From the article:<p>&quot;satellite observations and ground-based ecological work. The former gives an overall view of where forests do and don’t exist on the Earth’s surface. But the latter goes underneath the canopy to determine how many trees exist in a given area in a given type of forest.&quot;<p>It seems that satellites give a good measure of the acreage covered by forests but are not suitable for differentiating individual trees. To do that requires on the ground sampling and as more sampling is done it is being discovered that the actual number of trees differs substantially from previously established estimates.