Like herding scientists: one man and many many trees

A recent study by 143 international scientists revealed statistics that were surprising – mainly because they had never been collated before.

Who knew that we have never been able to count tropical tree species properly?

Who knew there were between 43,000 and 50,000 tropical tree species?

And who knew there were that many scientists eager to count them?

Coordinator and lead author of the report is scientist Ferry Slik, a Center for International Forestry Research (CIFOR) associate, based at Universiti Brunei Darussalam. He did this interview with Forests News.

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Why do we need this global count?

I was quite shocked when I initially started thinking about this study; I went through the existing literature: no one knew how many tropical tree species there were.

That’s amazing considering how many people work in the forestry sector who deal with these forests every day without really knowing what is out there.

How can you have good conservation practices and sustainable forest use if you don’t even know what is there, and what is being lost when cleared?

So there was obviously a real need for more information.

Also, since almost all tree species live in the tropics, an estimate of the number of tropical tree species immediately gives you a good idea of the number of tree species globally.

There’s a list of scientific contributors as long as your arm. How did you gather the data?

That was a major effort because, obviously, not everybody will just give you their data right away, considering how much energy, time and money it takes to do good forest inventories.

I started collecting data around the year 2000, so that’s about 15 years ago! First just to study the trees of Borneo, but it gradually expanded to Asia and then to the tropics as a whole.

Slowly we built up a track record of good studies in good journals, so more and more people became interested. Also, I think it is extremely important that all the data contributors were also involved in the writing of the paper itself and become co-authors.

I guess the journals are getting a bit crazy about our studies now with an author list that can cover the whole first page of the paper!

Many people don’t really realize what an enormous effort it is to do these forest inventories. Conditions are hard. You work in a humid, warm and rainy climate that tends to break all your electronic equipment – not to speak what it may do to your clothes and skin.

Often, these sites are far away from any roads so that you have to camp in the forest. You have to arrange logistics for supplies, find smart and motivated people to help with the fieldwork.

Things often don’t go as expected so it always involves some crisis management. It may sound all very adventurous, but in reality it is also just plain hard work.

Southeast Asia seems to have yielded the most interesting results – it competes with Latin America for the most tree species (both have between 19,000 and 25,000 species). Was that result surprising?

Personally, I was not surprised. I actually expected Asia to have higher diversity, given its very complex topography, geology and biogeographic history…. Africa and America basically represent huge flat areas with forests that have been connected most of the time.

Asia, on the other hand, is all mountainous (providing many opportunities for isolated populations to form new species), and is a mix of floras because different plates have collided (India, Southeast Asia, Austral-Asia, and probably even remnants of the now extinct tropical northern hemisphere forests).

READ 

• The research: An estimate of the number of tropical tree species
• A blog: Counting trees: 143 scientists just did

I think these ideas mainly developed because for many animal groups, like birds, America is clearly more diverse. Also, the Americas have the largest extent of tropical forest and more area often equates to more species.

Anyway, we have now shown that both areas are at least equally diverse.

The statistic from Africa sounds alarming (between 4,500 and 6,000 species). Is it right to be concerned or how do you explain the relatively small number of species?

Well, it’s all a question of comparison.

Africa has a low number of species compared to Asia and America, but a huge number of species when compared with any extra-tropical region – Europe for example has only 124 temperate tree species versus a minimum of 4000-6000 tropical tree species in Africa!

Also, the African tropical region is the smallest of the three and so you would expect lower tree diversity there. However, even when taking that into account, Africa has fewer species than you would expect given its current forest size.

What has very likely happened is that during the last 5 million years, when the climate cooled and became drier – especially during the Ice Ages – is that the savannas expanded at the expense of the forest, leading to forest loss and fragmentation that was much more extreme than in any of the other tropical regions.

Forest fragmentation and shrinking of forest area leads to species extinctions in the long run, so after 5 million years of repeated cycles of forest shrinkage and expansion, Africa just lost many more species than the Americas and Indo-Pacific region.

In that light there is an important lesson to learn for what humans are now doing because we have also fragmented and cleared many tropical forests, but this time at a global scale! If this lasts too long the whole tropics will follow Africa’s example …

How can this new information inform existing conservation efforts – particularly REDD+?

This study is important for the “plus” part of REDD+ as it gives first estimates of the tree diversity in tropical forests.

Now it becomes important to find how this diversity is linked to carbon content of forests. Results of this are contradictory. Some studies show a clear link between tree diversity and carbon storage, but many others don’t see this link at all. I personally think that the link is very weak in the tropics, if it exists at all.

On the other hand, almost all tropical forests store a lot of carbon, especially those in Asia and Africa. So a REDD+ strategy that may work is to first map out tree diversity patterns across the tropics, combine this with a carbon storage map for the same area, and then make a combined map that indicates the areas that have to be protected to give the optimum value for both carbon and diversity conservation.I don’t really see how diversity equates to high carbon content. For high carbon content it is more important to have large trees or trees with heavy wood. These are not always the forests that are the most diverse. Another example of mismatch are peat-swamps where you find low tree diversity, but extremely high carbon storage due to huge amounts of organic carbon in the soil.

This may sometimes include low-diversity forests because so much carbon is stored there (peat swamps), and other times high-diversity forests with low carbon storage.

What are you going to count next?

Ha, that’s secret, we don’t want to inform the competition! Just joking.

Obviously, the way to go now is to map out the tree diversity patterns more precisely.

This study provided the basic numbers, but did not yet provide a detailed map of how this diversity is distributed within each of the tropical regions. This is actually what we need for the REDD+ question. So that’s where we are going now.

For this new effort we will likely have more than 200 co-authors!

So that’s how many tropical tree species?

You can contact Ferry Slik at ferryslik@hotmail.com