Work for a forestry organization that collects and disseminates information about the forests of the Northeastern United States. We accomplish this through annual inventories of forest land in each state on both public and private land. This includes developing estimates of trends in forest area, species composition, growth, mortality, and harvesting. We also survey forest land owners for patterns and trends; conduct timber use studies to determine the amount of timber that is made into wood products; and measure a variety of ecological parameters including wildlife habitat, forest biomass, soil conditions, forest fragmentation, and damage due to insects, disease, and other destructive agents.

Thats the complicated answer. The simple answer is that basically we count trees ... lots of them.