Forest response to elevated CO2 is conserved across a broad range of productivity.
Climate change predictions derived from coupled carbon-climate models are highly dependent on assumptions about feedbacks between the biosphere and atmosphere. One critical feedback occurs if C uptake by the biosphere increases in response to the fossil-fuel driven increase in atmospheric [CO(2)] ("CO(2) fertilization"), thereby slowing the rate of increase in atmospheric [CO(2)]. Carbon exchanges between the terrestrial biosphere and atmosphere are often first represented in models as net primary productivity (NPP). However, the contribution of CO(2) fertilization to the future global C cycle has been uncertain, especially in forest ecosystems that dominate global NPP, and models that include a feedback between terrestrial biosphere metabolism and atmospheric [CO(2)] are poorly constrained by experimental evidence. We analyzed the response of NPP to elevated CO(2) ( approximately 550 ppm) in four free-air CO(2) enrichment experiments in forest stands. We show that the response of forest NPP to elevated [CO(2)] is highly conserved across a broad range of productivity, with a stimulation at the median of 23 +/- 2%. At low leaf area indices, a large portion of the response was attributable to increased light absorption, but as leaf area indices increased, the response to elevated [CO(2)] was wholly caused by increased light-use efficiency. The surprising consistency of response across diverse sites provides a benchmark to evaluate predictions of ecosystem and global models and allows us now to focus on unresolved questions about carbon partitioning and retention, and spatial variation in NPP response caused by availability of other growth limiting resources.
Norby
Richard J
RJ
Environmental Sciences Division, Oak Ridge National Laboratory, TN 37831-6422, USA.
Delucia
Evan H
EH
Gielen
Birgit
B
Calfapietra
Carlo
C
Giardina
Christian P
CP
King
John S
JS
Ledford
Joanne
J
McCarthy
Heather R
HR
Moore
David J P
DJ
Ceulemans
Reinhart
R
De Angelis
Paolo
P
Finzi
Adrien C
AC
Karnosky
David F
DF
Kubiske
Mark E
ME
Lukac
Martin
M
Pregitzer
Kurt S
KS
Scarascia-Mugnozza
Giuseppe E
GE
Schlesinger
William H
WH
Oren
Ram
R
D001272:Atmosphere / Q000737:chemistry*; D002245:Carbon Dioxide / Q000032:analysis*; D002980:Climate; D007558:Italy; D008027:Light; D008962:Models, Theoretical; D018515:Plant Leaves / Q000033:anatomy & histology / Q000502:physiology; D012044:Regression Analysis; D013997:Time Factors; D014197:Trees / Q000254:growth & development* / Q000378:metabolism*; D014481:United States
D002245:Carbon Dioxide