- This next talk is from Kelly French,
with the University of Maine,
School of Forest Resources.
Presenting Spruce: DRIED,
Density Reduction and Imposed Extreme Drought.
- I am a second year masters student
at the University of Maine.
So this is part of my thesis research
and I'm just gonna present some of the preliminary results
we have from this part of my study.
I can't get through everything today unfortunately,
'cause there's a lot of data
but I'm just gonna share some of the highlights with you.
I would like to mention all of the other study authors
that have been involved in this.
It was a great collaborative effort
between my team here at the University of Maine,
including my advisor Dr. Jay Waston
and Dr. Fraver and Dr. Kenefic on my committee,
and Heidi's lab at the University of New Hampshire.
So as the title of my study indicates,
this is kind of a two-part study
looking at thinning and drought.
So just a little bit of brief background.
We do know that forest density reduction
through thinning is beneficial for residual trees
because of the decreased competition
for resources like growing space,
light, water, and nutrients.
However, something that's not thought about
necessarily as commonly,
is the impact of this increased exposure and temperature
that can come from heavily thinning these stands
and really opening up the canopy
for all this increased light.
This can potentially lead to
what are called thinning shocks.
A lot of this is due to this abrupt transition
from shade to light in these non adapted needles.
And some things that have been quantified thus far
have been decreased growth,
reduced photosynthesis, leaf mortality,
all in these non adopted needles.
And the reason that I am interested
at looking at these effects,
is due to the fact that they may be worse
in projected future climates.
Which kind of ties into the second part of my study
which is looking at drought.
As we have already talked about earlier
in previous presentations that climate change
is expected to drive a lot of differences,
increased temperatures,
increased vapor pressure deficit,
which is a quantification of how dry the air actually is.
So sometimes it's termed like
atmospheric drought conditions.
And then also drought frequency,
as we talked about annual precipitation
is projected to increase
but so our risks of seasonal droughts
particularly in the summer and the fall.
So, as you can see from this figure above
spruce-fir forest types and other mesic forest types
in the Northeast are potentially going to have
large impacts just even due to small changes
in these different climactic variables
because they're not adapted to these changes in climate.
And my study species,
which is red spruce,
has been shown through growth climate correlations,
to decline in growth with increased summer heat,
and also has growth declines
with decreased spring and fall precipitation.
Showing that it is a particularly climate sensitive species.
So thinking about thinning and thinking about drought,
kind of drives the overall goal of my study.
Which was to improve our understanding of how thinning
and extreme drought influenced the physiology of red spruce
and then trying to interpret these patterns
to help better inform future management
of this species with climate change.
So the first objective
is investigating the immediate impacts of thinning
on mature red spruce,
trying to quantify any evidence of thinning shock.
And then the second objective is it looking at
how these red spruce respond physiologically over time
to this experimentally imposed extreme drought,
I'll talk a little bit more about.
So we had a study sample size
of eight focal red spruce trees.
They were all crown-suppressed on the same size,
16 to 26 centimeters in DBH.
And we created this factorial design of two by two
for our thinning treatment and our drought treatment.
So for our thin treatment,
we had a full kind of crop tree release of the canopy
or up the crown.
Everything was released on all four sides.
That's why we wanted to look at crown suppressed trees
because we wanted to really capture
as much effect as we could of thinning shock potentially.
So we wanted canopies that had previously been shaded
to fully open them up.
And for our drought treatment,
we did this treatment that is pretty novel
where we actually severed the sapwood.
So effectively cutting off the xylem
which is transporting water from the roots
up into the rest of the tree.
So if we severed all of the sapwood,
that would effectively mimic the effects of
if there was no water in the soil to begin with.
And our control for this treatment
was a phloem girdled control
because we expected that phloem girdling
might have some sort of physiological effect as well.
So we wanted to account for that.
And there was a lot of different measurements
that took place throughout the growing season.
This is a preliminary experiment.
So our sample size wasn't as large
as hopefully future studies will be.
But one of the limiting factors also
was installation of these sap flow sensors
which are what we use to monitor a tree water use
alongside all of these other physiological responses.
So let's talk first about some of the impacts
of thinning that we saw.
The first thing was that
we did notice thinning did drive
this increase in temperature and vapor pressure deficit.
So we have five days pre-thinning
and five days posts in these graphs.
And the red line indicates when we conducted the thinning.
And for the thinned trees,
we saw an increase in temperature
of about 2 1/2 degrees Celsius
and a vapor pressure a deficit increase
of about 0.64 kilopascals.
That indicates that air was drier than
for the non thinned trees.
So, (keyboard clicking).
There we go.
So these changes in microclimate
did end up driving some evidence of thinning shock.
And the first way that we saw this
was through reduced midday water potentials.
So water potentials,
for those who aren't really familiar with that term,
that's basically a quantification of the pressure
that builds up within a plant
in order to drive the flow of water up the xylem.
So a stressed plant that doesn't have a lot of water
is gonna continuously increase that pressure
to really try to drive the water up.
So for our thin trees we did see that
they were significantly decreased in water potential.
They weren't really stressed
on this cloudy and cool day as to be expected.
But on a sunny and hot day,
it's important to consider their turgor loss point
which is where these needles lose turgor
and may potentially start to wilt.
And the fact that the thin trees
were well below this range for turgor loss point.
Indicating that if they persist
in the state at this level,
it's going to have potentially harmful effects
on the leaves.
And the second evidence of thinning shock that we saw
was this decreased photosynthetic efficiency
of the photosystems within the chloroplasts.
So a healthy level for this is about 0.83.
This is kind of a a more complicated metric
using a little FluorPen that we used,
but we did see not only immediate effects here
in immediate decrease in photosynthetic efficiency
very shortly after the thinning,
but this persisted throughout the rest of the growing season
until the end of the experiment.
So that was quite surprising but very interesting
to see that that effect lasted so long.
Moving on to the effects of the imposed drought.
We imposed this drought two weeks after we did the thinning.
So went in with our circular saw
and cut the sapwood at that point.
It's important to know that we did not see
an interactive effect of thinning and drought.
So moving forward,
I'm just gonna be referring to these trees
by their drought treatment.
So the first thing that we saw was that
severing the sapwood did cause reductions in sap flow,
but the only 100% reduction in sap flow
actually imposed this extreme drought we were looking for.
I'll get into some of the evidence of that
in the next few slides.
This is a graph showing five days pre-treatment
versus post-treatment total daily water use changes,
the percent change.
So for two of our trees that we severed sapwood on,
we were successful at severing a 100% of the sapwood
which caused a 100% reduction sap flow.
For two of the trees,
we didn't successfully sever all the sap flows.
So they didn't experience
all of their water use being cut off.
And then the phloem girdled,
as we expected didn't have significant changes.
So this was really interesting to us to see,
that partially severing the sapwood
which did cause these large declines,
actually never caused water stress in these trees
throughout the duration of the experiment
which was really quite shocking to us.
We quantified the sapwood by taking increment cores
and measuring the area that was wet.
We also quantified this with relative water content analysis
and using a wood moisture meter.
And basically I have this little schematic of a cookie.
So if we went around,
this is our circular saw and cut the sapwood
cause we wanted to cut all the way through
potentially into the heartwood.
Some maybe thicker bark in certain areas
or just the saw like not being able to penetrate
all the way through caused these little inclusions
of residual sapwood remaining into these trees
which after we harvested the trees,
we quantified at about 2 to 5%.
Which is shockingly low for still being able to transport
about like 30% of previous sap flow volumes.
But this seemed to be enough
for the duration of this study at least
to prevent these trees from getting water stress.
They never experienced water potential declines,
trunk rot, relative water content declines
and there was no impact on their stomatal conductance
indicating they weren't closing their stomata at all.
So this was one of the more shocking findings
of this study but very interesting.
For the trees that
we did successfully sever all the sapwood on,
and I'm gonna refer to them moving forward as droughted
because they are experiencing the drought.
Another really really surprising thing was that
these droughted trees lasted at least 12 weeks
without showing evidence of water stress as well.
So I have a graph here showing midday leaf water potential,
the droughted trees are in red.
So you do see that they are starting to decline this week
since the drought was initiated.
They are declining,
but they don't reach these kinds of detrimental levels
until around the end of September.
And they don't reach their P50 value,
which is the the water potential at which 50% of the xylem
has lost embolism and can't transport water.
They didn't reach that level
until 15 weeks after we initiated the drought.
So this happened over a much longer period of time
than we were expecting this to happen,
kind of within a month or so.
So we ended up looking also at stomatal conductance
to try and see, you know,
what is actually going on with the mechanisms.
And we found these droughted trees
did have greatly reduced stomatal conductance.
So this is indicating their stomates were tightly closed.
So when the stomata are open,
active photosynthesis is happening,
transpiration is happening.
So water is being lost through these open stomata
and our droughted trees closed up their stomata
to prevent water loss.
But as a result their photosynthesis
is gonna be really greatly reduced.
Another evidence of the drought that we did see
was declines in relative water content in the trunk wood.
So here is a graph of all three of our treatments
showing there is some seasonal variation
in relative water content that always occurs
and that's to be expected.
However, again, at around week 12,
we see that the droughted trees
are not really able to recover potentially
due to so much of their xylem being filled with embolisms
and being unable to refill with water
which would cause their relative water content
to go up again.
It's important to note that
while the droughted trees lasted so long
without really true signs of stress,
they did eventually die.
And this is a graph showing water potentials
of branch wood and crown-wood
from the final harvest date which was in November,
showing that by the experiments,
and we did reach these catastrophic levels,
that the tree would not be able to recover from.
Whereas the other two treatments,
the partially severed and the phloem girdled
they never reached these low levels.
So our conclusions,
going back again to thinning shock.
Due to increase temperature and vapor pressure deficit,
we did see evidence of thinning shock
which was lowered midday water potentials
and this significantly decreased photosynthetic efficiency.
The as little as 2% of sapwood left intact
seems sufficient to transport
you know, 30% of prior sap flow volumes
and that prevented water stress
throughout the duration of the study.
And then our last big finding was that
there was no clear signs of water stress
in the needles or the trunk for about 12 weeks
even in these trees that had 100% reductions in sap flow.
So this has pretty big implications for future research
and just the future in general,
what to think about on heavily thinning stand
of spruce-fir forest and future productive climates
could further increase stand temperature
and vapor pressure deficit above of where it already is
due to chronic change.
So more studies needed
on how this is gonna affect tree physiology,
the water relations,
and just treat growth as a whole,
by looking at, you know, increment cores.
That's something that a longer term study doing this over,
you know, a few years would be able to look at
the actual effect on the growth itself.
And then secondly,
for the drought red spruce did show
very strong signs of drought resistance,
but it's important to consider that
this is due to this trade-off
between photosynthesis and water conservation.
So this is really major opportunity cost for for carbon gain
due to the fact they're closing their stomata.
So it's still important to consider that
the species while it may be resistant,
is very sensitive.
You know, they're closing their stomata
and that's preventing them from photosynthesizing
which is their means of survival.
So for a natural drought,
you know that they might be able to overcome,
surviving a natural drought is probably going to show
you know, a greatly reduced growth and like a CFX.
So thinking about this more long-term is important.
I just really quickly like to acknowledge everybody
that was involved in this study,
my funders, all my co-authors,
the university forest staff and my lab mates.
Without them I really could not have done all this work.
Thank you.
And I'll take any questions that you might have.
- Tony McCullough, "should managers of spruce-fir forest
rethink their approach to timber management
in light of your findings?"
- So I think that it's really important to keep in mind
like how important and essential thinning is,
you know, as a tool for forest managers.
I think that the benefits that come from thinning
due to reduced competition,
are more than likely going to outweigh
the negative effects of thinning shock in the future.
But I think that it kind of has more of an impact
on the actual mechanism of thinning that we might use.
You know, I don't know,
as I'm not a silviculturist,
I'm not sure how often
like a full crop tree release of individual red spruce is.
But maybe, you know,
considering using low light thinnings
versus a very heavy crown thinning.
Just thinking more along the terms of how can we
still get all of the beneficial effects of thinning
without exacerbating these potential thinning shock effects
in the future.
- Nancy Patch says "low thinning would have less stress."
- Yeah, exactly.
- You have any other questions?
I think they take a minute to come in on my computer too.
So I'm just giving them a second.
- I know that was a lot of graphs and a lot of data.
If anybody also thinks of questions later
and wants to email me,
that would be great as well.
- Can you put your email in the chat there?
- I'll do that, yes.
- I think it might be accessible other places too.
(keyboard clicking)
Okay, great.
I think we'll close it up then.
And people can email for more questions.
Thank you so much.
- Yeah