So do I have to start from the beginning again?
So the good news is, plants also follow thermodynamical rules,
basically.
They follow the same laws as they are there in physics.
This is the good news.
But there's also bad news today for most of you.
I won't provide any real equations today.
So basically it's mostly pictures, examples, and some background, at
least how I see it.
So I think it's also a personal interpretation of this concept of
stress physiology of plants.
First I'd like to start thinking about that everything that we see in
nature, in the ecosystem, in plants, in ourselves, is basically in a
sort of a flow.
So there's always movement.
We are constantly changing.
And so that's what I want to stress here, that we have seasonality.
So there's a seasonal change in every plant, in every ecosystem, that
can be even independent of temperature changes and so on, that's
intrinsic in our bodies.
There's also these circadian rhythms, so day and night rhythms.
Also if we see light or no light, we can also still have these kind of
rhythms that are occurring.
But of course, we also respond to environmental conditions, we respond
to changes in temperature and light and so on.
And when we now have some event that stress occurs, for example for us
or for plants, then we can have this sudden deviation from the normal
flow.
So basically we go into a certain state of decrease, and maybe at some
point we also recuperate from that stress.
So we have a sort of recovery from that stress.
And we might end at the same level as we had before, or we might end a
little bit lower.
And this is also the two lectures I want to give today.
So the first one is mainly on the stress physiology of plants, and the
second lecture is on the recovery of plants.
And together this concept is often called in ecology or in other
disciplines, it's called the stress resilience.
So the resilience to stress is once how do you respond to a stress,
and then how can you recover from that stress.
So looking on stress in general, there are a lot of forms of changes
in the ecosystem and the environment that can cause stress.
For example, disturbance.
Too much stability can also be a stress for little seedlings.
They don't see the light, they need disturbance to grow up to become
big trees.
Light and dark, drought.
But flood on the other side is also a problem for plants.
Too much water they don't like.
The roots don't see oxygen, they need oxygen to respirate.
Also heat and frost, so there's basically some opposites.
There are probably things missing and you might think about other
forms of stress.
But there are also biotic stresses.
Competition between plants, for example.
Insects, pathogens, but also like grazers, cows also.
This can all expose stress on plants.
But because of personal interest, I don't want to talk about all of
them today.
So I really like to focus on two forms of stress.
And these are drought stress, so the water limitation.
How do plants respond to water limitation?
And heat stress, very high temperatures.
So here I have just to give you a short relevance.
Why is it important?
So it's important globally because most plants are really...
or like the whole global plant productivity is limited by water
shortage.
And also high temperatures.
That's one of the main limitations to plant productivity is really
drought.
But we all know with climate change, these weather extremes are
increasing.
And we are probably facing more of these heat and drought events in
the future.
And they can really have large impacts on plants.
But also on how they cycle carbon and water from the biosphere to the
atmosphere.
And we also know that with increasing temperatures, these droughts can
become more severe.
Because it's a double stress on plants.
Water shortage and high temperatures.
And there might be also a consequence that less trees or plants might
survive these forms of stress.
And there might be an increased mortality because of increasing
abiotic stress.
One example that's very famous.
And I thought it's not long ago, but already now it's like 2017.
It's some time ago, like 14 years by now.
So it's the European summer heatwave.
Some of you have been a bit younger when this happened.
So there was this large temperature anomaly that happened between July
and September in Europe.
And this is from this famous paper from CIS that appeared in Nature.
And they found that temperature was really much increased compared to
normal.
And also that precipitation was... oh sorry, I didn't want that.
I don't know why that happened.
And precipitation was also very much decreased then.
And what they looked at in this paper is they looked on...
They have a lot of adequate variant sites.
So they looked on the CO2 and water exchange of these sites that they
have.
Mainly forest sites.
And they found that these exceptionally hot and dry weather periods
caused a large decrease in net primary productivity over Europe in
2003.
And this is now shown for the whole year, basically.
So you found that about 200 grams or something less carbon were taken
up by the ecosystems compared to a more normal condition period.
So we can see, okay, these ecosystems really respond strongly when
something is happening, when there's not enough water, when the
temperature is too hot.
But now I'd like to try to answer the question, why is water shortage
really critical for plants?
Why do they need so much water, basically?
What is it?
What can make them suffer?
And here I want to provide some background.
It's also important for the other lectures later, and I think it will
come more often.
It's about the water transport in a plant.
So basically, water is transported from the root along the xylem.
So the xylem is basically the wood in the plant transported up.
And then at some part, it will be transpired from the leaf.
It happens mainly via the stomata, but can also happen through the
critical.
And this water transport, basically the driving force for it, is
suction generated by the leaves.
That's at least the theory.
It's the cohesion-tension theory, and it has been proven to be valid.
And it happens that way when we now think about a normal atmosphere,
about relative water content of 50%, relative humidity.
And then the water potential of the atmosphere will be around 100
megapascal retention, so negative megapascal.
And this suction now, when the leaf is on, is losing water.
When the stomatas are open, this atmosphere kind of generates suction
and sucks out the water from the leaf.
And along this, we can now look into a plant.
For example, here we have normal conditions.
There's a lot of water there, so the soil can have a water potential
of minus 0.5 megapascal, for example, here.
The root will have a lower water potential, the stem.
And the tension increases with going up into the leaf.
So along this tension, the water is basically transported along in the
xylem of the plant.
But when we now look on drought periods, the soil is much drier, so
the water potential of the soil will be much lower.
It's more difficult for the plant to extract the water.
In this example, the atmosphere still is at 50% relative humidity.
And then the water potential from the soil going to the roots, stem,
and leaves will further decrease.
So a leaf water potential of minus 3 megapascal, at least for several
species here in Europe, might be already at the level where there will
be embolism starting in the xylem.
This is because when the water is transported under tension, it's
basically in a metastable state, this water that's transported in the
trees.
So it's really amazing how this works.
And these metastable states of this tension can draw air bubbles into
the xylem.
And when these air bubbles get big enough, they can... some of these
trachrides, that are basically called the parts, the smaller sections
of the xylem, can be trachrides in most plants.
And then in this more section, there will be like air blocked in
there, an embolism.
And if you now think about a xylem that's full of air, you might have
a problem.
There will be not much flow, and this can happen in catastrophic
hydraulic failure at some point.
So there's also a global convergence to this vulnerability to air
entry.
So they looked at a lot of tree species globally, and they found that
all of them, it doesn't matter under which conditions, will have a
point where this hydraulic failure will start.
There will be more and more embolisms, and there will be runaway
cavitation, and plants cannot recover from it.
And I'd like to show you an example, and I think it's really, really
amazing.
It's one of the most drought-resistant plants in the world, or at the
moment it's the most drought-resistant plant in the world.
It's Calitris tuberculata, from Australia.
The whole genus Calitris, are basically cypress trees, are very, very
drought -resistant.
And they grow in really, really dry areas, as shown here.
I think it has less than 200 millimeters of rain per year.
And they looked on the drought-resistance of the xylem, how resistant
is it to these embolism build-ups, how much tension can it endure.
And what they found, and this is typically done in this kind of
hydraulic research on xylem, you plot basically the xylem pressure
versus the loss of hydraulic conductance in the xylem.
So at a loss of 100%, there's only air in the xylem.
At zero, it's only water in the xylem.
And you increase the tension, you increase it, increase it, increase
it.
And you see, one of the traits ecologists are looking at, it's called
the PLC50, so the point where you have 50% of embolism, 50% of water.
It's often used to compare different plant species.
And this plant can endure minus 19 megapascals.
I mean, one megapascal is 10 bars.
So it's really amazing when you look at this.
And they compared it to other unused sperms and gymnosperms.
So unused sperms are mainly deciduous trees, and gymnosperms mainly
conifers.
And most of the other trees lie around minus 6 megapascals, for
example, for gymnosperms.
And unused sperms around minus 4 megapascals.
And what is interesting also for the physicists now is that, also in
the title of the paper, extreme aridity pushes trees to their physical
limits.
So there's classical nucleation theory.
It says that water cavitation should occur at minus 120 megapascals.
I don't know how they test it, but they do it somehow.
But this is with pure water.
Xylem water is never pure water.
It's impure water.
There are sugar dissolved minerals, nutrients.
And so they figured out, looking at synthetic trees, that the limit
should be around minus 22 megapascals for total cavitation.
So I think this plant, at least this species, seems to really operate
at the physical limit of life.
That's really, really interesting.
Okay, now we have seen one example how a tree really can operate at
this limit.
And it's really an exception.
But how can trees, in general, avoid to lose so much water?
Because, as we have seen, at some point, when they lose too much
water, the water potential goes down, and there will be all this
embolism, and it won't be good for them.
But what are their mechanisms they have?
And I think one of the most drastic ones is, for example, doing this
during a drought period.
Just getting rid of these leaves, and then you don't have a problem.
Then you wait, and when the drought is over, you get new leaves.
But there are a lot of trees that can't do that.
For example, coniferous trees, they cannot just shed their leaves.
So there have to be other mechanisms in the plant.
And I think most of you know them, I'm pretty sure, when we have
talked about this quickly before.
These are the stomata.
So the stomata on the leaf, mostly on the underside, but also on both
sides.
These little pores, these little pores, plants can regulate the water
loss.
When they close these pores, hardly any water loss occurs.
When they open these pores, a lot of water loss occurs.
So they can really operate.
And this is a cross-section of a leaf, and I wanted to show that to
you.
Just show, okay, this is the leaf, the typical cells in the leaf.
And here you have the areas where you have the stomata.
And this is an example for, like, these are the stomata cavities.
This is an area where a lot of, like, where the humidity is at 100%.
And from this area, water is lost to the atmosphere.
There's also the boundary layer there, the atmospheric boundary layer,
because it's also important to regulate the water loss.
But there can also be water loss from the cuticula, also when the
stomata are closed.
So during a drought period, the stomata, at some point, can be totally
shut down by the plant, but it will still lose some water.
It cannot, like, stay at 100%.
So, and that's also, I think, really interesting.
You can really say that stomata are togo-operated moths.
You know they can open and close.
You know a lot of mechanisms.
But there's still some questions we cannot really answer.
So we know, for example, that there is an indirect signal that can
come from the roots.
When the soil gets dry, the cells start to build up synthesized
abscisic acid.
And abscisic acid has been shown when it's, like, sprayed on plant
leaves, for example, to really start stomata closure.
But also, for example, also important is the pH of the xylem.
So if the xylem has a higher pH, this abscisic acid can operate much
better and close these stomata.
But that's only one side of the story.
There's also this more direct effect that we cannot observe.
And this is the effect.
The stomata not only respond to the soil water, but also to the
atmospheric water, to the atmospheric demand.
And this is a more quicker answer.
And there seems to be more and more indications that they can really
respond to water supply or water loss.
And somehow they can measure this.
And it's thought that it's through changes in the leaf toggle and then
through changes in the cells next to the stomata, so these cells are
called the guard cells, that they change their osmotic pressure.
And when the stomata shrinks, it closes.
So there's the change in the osmotic pressure.
There can be... some of these mechanisms might be answered there.
But still, it's not fully understood yet.
And that's very interesting, I think.
And I want to show you another example.
This is from a pine site in Oregon, where I did my poster there.
And we looked at how these trees, and this is a summer drought forest,
how these trees will respond if they get more water than normal.
So mainly atmospheric drought is making them dry.
And how will the stomata respond there?
And this is why I like to show this to you, to show you how basically
you can see stomata respond to changes in vapor pressure deficit.
So if you look at the darker lines, you see the stomata here was at
100 micromoles, they were... sorry, millimoles, they were totally
open, at least this was one of the maxima we measured there.
And then this increasing vapor pressure deficit, or you can also call
it atmospheric demand, these stomata are starting to close.
So these are like lines that have been synthesized from a lot of data
points.
And this is all true as long as it's very wet, they respond very
tightly.
And at some point you can see that when you go to 4 kilopascal vapor
pressure deficit, the stomata are nearly closed.
So a plant can really shut down the stomata, no water is being lost.
But when you then go to the more drier periods of the year, this was
for example done in August when the soil drought really kicked in, you
can see that the stomata are already closed at a very small
atmospheric demand.
So they just basically start in the morning already at a much more
closed baseline.
And from there, you don't see this dramatic response anymore, this
vapor pressure deficit, because they are already at a much lower
baseline there.
And after doing the study, we also did a modeling excursion on this,
and we figured out that basically for this forest at least, we figured
out that high temperatures and an increase in atmospheric demand might
be one of the dominant controls in the future for the water and the
carbon cycle of that forest.
And recently this has been also highlighted in Nature, they did a
synthesis of a lot of meri-flux sites, and they found that there's an
increasing importance of atmospheric demand for ecosystem water and
carbon fluxes.
It's not only about precipitation and soil drought, but it's a lot
about the atmospheric demand.
And with increasing temperatures, the atmospheric demand increases
exponentially, as you all know.
But yes, we talked a lot about water, but you all know it's only part
of the story.
There are more fluxes in a plant than water fluxes, and we live for
more than from water.
So a plant needs also some bread, and on this one I thought it's
really pretty cool, it shows most of the things that are basically
going on.
So the plant needs somehow CO2 and light, and it produces oxygen and
sugars.
And yeah, of course, the water, we talked all about the water before.
And this is the only chemical equation I will show here.
So this is very basic for all life that's there.
It's a simplified version.
CO2 and water and some light gives sugar and oxygen.
Very simple.
And who is responsible, at least for this transfer from the atmosphere
to the plants?
These are the stomata.
So through the stomata, we see this water loss.
And the water loss is a magnitude higher than the CO2 that's entered.
into the stomata.
I tried to show this by the arrows, but it's still not the correct
dimension.
So there's like one per thousand water molecules lost by one CO2
molecule is taken up.
So it's a pretty bad kind of view for the plants, at least it looks
like.
So CO2 is also regulated there.
And I'd like to show you another example that's from our research we
did here in the greenhouse outside.
We looked on how stomata conductance and photosynthesis are related to
each other.
Because we know stomata basically respond to this water, to this limit
in water supply.
And they might close.
So how does photosynthesis respond?
And we looked on two tree species and we did a lot of stress
treatments.
Because we like to see how plants suffer.
So we have always, what we always have is a control treatment.
So this is basically the well watered, they are doing good plants.
We did here also a heat treatment, a drought, and a combined heat and
drought treatment.
And what we can see is, very interestingly, it doesn't matter which
treatment we use.
On the x-axis, it's stomata conductance.
And on the y-axis, it's light saturated photosynthesis.
We can measure these on the leaf with this little guy here, a LICO
6400.
And we can see that when stomata start to close, here, basically,
photosynthesis goes down.
That's what we can see.
So it looks like a direct relationship, all nice and so on.
We thought also, we wanted to publish that, but then some reviewers
said, no, no, it's not that easy.
Because it looks like there's a real nice relationship, but it's only
an apparent relationship, probably.
Because often, when you come in this area, and the stomata are nearly
closed, they say around 0.05 mole stomata conductance, then you might
reach what's called the CI inflection point.
And inside the stomata cavities, normally, you see all this CO2 is
decreasing.
The plant takes it up, does photosynthesis with it, takes out the
sugar.
But at some point, when the photosynthesis might be impaired, then the
CO2 is not decreasing there anymore.
So you might have biochemical limitations.
So your photosynthetic machinery might have already been limited, or
be affected, impaired, by a stress that has been built up, and so on,
in the plant.
So then this relationship might be pretty true for this area here.
But at this point, and it looks also, it's much deeper, you can think
that a lot of biochemical impairment is already happening.
For example, when you have a lot of stress, one of the most important
enzymes in photosynthesis is rubisco.
It's not being recycled and regenerated anymore.
And so your photosynthetic system is also decreasing, independent of
the stomatal openness.
Another study, again, on pine trees.
And this is a pine seedling from the Yatir forest in Israel.
And we wanted to look also on the carbohydrates in the plant.
So, so far, we have looked on water, on the stomata, and on the CO2,
how it can enter the stomata.
But at some point, this carbon is also in a plant, and it's also in a
flow in the plant.
It's also still recycled and used for a lot of things.
These sugars have been unneeded there.
So, Pinosalipensis, yeah, we do a study together with colleagues from
Israel.
But we don't want to do that in Israel, because we want really to have
some nice control, and nice hands on the plants.
So we brought them to Garmisch, the seeds, and germinated seedlings
from them.
And we also imposed the stress treatment on these plants.
And here we looked on the soluble carbohydrates.
These are the water-soluble carbohydrates.
They are also very important for osmoregulation in plants.
I haven't talked much about this, but this is also one mechanism.
Plants can increase their water potential by putting more sugars in a
cell.
And there's also starch, that's more carbon reserve, more carbon
storage.
And we looked on the shoot of these little seedlings, and on the
roots.
And what you can see in the shoots is basically, and this is all shown
compared to a control.
So we just say, we don't show the control treatment here, we just
compare to the control.
If it's at zero, it's the same.
If it's lower, it's lower than the control concentrations.
So we have also three treatments, three stress treatments.
And we can see that nothing happens in the shoot.
But we know, photosynthesis is basically zero.
There's no CO2 entering the system anymore.
There's nothing happening there.
How can plants do that then, or why do that?
Why are they doing that?
When we look on the shoot, we can already see some indication.
During stress, the starch goes down.
So eventually, some of the starch has been used as a soluble sugar to
increase osmotic potential in the plant.
To decrease osmotic potential.
But in the roots, we see a very different picture.
In the roots, we see a total decrease in the soluble carbohydrates
under stress, and also in the starch.
And this is probably one thing that we often see during such a drought
or stress event.
That there's an uncoupling from the canopy of the plant to the roots.
So, this is something that still shows us that a lot of different
flows are affected in a plant.
There's also not only the watering, the water going through the plant.
Not only the CO2 entering the plant, but also the carbohydrates within
the plants are affected.
And also the coupling from the canopy to the roots.
And I haven't talked about this before, but there's another important
part of the vascular system.
It's not only the xylem, the wood, where the water goes up, the upward
flow of the pipeline.
But there's also a downward flow, where the sugar-enriched sap goes
from the leaves to the roots.
Ah, sorry, that was something I missed.
The sugars in the shoot might be actively retained to decrease osmotic
potential.
Which affects water potential and increases sugar, as I said before.
Another example, we looked on these shoot-root coupling under drought
conditions.
And one tool that you can use, for example, to look on how CO2 is
entering the plant, where it's entering it.
And how it goes through the plant, where it's allocated to.
Is using labeling with certain CO2.
So you basically impose the plants to a certain CO2-enriched
atmosphere.
And normally, our atmosphere, the CO2 that's there, it's mainly 12C.
So if you increase the certain CO2, you can really follow the certain
C isotope to the plant.
And it's a stable isotope, so it's not harmful at all.
And we did that in an experiment.
And it was very interesting also, and that has been found very often.
That there's a very quick flow in the plant, a very quick transport
normally.
So the CO2 is taken up by the leaf.
We found it in the phloem, so the vascular tissue that transports the
sugar to the roots.
Five hours later, it was there already, so immediately.
We found it down there in the base of the tree about a day later.
And we found it in the roots, and in the soil microbes, and in the CO2
that's entering and that's leaving the system.
So it's a very fast transfer, it's a very fast flow.
Because in one day, the CO2 can go back to the atmosphere, parts of
it.
And we now look under drought conditions.
I mean, you might have a guess already, I'm pretty sure what can
happen.
What we can see is that this transport is pretty much delayed.
It takes about four days, but we see a peak in the phloem.
It takes another six days until we find it down there.
We found it after about ten days in the roots.
And for the microbes, we didn't know because we didn't measure long
enough.
But we think it was even more delayed there.
So we can also see that water is also very important there.
Probably also for the transport of these carbohydrates within the
plant.
So here I come to the summary of my first presentation, or basically
the take-home message.
Yeah, plants respond to changes in environmental conditions.
And they really affect the fluid flow dynamics, I would say, in the
soil -plant-atmosphere continuum.
And this water flow in trees can be really finely tuned using the
stomata.
So they can really say, okay, I don't want a water potential of minus
three megapascal.
I can close the stomata, and as long as the soil is not too dry, I can
have only minus one megapascal or something.
So they can really finely tune that.
Or they can also shed all their leaves if they wanted, or if they can.
Yeah, we have seen that there are really some trees that are really
amazingly drought-resistant.
I mean, they can have endured silent tensions of more than minus 16
megapascal.
I mean, I think it's very amazing how wood is built in plants and how
plants can do that.
Yeah, and this might really be very close to the physical limit for
bulk cavitation of impure water.
So really the physical limit of life, of what the water transport is
in a plant.
And we see that these water and carbon fluxes are really ultimately
linked at the stomata of a plant.
When the stomata closes, there is less CO2 entering the plant.
So there is a really, really close link.
And these also link again to the carbohydrate status of the tree.
But a plant also has mechanisms to change its carbohydrate status.
They can also do some immobilization of sugars and starches and so on.
And then affect the osmotic gradient, which can also help them to
endure a drought event much longer.
Yeah, and really water seems to be really the solvent, I mean, as we
all know, for all biological reactions.
And plays really a very important role also in the transport in a
plant from the canopy to the soil.
So from the source organ to the sink organ.
Okay, let's start with the second part of my lecture.
It's on recovery dynamics.
So how can plants recover from heat and drought stress?
And so my main motivation there is that forest resilience to extreme
events is really linked to understand stress responses on one side.
But also to understand recovery responses.
Because otherwise we won't know if an ecosystem will really recuperate
from a stress event.
And can maintain on the long term.
But then I started to look into the literature and really figured out
that at least for woody plants, there's more known in crops and so on.
There's really not much known on how plants recover from such a stress
event.
There has been so much research on drought.
But very little research on how plants can recover from such events.
So this was from Web of Science for example.
I found 2700 papers on drought entry.
But then I looked on recovery entry.
I could only find 180 publications there.
So there's still some imbalance.
But I think both are similar important.
To come back to this picture I've shown before.
So we have again this, somehow our seasonal flow or our dynamics.
And then we have a stress event that kicks us out from our normal
situation.
And then we can recover from that stress to some level.
And there's a little equation I made up by myself even.
So it might not have a real physical background.
But I wanted to think about recovery.
What are the functions that affect recovery?
So there's a function of stress.
So the stress intensity, the stress duration will probably affect how
well can a plant recover from that stress.
But maybe also the seasonality.
If a stress hits early or late in the season.
If a plant has leaves out there or not, can affect it.
But also the species, as we have seen.
There are these super drought-resistant species.
So if you don't water them, they just don't care, I guess.
But also maybe the conditions during these recoveries.
So what are the conditions after a heat wave?
For example, after this 2003 heat wave.
What were the conditions afterwards?
Was there so much rain that all the soil was immediately recharged and
all the plants could restart again?
There might be probably more things you can think of.
But in this lecture today, I really don't want to think too much about
these conditions during the recovery period.
Because I think this will complicate the process pretty much.
And we don't know so much about actually what's going on from the
recovery from stress itself.
So I basically want really to see recovery as a response of the
stress.
So I want to focus on that.
And here another idea how we can see stress and recovery.
So we have initial conditions, 100% photosynthesis.
We have a stress, photosynthesis decreases.
We have a re-watering event or the heat wave stops.
And we have a recovery from that stress.
We might have even compensation of the stress period that has been
seen in literature.
So that plants that after a drought stress can do more photosynthesis
afterwards than their control trees.
Or we can have just a full recovery that the photosynthetic rates will
be the same as they have been before.
Then we have a more severe stress.
Photosynthesis declines by nearly 100%.
Then there's a heat release, a re-watering.
But maybe we cannot reach the levels we have reached before because
there has been really critical damage to our photosynthetic machinery,
leaf tissues, whatever.
And we only might have a partial recovery, photosynthetic rates will
be only half of what they have been before.
Or maybe very, very low recovery or no recovery at all, which would
lead probably to a mortality in the long term.
So it's a very important process to understand.
And I looked not too much on my own studies this time, but I looked
more into the literature.
On what is there, what do the published studies tell us about the
recovery from stress.
But I mainly looked on to exclude these conditions during the recovery
period.
So I mainly looked on experimental conditions.
Studies that have been conducted under controlled conditions to make
sure that when there was a re-watering, there was really 100% re
-watering, for example.
So temperatures were constant and so on.
And I synthesized some of these studies that are out there.
And I think this is one way of doing research nowadays.
There's so many publications out there.
If you have an idea, dig into the literature.
Maybe you can really make something out of it and synthesize the data
that's there.
So I looked on the studies that report recovery of pre-physiological
processes or woody plant processes over time.
So I didn't include studies that only had one time point of
measurement, but really over time.
And most of them report just leave physiological parameters.
Some also report some asylum conductivity and so on.
So I looked at how many of these studies report, for example, complete
recovery of photosynthesis or only partial recovery.
And I wanted to link this to the stress event here, to the drought
event.
So I linked most of these studies, luckily measured leaf water
potential.
So I could link that to leaf water potential measurements.
So what we can see, for example, is that when leaf water potential has
not been very critical, has been here largely above minus three
megapascals, or the tension in the xylem has been not very large, then
we can find that most processes that the studies have looked at
completely recovered.
If the drought has been more severe and average water potentials of
above minus four megapascals have been reached, then I find that most
studies say, OK, there's only partial recovery of this physiological
process.
There are two exemptions.
These are trees from the genus Callitris.
They can show both.
That's also interesting.
I don't say too much about this study, but it's interesting.
These are two different tree species.
They were both stressed to minus 10 megapascals water potential.
But one of these Callitris species had 80% conductivity in the xylem
and the other only 10%.
So the one with 10% showed complete recovery while having the same
leaf water potential, and the other one only partial recovery, very,
very slow recovery.
Let me now look on heat stress.
How can heat stress affect the recovery?
So before we have seen drought stress, I think to make it very simple,
we can think about maximum temperature might be a very good proxy, but
also the duration of the heat wave.
So how long has this maximum temperature been maintained over a
season?
There's an interesting study under controlled conditions.
An older study, but very simple.
They just exposed Zea mariana leaves to high temperatures for a
certain amount of time.
For example, they exposed them to 44 degrees for 110 minutes, and they
found 50% of the foliage being damaged afterwards.
So 44 degrees, 2 hours.
Some of the leaves survived that.
But then when you do 50 degrees, only after 10 minutes you have 50% of
foliage damaged.
It's very interesting.
I mean, it's very simple, but it's very interesting and
straightforward.
So you can really see how the duration of a high temperature and the
high temperature might damage tissue.
And then of course, if the tissue is really damaged, it cannot really
recover.
I mean, if the needles are brown, they cannot recover from that.
At least these 50% of these needles.
I looked also at our own study, where it fit in.
So one of our heat stress was a bit severe, and we reached about 46
degrees for about a period of 90 minutes.
And we did not find the whole Douglas fir branch to be brown or
wilted, but only the really, really fresh needles that have just been
emerged.
Because they don't have a very big cuticula yet, so they are more
prone to a high water loss.
And I think this just happened because high temperature, the leaves
just dehydrated them.
They couldn't be stomata closed, but there was probably a lot of
cuticular transpiration and they dehydrated them.
But we have now seen that the recovery can depend on the drought, and
probably also on the temperature or the exposure of the heat wave.
At least when a lot of tissue has been damaged, there will be not a
lot of recovery.
But can we also see that the pace of the recovery of different
physiological parameters differ?
So if you now look at this leaf water potential photosynthesis and
stomata reduction recover to the same speed.
Or do some of them grow quicker and others grow a bit slower?
And I looked on, again, the studies that have been looked on post
-drought recovery and studies that look on post-heat recovery.
And here we can see, this is on the x-axis, the recovery duration in
days.
So from zero days, basically quick recovery, to up to 80 days.
And we can see that after a drought and a heat event, leaf water
potential recovers basically immediately.
When you rewater the plants, it goes up to normal.
Like that.
Two exceptions.
But we can see a delayed recovery in photosynthesis, and we find a
delayed recovery of stomatal conductance.
So now we don't have this 10-minute response of the stomata and so on,
for some reason.
And we find pretty much the same picture under heat conditions.
The numbers behind indicate the numbers of species and the numbers of
studies.
No, sorry, the numbers of studies and the numbers of species.
So you see, for the heat period, there's only one study.
But you can also see the discrepancy.
There may be more studies following a drought recovery than a heat
recovery.
That's why I focus also a bit more on drought here.
This is why I forgot to mention this.
And then some studies also looked on the abscisic acid.
As I said before, abscisic acid, this phytohormone, can initiate
stomata closure.
So the one idea would be, okay, if the stomatas are closed or don't
recover quickly, maybe abscisic acid has been still upregulated.
But they couldn't find an indication for that.
It's only the first two or three days after a drought event, three
independent studies found abscisic acid to be upregulated.
But then afterwards, not at all.
So what are the underlying mechanisms, maybe, for this prolonged
reduction in stomata conductance?
And I think because stomata conductance is so important, because it
tells us about water loss from an ecosystem or from a plant, and it
tells us also something about CO2 uptake, there might be some ideas or
maybe some underlying mechanisms.
One that's very interesting and highly discussed at the moment, I
won't talk much about this, but I think maybe Konrad later or Kari
Jensen later, it's reduced xylem conductivity or conductance.
So there's one idea maybe, when the stress event was very severe, and
we have seen before that under certain tension, there might be
embolism building up in the xylem.
These embolisms are air that's basically in there.
And now this plant is re-watered.
It might happen that it cannot really dissolve the air that's in
there.
It cannot really dissolve the gas, because physically, it's impossible
as long as the system is under tension to really fill these air
bubbles, because you would need positive pressure.
But there are ideas behind this, maybe the sugars or osmoregulation,
that might work.
But there's also something, and now you come to the feed-forward, the
feedback loop from photosynthesis.
So if photosynthesis has been impaired through the stress event, for
example, and the plant is a bit slower in doing its photosynthesis,
the stomata also don't need to be so much open because the eye is
already high enough.
So that could be one of the underlying mechanisms.
Or there might be a change in leaf morphology or in the leaf axis, for
example.
At least, maybe there might be more ideas, but these are the ideas I
came up with.
To show you one example here, it's one of our studies with a heat
wave, and we found some more longer-term effects of that heat wave.
We conducted for three times 15 days a heat wave, and we found that
water potential, as you have seen before in this one study, recovered
immediately, but not photosynthesis and not stomata conductance.
So A is often the abbreviation for photosynthesis assimilation.
And we found that still about 50 days after the heat event,
photosynthesis was about 40% reduced and stomata conductance was only
half compared to our control trees at that moment.
So we found only a partial recovery after this heat wave.
And this is just a figure that shows you what we measured.
For example, we measured the electron transport within the
photosynthetic system, and we measured before the stress, during the
heat wave, and then during the recovery.
And you can see that these are averages of five trees that have been
measured, treatment averages.
The red color shows the heat.
You can see that also at the end of the recovery period, 45 days
afterwards, there was still a big discrepancy in electron transport
rates when you compare the control with the heat to the trees.
And we also found an effect on something we call minimum stomata
conductance.
At the point where the stomatas are closed, or we think they are
closed, because we never really, really know, this would be mainly
critical transpiration.
And we can also see that this critical or minimum stomata conductance
was also much lower in the plants that have been exposed to heat
events than in our control plants afterwards, which could also explain
some of the changes in stomata conductance.
There might have been a change in the baseline conductance that's
occurring.
Of course, also when a plant has the stomata closed, there's still
some water loss, if you will measure.
Okay, that's why I mentioned this already.
And very interesting, that's not from us, but it also brought me to
this idea of maybe a change in leaf morphology.
We saw that leaf and epicuticularexis might change, and this is a
study done by Gayet and Feller.
And they looked on Phagocelvatica, and they did a drought experiment.
And they looked on the stomata.
It's not very good to see, unfortunately.
I'm sorry, but here you see on the left side, you see the stomata are
all light-colored.
And on the right side, you see something inside the stomata that's
more dark-colored.
And they thought this might be stomata occlusions.
So basically, vexes that can build up within the stomata, and they can
affect the diffusion rates.
And what is very interesting also is that often these occlusions
affect transpiration more than photosynthesis.
That's also interesting.
There have been some studies, but not a lot.
And I think it's also a very interesting, often neglected topic.
But now we have seen that we found immediately after the heat event,
we found, and also in the other studies, we found immediately after a
drought, we can have a delayed recovery.
But we also find some responses that might carry over, really, into
the next year.
That's also called now stress legacies, because it sounds really cool.
And, yeah, we found something.
I haven't published it, though, but we found that our plants that have
been here, the ones that have been heat-stressed, showed a later bud
break the next year.
But not very long, just, I think, a week or something, a delay in bud
break.
And they showed a reduced leaf elongation.
So the shoot growth was much lower than in our control trees.
So there can be some carryover effects.
You can have a stress event, and things that you measure the next year
might have been caused by the stress that has happened before.
So it's always good to know the history of the plants or the
ecosystems you work in.
Maybe to understand some of the things that are happening.
But, of course, we're not the only ones.
I see something like this in other people.
This is, for example, here now.
It's not an experiment under controlled condition, but semi
-controlled.
It's done in a field.
And they found that experimental drought and heat delayed phenological
development and reduced foliar and shoot growth in semi-arid trees.
So they really found from this stress experiment that it has really an
implication onto the tree's phenological state and onto the growth
into the next year.
And even going a bit higher up there, looking on larger parts of the
globe, not only going higher up in impact factor, there's a study
where they looked on basically the dental chronological data from
North America and Europe.
And they looked on stress events and wanted to see if the growth rates
are the same after the stress period.
And when they looked on all the ecosystems and trees combined, they
found some slight response, some slower growth rates in the next year.
But then after the third and fourth year, a pretty quick recovery, but
then they then differentiated into arid and wet ecosystems.
They found that in arid ecosystems, there's really a pronounced stress
legacy.
There's still really lower growth rates the year after and in the
second year.
And then also when they divided into different tree classes, so into
phagocy and piniacea, they found for the pine trees that this response
seemed even to be the harshest.
So there might be the largest stress legacy there.
But here now, one of the problems with such a study, I think, is also
we don't really know if after the stress event or several stress
events here, of course, what are the conditions afterwards.
For example, for the European summer heatwave in 2003, there have been
some smaller droughts the year after the next year.
So there have been also some reduced growth being reported, but that
might have to do with other conditions that happened afterwards.
So it doesn't tell us too much about the mechanisms that are behind or
about the processes, but it gives us an indication, yes, there might
be really substantial effects, and none of the carbon models or
vegetation models account for this yet at all.
But also there's a caveat to the studies I just showed you, to these
experimental studies on the control conditions, and this is the
duration of the post-stress observation period.
When you look at the studies, the studies that find full recovery, you
can see that full recovery happened within the first 20 days, and
there are studies that show partial recovery, and that happened later,
between 20 to 80 days.
But when you look on the average study length, these studies are only
conducted for 36 days on average.
That means we might also miss a lot of effects.
I mean, if you have full recovery occurring after like five months,
you won't measure it, so you don't know exactly what does this partial
recovery mean.
It doesn't help you too much.
And the same is basically for these studies that are following a heat
event, they are on average only like about 16 days conducted.
What I want to stress here, to say, is that it's really important that
we consider the study length that has to be long enough.
It's complicated under controlled conditions, but to get a good enough
picture, we really also need some more longer term studies there.
Again, they are often too short to detect complete recovery of these
stress legacies.
The take-home message here is that for the tree physiology, the
recovery depends on the impact and the stress integral, so the
severity and the intensity of the stress.
And we found that water potential recovers much quicker than stomatal
conductance and photosynthesis, while we find still limitations of
photosynthesis and stomatal conductance to occur.
And they might have to do with leaf morphology or with not fully
refilled embolisms within the xylem.
So leaf water potential can be in some way very independent of the
embolisms in the xylem because you might still have, let's say if you
had whatever 30-40% embolisms, you might still have a water path of
the other 70% and along this water path, the water can be quickly up
transported and leaves there still can be rehydrated really quickly
because often after the stress their osmotic potential is still very
large.
But we still don't know a lot about how xylem embolism is refilled and
if this plays a certain role or not.
And the stress legacies can really carry over into the following years
as we have seen.
So they affect overall plant vitality and also ecosystems.
The larger scale relevance is that such long-term reductions can
really have potential effects on the forest, carbon and water cycle.
And low recovery rates I mean, we can have a stress event and might
think oh, this has been a severe stress but the plant survived it but
we really haven't looked afterwards.
I mean, maybe they survived it for another two or three weeks, but
then they might die.
So this is really important that we also see from which stress can
plants still recover or are they going to die there.
Yeah, and all these processes are totally not accounted for in any of
the vegetation models.
With this, I'd like to thank you and questions are highly welcome.