Tropical rainforests have been an obsession for me for as long as I can remember, and in particular how tropical rainforest plants have found solutions to living in a common environment.
Just like people having a diversity of professions to provide different services to their community, we find in a tropical rainforest a mind-numbing number of plant species in different ecological groups.
Trees occupy different strata in the forest. Sun-loving canopy species claim the prime spots in the well-illuminated canopy, while shorter trees make do with the shade of the subcanopy.
Then there are also “sun-fearing” shrubs that have found their place in the deep shade and barely grow taller than 2 meters.
Conspicuously in tropical rainforests also, there are thick-stemmed lianas whose gnarled-twisted stems tangle and literally tie the forest together.
And also, there are more open or marginal environments in tropical rainforests, where a suite of pioneer species reside. These species of trees and shrubs are more able to cope with disturbance and exposure, and are typically hate to be in the shade.
So the question arose as to whether plant from all these different lifeforms will have different ways to ecological strategies to deal with the needs to transport water.
Following some of investigations in plant water transport in tree species in an earlier work, I became smitten with the inner mechanisms of plant, and fascinated how the little pipes or the vascular system within a plant’s stem enable a plant to conduct water. This earlier study had looked at just 8 species of trees, and I just had to see more.
With my co-workers at the James Cook Unversity, we set out to investigate how wood anatomy may reveal the different ecological strategies of rainforest plants, which has just come out in Functional Ecology.
Basing our study at one of Australia’s prime lowland tropical rainforest – the Daintree, We collected stem wood from 90 species of plants (15 species each) from within six ecological groups: mature-phase trees, understorey trees, understorey shrubs, pioneer trees, pioneer shrubs and lianas.
One of the best part of the project was that we got to use the Daintree Rainforest Observatory canopy crane. This construction crane is one of a global network of dedicated cranes used for scientific research work. With the skillful operation of the crane operator Andrew Thompson, we were raised 30-35m above the canopy of the trees where we could collect leaf samples with ease.
We used the leaf material for biochemical analysis, looking at leaf carbon isotope ratios. Basically, this analysis will enable us to get an idea of how well our study plants are photosynthesizing and how efficiently it is using water. Our intention was to correlate plant performance with the anatomical characteristics of plant vessels.
The laboratory work to prepare stem sections for microscope analysis was laborious, but it was all worth it to see the beautiful inner structures of our plant stems.
For the science part of things, we measured vessel features such as vessel sizes, frequencies and their tendency of grouping. These features can be hypothesized to have a direct influence on water transport and plant performance.
And our results show that yes, plant performance (how well our plant ecological groups use water) is related to the size of the vessels. One interesting conclusion we could draw from our results was that vines are very much like pioneer trees in their potential efficiency in transporting water, and also in terms of their plant performance.
Before this, wood anatomy was just something I had studied in my undergraduate days, and memorised terms for, in order to pass examinations. I never imagined that I would one day become something of a plant anatomist. Now, I am coming to appreciate the diversity and the depth of artistry that tropical plants hide beneath their bark.
Wood sections are not just a random mess of holes (vessel cell cross-sections) in a matrix of other fibre and ground tissue (i.e parenchyma cells, which are the most basic cell types which often form ground tissue within plant leaves and stems).
Even though the wood is basically comprised of these simple cells types (vessel, fibre and parenchyma cells), the manner in which these cells are arranged is not.
Different species do indeed have different ways of organising their vessels, fibres and parenchyma.
Sometimes vessel cells grouped together in fours, and in some delightful cases in long chains.
In some species, the parenchyma cells crisscrossed the stem sections, forming an intricate network.
If nature could be described as a form of art, the hidden artistry within the deceptively simple anatomical plan of plant stems must be one of nature’s most sublime of artforms.
And indeed, if anyone ever thought that plant anatomy is boring. It is time to think again.