Exposure to an enriched CO₂ atmosphere alters carbon assimilation and allocation in a pine forest ecosystem

Abstract

We linked a leaf‐level CO2 assimilation model with a model that accounts for light attenuation in the canopy and measurements of sap‐flux‐based canopy conductance into a new canopy conductance‐constrained carbon assimilation (4C‐A) model. We estimated canopy CO2 uptake (AnC) at the Duke Forest free‐air CO2 enrichment (FACE) study. Rates of AnC estimated from the 4C‐A model agreed well with leaf gas exchange measurements (Anet) in both CO2 treatments. Under ambient conditions, monthly sums of net CO2 uptake by the canopy (AnC) were 13% higher than estimates based on eddy‐covariance and chamber measurements. Annual estimates of AnC were only 3% higher than carbon (C) accumulations and losses estimated from ground‐based measurements for the entire stand. The C budget for the Pinus taeda component was well constrained (within 1% of ground‐based measurements). Although the closure of the C budget for the broadleaf species was poorer (within 20%), these species are a minor component of the forest. Under elevated CO2, the C used annually for growth, turnover, and respiration balanced only 80% of the AnC. Of the extra 700 g C m−2 a−1 (1999 and 2000 average), 86% is attributable to surface soil CO2 efflux. This suggests that the production and turnover of fine roots was underestimated or that mycorrhizae and rhizodeposition became an increasingly important component of the C balance. Under elevated CO2, net ecosystem production increased by 272 g C m−2 a−1: 44% greater than under ambient CO2. The majority (87%) of this C was sequestered in a moderately long‐term C pool in wood, with the remainder in the forest floor–soil subsystem.

Modeling nighttime ecosystem respiration from measured CO2 concentration and air temperature profiles using inverse methods

“…Using this estimate, the modeled GPP computed from modeled R A varied from 2033 to 2471 gC m À2 for these 3 years. This range is comparable to other estimates [Hamilton et al, 2002;Lai et al, 2002a;Schafer et al, 2003] conducted earlier at the site (2371 to 2486 gC m À2 from 1998 to 2000). As for NPP, the modeled values here ranged from 915 to 1112 gC m À2 during the 3-year study period.…”

“…The plant area density (PAD, m 2 m À3 ) measurements were conducted at 1 m intervals from the bottom to the top of canopy. Calibration of PAI was done using allometric relationships derived from different individual species within the canopy volume [Lai et al, 2000b;Pataki et al, 1998;Schafer et al, 2003].…”

“…[28] LAI-2000 measurements, coupled with the abovementioned allometric functions, were used to estimate the vertical distribution of PAD at daily time steps [Schafer et al, 2003]. The range of total PAI and pine PAI in 2001, 2002, and 2003 are given in Table 1.…”

Carbon allocation in forest ecosystems

“…The response of partitioning to changes in stand age and resource availability is relatively consistent among studies, but none have examined the shape or duration of the response. Finally, other factors such as intraspecific genotypic diversity (Crutsinger et al, 2006), atmospheric CO 2 concentrations (Schäfer et al, 2003;Palmroth et al, 2006) and temperature (Raich et al, 2006) may influence carbon allocation but data are limited.…”

Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

“…The difference increases as A n increases. High A n occurs during the peak growing season at the time when the sun angle is high [Schäfer et al, 2003]. This is the time when the largest portion of gaps (in the GCR representation) in stands can be seen in the direction of the sunbeam, allowing more sunlight to reach the forest floor.…”