Foliar methane uptake capacity as a critical plant trait in the global carbon cycle

Until recently it has been assumed that the main site of methane exchange between the terrestrial biosphere and the atmosphere is soils. However, recent research has shown that tree stems can contribute substantial methane flux; moreover, the few studies that have examined foliar methane flux in situ have found non-negligible fluxes. Foliar methane uptake appears to be mediated by methanotrophic endophytes and is appreciable in upland forests where soils also show net methane oxidation. In contrast, foliar release of methane can occur in lowland forests in which soils show net methane release as a result of transport of dissolved methane through the xylem stream. There is evidence that both foliar methane uptake (and release) are mediated by stomatal conductance, suggesting that the capacity for foliar methane fluxes may be closely related to other gas-exchange-related functional traits of leaves that covary along the fast-slow leaf economics spectrum.  Here we compile data on reported rates of foliar methane uptake in upland forests to test this idea. Data from three northern forest sites in Canada and Sweden indicate that: (1) methane uptake capacity is generally higher in broadleaf angiosperms than in conifers; and (2) methane uptake capacity is positively correlated with leaf nitrogen content, but shows a saturating pattern with a maximum rate of ~0.6-0.7 nmol m^-2 s^-1. We contend that foliar methane uptake has been under-appreciated as an important process in the global carbon cycle, but that patterns suggest a close linkage to other plant traits that will permit integration of this process into existing carbon cycle models.