Pastore, M.A., Lee, T.D.,
Hobbie, S.E. and Reich, P.B.
2019
Strong photosynthetic acclimation
and enhanced water-use efficiency
in grassland functional groups
persist over 21 years
of CO2 enrichment,
independent of
nitrogen supply.
Global Change Biology 25: 3031-3044.
NOTE:
Long-term experiments
conducted in the field
are the gold standard
for examining
green plant responses
to higher CO2 levels.
But few studies have
examined plants beyond
one growing season.
Very few studies
lasted for one decade.
Pastore et al. (2019) provided
an update on their findings
after 21 years.
SUMMARY:
The scientists note
the positive responses
of the C4 species,
"though small, shows
that "C4 photosynthesis
is not always CO2-saturated
at ambient atmospheric
CO2 concentration"
and highlights
"the need for better
representation
of C4 species in the
CO2 enrichment literature."
The increased water
use efficiency
( iWUE )
of the grassland
species, which:
"could mitigate
the impact
of drought
in the future,
by reducing
plant water use
in a variety
of species,
and increasing
soil moisture
in some
ecosystems."
For net photosynthesis,
the four scientists report
that elevated CO2 stimulated
this parameter by an average of
+11.1% for three C3 perennial
grass species, and
+5.7% for three C4 grasses,
+11.6% for four C3 forbs, and
+14.8% for four C3 legumes
(see the left chart, below).
The authors note
"the positive CO2 effect
[on photosynthesis]
became greater over time"
for each functional group
of grassland species,
including the C4 grasses,
albeit the temporal trend
observed was described
as "modest."
There was no
interactive effect
between elevated CO2
and nitrogen supply,
on photosynthesis.
Pastore et al.
also observed
a CO2-induced
23.3% decline in
stomatal conductance
across all species,
which when coupled with
the photosynthesis gains,
helped explain
a near +50% average
increase in intrinsic
water use efficiency
( iWUE )
under elevated CO2.
As shown in
the right chart below,
iWUE increased by
+55% in C3 grasses,
+38.6% in C4 grasses,
+49.1% in C3 forbs and
+61.1% in C3 legumes.
DETAILS:
This experiment, in operation
for over two decades, was located
at the Cedar Creek Ecosystem
Science Reserve, in East Bethel,
Minnesota, USA.
Scientists have
been investigating
the interactive effects
of CO2, nitrogen
and biodiversity
on various perennial
grassland species
in a free-air
CO2 enrichment
( FACE )
environment.
CO2 enrichment at the site
is maintained at 180 ppm
above ambient, during
daylight hours, of the
growing season.
The charts below show
mean net photosynthesis (left)
and intrinsic water use efficiency
(right) in
(a) C3 grasses,
(b) C4 grasses,
(c) forbs, and
(d) legumes,
all grown at ambient (closed circles)
or elevated (open circles) CO2
over 21 years.
Data for each plant grouping
are pooled across species
and nitrogen treatments.
Hash marks indicate a gap
in years of data collection,
not an axis break.
Error bars represent standard error.
