Abstract
Abstract Leaf responses to elevated atmospheric CO 2 concentration (C a ) are central to models of forest CO 2 exchange with the atmosphere and constrain the magnitude of the future carbon sink. Estimating the magnitude of primary productivity enhancement of forests in elevated C a requires an understanding of how photosynthesis is regulated by diffusional and biochemical components and up‐scaled to entire canopies. To test the sensitivity of leaf photosynthesis and stomatal conductance to elevated C a in time and space, we compiled a comprehensive dataset measured over 10 years for a temperate pine forest of P inus taeda , but also including deciduous species, primarily L iquidambar styraciflua . We combined over one thousand controlled‐response curves of photosynthesis as a function of environmental drivers (light, air C a and temperature) measured at canopy heights up to 20 m over 11 years (1996–2006) to generate parameterizations for leaf‐scale models for the D uke free‐air CO 2 enrichment ( FACE ) experiment. The enhancement of leaf net photosynthesis ( A net ) in P . taeda by elevated C a of +200 μmol mol −1 was 67% for current‐year needles in the upper crown in summer conditions over 10 years. Photosynthetic enhancement of P . taeda at the leaf‐scale increased by two‐fold from the driest to wettest growing seasons. Current‐year pine foliage A net was sensitive to temporal variation, whereas previous‐year foliage A net was less responsive and overall showed less enhancement (+30%). Photosynthetic downregulation in overwintering upper canopy pine needles was small at average leaf N ( N area ), but statistically significant. In contrast, co‐dominant and subcanopy L . styraciflua trees showed A net enhancement of 62% and no A net – N area adjustments. Various understory deciduous tree species showed an average A net enhancement of 42%. Differences in photosynthetic responses between overwintering pine needles and subcanopy deciduous leaves suggest that increased C a has the potential to enhance the mixed‐species composition of planted pine stands and, by extension, naturally regenerating pine‐dominated stands.
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Claims 9
Claim · model-written summary
Elevated CO2 enhanced upper-crown current-year P. taeda needle photosynthesis by 67%.
- Taxon
- P . taedae0
- Organ / stage
- current‐year needlese0
- Treatment
- elevated C a of +200 μmol mol −1e0
- Outcome
- leaf net photosynthesis ( A net )e0
- Direction
- enhancemente0
- Value
- 67%e0
Not reportedComparator
Verbatim
e0“The enhancement of leaf net photosynthesis ( A net ) in P . taeda by elevated C a of +200 μmol mol −1 was 67% for current‐year needles in the upper crown in summer conditions over 10 years.”
Claim · model-written summary
P. taeda leaf-scale photosynthetic enhancement doubled from driest to wettest growing seasons.
Verbatim
e0“Photosynthetic enhancement of P . taeda at the leaf‐scale increased by two‐fold from the driest to wettest growing seasons.”
Claim · model-written summary
Current-year pine foliage photosynthesis was sensitive to temporal variation.
- Organ / stage
- Current‐year pine foliagee0
- Outcome
- Current‐year pine foliage A nete0
- Qualifier
- temporal variatione0
PinusNot reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Current‐year pine foliage A net was sensitive to temporal variation, whereas previous‐year foliage A net was less responsive and overall showed less enhancement (+30%).”
Claim · model-written summary
Previous-year pine foliage had 30% photosynthetic enhancement, less than current-year foliage.
- Outcome
- previous‐year foliage A nete0
- Direction
- less enhancemente0
- Value
- +30%e0
- Comparator
- Current‐year pine foliagee1
Not reportedTreatment
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“previous‐year foliage A net was less responsive and overall showed less enhancement (+30%).”
e1“Current‐year pine foliage A net was sensitive to temporal variation”
Claim · model-written summary
Overwintering upper-canopy pine needles had small but significant photosynthetic downregulation.
- Organ / stage
- overwintering upper canopy pine needlese0
- Outcome
- Photosynthetic downregulatione0
- Direction
- downregulatione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Photosynthetic downregulation in overwintering upper canopy pine needles was small at average leaf N ( N area ), but statistically significant.”
Claim · model-written summary
L. styraciflua photosynthesis was enhanced 62%.
Not reportedTreatmentComparator
Verbatim
e0“co‐dominant and subcanopy L . styraciflua trees showed A net enhancement of 62% and no A net – N area adjustments.”
Claim · model-written summary
L. styraciflua showed no photosynthesis-leaf nitrogen adjustment.
Not reportedDirection
Verbatim
e0“co‐dominant and subcanopy L . styraciflua trees showed A net enhancement of 62% and no A net – N area adjustments.”
Claim · model-written summary
Understory deciduous trees had average 42% photosynthetic enhancement.
Not reportedTreatmentComparator
Verbatim
e0“Various understory deciduous tree species showed an average A net enhancement of 42%.”
Claim · model-written summary
The authors suggest elevated CO2 could enhance mixed-species composition of pine stands.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“increased C a has the potential to enhance the mixed‐species composition of planted pine stands and, by extension, naturally regenerating pine‐dominated stands.”