I have often wondered how many tree species there are in the world. You would think that in this modern age of technological advances and super computers it would not be too difficult to have an accurate count of how many kinds of trees exist at the present time. I say present time because many tree species have in fact gone into extinction already.
From what I can tell nobody knows for sure how many there are. This is partly due to the fact that there is not a universally accepted definition of what a "tree" is exactly. I tend to think of trees as big plants with roots, trunks and leaves but I guess this is too loose of a definition.
Some big plants that are NOT generally accepted as trees are Banana "trees" and Giant Bamboo.
We generally think of trees having just one main trunk. As a matter of fact this is usually a part of the definition of what a tree is. Well, how about the Banyan "trees" in India that have hundreds or even thousands of trunks? Did you know that there is a Banyan tree in Calcutta that has 2880 "trunks"! Actually they are "prop-roots" but in the case of this tree the main trunk was removed in 1925 but the tree lives on and now spreads out over an area that is 420 meters in diameter! It has almost three thousand trunks but no main trunk so is it a tree?
Anyway, back to the question of how many tree species there are in the world. I have read estimates that go all the way up to 100,000 but the number that looks most believable to me is 10,000. I got this number in a article by James E. Reeb entitled "Scientific Classification of Trees: An Introduction for Wood Workers".
Another estimate that sounded interesting was to take the total number of known plant species and then make an estimate of what percentage of these are trees based on smaller sample areas. Using this method one person that I read had estimated that there are 25,000 tree species (10% of the 250,000 plant species).
The number of tree species in North America is estimated at around 1,000 (also from the article by James E. Reeb). Europe is one of the continents with the least number of tree species.