of 18
Enhanced
Carbon
Flux
Response
to
Atmospheric
Aridity
and
Water
Storage
Deficit
During
the
2015–2016
El
Niño
Compromised
Carbon
Balance
Recovery
in
Tropical
South
America
Junjie
Liu
1,2
,
Kevin
Bowman
1,3
,
Paul
I.
Palmer
1,4
,
Joanna
Joiner
5
,
Paul
Levine
1
,
A.
Anthony
Bloom
1
,
Liang
Feng
4,6
,
Sassan
Saatchi
1
,
Michael
Keller
1,7
,
Marcos
Longo
1,8
,
David
Schimel
1
,
and
Paul
O.
Wennberg
2
1
NASA
Jet
Propulsion
Laboratory,
California
Institute
of
Technology,
Pasadena,
CA,
USA,
2
California
Institute
of
Technology,
Pasadena,
CA,
USA,
3
Joint
Institute
for
Regional
Earth
System
Science
and
Engineering,
University
of
California
Los
Angeles,
Los
Angeles,
CA,
USA,
4
National
Centre
for
Earth
Observation,
University
of
Edinburgh,
Edinburgh,
UK,
5
Goddard
Space
Flight
Center,
Greenbelt,
MD,
USA,
6
School
of
GeoSciences,
University
of
Edinburgh,
Edinburgh,
UK,
7
USDA
Forest
Service,
International
Institute
of
Tropical
Forestry,
San
Juan,
PR,
USA,
8
Now
at
Climate
and
Ecosystem
Sciences
Division,
Lawrence
Berkeley
National
Laboratory,
Berkeley,
CA,
USA
Abstract
During
the
2015–2016
El
Niño,
the
Amazon
basin
released
almost
one
gigaton
of
carbon
(GtC)
into
the
atmosphere
due
to
extreme
temperatures
and
drought.
The
link
between
the
drought
impact
and
recovery
of
the
total
carbon
pools
and
its
biogeochemical
drivers
is
still
unknown.
With
satellite‐constrained
net
carbon
exchange
and
its
component
fluxes
including
gross
primary
production
and
fire
emissions,
we
show
that
the
total
carbon
loss
caused
by
the
2015–2016
El
Niño
had
not
recovered
by
the
end
of
2018.
Forest
ecosystems
over
the
Northeastern
(NE)
Amazon
suffered
a
cumulative
total
carbon
loss
of
0.6
GtC
through
December
2018,
driven
primarily
by
a
suppression
of
photosynthesis
whereas
southeastern
savannah
carbon
loss
was
driven
in
part
by
fire.
We
attribute
the
slow
recovery
to
the
unexpected
large
carbon
loss
caused
by
the
severe
atmospheric
aridity
coupled
with
a
water
storage
deficit
during
drought.
We
show
the
attenuation
of
carbon
uptake
is
three
times
higher
than
expected
from
the
pre‐drought
sensitivity
to
atmospheric
aridity
and
ground
water
supply.
Our
study
fills
an
important
knowledge
gap
in
our
understanding
of
the
unexpectedly
enhanced
response
of
carbon
fluxes
to
atmospheric
aridity
and
water
storage
deficit
and
its
impact
on
regional
post‐
drought
recovery
as
a
function
of
the
vegetation
types
and
climate
perturbations.
Our
results
suggest
that
the
disproportionate
impact
of
water
supply
and
demand
could
compromise
resiliency
of
the
Amazonian
carbon
balance
to
future
increases
in
extreme
events.
Plain
Language
Summary
The
carbon
storage
in
tropical
South
America
(SA,
15°S
10°N)
is
equivalent
to
approximately
one
third
of
the
carbon
currently
residing
in
the
atmosphere.
The
future
durability
of
this
carbon
reservoir
is
very
uncertain,
contributing
significantly
to
the
uncertainties
of
the
global
carbon
cycle
predictions.
The
soil
and
the
overlying
atmosphere
of
tropical
SA
is
expected
to
become
drier
in
the
future
so
it
is
critical
we
understand
how
the
carbon
cycle
will
respond
to
this
combined
atmospheric
and
soil
drought.
Using
several
carbon
flux
quantities
that
are
constrained
by
satellite
observations,
we
quantified
the
recovery
of
total
carbon
loss
after
the
2015–2016
drought
at
regional
scale.
Our
work
sheds
light
on
how
the
combined
effect
of
atmosphere
dryness
and
soil
drought
exacerbates
the
impact
of
either
atmosphere
dryness
or
soil
drought
alone
and
delays
the
recovery
of
total
carbon
storage
after
the
drought.
Consequently,
our
study
suggests
that
tropical
SA
may
eventually
become
a
source
of
carbon
to
the
atmosphere
due
to
increasing
drought
events
and
decreasing
soil
water
storage,
instead
of
its
current
net
carbon‐neutral
state.
1.
Introduction
With
the
carbon
storage
equivalent
to
approximately
one
third
of
the
carbon
currently
residing
in
the
atmosphere
(S.
S.
Saatchi
et
al.,
2011
),
tropical
South
America
(SA,
15°S
10°N)
is
a
primary
contributor
to
the
uncertainties
of
the
global
carbon
cycle
predictions
(Bonan
et
al.,
2019
)
(IPCC
AR5,
Chapter
6).
The
large
carbon
stock
and
poor
understanding
of
the
sensitivity
of
biogeochemical
processes
to
climate
stressors
highlight
the
need
to
better
understand
the
carbon
dynamics
over
the
region.
In
response
to
the
2015–2016
El
Niño,
this
region
experienced
RESEARCH
ARTICLE
10.1029/2024AV001187
Peer
Review
The
peer
review
history
for
this
article
is
available
as
a
PDF
in
the
Supporting
Information.
Key
Points:
The
total
carbon
loss
caused
by
the
2015–2016
El
Nino
had
not
recovered
by
the
end
of
2018
The
slow
recovery
is
attributed
to
the
unexpected
carbon
loss
caused
by
severe
atmospheric
aridity
and
water
storage
deficit
during
drought
The
attenuation
of
carbon
uptake
is
three
times
higher
than
expected
from
the
pre‐drought
carbon‐climate
sensitivity
Supporting
Information:
Supporting
Information
may
be
found
in
the
online
version
of
this
article.
Correspondence
to:
J.
Liu,
junjie.liu@jpl.nasa.gov
Citation:
Liu,
J.,
Bowman,
K.,
Palmer,
P.
I.,
Joiner,
J.,
Levine,
P.,
Bloom,
A.
A.,
et
al.
(2024).
Enhanced
carbon
flux
response
to
atmospheric
aridity
and
water
storage
deficit
during
the
2015–2016
El
Niño
compromised
carbon
balance
recovery
in
tropical
South
America.
AGU
Advances
,
5
,
e2024AV001187.
https://doi.org/10.1029/
2024AV001187
Received
15
JUL
2022
Accepted
4
JUL
2024
Author
Contributions:
Conceptualization:
Junjie
Liu
Data
curation:
Junjie
Liu,
Paul
I.
Palmer,
Joanna
Joiner,
Paul
Levine,
A.
Anthony
Bloom,
Liang
Feng
Formal
analysis:
Junjie
Liu
Funding
acquisition:
Junjie
Liu,
Kevin
Bowman,
Paul
I.
Palmer
Investigation:
Junjie
Liu,
Kevin
Bowman,
Paul
I.
Palmer,
Joanna
Joiner,
Paul
Levine,
A.
Anthony
Bloom,
Sassan
Saatchi,
Michael
Keller,
Marcos
Longo,
David
Schimel,
Paul
O.
Wennberg
©
2024.
The
Author(s).
This
is
an
open
access
article
under
the
terms
of
the
Creative
Commons
Attribution
License,
which
permits
use,
distribution
and
reproduction
in
any
medium,
provided
the
original
work
is
properly
cited.
LIU
ET
AL.
1
of
18
high
temperatures
coupled
with
one
of
the
most
severe
and
enduring
droughts
on
record
(Jimenez
et
al.,
2018
).
Compared
to
the
previous
2005
and
2010
Amazonian
drought,
the
drought‐stricken
area
was
most
widespread
in
2015–2016.
The
associated
reduction
of
above
ground
biomass
due
to
this
drought
had
not
recovered
by
the
end
of
2017
(Wigneron
et
al.,
2020
);
at
which
time
it
is
also
unclear
whether
the
total
carbon
loss
(including
both
the
above
and
below
ground
biomass
and
soil
carbon)
had
recovered.
In
spite
of
a
rich
literature
that
has
investigated
the
impact
of
drought
and
its
recovery
on
the
above
ground
biomass
with
plot
and
remote
sensing
data
over
the
Amazon
(Lewis
et
al.,
2011
;
Phillips
et
al.,
2009
;
S.
Saatchi
et
al.,
2013
),
studies
that
quantify
the
recovery
of
total
carbon
loss
after
drought
at
regional
scale
are
largely
absent
from
the
literature.
As
the
2015–2016
drought
was
such
a
large
perturbation
to
the
ecosystem,
it
is
critically
important
to
understand
the
link
between
the
drought
impact
and
recovery
of
the
total
carbon
pools
and
its
biogeochemical
drivers.
While
precipitation
is
a
key
measure
of
drought,
the
balance
of
water
supply
indicated
by
total
water
storage
(TWS)
and
atmospheric
water
demand
represented
by
vapor
pressure
deficit
(VPD;
difference
between
saturation
and
actual
water
vapor
pressure)
can
have
a
more
direct
impact
on
plant
functioning
(Sulman
et
al.,
2016
).
In
particular,
increasing
VPD
tends
to
suppress
photosynthesis
and
stimulate
transpiration
over
wet
tropics
(Grossiord
et
al.,
2020
).
While
ground
water
can
act
as
a
buffer
on
atmospheric
demand,
under
prolonged
or
severe
drought,
TWS
could
be
greatly
reduced.
High
VPD
coupled
with
large
reductions
in
TWS
can
accelerate
tree
mortality
through
either
hydraulic
failure
or
carbon
starvation
(Bonal
et
al.,
2016
;
Breshears
et
al.,
2013
).
Over
tropical
SA,
VPD
has
increased
in
recent
decades
(Barkhordarian
et
al.,
2019
)
and
is
projected
to
continue
increasing
(Jensen
et
al.,
2020
),
while
TWS
is
projected
to
decrease
(Y.
Pokhrel
et
al.,
2021
).
Consequently,
it
is
crucial
to
enhance
our
understanding
of
how
the
sensitivity
of
carbon
fluxes
to
atmospheric
aridity
can
change,
particularly
in
conjunction
with
TWS
anomalies,
and
how
these
factors
collectively
contribute
to
carbon
flux
anomalies
during
drought
and
subsequent
recovery.
Assessing
these
aspects
is
essential
for
mitigating
un-
certainties
associated
with
carbon‐climate
feedbacks
(Barkhordarian
et
al.,
2021
;
Cook
et
al.,
2019
;
Ukkola
et
al.,
2020
).
The
tropical
SA
exhibits
a
remarkable
diversity
of
plant
types,
shaped
by
varying
mean
climates
and
a
complex
history
of
human
impact.
Covering
about
two‐thirds
of
the
area,
the
Amazon
rainforests
boast
a
significantly
wetter
climate
compared
to
the
Cerrado
region,
which
occupies
most
of
the
remaining
one‐third
and
is
charac-
terized
by
savannas
and
dry
forests.
Recent
trends
in
deforestation
rates
highlight
contrasting
patterns
between
these
regions,
with
the
Amazon
experiencing
a
decrease
in
deforestation
while
the
Cerrado
is
seeing
an
alarming
increase,
primarily
due
to
demographic
pressures
and
expanding
agriculture.
Furthermore,
the
role
of
fires
in
these
ecosystems
differs
substantially;
while
fire
is
not
a
natural
disturbance
mechanism
in
the
Amazon,
the
Cerrado
has
evolved
with
fire‐adapted
vegetation
and
is
naturally
prone
to
fires.
Even
within
the
Amazon,
there
are
pronounced
variations
in
drought
severity
in
2015–2016.
For
instance,
the
northeast
region
experienced
the
most
severe
drought
in
2015–2016
(Jiménez‐Muñoz
et
al.,
2016
)
and
is
predicted
to
become
significantly
drier
compared
to
the
rest
of
the
Amazon
(Baker
et
al.,
2021
).
These
distinctions
suggest
that
the
response
of
these
regions
to
droughts
and
subsequent
recovery
would
be
regionally
distinct
(Stuart‐Haëntjens
et
al.,
2018
).
To
better
understand
these
nuances
and
dependencies,
we
divide
tropical
South
America
into
three
sub‐regions:
(a)
the
northeast
(NE‐Amazon)
and
(b)
the
west
and
southwest
(WSW‐Amazon)
moist
tropical
rainforest,
and
(c)
the
southeast
dry
tropical
forest
and
savanna
(SE‐Savanna)
(Figure
S1
in
Supporting
Information
S1
).
The
NE‐
Amazon
includes
those
grid
points
over
the
northeast
part
of
the
Amazon
Forest
that
have
annual
mean
pre-
cipitation
less
than
5
mm
/day
and
the
VPD
variability
more
than
0.4
hPa,
with
a
few
exceptional
points
over
each
region
to
ensure
continuity
(Figures
S1
and
S2
and
Table
S1
in
Supporting
Information
S1
).
The
SE‐Savanna
includes
the
Cerrado
region.
Among
these
three
regions,
the
precipitation
variability
in
the
NE‐Amazon
has
the
highest
correlation
(
r
=
0.6)
with
the
Oceanic
Nino
index,
implying
a
stronger
teleconnection
with
El
Nino
Southern
Oscillation
(ENSO)
cycle,
while
the
SE‐Savana
has
the
lowest
correlation
(
r
=
0.2).
Previous
studies
assessing
the
impact
of
drought
and
its
recovery
in
the
Amazon
have
primarily
relied
on
pre-
cipitation
throughfall
experiments
(Brando
et
al.,
2019
;
da
Costa
et
al.,
2010
;
Meir
et
al.,
2009
),
along
with
limited
plot
data
and
aboveground
biomass
data
inferred
from
remote
sensing
(Lewis
et
al.,
2011
;
Phillips
et
al.,
2009
;
S.
Saatchi
et
al.,
2013
;
Wigneron
et
al.,
2020
).
Satellite‐based
net
biosphere
exchange
(NBE,
the
net
carbon
flux
from
all
land–atmosphere
exchange
processes
except
fossil
fuel
emissions)
and
its
component
fluxes
provide
a
complementary
viewpoint
allowing
us
to
quantify
total
carbon
pool
changes
at
the
subregional
scale
over
tropical
SA.
Using
9‐years
(2010–2018)
of
monthly
satellite‐constrained
NBE
estimates
and
their
component
fluxes
(i.e.,
Gross
Primary
Production
(GPP),
ecosystem
respiration
and
wildfire
emissions),
we
aim
to
address
the
following
Methodology:
Junjie
Liu,
Kevin
Bowman,
Paul
I.
Palmer,
Joanna
Joiner,
Paul
Levine,
A.
Anthony
Bloom
Resources:
Junjie
Liu
Software:
Junjie
Liu
Validation:
Junjie
Liu,
Kevin
Bowman
Visualization:
Junjie
Liu,
Kevin
Bowman,
Paul
I.
Palmer,
Paul
O.
Wennberg
Writing
original
draft:
Junjie
Liu,
Kevin
Bowman,
Paul
I.
Palmer
Writing
review
&
editing:
Junjie
Liu,
Kevin
Bowman,
Paul
I.
Palmer,
Joanna
Joiner,
Paul
Levine,
A.
Anthony
Bloom,
Liang
Feng,
Sassan
Saatchi,
Michael
Keller,
Marcos
Longo,
David
Schimel,
Paul
O.
Wennberg
AGU
Advances
10.1029/2024AV001187
LIU
ET
AL.
2
of
18
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