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