Tree Proximity Matters: A Novel Framework for Soil Greenhouse Gas Emissions

We introduce triproximity, a conceptual framework that organizes tree–soil greenhouse gas (GHG) interactions across three spatial dimensions: (i) horizontal distance from tree stems, (ii) vertical soil profile depth, and (iii) structural position relative to tree components including the stem itself as a gas conduit. This addresses a critical and previously unquantified methodological gap in the literature. Despite the inherent spatial heterogeneity of tree-based agricultural systems, where molecular oxygen gradients structured by root macropore networks, rhizosphere demand, and canopy-mediated moisture redistribution govern CO2, N2O, and CH4 fluxes across distances of just a few meters from the stem, most studies report GHG emissions from single locations without documenting distance from trees, effectively assuming spatial homogeneity where none exists. Following PRISMA guidelines, we systematically reviewed 107 field-based studies identified through a Scopus search (December 2025) of tree-based systems published between 2010 and 2025. Only 37.4% of studies explicitly reported measurement distance from trees, a proportion that has not improved despite a nearly four-fold increase in publication volume since 2020. Through narrative synthesis, we show that CH4 uptake follows the most consistent spatial response, with higher oxidation rates in the near-tree zone across diverse system types; N2O responses are context-dependent and governed by competing substrate availability and moisture controls; and CO2 fluxes show no universal spatial pattern yet respond predictably to specific proximity dimensions once the dominant source term is identified. Stem-level gas transport remains virtually unmeasured across the dataset, likely biasing ecosystem GHG budgets systematically. We propose a minimum triproximity-based sampling protocol for five major tree-based system types and call for journals to adopt spatial reporting as a minimum submission standard. This review was not pre-registered and received no external funding.