Written by Dr. David Tng
Mature rainforest trees in the tropics play an unprecedented role in the water balance of the planet. Within the trunk of every tree is a sophisticated plumbing (hydraulic) system that acts like a fountain, drawing water up from the soil and putting it out into the atmosphere through the process of transpiration. A single large mature tree may put out well over 100,000 litres of water into the atmosphere a year.
It is easy to understand therefore how rainforest trees are important for maintaining local climate and watersheds, and by extension, the importance of this ecosystem service to agricultural systems.
However, some worrisome trends have been forecasted for the coming decades, where tropical rainforests in many parts of the world are likely to face more severe or frequent droughts. This is foreboding for rainforest trees that are adapted to a regime of high rainfall.
Rightly so, ecologists and conservation scientists are concerned about what climate change may herald for tropical rainforest trees and forests.
Research on how drought affects plants is not new, but most such studies are limited to glasshouse experiments using seedlings or saplings, or to trees in plantations.
How does one go about studying changes in fully developed mature trees in a tropical forest? For Professor Susan Laurance at the James Cook University, the answer was an elegant one.
Set up an “umbrella” in the rainforest
In 2015, Professor Susan Laurance established the Daintree drought experiment within a 1-hectare forest monitoring plot in the lowland tropics of northeast Australia.
Using an infrastructure of plastic sheets and metal troughs in the rainforest understorey, Prof. Laurance succeeded in reducing the rainfall that gets into half hectare of rainforest, to artificially create a drier dry season. The other half a hectare obtains the normal amount of rain.
Some drought experiments in the Brazilian Amazonian tropics have used similar setups, and there are also other smaller setups in Asia that had setups to exclude rainfall from individual trees. However, the Daintree drought experiment is the first which includes a canopy crane in the centre of the plot, allowing easy access to plant material 30m up in the forest canopy.
The purpose of the experiment is not to kill the trees, but to gain a better understanding of how mature trees respond to environmental stress. Specifically, we wanted to know whether there would be short-term changes in their hydraulic system when exposed drought in the field for two consecutive years.
Most work on plant responses to drought focus on making physiological measurements, but we chose an emphasis on wood anatomy, because stem vessels and associated tissues form the framework of the plumbing system in trees, and also because this aspect of drought response in plants has not been extensively studied. In previous work, we had demonstrated how wood anatomy can reveal the strategies trees use to conduct water, and so we hypothesized that different rainforest trees would respond differently to drought.
Using the canopy crane, we sampled tree branches from four species of trees which we could find individuals of in both the drought-affected and non-drought affected areas of the forest, and we made anatomical examinations from the cut ends of these branches. We also examined some leaf features, such as leaf thickness and leaf water potentials, which would give us an indication of how dehydrated leaves are.
The inner transformation in trees
The baseline result is that we found a that the trees exposed to drought were undergoing an internal transformation in their wood anatomy, consistent having less water to use, and we published these results in a recent volume of Ecology and Evolution.
As we expected also, not all the species were changing in their wood anatomy in the same way.
Some species showed a shift in their vessel sizes towards having smaller vessels. Others species showed a shrinkage in ground tissue (parenchyma tissues) in their wood, likely reflecting the use of water stores. And one of the species even started to show blockages in their water conducting vessels.
We also found thinner leaves in most of our drought-affected trees, and our physiological measures on these leaves confirmed their dehydrated status.
Although there is no doubt that trees will die if droughts are severe or prolonged, what we can conclude from our study is that mature rainforest trees can modify their anatomy to some degree to acclimatize to drought. Also, the changes in the hydraulic properties of these trees would certainly reduce the ability of trees to transport water, and hence are put out less water into the atmosphere. We can speculate therefore that prolonged drought will ultimately lead to the degradation of the forest.
Knowing the limits on how trees of different species in species-rich tropical forests can acclimatize to droughts is an important avenue for future research, and we look forward to deeper insights on these issues with the ongoing Daintree drought experiment. We believe also that extending drought experiments to other tropical forest ecosystems such as swamp forests, mangroves, and mountain forests would be a priority for future research.
Interested in learning more about the Daintree drought experiment follow this link to watch a short video.