# Presentation: 656622

## Forest-atmosphere exchange

- of carbon isotopes of CO2

- Dave Bowling
- Dept. of Biology
- University of Utah
- AmerFlux Annual Meeting, Oct. 2007

## Applications of CO2 isotopes at flux towers

- Partitioning NEE into GPP and TER (independent of environmental regression methods)
- Rapid C cycling: environmental factors, plant physiology, and 13C of ecosystem respiration (R)
- Ecosystem-level water-use efficiency (GPP/ET, independent of fluxes)
- Additional applications
-  additional constraints on ecosystem models (Rhet, Rauto, A/g, etc)
-  transport applications (complex canopy flows, diffusion/advection)
-  fossil fuel influence
- 13C  of GPP and TER at large scales are important to understand atmospheric patterns and for inverse modeling
-  labeling (FACE, web-FACE, C3-C4 rotation, etc.) and C allocation
- 143 hits on _Fluxnet _publication search for “isotope”

## AmeriFlux and Fluxnet Canada sites with

- _ongoing_ CO2 isotope research

- Harvard Forest	since 2001	Chibougamau black spruce	2004-2006
- Howland		2001	 	Southern old black spruce	2004-2006
- Wind River	2001		Southern old aspen		2004-2006
- Rannells Prairie	2002	 	Southern old jack pine	2004-2006
- Rosemount 	2002 (TDL) 	Campbell River 1949	2004-2006
- Niwot Ridge	2003 (TDL)
- SGP–ARM	2003
- Mary’s River	2005
- Metolius		2006
- Freeman Ranch	2007
- Chestnut Ridge	2007
- WLEF tall tower	a long time, weekly
- _Many others _including FL slash pine, Duke Forest, original Metolius, Walker Branch, other BOREAS sites, sites in other networks Euroflux, LBA etc

## Major technological advances

- Automated systems to fill flasks
- Combined with traditional mass spectrometry in the lab
- Optical isotope measurements
- Tunable diode lasers and other methods

- Flanagan, Fluxnet Canada

- NOAA GMD, U. Colorado Stable Isotope Lab

## Long records of 13C of whole-forest respiration

- also see Lai et al. (2005)

- Long records of 13C of whole-forest respiration
- from flasks
- 13CR is calculated from a mixing line called a Keeling plot

##  Niwot Ridge Project – funded by DOE TCP

-  **Niwot Ridge Project – funded by DOE TCP**
- D Bowling and J Miller
-  CO2 and C isotopes measured using tunable diode laser
-  roughly 80% data recovery
-  4 x 105 isotope measurements per year
-  mass spectrometer would cost $3.4M/yr at $8.5 /sample
-  data available on web: http://www.biology.utah.edu/bowling/research/niwot/nwt_overview/niwotoverview.html

## Stable isotopes of carbon dioxide

- 	12C	98.89 %	16O	99.763 %
- 	13C	1.11 %	17O	0.0375 %
- 			 18O	0.1995 %

- Average atmospheric content (375 mol mol-1)
- 	12C16O16O	     370 mol mol-1	(range 350 to 600)
**	**_**13**_C16O16O	     4 mol mol-1	(range 3.9 to 6.7)
- 	12C_**18**_O16O	     0.7 mol mol-1	(range 0.6 to 1.2)
**	**12C_**17**_O16O, doubly-labeled species, _**14**_CO2 – trace amounts

## Isotopic composition

-  V-PDB standard		0 ‰ by definition
-  CO2 in air		-8 ‰  (-7 to -15 ‰ )
-  C3 plant biomass		-24 to -30 ‰
-  Respired CO2		-24 to -30 ‰

- per mil

## Influence of photosynthesis on 13C of CO2 in air

- Influence of _photosynthesis_ on 13C of CO2 in air

## Influence of respiration on 13C of CO2 in air

- Influence of _respiration_ on 13C of CO2 in air

- air becomes
- depleted in 13CO2
- (13C decreases)

- respired CO2
- -20 to -30 ‰

- air -8 ‰

## Influence of respiration (R) and photosynthesis (P)

- Influence of respiration (R) and photosynthesis (P)

**A**

**R**

**R**

**A**

- CO2 and isotope patterns in Niwot Ridge forest air:
- 3-month average daily pattern in summer

## Applications of CO2 isotopes at flux towers

- Partitioning NEE into GPP and TER (independent of environmental regression methods)
- Rapid C cycling: environmental factors, plant physiology, and 13C of ecosystem respiration (R)
- Ecosystem-level water-use efficiency (GPP/ET, independent of fluxes)

## Partitioning NEE – done by at least 6 research groups

- 13CO2:	isoflux = R(R) + (a – )A**	**
**	**

- standard eddy covariance / storage for CO2 flux

- new techniques for 13CO2 flux
- eddy covariance now working (Griffis et al.)

- C  of CO2 in air
- directly measured

- C of respiration – from a Keeling plot

- photosynthetic discrimination: Penman-Monteith inversion and gas-exchange equations
- or canopy modeling

- Yakir and Wang (1996), Bowling et al. (2001)

##  estimated from water vapor fluxvia conductance derived from Penman-Monteith

-  estimated from water vapor fluxvia conductance derived from Penman-Monteith

- Leaf–level  relation

- Fick’s law

- Penman-
- Monteith

-     water vapor flux
- vapor pressure deficit

- Better approaches exist for determining  – see Knohl (2003) and Ogee et al. (2003, 2004)

## Partitioning NEE – comparison of independent methods at Niwot Ridge

- FA and FR derived from isotopes
- GEE and TER via Reichstein et al. (2005) environmental regression

- Zobitz et al., in review, Global Change Biology

## Applications of CO2 isotopes at flux towers

- Partitioning NEE into GPP and TER (independent of environmental regression methods)
- Rapid C cycling: environmental factors, plant physiology, and 13C of ecosystem respiration (R)
- Ecosystem-level water-use efficiency (GPP/ET, independent of fluxes)

## What controls variability in 13C of whole-forest respiration?

- What controls variability in 13C of whole-forest respiration?

## Longer time scales (years to decades)
C3/C4 content, pasture conversion
Ometto et al. 2002, Still et al. 2003
mean annual precipitation
Bowling et al. 2002, Pataki et al. 2003, many others
forest stand age
Fessenden and Ehleringer 2002

Shorter time scales (days to weeks) 
soil moisture content
Ometto et al. 2002, Fessenden and Ehleringer, 2003, Lai et al. 2005, Alstad et al. 2007
humidity
Ekblad and Högberg 2001, Bowling et al. 2002, Mortazavi et al. 2005, Alstad et al. 2007
air or soil temperature
Bowling et al. 2002, McDowell et al. 2004, Alstad et al. 2007
light or light coupled with humidity
Knohl et al. 2005, Alstad et al. 2007

- C3/C4 content, pasture conversion
- Ometto et al. 2002, Still et al. 2003
- mean annual precipitation
- Bowling et al. 2002, Pataki et al. 2003, many others
- forest stand age
- Fessenden and Ehleringer 2002
- Shorter time scales (days to weeks)
- soil moisture content
- Ometto et al. 2002, Fessenden and Ehleringer, 2003, Lai et al. 2005, Alstad et al. 2007
- humidity
- Ekblad and Högberg 2001, Bowling et al. 2002, Mortazavi et al. 2005, Alstad et al. 2007
- air or soil temperature
- Bowling et al. 2002, McDowell et al. 2004, Alstad et al. 2007
- light or light coupled with humidity
- Knohl et al. 2005, Alstad et al. 2007

- Environmental controls on 13CR(all discovered in last 5-6 years)

## Factors influencing isotopic content of ecosystem respiration

- precipitation
- input

- soil water

- leaf water potential

- foliar respiration

- 13C of
- fixed carbon

- 

- ci/ca

- transpiration

- stomatal conductance

- CO2

- light

- humidity

- root/rhizosphere
- respiration

- decomp of SOM

- heterotrophic
- turnover

- 13C of
- respired CO2

- faster (days)

- slower (years to decades)

- photosynthesis

- 13CR

**Notes:**
Need to stress the recalcitrant nature of SOM here
Years to decades to even centuries

Data from radiocarbon suggest 59% of soil respiration was derived from C that resided in plant/soil system < 1 year (Gaudinski et al. ____)

## Humidity and soil moisture influence nightly 13CR in many forests

- humid

- moist

- dry

- dry

- Humidity and soil moisture influence _nightly_ 13CR in many forests

## Humidity and soil water influences on 13CR observed in many forests

*Picea/Tsuga spp., Pseudotsuga menziesii, Pinus ponderosa, Juniperus occidentalis, (*Bowling et al. 2002)
*Pinus taeda* (Mortazavi et al. 2005)
*Pseudotsuga menziesii* (McDowell et al. 2004, Lai et al. 2005, Ponton et al. 2006)
*Picea engelmannii*, *Abies lasiocarpa*, *Pinus contorta* (Schaeffer et al. *in review*)
- temperate deciduous forest
*Populus tremuloides* (Ponton et al. 2006)
*Fagus sylvatica* (Scartazza et al. 2004)
*Fagus*, *Quercus* spp. (Steinmann et al. 2004)
*Fagus sylvatica *(Knohl et al. 2005)
- mixed *Quercus*, *Acer* spp. (Mortazavi et al. 2005)
- boreal coniferous forest
*Picea abies *(Ekblad et al. 2005)
- mixed species, (Lai et al. 2005)
- tropical forest
- mixed species, Brazil, Ometto et al. (2002)

- Humidity and soil water influences on 13CR observed in many forests

## Applications of CO2 isotopes at flux towers

- Partitioning NEE into GPP and TER (independent of environmental regression methods)
- Rapid C cycling: environmental factors, plant physiology, and 13C of ecosystem respiration (R)
- Ecosystem-level water-use efficiency (GPP/ET, independent of fluxes)

## Ponton et al. (2006)

## Ponton et al. (2006)

## Applications of CO2 isotopes at flux towers

- Partitioning NEE into GPP and TER (independent of environmental regression methods)
- Rapid C cycling: environmental factors, plant physiology, and 13C of ecosystem respiration (R)
- Ecosystem-level water-use efficiency (GPP/ET, independent of fluxes)
- Additional applications
-  additional constraints on ecosystem models (Rhet, Rauto, A/g, etc)
-  transport applications (complex canopy flows, diffusion/advection)
-  fossil fuel influence
- 13C  of GPP and TER at large scales are important to understand atmospheric patterns and for inverse modeling
-  labeling (FACE, web-FACE, C3-C4 rotation, etc.) and C allocation

## Ecosystem components differ in C isotope content

- Bowling et al. (*in review*) New Phytologist

- Plant C pools

- Forest C
- pools

- Respiratory
- CO2

## References

- Partitioning NEE into GPP and TER
- Bowling DR, Pataki DE, Ehleringer JR. 2003. Critical evaluation of micrometeorological methods for measuring ecosystem-atmosphere isotopic exchange of CO2. *Agricultural and Forest Meteorology 116(3-4): 159-179.*
-  Bowling DR, Tans PP, Monson RK. 2001. Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2. *Global Change Biology 7(2): 127-145.*
- Griffis TJ, Baker JM, Zhang J. 2005a. Seasonal dynamics and partitioning of isotopic CO2 exchange in C3/C4 managed ecosystem. *Agricultural and Forest Meteorology 132(1-2): 1-19.*
- Griffis TJ, Black TA, Gaumont-Guay D, Drewitt GB, Nesic Z, Barr AG, Morgenstern K, Kljun N. 2004. Seasonal variation and partitioning of ecosystem respiration in a southern boreal aspen forest. *Agricultural and Forest Meteorology 125(3-4): 207-223.*
- Griffis TJ, Lee X, Baker JM, Sargent SD, King JY. 2005b. Feasibility of quantifying ecosystem-atmosphere C18O16O exchange using laser spectroscopy and the flux-gradient method. *Agricultural and Forest Meteorology 135(1-4): 44-60.*
- Knohl A, Buchmann N. 2005. Partitioning the net CO2 flux of a deciduous forest into respiration and assimilation using stable carbon isotopes. *Global Biogeochemical Cycles 19(4): GB4008, doi:10.1029/2004GB002301.*
- Lai CT, Schauer AJ, Owensby C, Ham JM, Ehleringer JR. 2003. Isotopic air sampling in a tallgrass prairie to partition net ecosystem CO2 exchange. *Journal of Geophysical Research-Atmospheres 108(D18): 4566, doi:10.1029/2002JD003369.*
- Ogee J, Peylin P, Ciais P, Bariac T, Brunet Y, Berbigier P, Roche C, Richard P, Bardoux G, Bonnefond J-M. 2003. Partitioning net ecosystem carbon exchange into net assimilation and respiration using 13CO2 measurements: A cost-effective sampling strategy. *Global Biogeochemical Cycles 17(2): 1070, doi:10.1029/2002GB001995.*
- Ogee J, Peylin P, Cuntz M, Bariac T, Brunet Y, Berbigier P, Richard P, Ciais P. 2004. Partitioning net ecosystem carbon exchange into net assimilation and respiration with canopy-scale isotopic measurements: An error propagation analysis with (CO2)-C-13 and (COO)-O-18 data. *Global Biogeochemical Cycles 18(2): -.*
- Reichstein M, Falge E, Baldocchi D, Papale D, Aubinet M, Berbigier P, Bernhofer C, Buchmann N, Gilmanov T, Granier A, Grunwald T, Havrankova K, Ilvesniemi H, Janous D, Knohl A, Laurila T, Lohila A, Loustau D, Matteucci G, Meyers T, Miglietta F, Ourcival JM, Pumpanen J, Rambal S, Rotenberg E, Sanz M, Tenhunen J, Seufert G, Vaccari F, Vesala T, Yakir D, Valentini R. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. *Global Change Biology 11(9): 1424-1439.*
- Stoy PC, Katul GG, Siqueira MBS, Juang JY, Novick KA, Uebelherr JM, Oren R. 2006. An evaluation of models for partitioning eddy covariance-measured net ecosystem exchange into photosynthesis and respiration. *Agricultural and Forest Meteorology 141(1): 2-18.*
- Yakir D, Wang XF. 1996. Fluxes of CO2 and water between terrestrial vegetation and the atmosphere estimated from isotope measurements. *Nature 380: 515-517.*
- Zhang J, Griffis TJ, Baker JM. 2006. Using continuous stable isotope measurements to partition net ecosystem CO2 exchange. *Plant Cell and Environment 29(4): 483-496.*

## References

- Rapid C cycling: environmental factors, plant physiology, and 13C of ecosystem respiration (R)
- Ecosystem-level water use efficiency
- Alstad KP, Lai C-T, Flanagan LB, Ehleringer JR. 2007. Environmental controls on the carbon isotope composition of ecosystem-respired CO2 in contrasting forest ecosystems in Canada and the USA. *Tree Physiology 27: 1361-1374.*
- Bowling DR, Burns SP, Conway TJ, Monson RK, White JWC. 2005. Extensive observations of CO2 carbon isotope content in and above a high-elevation subalpine forest. *Global Biogeochemical Cycles 19(3): GB3023, doi:10.1029/2004GB002394.*
- Bowling DR, McDowell NG, Bond BJ, Law BE, Ehleringer JR. 2002. 13C content of ecosystem respiration is linked to precipitation and vapor pressure deficit. *Oecologia 131(1): 113-124.*
- Ekblad A, Bostrom B, Holm A, Comstedt D. 2005. Forest soil respiration rate and 13C is regulated by recent above ground weather conditions. *Oecologia 143: 136-142.*
- Ekblad A, Hogberg P. 2001. Natural abundance of 13C in CO2 respired from forest soils reveals speed of link between tree photosynthesis and root respiration. *Oecologia 127(3): 305-308.*
- Fessenden JE, Ehleringer JR. 2002. Age-related variations in 13C of ecosystem respiration across a coniferous forest chronosequence in the Pacific Northwest. *Tree Physiology 22(2-3): 159-167.*
- Fessenden JE, Ehleringer JR. 2003. Temporal variation in 13C of ecosystem respiration in the Pacific Northwest: links to moisture stress. *Oecologia 136(1): 129-136.*
- Flanagan LB, Brooks JR, Varney GT, Berry SC, Ehleringer JR. 1996. Carbon isotope discrimination during photosynthesis and the isotope ratio of respired CO2 in boreal forest ecosystems. *Global Biogeochemical Cycles 10: 629-640.*
- Hemming D, Yakir D, Ambus P, Aurela M, Besson C, Black K, Buchmann N, Burlett R, Cescatti A, Clement R, Gross P, Granier A, Grunwald T, Havrankova K, Janous D, Janssens IA, Knohl A, Kostner B, Kowalski A, Laurila T, Mata C, Marcolla B, Matteucci G, Moncrieff J, Moors EJ, Osborne B, Periera JS, Pihlatie M, Pilegaard K, Ponti F, Rosova Z, Rossi F, Scartazza A, Vesala T. 2005. Pan-European 13C values of air and organic matter from forest ecosystems. *Global Change Biology 11: 1065-1093.*
- Keeling CD. 1958. The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas. *Geochimica et Cosmochimica Acta 13(4): 322-334.*
- Keeling CD. 1961. The concentration and isotopic abundances of carbon dioxide in rural and marine air. *Geochimica et Cosmochimica Acta 24(3-4): 277-298.*
- Knohl A, Werner RA, Brand WA, Buchmann N. 2005. Short-term variations in 13C of ecosystem respiration reveals link between assimilation and respiration in a deciduous forest. *Oecologia 142(1): 70-82.*
- Lai C-T, Ehleringer JR, Schauer AJ, Tans PP, Hollinger DY, Paw U KT, Munger JW, Wofsy SC. 2005. Canopy-scale 13C of photosynthetic and respiratory CO2 fluxes: observations in forest biomes across the United States. *Global Change Biology 11(4): 633-643.*
- Lai CT, Ehleringer JR, Tans P, Wofsy SC, Urbanski SP, Hollinger DY. 2004. Estimating photosynthetic 13C discrimination in terrestrial CO2 exchange from canopy to regional scales. *Global Biogeochemical Cycles 18(GB1041): doi:10.1029/2003GB002148.*

## References

- Rapid C cycling: environmental factors, plant physiology, and 13C of ecosystem respiration (R)
- Ecosystem-level water use efficiency
- (continued from last page)
- McDowell NG, Bowling DR, Bond BJ, Irvine J, Law BE, Anthoni P, Ehleringer JR. 2004a. Response of the carbon isotopic content of ecosystem, leaf, and soil respiration to meteorological and physiological driving factors in a *Pinus ponderosa ecosystem. Global Biogeochemical Cycles 18(1): GB1013, doi:10.1029/203GB00249.*
- McDowell NG, Bowling DR, Schauer A, Irvine J, Bond BJ, Law BE, Ehleringer JR. 2004b. Associations between carbon isotope ratios of ecosystem respiration, water availability and canopy conductance. *Global Change Biology 10(10): 1767-1784.*
- Mortazavi B, Chanton JP, Prater JL, Oishi AC, Oren R, Katul G. 2005. Temporal variability in 13C of respired CO2 in a pine and a hardwood forest subject to similar climatic conditions. *Oecologia 142(1): 57-69.*
- Ometto JPHB, Flanagan LB, Martinelli LA, Moreira MZ, Higuchi N, Ehleringer JR. 2002. Carbon isotope discrimination in forest and pasture ecosystems of the Amazon Basin, Brazil. *Global Biogeochemical Cycles 16(4): 1109, doi:10.1029/2001GB001462.*
- Pataki DE, Ehleringer JR, Flanagan LB, Yakir D, Bowling DR, Still CJ, Buchmann N, Kaplan JO, Berry JA. 2003. The application and interpretation of Keeling plots in terrestrial carbon cycle research. *Global Biogeochemical Cycles 17(1): 1022, doi:10.1029/2001GB001850.*
- Ponton S, Flanagan LB, Alstad KP, Johnson BG, Morgenstern K, Kljun N, Black TA, Barr AG. 2006. Comparison of ecosystem water-use efficiency among Douglas-fir forest, aspen forest and grassland using eddy covariance and carbon isotope techniques. *Global Change Biology 12(2): 294-310.*
- Scartazza A, Mata C, Matteucci G, Yakir D, Moscatello S, Brugnoli E. 2004. Comparisons of 13C of photosynthetic products and ecosystem respiratory CO2 and their response to seasonal climate variability. *Oecologia 140(2): 340-351.*
- Steinmann K, Siegwolf RTW, Saurer M, Korner C. 2004. Carbon fluxes to the soil in a mature temperate forest assessed by 13C isotope tracing. *Oecologia 141(3): 489-501.*
- Still CJ, Berry JA, Ribas-Carbo M, Helliker BR. 2003. The contribution of C3 and C4 plants to the carbon cycle of a tallgrass prairie: an isotopic approach. *Oecologia 136(3): 347-359.*
- Werner C, Unger S, Pereira JS, Maia R, David TS, Kurz-Besson C, David JS, Maguas C. 2006. Importance of short-term dynamics in carbon isotope ratios of ecosystem respiration (13CR) in a Mediterranean oak woodland and linkage to environmental factors.  172(2): 330-346.

## References

- constraints on ecosystem models (Rhet, Rauto, A/g, etc)
- Aranibar JN, Berry JA, Riley WJ, Pataki DE, Law BE, Ehleringer JR. 2006. Combining meteorology, eddy fluxes, isotope measurements, and modeling to understand environmental controls of carbon isotope discrimination at the canopy scale. *Global Change Biology 12(4): 710-730.*
- Baldocchi DD, Bowling DR. 2003. Modelling the discrimination of 13CO2 above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales. *Plant Cell and Environment 26(2): 231-244.*
- Ponton S, Flanagan LB, Alstad KP, Johnson BG, Morgenstern K, Kljun N, Black TA, Barr AG. 2006. Comparison of ecosystem water-use efficiency among Douglas-fir forest, aspen forest and grassland using eddy covariance and carbon isotope techniques. *Global Change Biology 12(2): 294-310.*
-  fossil fuel influence
- Lai C-T, Ehleringer JR, Schauer AJ, Tans PP, Hollinger DY, Paw U KT, Munger JW, Wofsy SC. 2005. Canopy-scale 13C of photosynthetic and respiratory CO2 fluxes: observations in forest biomes across the United States. *Global Change Biology 11(4): 633-643.*
- Pataki DE, Bowling DR, Ehleringer JR. 2003. Seasonal cycle of carbon dioxide and its isotopic composition in an urban atmosphere: Anthropogenic and biogenic effects. *Journal of Geophysical Research-Atmospheres 108(D23): -.*
- Pataki DE, Bowling DR, Ehleringer JR, Zobitz JM. 2006. High resolution atmospheric monitoring of urban carbon dioxide sources. *Geophysical Research Letters 33(3): L03813, doi:10.1029/2005GL024822.*
- 13C  of GPP and TER at large scales are important to understand atmospheric patterns and for inverse modeling
- Fung I, Field CB, Berry JA, Thompson MV, Randerson JT, Malmstrom CM, Vitousek PM, Collatz GJ, Sellers PJ, Randall DA, Denning AS, Badeck F, John J. 1997. Carbon 13 exchanges between the atmosphere and biosphere. *Global Biogeochemical Cycles 11(4): 507-533.*
- Randerson JT, Collatz GJ, Fessenden JE, Munoz AD, Still CJ, Berry JA, Fung IY, Suits N, Denning AS. 2002a. A possible global covariance between terrestrial gross primary production and 13C discrimination: Consequences for the atmospheric 13C budget and its response to ENSO. *Global Biogeochemical Cycles 16(4): 1136, doi:10.1029/2001GB001845.*
- Randerson JT, Still CJ, Balle JJ, Fung IY, Doney SC, Tans PP, Conway TJ, White JWC, Vaughn B, Suits N, Denning AS. 2002b. Carbon isotope discrimination of arctic and boreal biomes inferred from remote atmospheric measurements and a biosphere-atmosphere model. *Global Biogeochemical Cycles 16(3): 1028, 10.1029/2001GB001435, 2002.*
-  labeling (FACE, web-FACE, C3-C4 rotation, etc.) and C allocation
- Leake JR, Ostle NJ, Rangel-Castro JI, Johnson D. 2006. Carbon fluxes from plants through soil organisms determined by field 13CO2 pulse-labelling in an upland grassland. *Applied Soil Ecology 33(2): 152-175.*
- Steinmann K, Siegwolf RTW, Saurer M, Korner C. 2004. Carbon fluxes to the soil in a mature temperate forest assessed by 13C isotope tracing. *Oecologia 141(3): 489-501.*
- Taneva L, Pippen JS, Schlesinger WH, Gonzalez-Meler MA. 2006. The turnover of carbon pools contributing to soil CO2 and soil respiration in a temperate forest exposed to elevated CO2 concentration. *Global Change Biology 12(6): 983-994.*