Abstract
Abstract In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce ( Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75‐year tree ring chronologies of basal tree growth, carbon isotope discrimination (∆ 13 C, a proxy for leaf gas exchange), and δ 15 N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca . 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO 2 , while reductions in pollutant emissions of NO x and warmer springs played smaller, but significant roles. Tree ring ∆ 13 C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ 15 N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency ( i WUE ) of the red spruce trees increased by ~51% across the 75‐year chronology, and was driven by changes in atmospheric CO 2 and acid pollution, but i WUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.
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Claims 10
Claim · model-written summary
Reductions in acidic sulfur pollution were among the two most important factors driving increased red spruce tree growth since ca. 1989.
- Taxon
- red spruce
Picea rubense1 - Treatment
- reductions in acidic sulfur pollutione2
- Outcome
- tree growthe2
- Direction
- increasede2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“red spruce ( Picea rubens Sarg.) trees”
e2“Results reveal the two most important factors driving increased tree growth since ca . 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO 2”
Claim · model-written summary
Increases in atmospheric CO2 were among the two most important factors driving increased red spruce tree growth since ca. 1989.
- Taxon
- red sprucePicea rubense1
- Treatment
- increases in atmospheric CO 2e2
- Outcome
- tree growthe2
- Direction
- increasede2
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“red spruce ( Picea rubens Sarg.) trees”
e2“Results reveal the two most important factors driving increased tree growth since ca . 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO 2”
Claim · model-written summary
Tree ring Δ13C signatures of red spruce increased significantly since 1989.
- Taxon
- red sprucePicea rubense1
- Outcome
- tree ring ∆ 13 C signaturese4
- Direction
- increasede4
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“red spruce ( Picea rubens Sarg.) trees”
e4“Tree ring ∆ 13 C signatures increased significantly since 1989”
Claim · model-written summary
Tree ring δ15N signatures of red spruce declined significantly since 1989.
- Taxon
- red sprucePicea rubense1
- Outcome
- tree ring δ 15 N signaturese5
- Direction
- declinese5
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“red spruce ( Picea rubens Sarg.) trees”
e5“concurrently with significant declines in tree ring δ 15 N signatures”
Claim · model-written summary
Intrinsic water use efficiency of red spruce trees increased by about 51% across the 75-year chronology.
- Taxon
- red sprucePicea rubense6
- Outcome
- Intrinsic water use efficiencye6
- Direction
- increasede6
- Value
- ~51%e6
- Qualifier
- across the 75‐year chronologye6
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e6“Intrinsic water use efficiency ( i WUE ) of the red spruce trees increased by ~51% across the 75‐year chronology”
Claim · model-written summary
Intrinsic water use efficiency of red spruce was driven by changes in atmospheric CO2 and acid pollution, but was not linked to recent tree growth increases.
- Taxon
- red sprucePicea rubense6
- Treatment
- changes in atmospheric CO 2 and acid pollutione7
- Outcome
- Intrinsic water use efficiencye6
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e6“Intrinsic water use efficiency ( i WUE ) of the red spruce trees increased by ~51% across the 75‐year chronology”
e7“was driven by changes in atmospheric CO 2 and acid pollution”
Claim · model-written summary
Intrinsic water use efficiency (iWUE) of red spruce was not linked to recent increases in tree growth.
- Taxon
- red sprucePicea rubense1
- Outcome
- i WUEe0
- Comparator
- recent increases in tree growthe0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“red spruce ( Picea rubens Sarg.) trees”
e0“but i WUE was not linked to recent increases in tree growth”
Claim · model-written summary
Greater photosynthesis and stomatal conductance of trees, with increases in ecosystem N retention, were related to recent increases in red spruce tree growth.
- Taxon
- red sprucePicea rubense0
- Treatment
- greater photosynthesis and stomatal conductance of treese0
- Outcome
- tree growthe0
- Direction
- increasese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth”
Claim · model-written summary
Reductions in pollutant emissions of NOx played a smaller but significant role in the increase of red spruce tree growth.
- Taxon
- red sprucePicea rubense1
- Treatment
- reductions in pollutant emissions of NO xe3
- Outcome
- increased tree growthe2
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“red spruce ( Picea rubens Sarg.) trees”
e2“driving increased tree growth since ca . 1989”
e3“while reductions in pollutant emissions of NO x and warmer springs played smaller, but significant roles”
Claim · model-written summary
Warmer springs played a smaller but significant role in the increase of red spruce tree growth.
- Taxon
- red sprucePicea rubense1
- Treatment
- warmer springse3
- Outcome
- increased tree growthe2
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e1“red spruce ( Picea rubens Sarg.) trees”
e2“driving increased tree growth since ca . 1989”
e3“while reductions in pollutant emissions of NO x and warmer springs played smaller, but significant roles”