Are tropical trees really growing faster as a result of climate change? Wageningen is looking for the answer to this question by counting tree rings. A look behind the scenes at the Dendrolab.
Tropical forests are gigantic carbon traps. Trees capture carbon dioxide from the atmosphere in biomass: one cubic metre of wood contains about one tonne of CO2. One quarter of the carbon stocks on land are stored in tropical forest. But do tropical forests grow faster when there is more CO2 in the atmosphere?
Scientists think they do. In recent decades the biomass in tropical forests has increased, according to measurements of the circumference of trees. If this is the case, tropical forest will temper the effects of climate change. What is not clear is whether this has already been going on for centuries. Has biomass been increasing since the start of the industrial revolution?
To find an answer to that question you need to go back in time to a couple of hundred years ago. To do this, Pieter Zuidema makes use of a method that is little used in the tropics. 'A cross section of a tree is a kind of database that can give you a lot of information', says Zuidema. 'Only it does take quite an effort.' So do tropical trees have annual growth rings, then? They certainly do, says Zuidema, putting paid to a common misconception. 'Textbooks have always claimed that tropical trees do not have clear growth rings. After all, there are no clear seasons in the tropics, so for a long time people thought counting growth rings was difficult and would not be applicable. But in a number of species you do see very clear rings.'
Zuidema and his group - three PhD researchers and one postdoc - use these species to get a picture of the driving forces behind the growth of tropical trees. Groenendijk and his colleagues flew out to Bolivia, Cameroon and Thailand to collect thousands of samples. Cross sections of felled trees and core samples of living trees. Lovely work, but heavy going, says Groenendijk. The drilling is done with a handheld drill, so it can take a while to get through to the core of a giant forest tree. 'After the first few days your arms are worn out.'
Groenendijk spent seven months, spread over several periods, drilling into trees in a forestry concession in Cameroon. In Thailand and Bolivia the project piggybacked with commercial felling teams - which delivered slices of tree trunk with diameters almost equivalent to a man's height.
Back in time
Groenendijk now spends his days in the Dendrolab. Counting growth rings (see box) is painstaking work, he admits. 'Not really fun. But it is the sort of spade work that you get in almost every scientific field. It does get pretty tedious.' It is rewarding, however. 'Because it is nice to be able to go back 200 years like this. It is incredible, really. If you wanted to get these data using ring measurements you would have had to send Dr Livingstone off with a tape measure.'
But even when you've got your growth ring measurement, you are not there yet. The rings only tell part of the story. There is a lot more information to be gained from a cross section of a tree, says Zuidema. As an example, isotope research is going on with the aim of gauging the effect of drought on growth. To avoid losing moisture during dry seasons, the stoma in the leaves close faster and breathe less often, explains Zuidema. This means the plant goes longer on the same air, which means that more 13C (an isotope of carbon) is captured than under wet conditions. And that can be measured. Zuidema: 'This enables you to see what sort of growth conditions the tree has been subjected to. You see a similar thing with 180, the heavy isotope of oxygen. The 180 in the plant correlates with the 180 in rain water that is absorbed through the roots. If there is little rainfall, the proportion of heavy oxygen 180 in the plant is higher. So using isotope research on growth rings we can reconstruct rainfall patterns over long periods of time. If the frequency of drought periods goes down due to climate change, for example, that can cause changes in the growth in that period.'
And so Zuidema and his group are gradually putting pieces of the puzzle together. It is too early for answers at this stage. But early indications suggest that some species have indeed been growing at a faster rate over the past 150 years, says Zuidema. The question is, is this because of increased CO2 in the air? Or is it due to the warming up of the earth, the rainfall or the availability of nutrients? That remains to be seen. Zuidema calls it detective work. 'And I'm afraid there is no smoking gun to give us a clue. It will always come down to a combination of factors. And we have so little understanding of how trees react to changing conditions. It is difficult to trace patterns because so many different factors determine growth. In that sense it is a battle with variation.'
In 2012 counting growth rings is done on the computer using a special programme (WinDendro) and a good scanner. It looks quite funny to see a cross section of a tree or a core sample on a scanner. At the press of the button you have a high resolution (1600 dpi, 200-300 MB) cross section of a tree on your screen. Or part of it at least. Scanning the entire disk would take up too much memory. The high resolution makes it possible to magnify the wood forty times. Then the counting begins. The oldest tree Peter Groenendijk has had under his scanner dated back to 1750. That means 260 rings, times the four angles from which the rings are counted. But a tree as old as that is an exception. Samples have been taken from 1500 trees representing 15 species and three countries. From most of the trees, core samples were taken rather than cross sections. The advantage of this is that the tree does not die. Before the slice of tree is put under the scanner, it has to be sanded down. For the do-it-yourselfers among us, this is done with grit size 600. 'If you do it really well, it is like a mirror. That is necessary because those growth rings are very fine.'