Where does the carbon go? A model-data intercomparison of vegetation carbon allocation and turnover processes at two temperate forest free-air CO2 enrichment sites.
Elevated atmospheric CO2 concentration (eCO2) has the potential to increase vegetation carbon storage if increased net primary production causes increased long-lived biomass. Model predictions of eCO2 effects on vegetation carbon storage depend on how allocation and turnover processes are represented. We used data from two temperate forest free-air CO2 enrichment (FACE) experiments to evaluate representations of allocation and turnover in 11 ecosystem models. Observed eCO2 effects on allocation were dynamic. Allocation schemes based on functional relationships among biomass fractions that vary with resource availability were best able to capture the general features of the observations. Allocation schemes based on constant fractions or resource limitations performed less well, with some models having unintended outcomes. Few models represent turnover processes mechanistically and there was wide variation in predictions of tissue lifespan. Consequently, models did not perform well at predicting eCO2 effects on vegetation carbon storage. Our recommendations to reduce uncertainty include: use of allocation schemes constrained by biomass fractions; careful testing of allocation schemes; and synthesis of allocation and turnover data in terms of model parameters. Data from intensively studied ecosystem manipulation experiments are invaluable for constraining models and we recommend that such experiments should attempt to fully quantify carbon, water and nutrient budgets.
Department of Biological Sciences, Macquarie University, Sydney, New South Wales, 2109, Australia.
Martin G
MG
De Kauwe
Belinda E
BE
Medlyn
Sönke
S
Zaehle
Anthony P
AP
Walker
Michael C
MC
Dietze
Ying-Ping
YP
Wang
Yiqi
Y
Luo
Atul K
AK
Jain
Bassil
B
El-Masri
Thomas
T
Hickler
David
D
Wårlind
Ensheng
E
Weng
William J
WJ
Parton
Peter E
PE
Thornton
Shusen
S
Wang
I Colin
IC
Prentice
Shinichi
S
Asao
Benjamin
B
Smith
Heather R
HR
McCarthy
Colleen M
CM
Iversen
Paul J
PJ
Hanson
Jeffrey M
JM
Warren
Ram
R
Oren
Richard J
RJ
Norby
D000388:Air / Q000032:analysis*; D018533:Biomass; D002244:Carbon / Q000032:analysis*; D002245:Carbon Dioxide / Q000032:analysis*; D003198:Computer Simulation; D017753:Ecosystem; D065928:Forests; D008962:Models, Theoretical; D014197:Trees / Q000737:chemistry*; D014934:Wood / Q000502:physiology
D002245:Carbon Dioxide; D002244:Carbon
CO2 fertilisation; allocation; carbon (C); climate change; elevated CO2; free-air CO2 enrichment (FACE); models; phenology