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Harvard Forest Data Archive
Estimating Forest Productivity Using Carbon Isotopes in Tree Rings at Harvard Forest 1992-2010Related Publications
- Lead: Soumaya Belmecheri, R. Stockton Maxwell, Kenneth Davies, Alan Taylor
- Contact: Soumaya Belmecheri
- Start date: 1992
- End date: 2010
- Status: completed
- Location: Prospect Hill Tract (Harvard Forest)
- Latitude: +42.53 to +42.55
- Longitude: -72.20 to -72.17
- Elevation: 280 to 420 meter
- Taxa: Quercus rubra (red oak), Tsuga canadensis (hemlock)
- Release date: 2015
- EML file: knb-lter-hfr.235.2
- DOI: digital object identifier
- Related links:
- Biomass Inventories at Harvard Forest EMS Tower since 1993
- Canopy-Atmosphere Exchange of Carbon, Water and Energy at Harvard Forest EMS Tower since 1991
- Tree Growth and Above-Ground Biomass at Harvard Forest HEM and LPH Towers since 2001
- Study type: historical, paleological
- Research topic: forest-atmosphere exchange; historical and retrospective studies; physiological ecology, population dynamics and species interactions
- LTER core area: organic matter, disturbance
- Keywords: carbon, cellulose, climate change, photosynthesis, stable isotopes, tree physiology, tree rings
We investigated relationships between tree-ring δ13C and growth, and flux tower estimates of gross primary productivity (GPP) at Harvard Forest from 1992 to 2010. Seasonal variations of derived photosynthetic isotope discrimination (Δ13C) and leaf intercellular CO2 concentration (ci) showed significant increasing trends for the dominant deciduous and coniferous species. Δ13C was positively correlated to growing-season GPP and is primarily controlled by precipitation and soil moisture indicating that site conditions maintained high stomatal conductance under increasing atmospheric CO2 levels. Increasing Δ13C over the 1992-2010 period is attributed to increasing annual and summer water availability identified at Harvard Forest and across the region. Higher Δ13C is coincident with an enhancement in growth and ecosystem-level net carbon uptake. This work suggests that tree-ring δ13C could serve as a measure of forest GPP and be used to improve the calibration and predictive skill of ecosystem and carbon cycle models.
Ring Width Chronology and Basal area increment
To identify interannual variability in tree growth and tree-ring δ13C, we cored Q. rubra and T. canadensis with a 5-mm increment borer at 1.37 m stem height (Table 1). Two cores were taken from each tree to build ring-width chronologies. Additionally, we cored five and four trees respectively of each species at the same height with a 12-mm increment borer. The large cores were used to analyze the δ13C in the LW of the tree rings. A sample of four trees is sufficient to achieve good precision of the δ13C sample mean (McCarroll and Loader; 2004; Leavitt, 2008).
Ring-width chronologies were developed for each species using standard dendrochronological techniques (Speer, 2010). For the site, Q. rubra mean age was 97 years (range 71-115 yrs) and T. canadensis mean age was 145 years (range 89-221 yrs). For isotope cores, Q. rubra mean age was 103 years (range 87-113 yrs) and T. canadensis mean age was 142 years (range 93-188 yrs). All ages were estimated from inner ring dates.
Basal area increment (BAI) was calculated for each tree and species using the dplR package in R (Bunn, 2008). We used the “outside in” function to convert raw ring-width measurements to BAI based on the diameter of the tree and the width of each ring moving towards the pith of the tree. The method assumes a circular growth pattern. BAI was used instead of ring width as a surrogate of radial growth and carbon gain because it drepresents more accurately tree annual biomass increment without the need for standardization (Biondi and Qeadan 2008). A mean BAI value was computed for each species at an annual resolution, averaged over all the trees cored for both carbon isotope analyses and ring-width chronologies (i.e., 16 trees/42 cores for Q. rubra and 22 trees/55 cores for T. canadensis).
Stable istope analyses
We analyzed the LW portion of each tree ring from individual trees over the 1992-2010 period. The samples were milled using an ultra-centrifugation mill (Qiagen TissueLyserII) and α-cellulose was extracted from each wood sample following the Soxhlet method elaborated by Green (1963) and modified by Leavitt and Danzer (1993). δ13C ratios were measured on the CO2 produced by α-cellulose combustion in a Costech elemental analyzer (EA) coupled with a Thermo Delta V-IRMS. The sample accuracy was determined to be within 0.08‰ (1 standard deviation calculated from the average difference between measured and true internal standard, n = 13). The isotopic value is expressed in the delta (δ) notation relative to the VPDB (‰ VPDB).
We used δ13C to determine carbon isotope discrimination (Δ) by the plant against atmospheric δ13C, and variation in plant iWUE. The Δ describes the isotopic difference between the δ13C of air (δ13Cair) and the plant (δ13Cplant) and results from the preferential use of 12C over 13C during photosynthesis. Δ is calculated using:
Δ ‰ = (δ13Cair - δ13Cplant)/(1 + δ13Cplant/1000) 
Records of δ13Cair were obtained from Mauna Loa from 1992 to 2002, and from aircraft measurements collected in Worcester, Massachusetts at 500 m above ground and available from 2003-2010 (White and Vaughn 2011). The model of Farquhar, O’Leary, and Berry 1982 describes isotopic Δ for C3 plants via:
Δ ‰ = a + (b - a)ci/ca 
where a is the fractionation during CO2 diffusion through the stomata (4.4‰: O'Leary, 1981); b is the fractionation by RuBP carboxylase (27‰: Farquhar and Richards, 1984); and ci and ca, are the leaf intercellular space and ambient CO2 concentrations (μmol. Mol-1), respectively. We used the data of atmospheric CO2 concentration (ca) measured at 29 m height on the eddy-covariance tower to calculate ci from Equation . The fractionations due to diffusion and carboxylation are constant but additive; therefore, the δ13C records variations in ci as regulated by two main processes: stomatal conductance (ca/ci) and photosynthetic assimilation rate (A). The iWUE is the ratio of the net photosynthetic assimilation rate (A) and water vapor conductance (gH2O) and is described by Ehleringer and Cerling (1995) as:
iWUE = A/gH2O = ca - ci(1/1.6) 
We used summer (June, July and August) values of δ13Cair and ca to calculate Δ, and to reconstruct ci and iWUE for the period corresponding to LW formation.
This dataset is released to the public under Creative Commons license CC BY (Attribution). Please keep the designated contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement.
Belmecheri S, Maxwell R, Davies K, Taylor A. 2015. Estimating Forest Productivity Using Carbon Isotopes in Tree Rings at Harvard Forest 1992-2010. Harvard Forest Data Archive: HF235.
hf235-01: carbon isotope data
- year: year
- d13c.tsca: δ13C of isotope analysis of Tsuga canadensis (TSCA) measured from four trees/cores (‰) (unit: dimensionless / missing value: NA)
- d13c.quru: δ13C of Quercus rubra (QURU) measured from five trees (‰) (unit: dimensionless / missing value: NA)
- cid.tsca: carbon isotope discrimination (Δ) by the plant against atmospheric δ13C for Tsuga Canadensis (‰) (unit: dimensionless / missing value: NA)
- cid.quru: carbon isotope discrimination (Δ) by the plant against atmospheric δ13C for Quercus rubra (‰) (unit: dimensionless / missing value: NA)
- ci.tsca: intercellular space CO2 concentration for Tsuga canadensis (ppm) (unit: dimensionless / missing value: NA)
- ci.quru: intercellular space CO2 concentration for Quercus rubra (ppm) (unit: dimensionless / missing value: NA)
- iwue.tsca: intrinsic water use efficiency for Tsuga canadensis (µmol/mol) (unit: dimensionless / missing value: NA)
- iwue.quru: intrinsic water use efficiency for Quercus rubra (µmol/mol) (unit: dimensionless / missing value: NA)
- cica.tsca: stomatal conductance for Tsuga canadensis (ci/ca) (unit: dimensionless / missing value: NA)
- cica.quru: stomatal conductance for Quercus rubra (ci/ca) (unit: dimensionless / missing value: NA)
- bai.tsca: basal area increment for all sampled trees of Tsuga canadensis (unit: centimeterSquaredPerYear / missing value: NA)
- bai.quru: basal area increment for all sampled trees of Quercus rubra (unit: centimeterSquaredPerYear / missing value: NA)