What was done
The authors used the Dynamic Land Ecosystem Model (DLEM) to estimate carbon (C) storage and analyze the impacts of environmental changes on C dynamics from 1971 to 2001 in the USA's Great Smoky Mountains National Park (GRSM). This model, in the words of Zhang et al., "couples biophysical processes (energy balance), biogeochemical processes (water cycles, carbon cycles, nitrogen cycles, and trace gas-related processes), community dynamics (plant distribution and succession), and disturbances (land conversion, agriculture management, forest management, and other disturbances such as fire, pests, etc.) into one integral model system."

What was learned
Noting that ozone data showed the annual average AOT40 (the accumulated ozone dose over a threshold of 40 ppb during daylight hours) to have increased from 1418 ppb-h in 1971 to 3194 ppb-h per 30 days in 2001, the ten researchers report that "ozone stress could have reduced the C sequestration rate by 50% (about 0.9 Tg) in GRSM," and that "the combination effects of climate and ozone result in a 1.4 Tg loss of C from 1971 to 2001." However, they found that "the positive CO2 fertilization effect on C sequestration dominated [our italics] the ozone stress and the negative effect of climate stress," such that there was a net accumulation of 0.9 Tg C in the GRSM ecosystem over the three-decade study period.

What it means
When changes in climate and atmospheric ozone concentration are working together to degrade an ecosystem, it is heartening to know that the ongoing rise in the air's CO2 content is working for the ecosystem, and that the steadily rising CO2 concentration is totally overpowering the adverse effects of the contrary factors.

Nice work, CO2!