That was my first thought too. Some years ago I hiked 40 acres with a forester for 4 1/2 hours. My husband and I were trying to decide whether to buy out his brother’s half, or sell our 20 too. He showed us marketable trees, and “useless” trees, and how to estimate amount of usable areas within a marketable tree. I think he said they use waist height to determine diameter or circumference of a tree. There were many small trees with trunks 3 to 5 inches in diameter. He showed us how to select the best tree in a cluster, and then the 10 to 20 trees surrounding it that should be cut to allow the chosen one to grow into a good timber tree. If we had bought we would have wanted to sell some mature trees, and he gave us the name of a reputable guy who would remove trees carefully without clearcutting. Then we went to a lumber yard to find out what prices were being paid for various species of trees on this land. We ended up doing an IRS 1031 “Starker” exchange for 12 acres a lot closer to home.
So to answer the question, I am sure that each of the areas of state, national, etc. forest that were used in preparing these estimates had their own standards of what size tree counted as a tree, and without that information, it is hard to say what is meant by a tree, versus a sapling. Also, with aerial counting, there is no way to know what diameter of foliage was considered a tree. I strongly suspect that a forest of much taller trees produces more CO2 than does one composed of second growth saplings or small trees. As any farmer can tell you weeding allows much better production of the main crop. The other issue about mature forests or jungles like the Amazon is that there is often more diversity of plants and animals. One of the arguments on the Amazon is that they have a huge number of species many of which have not been identified. A number probably have medicinal properties which we may never learn about if the Amazon is largely destroyed.
The US forests were no doubt more dense and extensive than they are now before Europeans came. There are descriptions of extensive forests in the East with huge trees 6 and more feet in diameter, and relative clear forest floors. This would be one result of natural burning. The Shenandoah Valley was kept burnt off by the Indians and was used as a Buffalo common prairie where the various tribes suspended warfare for the hunt. Another Buffalo prairie was north of the Green River near Mammoth Cave. There were probably fewer trees, but much larger ones covering more total acres.
“. There are descriptions of extensive forests in the East with huge trees 6 and more feet in diameter,”
Those large trees were American Chestnut trees and they were still very common as late as 1900, after which the chestnut blight eliminated virtually every one of them.
Here in The South, we have ‘Tree Farms’ of pine trees used for pulp wood. They are planted in rows just like any other crops and are way denser than the normal forest.
They are genetically selected trees provided by the paper companies, for fast growth and straightness of trunks.
My brother-in-law had a summer job back in 1977 when he was 16, planting these types of trees on recently cleared swampy forest land.
Those trees he planted have grown, been harvested and new trees planted in their place and are ready to be harvested again.........................