Background
Ice storms, which in the southeastern United States have an average return time of six years, can have huge effects on carbon sequestration. Most immediately, their breaking of branches and felling of whole trees stops the primary production of affected trees in its tracks. Thereafter, "reduced post-storm productivity of damaged trees, which may last upward of ten years following severe ice storms," in the words of McCarthy et al., "decreases future carbon sequestration in woody biomass." Last of all, they report that "ice damage to upper crowns decreases the reproductive capacity of species that rely on wind for seed dispersal and therefore produce seeds exclusively in the upper crown," which can limit the range expansion capacities of such trees and the creation of new forests.

What was done
The authors determined that the non-intensively managed pine plantation that is home to the Duke Forest FACE study experienced an approximate 254 g C m^-2 reduction in living biomass carbon during the single severe ice storm that affected much of the southeastern United States between 4 and 5 December of 2002; and drawing on weather and damage survey data from the entire storm cell, they calculated that the amount of carbon that was transferred from the living to the dead biomass pool during that event was equivalent to about 10% of the annual carbon sequestration of all forests in the conterminous United States. Taking advantage of this unique situation, they also compared how the ambient-air and CO2-enriched loblolly pine trees stood up to the devastating effects of the storm.

What was learned
After carefully tabulating all of the damage done to the trees in their experimental plots, McCarthy et al. found that the loblolly pine trees growing in ambient air experienced an ice-storm-induced live biomass carbon reduction of 254 g C m^-2, while those growing in air enriched with an extra 200 ppm of CO2 (about a 50% increase above the ambient concentration) suffered a live biomass carbon reduction of only 80 g C m^-2, which loss was about 70% less than the loss experienced by the trees growing in ambient air. What is more, they found that because of the lesser leaf area reduction caused by the storm in the CO2-enriched plots, the trees in those plots "also exhibited a smaller reduction in biomass production the following year."

What it means
In the words of the seven researchers who conducted the analysis, "these results suggest that forests may suffer less damage during each ice storm event of similar severity in a future with higher atmospheric CO2." In addition, they say that "the lessening of crown breakage by ice storms in a future CO2-enriched atmosphere may allow loblolly pine to expand its range northerly."