Linking Soil CO2 Efflux to Individual Trees: Size-Dependent Variation and the Importance of the Birch Effect

Soil CO₂ efflux (F_CO2) is a major component of the terrestrial carbon (C) cycle but challenges in explaining local variability hamper efforts to link broad-scale fluxes to their biotic drivers. Trees are the dominant C source for forest soils, so linking tree properties to F_CO2 could open new avenues to study plant-soil feedbacks and facilitate scaling; furthermore, F_CO2 responds dynamically to meteorological conditions, complicating predictions of total F_CO2 and forest C balance. We tested for proximity effects of individual Acer saccharum Marsh. trees on F_CO2, comparing F_CO2 within 1 m of mature stems to background fluxes before and after an intense rainfall event. Wetting significantly increased background F_CO2 (6.4 ± 0.3 vs. 8.6 ± 0.6 s.e. μmol CO₂ m⁻²s⁻¹), with a much larger enhancement near tree stems (6.3 ± 0.3 vs. 10.8 ± 0.4 μmol CO₂ m⁻²s⁻¹). F_CO2 varied significantly among individual trees and post-rain values increased with tree diameter (with a slope of 0.058 μmol CO₂ m⁻²s⁻¹cm⁻¹). Post-wetting amplification of F_CO2 (the ‘Birch effect’) in root zones often results from the improved mobility of labile carbohydrates and further metabolization of recalcitrant organic matter, which may both occur at higher densities near larger trees. Our results indicate that plant-soil feedbacks change through tree ontogeny and provide evidence for a novel link between whole-system carbon fluxes and forest structure.