Trees in tropical rainforests play an important role in the water balance of the planet. Every tree is like a fountain, drawing water up from the soil and putting it out into the atnosphere.
Just one large mature tree may transpire well over 100,000 litres of water a year.
Imagine how much water a forest of trees would put out into the atmosphere, and the importance of this in maintaining the local climate.
Yet, tropical rainforests in many part of the world are likely to face more frequent or more severe droughts due to climate change.
What is going happen to rainforest trees?
As part of Professor Susan Laurance´s research team at the James Cook University in Australia and collaborators University of Edinburgh and Imperial College in the UK, we set out to investigate how drought would affect rainforest trees.
But how does one go about studying changes in mature trees in a tropical forest?
The Daintree Drought Experiment
In 2015, Professor Susan Laurance established a large scale field experiment in the wet tropics of Australia to do just that.
In a preexisting 1-hectare rainforest monitoring plot in the dense lowland tropical rainforests of Cape Tribulation, northeast Queensland, Australia, Susan has set up the Daintree Drought experiment.
This setup includes an infrastructure of plastic sheets in the understorey of the rainforest which reduces the rainfall that gets into half hectare of rainforest to artificially create a drier dry season, while the other half a hectare obtains the normal amount of rain.
The purpose of the experiment is not to kill the trees, but to understand how trees are changing in the short term by simulating two years of drought in the field.
Specifically, we wanted to know if trees are changing in their ability to conduct water. In previous work, we showed how wood anatomy can reveal the strategies trees use to conduct water, and so we focused again on wood anatomy.
The wonderful advantage of this plot is also that it has a canopy crane that enables researchers to access the rainforest canopy. The crane allowed us to reach 30m up into the canopy to sample 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 from the cut ends of these branches, we made anatomical sections to examine more closely. We also examined some leaf features, such as leaf thickness, to see if there are any changes.
Inner transformations in trees
Our results have just been published in the journal Ecology and Evolution. Basically, we found that the trees were changing in their wood anatomy, consistent with the the fact that they have less water to use.
Some species developed 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 some of the drought-affected individuals.
We can conclude from our study is that mature rainforest trees can modify their anatomy to some degree as an adaptation to drought.
We do not know yet what is the limits of this plasticity, but the changes in the hydraulic properties of these trees would certainly reduce the ability of trees to transport water, and that would also mean the trees are less able to put out water into the atmosphere.
What we can speculate is that in the long term, if real and prolonged drought occurs, the reduced ability of trees to transport water may lead to local changes hydrological cycles, and ultimately changes in the forest vegetation.
We expect to get more insights on these issues with the ongoing Daintree Drought experiment.
Tng DYP, Apgaua DMG, Ishida YF, Mencuccunu M, Lloyd J, Laurance WF, Laurance SGW (Accepted) Rainforest trees respond to drought by modifying their hydraulic architecture. Ecology and Evolution doi.org/10.1002/ece3.4601
Apgaua DMG, Tng DYP, Cernusak LA, Cheesman AW, Santos RM, Edwards WJ, Laurance SGW (2017) Plant functional groups within a tropical forest exhibit different wood functional anatomy. Functional Ecology 31, 582-591.
Tng DYP, Apgaua DMG, Campbell MJ, Cox CJ, Crayn DM, Ishida FY, Laidlaw MJ, Liddell MJ, Seager M, Laurance SGW (2016) Vegetation and floristics of a lowland tropical rainforest in northeast Australia. Biodiversity Data Journal 4, e7599.
Laurance, S. (2015). A raincoat for a rainforest. Australasian Science, 36, 20–22.