In general Rainforest Biomass studies try to determine the density, the character and the mass of the aboveground flora. It is useful for many reasons, partially because the levels of carbon in the biomass can be estimated. Thus the carbon sink of the rainforest, and the flora can be quantified. That can lead to studies on air quality, burning rates, CO2 emmisions etc. The understanding of this crucial aspect may be a sufficiently adequate proxy for many other studies.
 

Rainforest assay1
 

And the second with a bit more depth, more tables and a specific interest on palms.