High Carbon Dioxide Levels Spur Southern Pines to Grow More Needles
MONTREAL — A Duke University study has found that maturing stands of pines exposed to the higher levels of carbon dioxide expected by mid-century produce more needles than those absorbing today’s levels of the gas, even under drought conditions. However, the study also found that lack of soil nutrients may impose limitations in many forests.
Duke graduate student Heather McCarthy will describe results she obtained from a futuristic open-air experimental forest site at 8 a.m. Eastern Time on Thursday, Aug. 11, 2005 during the 2005 annual meeting of the Ecological Society of America in Montreal.
Her work was supported by the United States Department of Energy and the U.S. Department of Agriculture’s Forest Service.
McCarthy, who has just competed her fifth year as a doctoral student in environmental studies at Duke’s Nicholas School of the Environment and Earth Sciences, analyzed 10 years of pine needle data collected at the Free-Air Carbon Dioxide Enrichment (FACE) experiment in Duke Forest, a near-campus research reserve.
At FACE, some stands of fast-growing loblolly lines are being exposed to the higher levels of CO2 expected by 2050 due to human activities such as fossil fuel burning. Other stands are left as untreated controls for comparison. The elevated carbon dioxide is delivered from rings of towers in the open air setting of a Southern forest ecosystem.
McCarthy found that, over the most recent six years of the FACE experiment, the pines receiving elevated CO2 had on average about 17 percent more needles than untreated pines. Higher needle percentages in trees receiving the gas were recorded even during years when forest soils were driest — when both treated and untreated trees suffered dryness-related needle losses and less leaf growth.
"This would imply that, even under drought conditions, there would probably be an enhancement with elevated CO2," McCarthy said in an interview.
Her analysis singled out the last six years because "that was after the canopy had closed, meaning that the trees had reached the maximum leaf areas than can be achieved for that forest," she said.
All trees undergo their highest rates of leaf area accretions before canopy closure, and that could lead to uncertainty about whether normal needle growth spurts or CO2 effects were responsible for higher counts, she explained.
"I’m trying to draw the distinction between closed canopy versus non-closed canopy, because when a canopy is not closed you’re mixing several issues," she said.
She also noted that experiments with other non-conifer broad-leaf species have made some scientists conclude that CO2-treated trees would not retain higher leaf counts after their canopies close. "These results are disputing that conclusion somewhat," she said.
McCarthy’s findings showed that the factor most affecting needle volumes was the amount of nitrogen present in the soils.
Since much of the forested area was once overused farmland, local soils tend to be nitrogen deficient. Her results showed that needle enhancements in CO2-treated trees were insignificant when soil nitrogen was low, but increased with the nitrogen levels. She could gauge the effects of nitrogen on needle volumes because one area of the FACE experiment had been fertilized during her study period. "The viability of leaf area enhancement is really driven by the nitrogen availability," McCarthy said.
While nitrogen fertilization enhanced leaf counts in CO2-treated trees, the most heavily fertilized sites conversely suffered the highest needle losses under drought conditions. "They put out a lot of leaf area, but then they get hit very hard under water stress because they have they extra leaf area they are no longer able to support," she said.
Much of the leaf volume information for her study came after she and others weighed basketfuls of needles that had fallen into collection baskets at the FACE site, she said.
While those needles had already dropped from the trees, she could backtrack to the time the needles were produced "by making some assumptions about foliage longevity," she explained.
She and others also measured and counted growing needles and branches by ascending climbable towers at the FACE site.
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