Leaf dynamics of a deciduous forest canopy: no response to elevated CO 2

Norby, Richard J.; Sholtis, Johnna D.; Gunderson, Carla A.; Jawdy, Sara

doi:10.1007/s00442-003-1296-2

Cited by 113 publications

(130 citation statements)

References 43 publications

(55 reference statements)

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“…Our initial assumption was that the sweetgum trees had fully occupied the site. This assumption was verified above ground in that peak leaf area index did not increase from 1998 to 2003 (12), but the root system may have still been expanding during this period. Peak root length (Fig.…”

Section: Resultsmentioning

confidence: 83%

“…When pretreatment measurements were made in 1997, the trees (Ϸ90 per plot) were 12 m tall with an average diameter of 11 cm and a stand basal area of 28 cm 2 ⅐m Ϫ2 (11). The trees were in a linear growth phase, leaf area index was 5.5 m 2 ⅐m Ϫ2 , and the canopy was no longer expanding (12). Exposure to elevated CO 2 commenced in two plots in April 1998 and has continued during the growing season (April to November) since then.…”

Section: Methodsmentioning

confidence: 99%

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Fine-root production dominates response of a deciduous forest to atmospheric CO ₂ enrichment

Norby

Ledford

Reilly

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

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Fine-root production and turnover are important regulators of the biogeochemical cycles of ecosystems and key components of their response to global change. We present a nearly continuous 6-year record of fine-root production and mortality from minirhizotron analysis of a closed-canopy, deciduous sweetgum forest in a free-air CO 2 enrichment experiment. Annual production of fine roots was more than doubled in plots with 550 ppm CO 2 compared with plots in ambient air. This response was the primary component of the sustained 22% increase in net primary productivity. Annual fine-root mortality matched annual production, and the mean residence time of roots was not altered by elevated CO 2, but peak fine-root standing crop in midsummer was significantly higher in CO 2-enriched plots, especially deeper in the soil profile. The preferential allocation of additional carbon to fine roots, which have a fast turnover rate in this species, rather than to stemwood reduces the possibility of long-term enhancement by elevated CO 2 of carbon sequestration in biomass. However, sequestration of some of the fine-root carbon in soil pools is not precluded, and there may be other benefits to the tree from a seasonally larger and deeper fine-root system. Root-system dynamics can explain differences among ecosystems in their response to elevated atmospheric CO 2; hence, accurate assessments of carbon flux and storage in forests in a globally changing atmosphere must account for this unseen and difficult-to-measure component.R esearchers who attempt to quantify C pools and fluxes in terrestrial ecosystems often find it expedient to measure only the aboveground components despite the wide recognition that a substantial fraction of net primary production occurs belowground (1). Fine roots (ephemeral, nonwoody roots generally of diameters Ͻ1 mm) are important regulators of biogeochemical cycling (2), and their turnover is a key component of C sequestration in soil (3). Root production often is enhanced when the plants are grown in a CO 2 -enriched atmosphere (4, 5). However, the difficulty in measuring these roots, which cannot generally be seen, are not easily sampled, and can grow and die rapidly and unpredictably, means that they are left out of both retrospective (6) and prospective (7) studies of the potential for terrestrial ecosystems to sequester additional C in response to a changing atmosphere and climate. Fine-root production and distribution in forests exhibit high spatial and temporal variation, making their characterization especially difficult, and most of the experimental evidence on tree root responses to elevated CO 2 comes from static or short-term observations that are strongly confounded with plant developmental trajectories (5). Free-air CO 2 enrichment (FACE) experiments (8) provide an opportunity for longer-term observations under realistic conditions in fully developed forests stands.We used minirhizotron observation tubes and video imaging (9, 10) to quantify fine-root production and mortality in a FACE experim...

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Section: Resultsmentioning

confidence: 83%

Section: Methodsmentioning

confidence: 99%

Fine-root production dominates response of a deciduous forest to atmospheric CO ₂ enrichment

Norby

Ledford

Reilly

et al. 2004

Proc. Natl. Acad. Sci. U.S.A.

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352

305

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“…The dotted line represents the 1:1 relationship. A useful analytical tool for interpreting plant responses that are confounded with ontogenetic or stand development has been to separate the response into structural and functional components (25), which also is an important analytical framework used in ecosystem models (34) and remote sensing analysis (32). Here, the structural component is leaf area by which light is absorbed, and the functional component is the conversion of light energy to organic matter.…”

Section: Discussionmentioning

confidence: 99%

“…In the pine forest at DukeFACE, I leaf also was estimated from allometric regression (22) applied at different intervals (18); I leaf ϭ 0 for the deciduous forests. The production of leaf litter (D litterfall ) was estimated from litter baskets at all sites (23)(24)(25). In most cases biomass measurements were converted to C units by using the measured C content of different tissues, which varied between 0.4 and 0.5 (26,27); when measured values were not available, C content was assumed equal to 0.5.…”

Section: Methodsmentioning

confidence: 99%

“…The combined contribution of herbivory and dissolved organic C in the DukeFACE experiment was only 3% of NPP (18), so it is unlikely that their exclusion would alter our results. Absorbed photosynthetically active radiation (APAR, or the light energy in megajoules absorbed by the canopy) was estimated from the difference between quantum sensor measurements of down-welling radiation (400-700 nm) and the amount reaching the forest f loor (POP-EUROFACE and ORNL-FACE) (19,25) or by applying a Beer's Law approximation to measurements of leaf area index (LAI) (21). Values of APAR were integrated over the growing season.…”

Section: Methodsmentioning

confidence: 99%

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Forest response to elevated CO₂is conserved across a broad range of productivity

Norby

DeLucia

Gielen

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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746

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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] (''CO2 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 (Ϸ550 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.CO2 fertilization ͉ global change ͉ leaf area index ͉ net primary productivity A nalysis and prediction of the effects of human activities, particularly the combustion of fossil fuels, on climate and the biological, physical, and social responses to changing climate require an integrated view of the complex interactions between the biosphere and atmosphere. Carbon cycle models are now being coupled to atmosphere-ocean general circulation climate models to achieve a dynamic analysis of the relationships between C emissions, atmospheric chemistry, biosphere activity, and climatic change (1-3).Exchanges between the terrestrial biosphere and atmosphere are represented in models using empirical and theoretical expressions of net primary productivity (NPP), the net fixation of C by green plants into organic matter, or the difference between photosynthesis and plant respiration. Because the photosynthetic uptake of carbon that drives NPP is not saturated at current atmospheric concentrations (4), NPP should increase as fossilfuel combustion adds to the atmospheric [CO 2 ]. Increased C uptake into the biosphere in response to rising [CO 2 ] (''CO 2 fertilization'') can create a negative feedback that slows the rate of increase in atmospheric [CO 2 ] (3, 5). Hence, assumptions regarding CO 2 fertilization of the terrestrial biosphere greatly affect predictions of future atmospheric [CO 2 ] (3)...

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Comprehensive ecosystem model‐data synthesis using multiple data sets at two temperate forest free‐air CO₂ enrichment experiments: Model performance at ambient CO₂ concentration

et al. 2014

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Free-air CO 2 enrichment (FACE) experiments provide a remarkable wealth of data which can be used to evaluate and improve terrestrial ecosystem models (TEMs). In the FACE model-data synthesis project, 11 TEMs were applied to two decadelong FACE experiments in temperate forests of the southeastern U.S.-the evergreen Duke Forest and the deciduous Oak Ridge Forest. In this baseline paper, we demonstrate our approach to model-data synthesis by evaluating the models' ability to reproduce observed net primary productivity (NPP), transpiration, and leaf area index (LAI) in ambient CO 2 treatments. Model outputs were compared against observations using a range of goodness-of-fit statistics. Many models simulated annual NPP and transpiration within observed uncertainty. We demonstrate, however, that high goodness-of-fit values do not necessarily indicate a successful model, because simulation accuracy may be achieved through compensating biases in component variables. For example, transpiration accuracy was sometimes achieved with compensating biases in leaf area index and transpiration per unit leaf area. Our approach to model-data synthesis therefore goes beyond goodness-of-fit to investigate the success of alternative representations of component processes. Here we demonstrate this approach by comparing competing model hypotheses determining peak LAI. Of three alternative hypotheses-(1) optimization to maximize carbon export, (2) increasing specific leaf area with canopy depth, and (3) the pipe model-the pipe model produced peak LAI closest to the observations. This example illustrates how data sets from intensive field experiments such as FACE can be used to reduce model uncertainty despite compensating biases by evaluating individual model assumptions.

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Leaf dynamics of a deciduous forest canopy: no response to elevated CO 2

Cited by 113 publications

References 43 publications

Fine-root production dominates response of a deciduous forest to atmospheric CO ₂ enrichment

Fine-root production dominates response of a deciduous forest to atmospheric CO ₂ enrichment

Forest response to elevated CO₂is conserved across a broad range of productivity

Comprehensive ecosystem model‐data synthesis using multiple data sets at two temperate forest free‐air CO₂ enrichment experiments: Model performance at ambient CO₂ concentration

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Leaf dynamics of a deciduous forest canopy: no response to elevated CO 2

Cited by 113 publications

References 43 publications

Fine-root production dominates response of a deciduous forest to atmospheric CO 2 enrichment

Fine-root production dominates response of a deciduous forest to atmospheric CO 2 enrichment

Forest response to elevated CO2is conserved across a broad range of productivity

Comprehensive ecosystem model‐data synthesis using multiple data sets at two temperate forest free‐air CO2 enrichment experiments: Model performance at ambient CO2 concentration

Contact Info

Product

Resources

About

Fine-root production dominates response of a deciduous forest to atmospheric CO ₂ enrichment

Fine-root production dominates response of a deciduous forest to atmospheric CO ₂ enrichment

Forest response to elevated CO₂is conserved across a broad range of productivity

Comprehensive ecosystem model‐data synthesis using multiple data sets at two temperate forest free‐air CO₂ enrichment experiments: Model performance at ambient CO₂ concentration