Tropical trees do not grow any faster when there is more CO2 in the air. This surprising conclusion is drawn by Peter van der Sleen in his thesis on the forces driving the growth of tropical trees.
Van der Sleen hereby disputes the popular hypothesis that tropical forests mitigate climate change by absorbing extra CO2 from the air: a process climate scientists dub the CO2 fertilization effect. Carbon dioxide as a kind of fertilizer for vegetation. More CO2 stimulates photosynthesis, causing trees and plants to grow faster. Higher concentrations of CO2 in the air also cause leaves to open their stomata less wide so less water evaporates and the trees go on growing longer in times of relative drought or in the heat of the day.
Van der Sleen more or less demolishes the hypothesis of the fertilization effect of carbon dioxide. He correlated the data on growth rings on 1100 trees in Cameroon, Bolivia and Thailand with the figures for C-13 isotopes in those growth rings. Growth rings are a tree’s historical archive, indicating exactly how old a tree is. The presence in the wood of C-13, a stable and heavier form of standard carbon, is a result of processes in the leaf. Van der Sleen used C-13 to calculate the efficiency of water use, or: how much photosynthesis took place per unit of water consumed by the tree.
This efficiency turned out to increase enormously with the increase in CO2 in the air. In the last 150 years the trees in the study have become 35 percent more efficient in their water consumption. But that increased efficiency was not reflected in the width of the growth rings. It appeared that the tropical trees had not grown in the long term. ‘So I have not found any evidence of CO2 fertilization. The question is of course, how is that possible? I reckon the most likely explanation is that the growth of these tropical trees is not limited by the availability of carbon dioxide or water, but by nutrients in the soil such as phosphate.’
The lack of a growth response to the increase in CO2 in the atmosphere has far-reaching implications. The absorption of carbon by tropical forests is probably much lower than has been assumed, says Van der Sleen.
Climate models assume a strong CO2 fertilization effect. Tropical forests are therefore seen as carbon sinks: they extract carbon from the air, forming a buffer against climate change. Van der Sleen’s work undermines this theory. Supervisor Pieter Zuidema backs his student’s findings. ‘This research was done in three different countries. But the results are consistent. I would bet my bottom dollar that you will find the same in other places too.’ He does add, however, that it is tricky to translate this result to the growth of forests and the changes to biomass over time. ‘Whether a forest increases in biomass depends on the rates of new growth and death, which are influenced by many different factors. So we have to bear that in mind. But we have not found any evidence of CO2 fertilization.’