What was done
Data on tree basal area - which is a well-substantiated surrogate measure of total biomass in tropical forests - were obtained for the period 1958 to 1996 for several hundred plots of mature tropical trees scattered about the world.  These data were then used to calculate total tree biomass changes over this period for several geographical regions of the planet.

What was learned
Average forest biomass for the tropics as a whole increased substantially over the period of record.  In fact, in the Neotropics, the increase amounted to approximately 40% of the missing terrestrial carbon sink of the entire globe.

What it means
In the words of the authors, their results suggest that "intact forests may be helping to buffer the rate of increase in atmospheric CO2, thereby reducing the impacts of global climate change."  They also identify the aerial fertilization effect of the ongoing rise in the air's CO2 content as one of the factors that may be responsible for this phenomenon.

Update
According to Clark (2002), methodological artifacts affected the analysis of Phillips et al. (1998); and her reanalysis of the data on which their study was based suggests that when analyzed properly, the data "do not indicate a significant biomass carbon sink in old-growth forests of the humid Neotropics."

Reference
Clark, D.A.  2002.  Are tropical forests an important carbon sink? Reanalysis of the long-term plot data.  Ecological Applications 12: 3-7.