The trees of the Amazon: how many are there?

Imagine a botanist given the task of identifying trees on 3.5% of the land area of world (equivalent to about 60% of the size of Europe). Furthermore, the botanist is asked to do this in one of the most diverse forest, where access is very difficult – or simply putting, in some of the most uncharted areas of the planet. Of course, the botanist will not count or identify every single tree, but still a vast sampling would be needed to extrapolate meaningful results. A sampling that, to anyone in his right mind, would appear as “mission impossible”.

This “impossible mission” has been accomplished now. Not by one, but still, by a small group of 122 scientist (ter Steege et al. 2013 ) that shared the results of their surveys where they counted and identified almost half a million trees within a bit less than 1,200 ha distributed over an area of about 6 million km2 in the 9 countries that encompasses the Amazon forest. This massive dataset, centralised in the Amazon Tree Diversity Network , was processed and fitted into different models to estimate the total number of trees and species for the whole Amazon.

The results are mind-blogging: the largest tropical forest of the world has around 390 billion trees reaching a diameter of 10 cm, or around 565 trees per hectare. All these trees were estimated to belong to 16,000 different species. More interestingly, about half of these trees belong to just about 230 species – or only 1.4% of all tree species estimated to exist in the Amazon. Although a few species hyper-dominate the landscape, the forest is not homogenous across this vast region because these very common species (apart one) are not present in all types of forest that make up the Amazon. On the other hand, the rare species, 11,000 of them, sum to a mere 0.12% of the total number of trees. In fact, a large portion of these rare species (some 6,000) is even rarer, with estimated populations of just about or below 1,000 plants. Some of these exceptionally rare species have already been collected but need to be formally described (I will talk about this in another post), but most of them are yet be collected, and these would be an even more challenging task. This shows how little we know about one of the most important ecosystems of the world, as trees are the most well-known of all types of plants and by far the most conspicuous. Now, we can only wonder about the diversity of other types of plants, insects, and not to mention all the interactions between all these species.

This is to say how much diversity we stand to lose with the ongoing onslaught of large-scale tropical deforestation. This situation worsened by habitat fragmentation (i.e. breaking up of tracks of uninterrupted forest into smaller bits), that improves human access and increases hunting and logging, which are factors that further accelerate the haemorrhage of biodiversity that occurs in forest fragments. Not to mention climate change, a threat that is upon us with effects that are still mostly unknown.

Therefore, the results of this study imply that preserving the diversity of the Amazon tree forest would be challenging, as most species are rare and localised. In contrast, the fact that the Amazon is now known to be dominated by just a few species, should make the task of further studies such as those investigating ecology or biogeochemistry, including some of the ecosystems services that this forest provides at regional or global levels, less daunting.



1. ter Steege et al 2013 Hyperdominance in the Amazonian tree flora.Science DOI: 10.1126/science.1243092