High Carbon Dioxide Levels Spur Southern Pines To Grow More Needles

Duke graduate student Heather McCarthy will describe results sheobtained from a futuristic open-air experimental forest site at 8 a.m.Eastern Time on Thursday, Aug. 11, 2005 during the 2005 annual meetingof 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 inenvironmental studies at Duke's Nicholas School of the Environment andEarth Sciences, analyzed 10 years of pine needle data collected at theFree-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 exposedto the higher levels of CO2 expected by 2050 due to human activitiessuch as fossil fuel burning. Other stands are left as untreatedcontrols for comparison. The elevated carbon dioxide is delivered fromrings of towers in the open air setting of a Southern forest ecosystem.

McCarthy found that, over the most recent six years of the FACEexperiment, the pines receiving elevated CO2 had on average about 17percent more needles than untreated pines. Higher needle percentages intrees receiving the gas were recorded even during years when forestsoils were driest -- when both treated and untreated trees suffereddryness-related needle losses and less leaf growth.

"This would imply that, even under drought conditions, there wouldprobably be an enhancement with elevated CO2," McCarthy said in aninterview.

Her analysis singled out the last six years because "that was after thecanopy had closed, meaning that the trees had reached the maximum leafareas than can be achieved for that forest," she said.

All trees undergo their highest rates of leaf area accretions beforecanopy closure, and that could lead to uncertainty about whether normalneedle growth spurts or CO2 effects were responsible for higher counts,she explained.

"I'm trying to draw the distinction between closed canopy versusnon-closed canopy, because when a canopy is not closed you're mixingseveral issues," she said.

She also noted that experiments with other non-conifer broad-leafspecies have made some scientists conclude that CO2-treated trees wouldnot retain higher leaf counts after their canopies close. "Theseresults 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 thatneedle enhancements in CO2-treated trees were insignificant when soilnitrogen was low, but increased with the nitrogen levels.She could gauge the effects of nitrogen on needle volumes because onearea of the FACE experiment had been fertilized during her studyperiod. "The viability of leaf area enhancement is really driven by thenitrogen availability," McCarthy said.

While nitrogen fertilization enhanced leaf counts inCO2-treated trees, the most heavily fertilized sites converselysuffered the highest needle losses under drought conditions. "They putout a lot of leaf area, but then they get hit very hard under waterstress because they have they extra leaf area they are no longer ableto support," she said.

Much of the leaf volume information for her study came aftershe and others weighed basketfuls of needles that had fallen intocollection baskets at the FACE site, she said.

While those needles had already dropped from the trees, she couldbacktrack to the time the needles were produced "by making someassumptions about foliage longevity," she explained.

She and others also measured and counted growing needles and branches by ascending climbable towers at the FACE site.