Rhythms written in the wood of tropical trees

Do tree rings mark the beating of the seasons over time, or is it more complicated than that?

The seasons turn, and temperate forests change. With spring the leaves flush out from the trees and fall in the autumn. As spring returns, trees grow another ring to carry sap up to the branches, and as autumn returns the tree returns to dormancy. With the next spring, the leaves and sap return and the cycle continues as an annual rhythm. But is this rhythm like a universal metronome, beating out the seasons, or is it more haphazard?

Tom de Mil and colleagues have been looking closely at the seasons and the rhythm of tropical trees. The specific trees are found in the Congo Basin. This rhythm has consequences for the tree rings, as Tom de Mil explained: “In extratropical (temperate) regions, one ring is mostly one year. In tropical regions, there is an annual drought, and this can give the formation of growth rings as well. Nevertheless, they are less clearly defined, and more challenging and interesting to get a climate signal from. Therefore additional traits (such as water transporting vessels, etc.), measured between the ring boundaries, should be further investigated. But we simply do not know when these trees actually start forming their wood during the season!”

Looking up to the sky
Trees of the Congo. Photo: Tom de Mil.

In the Congo, trees do not follow such predictable rhythms. The team found between-tree differences in green-up of 45 days, with trees flushing before and after the rainy season. There was also a lag between leaves arriving on branches and xylem growing. This lag was around two months, plus or minus three weeks.

“It is important both for measuring climate change in the past and thus to model responses in the future under different global change scenarios,” de Mil. But this isn’t simply about modelling the climate, de Mil also sees the importance for the trees themselves. “Tropical forests are pushed into unprecedented climatic conditions, and we need to know if they will still be able to grow or perform their function in the future.”

The based on de Mil’s PhD research de mil said: “I did my PhD (WoodLab Ghent University) on Congo Basin trees, this is thanks to the AfricaMuseum and the INERA RDCongo, where a team of both Congolese and Belgian researchers do forest ecology research. The Democratic Republic of Congo has the biggest share of the Congo Basin forest, and is still largely understudied.”

“Currently, Congolese researchers are working in UNESCO MAB biosphere reserves of Luki and Yangambi.”

“It has helped me a lot in my understanding of trees. Working in the Congo forest, with its low seasonality and its challenging tree rings, helped me to see how trees grow. Currently, I work at the Laboratory of Tree-Ring Research at the University of Arizona, which is a world-leading institute of tree-ring analysis, and home to the famous analysis of the Bristlecone pines, and my Congo experience somehow helps me to read rings more easily in temperate regions. It all seems much easier now!”

I asked de Mil what he would say to encourage someone else to work in the region. He replied: “Basically, saying that you can work in the second biggest tropical forest in the world, and knowing that only a few researchers are studying trees in the DRC, should be enough as an encouragement! Moreover, there are excellent Congolese research teams, with highly qualified Congolese PhD researchers, that are open to collaboration at any time! Among the socio-economical and political struggles, we should not forget the forest in Congo. Given the enormous carbon stocks that the country currently holds, the services it provides for the people, and the threat of future non-beneficial climate scenarios and largescale logging in this country, all eyes should be on the Democratic Republic of the Congo!”

Research is needed on trees in the African rain forest, as their responses to seasons will impact how much carbon they can sequester. The same reactions will also tell us how resilient the trees are likely to be to rising temperatures.

De Mil said: “Unlike in Amazonia, only a little research on phenology is being done, but many projects are promising such as the COBECORE project, the African Phenology monitoring Network, and I installed several time-lapse cameras and dendrometers in the Congo Basin, both in the UNESCO Biosphere reserves of Luki, and Yangambi, both in the Democratic Republic of Congo.”

“But leaf phenology alone is not enough: we need to know exactly how much and when carbon is sequestered by these trees. Most of the carbon is sequestered in wood, so within the CIFOR FORETS project, colleagues at my department established a “true wood biology/Tree-Ring laboratory” at the heart of the Congo Basin.”

“Moreover the importance of old collections such as herbariums and xylariums, that are currently being digitized in projects such as HerbaXylaRedd, is another major improvement in the field.”

Putting together the mechanisms of how trees sprout leaves and grow rings will open new opportunities for research. De Mil concluded: “Once we have a clear view of rhythms present both in the leaves and in the wood of the trees, we will be able to predict their fate under future climate change. Also, we will be able to see how these trees reacted to the previous climate, centuries before we actually started measuring in the 19th Century with instruments.”

Further reading

De Mil T. Hubau W., Ilondea B.A., Rocha Vargas M.A., Boeckx P., Steppe, K,. van Acker, J., Beeckman, H., & ven den Bulcke, J. (2019). Asynchronous leaf and cambial phenology in a tree species of the Congo Basin requires space–time conversion of wood traits. Annals Of Botany. https://doi.org/10.1093/aob/mcz069