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

Jonathan S Schurman,Sean C Thomas

doi:10.3390/soilsystems5010007

Jonathan S Schurman, Sean C Thomas

Open Access

https://doi.org/10.3390/soilsystems5010007

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Abstract

Soil CO2 efflux (FCO2) 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 FCO2 could open new avenues to study plant-soil feedbacks and facilitate scaling; furthermore, FCO2 responds dynamically to meteorological conditions, complicating predictions of total FCO2 and forest C balance. We tested for proximity effects of individual Acer saccharum Marsh. trees on FCO2, comparing FCO2 within 1 m of mature stems to background fluxes before and after an intense rainfall event. Wetting significantly increased background FCO2 (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). FCO2 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 FCO2 (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.

Highlights

Such feedbacks between plants and soil biota play a stabilizing role in numerous ecosystem functions and allow for plant regulation of biogeochemical cycles [3,4]; the specific role of biota is often neglected in studies of FCO2 conducted at higher levels of aggregation, because belowground sources are difficult to disentangle, and because abiotic signals, especially temperature, may be more clearly expressed on spatially integrated data [2]
Total plot area is approximately 13.5 ha, of which approximately two thirds is occupied by a stand of shade-tolerant hardwoods. These interior, upland communities are dominated by sugar maple (Acer saccharum Marsh.), followed by American beech
Prior studies examining tree-size effects on FCO2 have not explicitly tested for Birch-effect patterns, but Søe and Buchmann [26] found that a best-fitting model describing variation in FCO2 included both tree size, and volumetric soil moisture content; Schwendenmann and Macinnis-Ng [30] noted relatively dry soil conditions near the base of large trees that could enhance the gas diffusion, and so increase FCO2

Summary

Introduction

Recent decades have seen a shift in how FCO2 is conceptualized—from a flux largely reflecting ecosystem-specific decomposition and its response to soil temperature and moisture, to a process highly influenced by active and mutual exchanges between plants and soil biota [2]. Such feedbacks between plants and soil biota play a stabilizing role in numerous ecosystem functions and allow for plant regulation of biogeochemical cycles [3,4]; the specific role of biota is often neglected in studies of FCO2 conducted at higher levels of aggregation, because belowground sources are difficult to disentangle, and because abiotic signals, especially temperature, may be more clearly expressed on spatially integrated data [2]. Without deeper insight into the biotic drivers of soil FCO2 , it will remain extremely challenging to link localized observations to broader scale, ecosystem-level fluxes and to develop process-driven predictions of future FCO2 dynamics [2,4]

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