title: Global patterns and predictors of stem CO2 efflux in forest ecosystems.
abstract: Stem CO2 efflux (ES) plays an important role in the carbon balance of forest ecosystems. However, its primary controls at the global scale are poorly understood and observation-based global estimates are lacking. We synthesized data from 121 published studies across global forest ecosystems and examined the relationships between annual ES and biotic and abiotic factors at individual, biome, and global scales, and developed a global gridded estimate of annual ES . We tested the following hypotheses: (1) Leaf area index (LAI) will be highly correlated with annual ES at biome and global scales; (2) there will be parallel patterns in stem and root CO2 effluxes (RA) in all forests; (3) annual ES will decline with forest age; and (4) LAI coupled with mean annual temperature (MAT) and mean annual precipitation (MAP) will be sufficient to predict annual ES across forests in different regions. Positive linear relationships were found between ES and LAI, as well as gross primary production (GPP), net primary production (NPP), wood NPP, soil CO2 efflux (RS), and RA . Annual ES was correlated with RA in temperate forests after controlling for GPP and MAT, suggesting other additional factors contributed to the relationship. Annual ES tended to decrease with stand age. Leaf area index, MAT and MAP, predicted 74% of variation in ES at global scales. Our statistical model estimated a global annual ES of 6.7 ± 1.1 Pg C yr(-1) over the period of 2000-2012 with little interannual variability. Modeled mean annual ES was 71 ± 43, 270 ± 103, and 420 ± 134 g C m(2) yr(-1) for boreal, temperate, and tropical forests, respectively. We recommend that future studies report ES at a standardized constant temperature, incorporate more manipulative treatments, such as fertilization and drought, and whenever possible, simultaneously measure both aboveground and belowground CO2 fluxes.
authors.affiliation: Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA, 30602, USA.
authors.forename: Jinyan
authors.initials: J
authors.lastname: Yang
authors.affiliation: Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, 47907, USA.|Department of Earth System Science, University of California Irvine, Irvine, CA, 92697, USA.
authors.forename: Yujie
authors.initials: Y
authors.lastname: He
authors.affiliation: Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA, 30602, USA.|Savannah River Ecology Laboratory, University of Georgia, Aiken, SC, 29802, USA.
authors.forename: Doug P
authors.initials: DP
authors.lastname: Aubrey
authors.affiliation: Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, 47907, USA.|Department of Agronomy, Purdue University, West Lafayette, IN, 47907, USA.
authors.forename: Qianlai
authors.initials: Q
authors.lastname: Zhuang
authors.affiliation: Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA, 30602, USA.
authors.forename: Robert O
authors.initials: RO
authors.lastname: Teskey
mesh_terms: D002245:Carbon Dioxide; D065928:Forests; D018515:Plant Leaves; D018517:Plant Roots; D018547:Plant Stems; D014197:Trees; D014934:Wood
chemical_list: D002245:Carbon Dioxide
keywords: gross primary production; leaf area index; respiration; root CO2 efflux; stem CO2 efflux