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Elevated CO
2
does not increase eucalypt forest
productivity on a low-phosphorus soil
Journal Item
How to cite:
Ellsworth, David S.; Anderson, Ian C.; Crous, Kristine Y.; Cooke, Julia; Drake, John E.; Gherlenda, Andrew
N.; Gimeno, Teresa E.; Macdonald, Catriona A.; Medlyn, Belinda E.; Powell, Jeff R.; Tjoelker, Mark G. and Reich,
Peter B. (2017). Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil. Nature
Climate Change, 7(4) pp. 279–282.
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2017 Macmillan Publishers Limited
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1
Elevated CO
2
does not increase eucalypt forest productivity on a low-
phosphorus soil
David S. Ellsworth
1
, Ian C. Anderson
1
, Kristine Y. Crous
1
, Julia Cooke
2
, John E.
Drake
1
, Andrew N. Gherlenda
1
, Teresa E. Gimeno
3
, Catriona A. Macdonald
1
, Belinda
E. Medlyn
1
, Jeff R. Powell
1
, Mark G. Tjoelker
1
, Peter B. Reich
1,4
1
Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW 2751,
Australia,
2
School of Environment, Earth and Ecosystem Science, The Open University,
Milton Keynes MK7 6AA, UK,
3
ISPA, Bordeaux Science Agro, INRA, 33140 Villenave
d'Ornon, France,
4
Department of Forest Resources, University of Minnesota, St. Paul, MN,
55108 USA.
e-mail: D.Ellsworth@westernsydney.edu.au
2
Rising atmospheric CO
2
stimulates photosynthesis and productivity of forests, offsetting 1
CO
2
emissions
1,2
. Elevated CO
2
experiments in temperate planted forests yielded ~23% 2
increases in productivity
3
over the initial years. Whether similar CO
2
stimulation occurs 3
in mature evergreen broadleaved forests on low-phosphorus (P) soils is unknown, 4
largely due to lack of experimental evidence
4
. This knowledge gap creates major 5
uncertainties in future climate projections
5,6
as a large part of the tropics is P-limited. 6
Here, we increased atmospheric CO
2
concentration in a mature broadleaved evergreen 7
eucalypt forest for three years, in the first large-scale experiment on a P-limited site. We 8
show that tree growth and other aboveground productivity components did not 9
significantly increase in response to elevated CO
2
in three years, despite a sustained 10
19% increase in leaf photosynthesis. Moreover, tree growth in ambient CO
2
was 11
strongly P-limited and increased by ~35% with added phosphorus. The findings suggest 12
that P availability may potentially constrain CO
2
-enhanced productivity in P-limited 13
forests; hence, future atmospheric CO
2
trajectories may be higher than predicted by 14
some models. As a result, coupled climate-carbon models should incorporate both 15
nitrogen and phosphorus limitations to vegetation productivity
7
in estimating future 16
carbon sinks. 17
18
Limited understanding of the size of the CO
2
-induced fertilisation effect on forest carbon 19
sinks remains among the largest quantitative uncertainties in terms of terrestrial feedbacks to 20
the carbon (C) cycle-climate system
6,8,9
. Coupled climate-C cycle models project a 24-80% 21
increase of net primary productivity (NPP) for forests in the next 50 years with rising 22
atmospheric CO
2
concentration, with substantial atmospheric CO
2
responses expected for 23
forests in the tropics
4,10
. These model projections are partly based on elevated CO
2
(eCO
2
) 24
experiments in young temperate planted forests, which have yielded on average ~23% 25
3
increases in production
3
over several years with 200 µmol mol
-1
increases in atmospheric 26
CO
2
concentrations
4,11
. Due to the lack of experimental evidence, we presently do not know 27
how large the eCO
2
fertilisation response is for mature forests that grow on soils where 28
phosphorus (P) is limiting productivity
4,10
, as is the case for many evergreen broadleaved 29
forests. This knowledge gap creates major uncertainties in future climate projections
9
because 30
evergreen broadleaved forests comprise over a third of global forest area, and dominate the 31
atmospheric CO
2
sink at lower latitudes
5,6
. Many eCO
2
experiments have taken place in 32
young tree plantations
3
on relatively P-rich soils, but unlike aggrading forests, mature forests 33
are more likely near nutritional equilibrium with their underlying soils. Hence mature forests 34
may be more appropriate for understanding in situ nutrient limitations to productivity and C 35
storage with rising atmospheric CO
2
. Without clear understanding of this nutrient feedback to 36
the C cycle in evergreen broadleaved forests, we cannot accurately estimate the trajectory of 37
future atmospheric CO
2
, thus limiting our ability to estimate climate change mitigation by 38
such forests and constrain internationally-allowable CO
2
emissions
9,12
. 39
40
Soil nutrient limitation may restrict eCO
2
-induced biomass enhancement and related C 41
storage processes
11
, but it is unclear if the type of nutrient limitation is important. Studies in a 42
temperate grassland and a forest ecosystem under contrasting CO
2
and N supply suggest a 43
large initial stimulation in productivity, often followed by reduced CO
2
stimulation when N is 44
limiting
13,14
. Limited P supply might affect tree growth and ecosystem C sequestration 45
processes differently than the N-supply limitation
15
that has thus far been demonstrated in 46
eCO
2
experiments on N-poor soils. In heavily weathered soils common in tropical and 47
subtropical regions, P is typically bound to Fe and Al oxides, hydroxides and secondary 48
minerals and not available to plants. One possibility is that increased plant carbohydrate 49
availability from eCO
2
leads to increased plant investment in the secretion of organic acids 50
4
from roots
16
or the investment in P-acquisition by mycorrhizal symbionts. This would thereby 51
reduce P-limitation to broadleaved evergreen forest productivity
17
by increasing plant access 52
to scarce soil P. Consistent with this idea, there is evidence that recent rising CO
2
may have 53
driven a substantial portion of the observed historical increase in tropical forest carbon 54
stocks
18
though future increases remain in question. 55
56
Although there is considerable variation in soil fertility across the world, tree growth in 57
highly weathered tropical and sub-tropical soils may be limited by P availability in addition 58
to, or rather than, N availability
19,20
. Hence nutrient availability and the type of nutrient 59
limitation may both be important in regulating forest CO
2
fertilisation responses in those 60
regions
7,17
. There is still little agreement on how to appropriately represent P limitations to 61
productivity in Earth systems models
7,21
, and there has been no direct experimental test of the 62
CO
2
fertilisation effect in P-limited forests (Supplementary Fig. 1). 63
64
To help fill this gap, we established a free-air CO
2
enrichment experiment on six circular 25m 65
diameter plots in mature Eucalyptus forest (EucFACE) on a low P soil near Sydney, Australia 66
(23 m elevation; 33° 37' 4" S, 150° 44' 25" E) (Supplementary Fig. 2). The main canopy 67
species, Eucalyptus tereticornis, has a distribution through tropical and temperate zones. 68
EucFACE has unique characteristics compared to prior forest elevated CO
2
experiments: the 69
presence of mature broadleaved evergreen trees in natural unmanaged forest, and nutrient-70
poor soil with a demonstrated P limitation to tree growth
22
. A gradual CO
2
enrichment began 71
in Sept 2012 at 30 µmol mol
-1
above ambient CO
2
concentration, and slowly ramped up to 72
the full-strength eCO
2
treatment of 150 µmol mol
-1
above ambient CO
2
concentration
23
, 73
which began on 6 Feb 2013. This full CO
2
treatment was maintained throughout the 74
following three years (Feb. 2013-Feb. 2016) that are the focus of this report. We 75