites and stem N O fluxes ₄) and nitrous oxide (N₂O) have ₂O) have also differed between species at the tropical site. However, there was little variation in soil CH or ₄) and nitrous oxide (N₂O) have N O fluxes. At both sites, tree-mediated CH fluxes declined from positive values (emission) at the ₂O) have ₄) and nitrous oxide (N₂O) have stem base to negative values (uptake) higher up. Stem CH fluxes generally increased significantly ₄) and nitrous oxide (N₂O) have with solar radiation, suggesting a link to photosynthetic activity mediated by tree water transport.Collectively, these results show that trees on free-draining soils could act as net sinks for CH₄) and nitrous oxide (N₂O) have and N O. These findings will improve GHG budgets because tree stem uptake is currently ₂O) have unaccounted for. In particular, if uptake of CH by tree stems on free-draining soils is widespread, ₄) and nitrous oxide (N₂O) have the global terrestrial CH sink could be much larger than currently estimated.