Prof. Susan Laurance from James Cook University and her research postdoc Dr. Yoko Ishida has set up a rainfall exclusion experiment at the Daintree Rainforest Observatory, where some of Australia’s premium lowland tropical rainforest still predominate. As one climate change scenario predicts an extended dry season, it is important to understand how tropical rainforest trees will adapt to such conditions.
Prof. Laurance’s cleverly designed project builds on the pre-existing infrastructure – a one-hectare plot with a canopy crane towering over the rainforest canopy. These allows researchers to monitor trees both from the bottom and from the canopy. Little devices known as sap flow meters have been attached onto the eight trees species selected for the study. These trees include a palm, two pioneer trees and five mature-phase rainforest trees. “Shelters” have been erected in the understorey of roughly half of this plot, with the intention to channel rainfall away from the ground.
In the past year I have been helping a PhD student Deborah Apgaua with an axillary study on these trees. Deborah measured baseline data on the leaf and wood traits of these species with the aim to determine how these feature relate to their water transport strategies.
One aspect of this study involved looking at the vessels inside the wood. Big vessels would logically mean more water can be transported, and faster. This was certainly true to some degree, but looking more closely at other wood features revealed a whole host of other strategies that trees might use to regulate their water relations.
For instance, the number of vessels in a given area might also play an important role in the rate and amount of water a tree’s stem can move. One of the mature-phase species Syzygium graveolens, had a large number of vessels in a given area. More vessels might be an insurance policy that could help plants these trees cope when water becomes limiting. When the transpiration demand in the canopy becomes too large relative to the amount of water available in the soil. It is likely that some vessels will become embolized (an air bubble will form inside). Having many vessels might help with insuring that at least some vessels will still be functioning.
Also, there are other tissues inside the wood that could give plants an edge when conditions are limiting. We found that the wood of the Black Bean tree (Castanospermum australe) for instance, had a lot of parenchyma tissue surrounding the vessels. Parenchyma tissue is the basic undifferentiated plant tissue that is found in the stems, leaves and roots of plants. These tissues can hold water in the cells, and it is possible that these tissues could be a water-storage facility for Black Bean trees.
The single palm in the study, the endemic Black Palm (Normanbya normanbyi) had very huge vessels and widely spaced vessels. Big vessels demand lots of water. Given it’s restricted distribution to some of the wettest forests in Australia, this could mean that the species is rather vulnerable to dry conditions.
The story becomes all the more complex when we consider leaf traits as well. Leaves are the gatekeepers of a plants’ water. The dimensions and chemistry of leaves can tell us a lot about how leaves manage their water budget. Trying to relate this to the function of wood vessels required the use of complicated analysis techniques that can look at data multidimensionally. Not to mention that individuals within a species can exhibit a lot of variability in their leaf and wood traits. In general however, it can be concluded that the study species exhibited different strategies to do the job of bringing water to their canopies.
All said and done, there is more to say and to do. Deborah has published her results in PLoS One, and is perhaps one of the first ecological studies in a field setting that attempts to relate sap flow with leaf and wood traits. However, we are still far from getting a full picture as to how the variability if these traits in plants will help them cope with environmental stresses.
This dry season, half of the rainforest plot will receive much less rain than it normally would, thanks to the rainfall-excluding “shelters” that Prof. Laurance has installed. How the rainforest trees will respond, I am waiting to see.