"UVED Presents"
"Drawings by Francis Hallé"
"Trees"
"Biophysical Challenges of Trees"
-Hello, I am going to speak to you about trees,
or, in particular, their specifications.
What must a tree accomplish?
What are the essential functions which allowed trees to evolve as such
and be part of the landscape everywhere around us
in the majority of Earth's ecosystems?
What is a tree?
They are living organisms, that breathe, grow, and reproduce.
They are an adaptation to life on dry ground.
The first trees appeared in the mid-Devonian period,
more than 390 million years ago.
Many lines of plants evolved, and converged into the tree form.
Most of these are extinct.
One line led to the wide shape diversity, including the trees we see today.
Tree form is due to evolutionary convergence.
This means species which are not related being morphologically similar.
They converged into comparable solutions to life requirements,
which caused the tree form to unify.
A tree can be recognized by its shape, no matter its species.
One of the drivers behind tree evolution is competition,
such as for light, essential for photosynthesis.
To grow tall as part of this competition for light,
a source of energy for the plant to survive,
trees have a central support structure.
This is the trunk, which rises from the ground.
A certain distance above the ground, branches spread from the trunk,
to capture the most sunlight possible for the tree and species to survive,
and to facilitate the dispersion and fertilization of seeds.
Trees grow upwards, from their meristems,
which are growth points found in the tree's crown.
A tree is therefore both a large structure,
a very large structure, even,
which must support the weight of the foliage and branches.
It must also direct water and minerals, essential for photosynthesis,
towards the foliage.
This can sometimes be up to 100 meters tall.
Trees must also build on themselves, and continue to function while doing so.
Even though trees prune themselves naturally,
to replace aging structures with new ones,
in the trunk,
wood accumulates in the tree's center, as functional wood develops around it.
All of this suggests new, biophysical limits,
imposed by their environment.
The tree's specifications are therefore complex and demanding.
It would be a big challenge for an architect or engineer
to re-create such a structure.
We can list out these challenges.
The first is the acquisition of and search for water.
This requires being set in the ground to draw water from the soil.
So, the result is that trees are in fixed positions.
Next, there is the search for light, resulting in height competition.
By building its trunk, the tree faces up to a significant force, gravity,
as well as major disturbances like wind.
Trees must therefore always be stable during their construction,
requiring what is called a gravitropic response,
a response to gravity.
It must also be functional to assure sap distribution.
Raising a massive column over tens of meters
is a physical challenge,
especially without lateral reinforcement or scaffolding during construction.
This implies constant load correction, or gravitropism,
to mechanically compensate for biomass accumulation,
and not collapse under its own weight.
Here, we see the reaction wood, which forms on the tree's periphery
and is part of its functional wood.
It is produced by the cambium, and is a sort of muscle
letting the tree correct its orientation,
as well as recover and reinforce itself.
The tree must also size itself,
by adjusting its diameter depending on its height,
as well as its weight.
Some species do not have this strategy, they are, however, still trees,
and are supported by surrounding plants,
not needing to adjust their dimensions or mechanical properties while growing.
These trees, if removed from their environment,
collapse under their own weight.
So, excluding the support of its own biomass,
trees also have a resistance to environmental stresses,
such as abiotic stress: wind, the weight of snow,
or the weight of epiphytic plants which grow on the trees.
These are substantial volumes and masses for trees and their branches.
We know today that wind is a very important stimulus
in tree diameter growth.
Trees are constantly acclimating to wind,
regulating their resource allocation towards increasing diameter.
Another significant challenge is transporting water
from the ground to the canopy.
For example, to transport it over 10 meters
is not a trivial thing.
It is physically impossible to pump water past 10 meters,
without an air bubble or a vacuum forming in the system
and halting transport.
Water circulates in trees under pressure,
making many physicists dubious.
Water under pressure is subject to the effect of cavitation.
Trees have got around this problem by segmenting their transport systems,
using what are called vessels or tracheids.
These are elements of the wood which deal with water transport,
and are a major plant innovation.
The vessels are linked by pits or micropores,
which let water through, but not air bubbles.
This is an ingenious system
which trees put in place during their evolution.
The biophysical challenges overcome by trees
always have and always will fascinate us.
However, their properties evolved over millions of years,
and are due to adapting their structures and functions to the environment.
They are therefore sensitive to sudden changes to their environment,
and especially to their water resources,
as well as extreme events like storms or prolonged droughts.
So, to conclude, although the specifications of trees
can be summed up in a few basic points common to all trees,
we are currently witnessing an explosion of different kinds of strategy
and an extraordinary diversity reflecting diverse life conditions
and evolutionary trajectories.
This diversity is essential to the survival of trees
and their ability to change and to endure in a changing landscape.