title: Aboveground sink strength in forests controls the allocation of carbon below ground and its [CO2]-induced enhancement.
abstract: The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO2 concentration ([CO2]) determines the enhancement in C sequestration, yet no clear partitioning rules exist. Here, we used first principles and published data from four free-air CO2 enrichment (FACE) experiments on forest tree species to conceptualize the total allocation of C to below ground (TBCA) under current [CO2] and to predict the likely effect of elevated [CO2]. We show that at a FACE site where leaf area index (L) of Pinus taeda L. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, reflecting the aboveground sink for net primary productivity, were accompanied by opposite changes in TBCA. A similar pattern emerged when data were combined from the four FACE experiments, using leaf area duration (LD) to account for differences in growing-season length. Moreover, elevated [CO2]-induced enhancement of TBCA in the combined data decreased from approximately 50% (700 g C m(-2) y(-1)) at the lowest LD to approximately 30% (200 g C m(-2) y(-1)) at the highest LD. The consistency of the trend in TBCA with L and its response to [CO2] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that use L to estimate components of the terrestrial C balance.
authors.affiliation: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708, USA.
authors.forename: Sari
authors.initials: S
authors.lastname: Palmroth
authors.forename: Ram
authors.initials: R
authors.lastname: Oren
authors.forename: Heather R
authors.initials: HR
authors.lastname: McCarthy
authors.forename: Kurt H
authors.initials: KH
authors.lastname: Johnsen
authors.forename: Adrien C
authors.initials: AC
authors.lastname: Finzi
authors.forename: John R
authors.initials: JR
authors.lastname: Butnor
authors.forename: Michael G
authors.initials: MG
authors.lastname: Ryan
authors.forename: William H
authors.initials: WH
authors.lastname: Schlesinger
mesh_terms: D001272:Atmosphere / Q000737:chemistry; D002244:Carbon / Q000378:metabolism*; D002245:Carbon Dioxide / Q000032:analysis / Q000378:metabolism*; D017753:Ecosystem; D008954:Models, Biological; D009657:North Carolina; D010788:Photosynthesis / Q000502:physiology*; D028223:Pinus / Q000378:metabolism*; D018515:Plant Leaves / Q000033:anatomy & histology / Q000378:metabolism*; D012621:Seasons; D014197:Trees / Q000378:metabolism*
chemical_list: D002245:Carbon Dioxide; D002244:Carbon