Alan,
From the article...
In addition, we calculated a ratio of the current vs. future modeled biomass in a geographic context, which summarizes the extent of current biomass approaching the future biomass under the best-case scenario (no disturbance). As an example, this ratio is 0.780 ± 0.438 for the current vs. the 2080s RCP8.5 future (Fig. 4), that is, the current forest biomass is on average 78% relative to the future biomass for the 2080s under the RCP8.5 best-case scenario. Because of the no-disturbance assumption, the actual future biomass is likely to be lower, and the actual ratio is likely to be higher, indicating that the biomass is likely to be even more saturated than 78%. In other words, under the unlikely best circumstances of no disturbances, North American forest carbon will only increase at most 22% over the current level to the 2080s