There seems to be some interesting sourcing in OPs post. The 15bn felled trees per year actually comes from the 3tn paper (it's in the abstract). So does the 46% decline since the start of civilisation:
> Based on our projected tree densities, we estimate that over 15 billion trees are cut down each year, and the global number of trees has fallen by approximately 46% since the start of human civilization.
So I'm not sure why the figure of 400bn was used! The paper points out that a recent Amazon study estimated the number of trees in the rainforest to be ~390bn alone. From the paper, the number of trees in the US is probably in the low hundreds of billions.
The scientists that wrote the paper specifically said (in an interview I think) that you shouldn't interpret this as that we've magically discovered 2.6tn more trees. It's not like this changes our view on carbon sinks.
The difference is that instead of taking satellite images and approximating how many trees are in a particular area, they went out and measured typical numbers of stems within an area and (I would guess) correlated that to satellite data. Some trees are more densely packed than others and the distribution of numbers of stems varies wildly with biome. There's so much variation in what constitutes a "tree" that it makes little sense to use the numbers of trees cut down for anything climate related except snappy headlines. Clearly more trees cut down = bad, but you shouldn't relate an abstract, ideal, "tree" to e.g. one ton of carbon dioxide.
A better metric might be area of forest or the amount of biomass. That smooths out variations in species and gives a nice number.
> Based on our projected tree densities, we estimate that over 15 billion trees are cut down each year, and the global number of trees has fallen by approximately 46% since the start of human civilization.
So I'm not sure why the figure of 400bn was used! The paper points out that a recent Amazon study estimated the number of trees in the rainforest to be ~390bn alone. From the paper, the number of trees in the US is probably in the low hundreds of billions.
The scientists that wrote the paper specifically said (in an interview I think) that you shouldn't interpret this as that we've magically discovered 2.6tn more trees. It's not like this changes our view on carbon sinks.
The difference is that instead of taking satellite images and approximating how many trees are in a particular area, they went out and measured typical numbers of stems within an area and (I would guess) correlated that to satellite data. Some trees are more densely packed than others and the distribution of numbers of stems varies wildly with biome. There's so much variation in what constitutes a "tree" that it makes little sense to use the numbers of trees cut down for anything climate related except snappy headlines. Clearly more trees cut down = bad, but you shouldn't relate an abstract, ideal, "tree" to e.g. one ton of carbon dioxide.
A better metric might be area of forest or the amount of biomass. That smooths out variations in species and gives a nice number.