Abstract
SUMMARY Two important processes which may limit productivity gains in forest ecosystems with rising atmospheric CO 2 are reduction in photosynthetic capacity following prolonged exposure to high CO 2 and diminution of positive growth responses when soil nutrients, particularly N, are limiting. To examine the interacting effects of soil fertility and CO 2 enrichment on photosynthesis and growth in trees we grew hybrid poplar ( Populus × euramericana ) for 158 d in the field at ambient and twice ambient CO 2 and in soil with low or high N availability. We measured the timing and rate of canopy development, the seasonal dynamics of leaf level photosynthetic capacity, respiration, and N and carbohydrate concentration, and final above‐ and belowground dry weight. Single leaf net CO 2 assimilation (A) increased at elevated CO 2 over the majority of the growing season in both fertility treatments. At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO 2 , leading to a 49% increase in total dry weight. In contrast, at low fertility leaf area growth was unaffected by CO 2 treatment. Total dry weight nonetheless increased 25% due to CO 2 effects on A. Photosynthetic capacity (A at constant internal p(CO 2 ), (( C 1 )) was reduced in high CO 2 plants after 100 d growth at low fertility and 135 d growth at high fertility. Analysis of A responses to changing C 1 indicated that this negative adjustment of photosynthesis was due to a reduction in the maximum rate of CO 2 fixation by Rubisco. Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO 2 . Carbon dioxide effects on leaf respiration were most pronounced at high fertility, with increased respiration mid‐season and no change (area basis) or reduced (mass basis) respiration late‐season in elevated compared to ambient CO 2 plants. This temporal variation correlated with changes in leaf N concentration and leaf mass per area. Our results demonstrate the importance of considering both structural and physiological pathways of net C gain in predicting tree responses to rising CO 2 under conditions of suboptimal soil fertility.
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Claims 16
Claim · model-written summary
Elevated CO2 increased single-leaf net assimilation through most of the season in both fertility treatments.
- Treatment
- elevated CO 2e0
- Outcome
- Single leaf net CO 2 assimilation (A)e0
- Direction
- increasede0
- Qualifier
- both fertility treatmentse0
PopulusNot reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Single leaf net CO 2 assimilation (A) increased at elevated CO 2 over the majority of the growing season in both fertility treatments.”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At high fertility, elevated CO2 increased maximum individual leaf size.
- Treatment
- elevated CO 2e0
- Outcome
- maximum size of individual leavese0
- Direction
- increasede0
- Qualifier
- At high fertilitye0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO 2”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At high fertility, elevated CO2 increased total leaf number.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO 2”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At high fertility, elevated CO2 increased seasonal leaf area duration.
- Treatment
- elevated CO 2e0
- Outcome
- seasonal leaf area duration (LAD)e0
- Direction
- increasede0
- Qualifier
- At high fertilitye0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO 2”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At high fertility, elevated CO2 increased total dry weight by 49%.
- Treatment
- elevated CO 2e0
- Outcome
- total dry weighte0
- Direction
- increasee0
- Value
- 49%e0
- Qualifier
- At high fertilitye0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO 2 , leading to a 49% increase in total dry weight.”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At low fertility, leaf area growth was unaffected by CO2 treatment.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“at low fertility leaf area growth was unaffected by CO 2 treatment.”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At low fertility, CO2 increased total dry weight by 25%.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Total dry weight nonetheless increased 25% due to CO 2 effects on A.”
e1“at low fertility leaf area growth was unaffected by CO 2 treatment.”
e2“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
Elevated CO2 reduced photosynthetic capacity after 100 days at low fertility.
- Treatment
- high CO 2e0
- Outcome
- Photosynthetic capacitye0
- Direction
- reducede0
- Value
- 100 de0
- Qualifier
- low fertilitye0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Photosynthetic capacity (A at constant internal p(CO 2 ), (( C 1 )) was reduced in high CO 2 plants after 100 d growth at low fertility and 135 d growth at high fertility.”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
Elevated CO2 reduced photosynthetic capacity after 135 days at high fertility.
- Treatment
- high CO 2e0
- Outcome
- Photosynthetic capacitye0
- Direction
- reducede0
- Value
- 135 de0
- Qualifier
- high fertilitye0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Photosynthetic capacity (A at constant internal p(CO 2 ), (( C 1 )) was reduced in high CO 2 plants after 100 d growth at low fertility and 135 d growth at high fertility.”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
The authors attribute photosynthetic downregulation to reduced maximum Rubisco CO2 fixation.
- Gene
- Rubiscoe0
- Outcome
- negative adjustment of photosynthesise0
- Qualifier
- reduction in the maximum rate of CO 2 fixatione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“this negative adjustment of photosynthesis was due to a reduction in the maximum rate of CO 2 fixation by Rubisco.”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
Maximum electron transport was unaffected or declined under elevated CO2.
- Treatment
- elevated CO 2e0
- Outcome
- Maximum rate of electron transporte0
- Direction
- unaffected or declinede0
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO 2 .”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
Phosphate regeneration capacity was unaffected or declined under elevated CO2.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO 2 .”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At high fertility, elevated CO2 increased mid-season leaf respiration.
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Carbon dioxide effects on leaf respiration were most pronounced at high fertility, with increased respiration mid‐season”
e1“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At high fertility, late-season leaf respiration per area was unchanged under elevated CO2.
- Treatment
- elevatede0
- Outcome
- respiratione0
- Direction
- no changee0
- Qualifier
- area basise0
- Comparator
- ambient CO 2 plantse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“no change (area basis) or reduced (mass basis) respiration late‐season in elevated compared to ambient CO 2 plants.”
e1“Carbon dioxide effects on leaf respiration were most pronounced at high fertility”
e2“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
At high fertility, late-season leaf respiration per mass declined under elevated CO2.
- Treatment
- elevatede0
- Outcome
- respiratione0
- Direction
- reducede0
- Qualifier
- mass basise0
- Comparator
- ambient CO 2 plantse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“no change (area basis) or reduced (mass basis) respiration late‐season in elevated compared to ambient CO 2 plants.”
e1“Carbon dioxide effects on leaf respiration were most pronounced at high fertility”
e2“hybrid poplar ( Populus × euramericana )”
Claim · model-written summary
Temporal respiration variation correlated with leaf nitrogen and leaf mass per area.
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“This temporal variation correlated with changes in leaf N concentration and leaf mass per area.”
e1“hybrid poplar ( Populus × euramericana )”
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