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Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO(2) efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204+/-53 ton C ha(-1), with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0-1 m) was 151+/-33 ton C ha(-1), accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53+/-20 ton C ha(-1), 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha(-1), of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha(-1) year(-1), of which 8.0 ton C ha(-1) (73%) was contributed by below-ground net primary production and 3.0 ton C ha(-1) (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha(-1) year(-1). Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5-6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha(-1) year(-1).
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Oecologia (2003) 137:405–416
DOI 10.1007/s00442-003-1358-5
Xiaoyong Chen · Lindsay B. Hutley · Derek Eamus
Carbon balance of a tropical savanna of northern Australia
Received: 13 September 2002 / Accepted: 16 July 2003 / Published online: 26 August 2003
Springer-Verlag 2003
Abstract Through estimations of above- and below-
ground standing biomass, annual biomass increment, fine
root production and turnover, litterfall, canopy respiration
and total soil CO
efflux, a carbon balance on seasonal
and yearly time-scales is developed for a Eucalypt open-
forest savanna in northern Australia. This carbon balance
is compared to estimates of carbon fluxes derived from
eddy covariance measurements conducted at the same
site. The total carbon (C) stock of the savanna was
204€53 ton C ha
, with approximately 84% below-
ground and 16% above-ground. Soil organic carbon
content (01 m) was 151€33 ton C ha
, accounting for
about 74% of the total carbon content in the ecosystem.
Vegetation biomass was 53€20 ton C ha
, 39% of which
was found in the root component and 61% in above-
ground components (trees, shrubs, grasses). Annual gross
primary production was 20.8 ton C ha
, of which 27%
occurred in above-ground components and 73% below-
ground components. Net primary production was 11 ton C
, of which 8.0 ton C ha
(73%) was
contributed by below-ground net primary production
and 3.0 ton C ha
(27%) by above-ground net primary
production. Annual soil carbon efflux was 14.3 ton C ha
. Approximately three-quarters of the carbon flux
(above-ground, below-ground and total ecosystem) occur
during the 5–6 months of the wet season. This savanna
site is a carbon sink during the wet season, but becomes a
weak source during the dry season. Annual net ecosystem
production was 3.8 ton C ha
Keywords CO
· Carbon cycling · Wet-dry tropics ·
Carbon source-sink relationships · Net ecosystem
Savannas, covering at least 16 million km
of the earth’s
land surface, are found in Africa, Australia, South
America, India and Southeast Asia and occupy the
latitudinal zone between evergreen tropical rainforest
and mid-latitude deserts (Scholes and Hall 1996). Savan-
nas are characterised by climates with distinct wet and dry
seasons and this has induced correspondingly strong
patterns in physiological and eco-physiological processes
(Eamus and Prior 2001). Savannas account for approxi-
mately 58.7 Pg of biomass, approximately 30% the global
carbon store of terrestrial ecosystems and savannas
therefore have the potential to significantly influence
global carbon cycling. Scurlock and Hall (1998) and Lal
(2002) suggest that tropical savannas and grasslands play
a more significant role in global carbon sequestration than
previously thought, with soil carbon storage of particular
Also of global importance is the extensive annual
biomass burning that occurs in savanna ecosystems
during the dry season, which results in a large quantity
of carbon and other trace greenhouse gases (methane,
) being released to the atmosphere (Andreae et al.
1996; Scholes et al. 1996). In the wet-dry tropics of
northern Australia, tropical savanna is the dominant
vegetation type and approximately 75% of Australia’s
total land area that is burnt annually occurs in this region
(AGO-NGGI 2000). These Australian savannas occupy an
area of almost 2 million km
, which is 12% of the worlds
savannas biome and some of the world’s most extensive
and intact Eucalypt open forest is located here. Given the
X. Chen · L. B. Hutley (
) · D. Eamus
Cooperative Research Centre for the Sustainable
Development of Tropical Savannas, Faculty of Science,
Information Technology and Education,
Northern Territory University, NT 0909 Darwin, Australia
Fax: +61-8-89466847
Present address:
X. Chen, Department of Geography,
University of Toronto, M5S 3G3 Toronto, Ontario, Canada
Present address:
D. Eamus,
Institute for Water and Environmental Resource Management,
University of Technology—Sydney,
Broadway, P.O. Box 123, NSW 2007 Sydney, Australia
size of this ecosystem and the extent of burning, it is
likely that savannas will have a major impact on
continental-scale carbon balance.
North Australian savannas are dominated by Eucalyp-
tus tree species which form an open overstorey canopy
(<50% cover) and a variety of annual and perennial C
grasses dominating the understorey (Williams et al.
1997). These savannas have been subjected to minimal
European disturbance when compared to Eucalypt dom-
inated ecosystems of southern Australia (Tothill at al.
1985). While there is an extensive ecological literature
describing savannas of Australia, plus knowledge of
ecophysiological processes at leaf (Eamus et al. 1999,
2000), tree (O’Grady et al. 1999; Eamus et al. 1999;
Myers et al. 1997), canopy and stand scale (Hutley et al.
2000, 2001; O’Grady et al. 2000; Eamus et al. 2001),
there are no detailed studies of the carbon balance for
these savannas. Most productivity studies of Australia’s
tropical savanna have concentrated on the herbaceous
layer, with a focus on agricultural potential (e.g. Mott et
al.1985; Williams et al. 1985). There are also no studies
of savanna carbon dynamics where both stocks and fluxes
have been measured at the same site and over the same
time period (House and Hall 2001).
In this paper, a range of measurements has been
integrated to establish a carbon balance for a tropical
savanna site of coastal northern Australia. Carbon pool
size and fluxes have been estimated on a seasonal and
annual basis to address the following questions: What are
the fluxes of carbon to and from these tropical savannas?
What are the allocation patterns of carbon among above-
and below-ground components? Are there seasonal
differences in carbon storage and carbon distribution?
What are the seasonal and annual budgets of carbon for
this ecosystem? For comparative purposes, we have
generated data tables, providing values for a wide range
of parameters, using a similar approach to that of Malhi et
al. (1999).
Materials and methods
Study sites
To obtain a typical range of values for the carbon balance
components, four study sites were used, all located within a 65 km
radius of Darwin, Northern Territory, Australia. Sites were located
at (1) Howard Springs (1228
S, 13108
E), (2) Humpty Doo
S, 13110
E), (3) the Territory Wildlife Park (1242
E) and (4) a fourth site at Gunn Point (1214
S, 13105
The vegetation at all sites was Eucalypt open-forest savanna with
an overstorey dominated by Eucalyptus tetrodonta (F. Muell) and
Eucalyptus miniata (Cunn. Ex Schauer). These two species
contribute >70% to the overstorey leaf area index (LAI) and
standing biomass (O’Grady et al. 2000). Sub-dominant tree species
include Erythrophyleum chlorostachys (F. Muell), Terminalia
ferdinandiana (F. Muell), Eucalyptus porrecta (S.T. Blake) and
Eucalyptus bleeseri (Blakely). The understorey is comprised of
semi-deciduous and deciduous small trees and shrubs with a
seasonally continuous cover of annual and, to a small extent,
perennial C4 grasses. Overstorey LAI of these sites typically ranges
from 0.6 to 1 with basal areas approximately 8–12 m
(O’Grady et al. 2000), with understorey LAI being far more
seasonally dynamic and ranging from 0.2 to 1.5.
Sites used in this study are representative of the open-forest
savannas of the coastal regions of the Northern Territory, which
receive annual rainfalls in excess of 1,200–1,400 mm and are
dominated by E. tetrodonta and E. miniata with Sorghum spp
frequently occurring in the understorey (Wilson et al. 1990). This
vegetation type occupies an area of approximately 135,000 km
This and other closely associated Eucalypt dominated savanna
types also occur in north-western Western Australia and the Gulf of
Carpentaria region of northern Queensland and occupy up to
200,000 km
in northern Australia (Fox et al. 2001). Low intensity
grass fires (fire line intensities of 2,000–8,000 kW m
, Williams et
al. 1999) are common, with sites burnt approximately 2 in every
3 years and annually at sites near urban settlements.
Eucalyptus tetrodonta and E. miniata open-forest savanna are
commonly associated with well drained lateritic red and yellow
earth soils, which tend to have A horizons of well drained, highly
weathered sands (clay content typically <5%, Calder and Day
1982) of low nutrient status, with a massive and earthy structure.
Transition at 15–30 cm to a sandy loam B horizon is gradational
and can extend up to 1–2 m, where ferricrete boulders occur in a
matrix of mottled, heavy clays forming a duricrust of low
permeability and variable depth (Calder and Day 1982). Prominent
macropores, often containing tree roots, are found in this layer.
Rounded ferricrete gravels can occur on the sandy soil surface and
throughout the profile up to 20% by volume. These soils are
generally acidic (pH approximately 5.5, Calder and Day 1982) and
low in available N and P (total N, 0.1% Schmidt et al. 1998). Dry
bulk densities of these soils range from approximately 1.4 kg m
the surface to 1.7 kg m
at depth (2 m).
The climate of the region is wet-dry tropical and rainfall is
distinctly seasonal, with a wet season occurring from November to
April (Cook and Heerdegen 2001). During this period, greater than
90% of the 1,700 mm annual rainfall occurs. The dry season occurs
from May to October with little or no rainfall. Temperatures remain
high throughout the year with mean daily maximum temperatures
at the Darwin Airport (35 km from the field site) ranging from
30.4C (July) to 33.1C (October and November). Maximum and
minimum temperatures have a range of 7C (wet season) to 11C
(dry season) (McDonald and McAlpine 1991).
Measurements and calculations
Carbon stocks
All measures of carbon pools and fluxes were derived from the
suite of measurements conducted at the four sites. A brief
description of these parameters, methods and their estimation is
given in Table 1. The magnitude of carbon pools (above- and
below-ground biomass, soil organic matter) and fluxes between
these pools (litterfall, soil respiration, root turnover, growth
increment) have been integrated to calculate above- and below-
ground allocation and derive a carbon balance. The range of
methods used is briefly described below, with further details
available in cited publications.
Above-ground biomass (AGB) was estimated from plot-based
measurements of tree diameter at breast height (DBH) and tree
height. Three 2020 m plots were located at each site and biomass
components (wood, bark, branch, leaf) were estimated from 48
harvested trees from 6 dominant tree species (Eucalyptus tetrodon-
ta, E. miniata, Erythrophyleum chlorostachys, T. ferdinandiana,
Eucalyptus porrecta and E. bleeseri). These species account for
95% of the standing biomass in these open-forest savannas
(O’Grady et al. 2000). Allometric regression equations (power
functions) relating tree DBH and biomass have been developed for
these and other species at these sites (O’Grady et al. 2000; Chen
2002). The carbon content of AGB was assumed to be 49% of dry
weight (Gifford 2000a) and 43% for grass fine roots.
Below-ground biomass (BGB) was estimated using the trench
method (Komiyama et al. 1987). Eamus et al. (2002) provides
details of these measurements for the Humpty Doo site, which
involved root excavation from 16 trees in 8 soil trenches (4–5 m in
length, up to 2 m depth). Roots were removed from excavated soil
blocks of known volume and sorted into coarse (>2 mm diameter)
and fine (<2 mm diameter) root fractions. The carbon content of
below-ground biomass was assumed to be 49% of dry weight
(Gifford 2000b).
Soil carbon stock was estimated using soil organic carbon
(SOC) content and soil bulk density. At each study site, three plots
were chosen for soil sampling with samples collected at 5, 20, 30,
50, 80 and 100 cm depths. SOC was determined using an improved
Walkley-Black wet digestion method (Heanes 1984). Percent SOC
values were converted to soil carbon stock (ton C ha
) using a bulk
density for each soil layer. Bulk density was determined from pits
dug at the Howard Springs site by taking 3–5 replicate soil samples
using 10 cm diameter metal rings of 100 cm
volume (A. O’Grady,
personal communication). Bulk densities ranged from 1.42 at 5 cm
depth to 1.7 g cm
at 1 m.
Above-ground carbon flux
Above-ground net primary production (ANPP) was estimated by
summing annual increments of all components of biomass plus
litterfall (Table 1). AGB was calculated from radial increments of
tree diameter and an allometric regression equation relating tree
diameter to AGB (O’Grady et al. 2000). Annual increment of
understorey biomass was calculated from seasonal maximum and
minimum values of understorey biomass, measured monthly for
one year, using destructive harvests of five randomly located
replicate 1 m
plots sampled at three different locations at the
Humpty Doo site. Litterfall was measured using 18 litter traps over
a 2-year period (1998–2000) at the Wildlife Park site. Traps, with
an area of 2,463 cm
for each, were set 80 cm above the ground and
litter (leaves, bark and fruit) were collected at monthly intervals and
dry weight determined.
Above-ground tree respiration was divided into four sources
(Table 1): leaf construction respiration (R
), leaf maintenance
respiration (R
), woody components construction respiration (R
and woody components maintenance respiration (R
). Construc-
tion respiration of leaf and woody components was calculated using
leaf and woody biomass increment multiplied by the construction
constant of 0.25 g C g C
(Keith et al. 1997). This assumed that
construction respiration consumes 25% of the carbon allocated
annually to each biomass component (Ryan 1991). Leaf mainte-
nance respiration (R
) was calculated using the following equation
from Ryan (1991):
¼ N
ðÞ27 exp 0:07T
ðÞ½ ð1Þ
where N
(g N m
) is the total leaf nitrogen content obtained from
leaf nitrogen concentrationleaf biomass, and T
(C) is the average
annual temperature, although mean dry season and wet season
temperatures were used. Leaf nitrogen concentration data was taken
from Eamus and Prichard (1998) for both E. tetrodonta and E.
miniata. Wood maintenance respiration (R
) was calculated using
the following equation developed by Ryan and Waring (1992):
¼ 0:00486V
exp 0:0663T
ðÞ ð2Þ
where V
) is sapwood volume. Sapwood volume for a stand
was calculated using regression equations derived between sap-
wood basal area and tree diameter for each dominant Eucalypt
species present in plots at the Howard Springs and Humpty Doo
sites. These relationships (sapwood area and DBH) have been
previously established at these sites for the dominate tree species by
O’Grady et al. (1999). Above-ground gross primary production
(AGPP) is then the sum of ANPP and above-ground tree
Below-ground carbon flux
Below-ground carbon fluxes were estimated from measures of
coarse and fine root production. Fine root production was estimated
using in-growth bags and coarse root production was estimated
from a simple allometric equation which assumes that coarse root
production is proportional to above ground NPP (Johnson and
Risser 1974):
where NPP
is coarse root net primary production, ANPP is above-
ground net primary production, AGB is above-ground biomass, and
is coarse root biomass. Root respiration was assumed to be 50%
of total soil respiration (Ewel et al. 1987; Keith et al. 1997; Haynes
and Gower 1995).
Table 1 Sources of data used to calculate components of the savanna carbon balance. Measurements were divided into wet (November–
April) and dry (May–October) seasons
Definition Data source
Above-ground biomass production Annual increment in DBH of 20 trees monitored using dendrometers
Biomass components (leaf, branch and stem) calculated using regression equations
(O’Grady et al. 2000)
Production of all components calculated from tree DBH increment
Coarse root production Derived from Johnson and Risser (1974), using AGB, DB
and BGBAGB from tree
harvesting (Satoo and Madgwick
1982, O’Grady et al. 2000) and plot biomass sampling
BGB from root trench method (Komiyama et al. 1987, Eamus et al 2002)
Fine root production Root ingrowth bags and root window methods (Vogt et al. 1998)
Leaf construction respiration Derived from Ryan (1991) and Keith et al. (1997) using leaf production
Leaf maintenance respiration Derived from Ryan (1991) using average temperature (Darwin airport) and total leaf N
content for dominant Eucalypt tree species (Eamus and Prichard, 1998)
Wood construction respiration Derived from Ryan (1991), Keith et al. (1997) using woody (branch and stem)
Wood maintenance respiration Derived from Ryan and Waring (1992) using an average temperature (Darwin airport)
and sapwood volume (O’Grady et al. 1999)
Soil CO
flux Soil respiration was measured using close chamber technique (Chen et al. 2002)
Root respiration Assumed to be 50% of total soil respiration (Keith et al. 1997)
Fine root production (NPP
) was estimated using in-growth
bags (Smit et al. 2000) with details given by Chen et al (2003). A
total of 72 in-growth bags were installed at the Howard Springs
site. Soil cores were dug to a depth of 50 cm with fine roots
removed and the resulting root-free soil used to fill in-growth mesh
bags. Bags filled with root-free soil were then inserted into
750 cm deep holes and the rate of in-growth of new fine roots
(productivity) determined by sequential re-sampling at 2-monthly
intervals from October 1999 to January 2001.
Soil carbon efflux was measured using a closed chamber
technique (Rochette et al. 1997). Chen et al. (2002) provides
detailed description of measurements at the Howard Springs site,
which involved the use of a polythene chamber (2021.512 cm) in
conjunction with portable infra-red gas analyzer (LI-6200, Licor,
Lincoln, Neb., USA). Soil CO
efflux (F
) was calculated as the
rate of change over time of CO
concentration in the chamber
(Chen et al. 2002). Estimates of F
were made every 4 h over a 2–
3 day period each month for over 2 years (September 1998 to
January 2001). The 2–3 day measurement period was assumed to
represent mean monthly F
and was used to calculate monthly and
annual rates of soil CO
Production indices
Gross primary production (GPP) is defined as the total carbon
assimilated by photosynthesis, minus photorespiration. Net primary
production (NPP) is defined as the difference between GPP and
autotrophic respiration (R
), representing the net result of CO
fixation by photosynthesis and CO
loss via plant respiration. Net
ecosystem production (NEP) is the net carbon balance of an
ecosystem over some time period (usually a year) and represents
net carbon fixation by photosynthesis and losses by autotrophic
plus heterotrophic respiration (respiration of soil organisms, R
(Kirschbaum 2001). As NEP reflects the annual change in C stored
at an ecosystem scale, it indicates whether the ecosystem is a
carbon “sink” or “source” for CO
relative to the atmosphere.
Carbon stocks in tropical savanna of northern Australia
Table 2 gives the carbon stocks of different components
of the savanna ecosystem. Data have been averaged using
data from the three sites (Howard Springs, Humpty Doo
and Territory Wildlife Park). The mean total carbon pool
was 204 ton C ha
(range 136 to 286 ton C ha
) with
approximately 84% of the carbon stored below-ground
(soil plus roots). Approximately 74% of the total C was
stored in the mineral soil as SOC (mean 151.3 ton ha
Table 2). Carbon stored in the tree component was the
next largest pool, which accounted for 24% of the total
carbon, followed by understorey (0.5%), litter-layer
(0.5%) and dead stems (0.5%). Eucalypt species domi-
nated the total carbon stored in vegetation, which was
50 ton C ha
(range 23–76.0 ton C ha
). Above-ground
woody components accounted for 64% of the total
vegetation pool (53 ton C ha
, live plus dead compo-
nents) with total root carbon at 19 ton ha
or 36% of the
total vegetation pool.
Above-ground carbon flux
Table 3 provides wet and dry season and annual estimates
of carbon fluxes between the various carbon pools. Total
carbon flux above-ground was calculated by summing the
carbon fluxes associated with tree biomass increment,
litterfall, understorey biomass increment, plus construc-
tion and maintenance respiration. This sum is AGPP
(Table 3). In the present study, the total carbon flux
above-ground was 5.7 ton C ha
, of which tree
biomass increment accounted for 28%, foliage respiration
accounted for 26%, wood respiration accounted for 21%,
litterfall accounted for 16% and understorey biomass
increment accounted for 9%.
There may be uncertainty associated with seasonal and
annual estimates of leaf and stem respiration for these
Eucalypt species, which were derived from equations of
Ryan (1991) and Ryan and Waring (1992). These
respiration equations we used in combination with site
specific estimates of LAI, specific leaf area, tree sapwood
area and volumes, which were derived from previous
studies of leaf photosynthetic properties (Eamus and
Pritchard 1998), biomass distribution and tree water use at
this site (O’Grady et al. 1999, 2000; Hutley et al 2000).
Stem respiration estimates using Ryan and Waring (1992)
Table 2 Estimated stocks of carbon (ton C ha
) in Eucalypt open
forest savannas of Northern Australia based on measurements at a
range of sites, Howard Springs, Humpty Doo and the Wildlife Park.
Multiply by 100 to convert values to g m
Parameter Range Mean (SD)
Above ground
(1) Tree foliage 0.6–1.1 0.9 (0.2)
(2) Tree branches 4.2–12.2 8.2 (2.5)
(3) Tree stems 12.9–28.0 21.7 (4.8)
(4) Above-ground live tree = (1)+(2)+(3) 17.7–41.2 30.7 (7.3)
(5) Dead stems 0.2–3.2 0.9 (0.9)
(6) Understorey 0.7–1.5 1.0 (0.3)
(7) Litter-layer 0.8–1.4 1.0 (0.2)
(8) Total above-ground = (4)+(5)+(6)+(7) 19.4–47.3 33.6 (7.7)
(9) Fine roots 0.2–0.8 0.5 (0.2)
(10) Coarse roots 5.2–38.8 18.9 (12.4)
(11) Total roots = (9)+(10) 5.4–39.6 19.3 (12.6)
(12) Soil organic matter 111.5–198.9 151.3 (32.9)
(13) Total below-ground = (11)+(12) 116.9–238.5 170.6 (45.5)
(14) Total live tree = (4)+(11) 23.1–80.8 50.0 (19.9)
(15) Total vegetation = (8)+(11) 24.8–86.9 52.9 (20.3)
(16) Ecosystem total = (8)+(13) 136.3–285.8 204.2 (53.2)
Above-ground live tree/ Total live tree =
Total above-ground/ Total vegetation =
Total above-ground/ Ecosystem total =
Total below-ground/ Ecosystem total =
Total live tree/ Ecosystem total = (14)/(16) 0.24
Soil organic matter/ Ecosystem total =
was compared to data of Tame (2002, unpublished data),
who measured stem respiration of Eucalyptus miniata, E.
tetrodonta and Erythrophyleum chlorostachys at the
Howard Springs site. Chambers were attached to stems
of a range of individuals (13–36 cm DBH) with a mean
rate of CO
efflux per unit stem area of approximately
0.05 mg CO
observed, with no statistical
difference evident between species. Using our plot data
(tree density, mean tree stem surface area, mean annual
air temperature for the site), in combination with this stem
efflux, gives a stand-scale estimate of approximately
1.3 ton C ha
, in good agreement with the estimate
provided by Ryan and Waring (1992) of 1.2 ton C ha
. Similarly, leaf maintenance respiration as calcu-
lated using Ryan (1991) was comparable to typical dark
respiration rates of E. tetrodonta and E. miniata leaves,
which range from 1 to 20 nmol CO
leaf area s
(Eamus and Pritchard, unpublished data). While leaf dark
respiration rates have a component of growth respiration
in addition to maintenance respiration, some check of our
extrapolation using Ryan (1991) is required. Converting
our R
estimate to units of CO
efflux per unit leaf area
per second using overstorey LAI (0.9 wet season, 0.6 dry
season) and specific leaf area for these species (6.1 kg
), gives a value for R
of 2.3 nmol CO
the wet season and 1.4 nmol CO
during the dry
season. These values are within the range observed by
Eamus and Prichard, although they may be an underes-
timate of the true value.
Mean annual tree increment (radial DBH increment)
was 4.3€0.95 mm year
(0.014€0.002 m
basal area ha
), although this estimate is based on records from 10
of the original 20 stems, as dendrometers were damaged
by fire during the dry season. During the dry season there
was no tree growth and biomass increment was zero and
on some stems, shrinkage was observed. By contrast, wet
season mean stem increment was maximal at 0.71 mm
during December–January, and in terms of
seasonal C flux above-ground, the wet season accounted
for approximately 75% of the annual total. Only litterfall
was larger in the dry season than in the wet season, due to
canopy leaf area reductions of semi-, brevi- and fully
Table 3 Seasonal and annual C
fluxes (ton C ha
or ton C ha
) for a
Eucalypt open forest savanna of
northern Australia
Processes Dry season Wet season Annual
(1) Net tree biomass increment 0.0 1.6 1.6
(2) Litter-fall 0.6 0.3 0.9
(3) Net understorey biomass increment 0.0 0.5 0.5
(4) Respiration of tree foliage 0.7 0.8 1.5
(5) Respiration of tree wood 0.2 1.0 1.2
(6) C allocation above-ground = (1)+(2)+(3)+(4)+(5) 1.5 4.2 5.7
(7) Net coarse root biomass increment 0.0 1.0 1.0
(8) Net fine root production 1.3 5.7 7.0
(9) Respiration of roots 2.1 5.0 7.1
(10) Total soil respiration 4.2 10.1 14.3
(11) C allocation below-ground = (7)+(8)+(9) 3.4 11.7 15.1
(12) C input = (2)+(7)+(8) 1.9 7.0 8.9
(13) C output = (17) 2.1 5.1 7.2
(14) Net soil C exchange = (12)(13) 0.2 1.9 1.7
(15) Total respiration = (4)+(5)+(10) 5.1 11.9 17.0
(16) Autotrophic respiration = (4)+(5)+(9) 3.0 6.8 9.8
(17) Heterotrophic respiration = (15)-(16) 2.1 5.1 7.2
(18) C allocation ecosystem total = (6)+(11) 4.9 15.9 20.8
(20) C allocation above-ground/ecosystem = (6)/(18) 30.6% 26.4% 27.4%
(21) C allocation below-ground/ecosystem = (11)/(18) 69.4% 73.6% 72.6%
(22) C allocation above-ground/below-ground = (6)/(11) 44.1% 35.9% 37.7%
(23) NPP = (1)+(2)+(3)+(7)+(8) 1.9 9.1 11.0
(24) GPP = (23)+(16) 4.9 15.9 20.8
(25) NEP = (23)(17) 0.2 4.0 3.8
(26) NPP/GPP = (23)/(24) 38.8% 57.2% 52.9%
Mean residence time (years)
(27) Biomass = (8+11)
/(1+3+7+8} 4.8
(28) Soil and litter = (12)
/(2+7+8) 17
(29) Total ecosystem = (16)
/(1+2+3+7+8) 19
Values taken from Table 2, mean resident time calculated as stock/productivity
deciduous trees and shrubs and annual grass senescence
during the dry season. For all other components, fluxes
during the wet season were larger than during the dry
season (Table 3).
Below-ground carbon flux
Total C flux below-ground (BGPP) was 15.1 ton C ha
(Table 3). This was calculated by summing root
production of both coarse and fine root, plus root
respiration. Root production and root respiration com-
prised approximately 53% and 47% of BGPP respective-
ly. More than 70% of root respiration occurred during the
wet season. The fine root component was the dominant
contributor to total root biomass increment and accounted
for more than 87% of the total root biomass production of
8 ton C ha
Tropical savanna carbon balance
The savanna carbon balance is summarised in Fig. 1,
using data from Table 3. Integrating all above and below
ground fluxes, production indices can be calculated. The
total ecosystem carbon flux (GPP) was 20.8 ton C ha
, of which 76% occurred in the wet season and 24%
in the dry season (Table 3). Carbon flux below-ground
was higher than carbon flux above-ground, and the former
accounted for approximately 70% of total carbon flux.
NEP was calculated by subtracting heterotrophic respira-
tion (R
) from NPP, which gave a value of 3.8 ton C ha
(Table 3). Although the NEP was positive over the
entire year, net productivity was strongly seasonal and
was dominated by wet season fluxes. The ecosystem was
a weak carbon source during the dry season, with NEP
0.2 ton C ha
(Table 3). The mean residence time
for carbon for biomass, soil and the ecosystem as a whole
was calculated by dividing the total carbon stock (Table 2)
by rates of carbon input (Table 3). Mean carbon residence
times for biomass, soil and the ecosystem were 5, 17 and
19 years respectively (Table 3).
Fig. 1a–c Estimated seasonal
and annual carbon flux in the
savanna studied. All units
are ton C ha
or ton C
. a Dry season, b wet
season and c annual. GPP Gross
primary production, R
respiration, R
respiration R
respiration, R
leaf respiration,
woody respiration, R
respiration, L litterfall, D B
above-ground biomass incre-
ment, DB
fine root biomass
increment, DB
coarse root
biomass increment, D
organic carbon change mea-
Savanna carbon stocks
Carbon stocks (as opposed to biomass) of the vegetation
component in these savannas was 53 ton C ha
(Table 2)
and is on the lower end of the global range of carbon
stocks in vegetation estimated for tropical savannas (20–
150 ton C ha
, Tiessen et al. 1998). The value is
significantly lower than estimates for tropical forests,
where AGB carbon stocks range from 70 to 179 ton C
(Delaney et al. 1997; Malhi et al. 1999). Values from
the present study are closer to those given by Scholes and
Hall (1996) for tropical dry forest (74.7 ton ha
reflecting the significant woody component of these
savannas. Scholes and Hall (1996) report 37.4 ton ha
carbon density for woodlands and 11.3 ton C ha
for ’dry
Below-ground biomass was 19 ton C ha
, approxi-
mately 35% of the total biomass carbon stock, which is a
higher percentage than that commonly observed in
drought deciduous forests (20%) or moist, broad-leaved
woodlands and savannas (25%) (Scholes and Hall 1996).
Like the seasonal patterns of ANPP, high below-ground
carbon allocation relative to above-ground at these sites
reflects the annual drought of this wet-dry climate zone.
There is significant investment of carbon in root systems
of the dominant tree species (Eucalyptus tetrodonta, E.
miniata, Erythrophyleum chlorostachys), via the devel-
opment of large lignotubers that enable carbon storage
and vegetative re-growth following frequent fires (burn-
ing 2 in 3 years, Williams et al. 2002) occurring in these
savannas (Williams et al. 1999). Root biomass tends to be
concentrated in the upper 50 cm of soil (Werner and
Murphy 2001; Eamus et al. 2002), roots of mature trees
can grow to 5 m depth (Kelley et al. 2002) and we have
observed roots to 9 m (A. O’Grady, personal communi-
cation), although the biomass at these depths was small.
These root systems enable extraction of water from the
sub-soil (1–5 m depth, Kelley et al. 2002) during the
6 month dry season and maintain tree stand transpiration
at a constant rate all year (O’Grady et al.1999).
Dry season dormancy in stem growth of these species
occurs despite the maintenance of dry season photosyn-
thesis (Eamus et al. 1999) and transpiration (O’Grady et
al. 1999). Eamus et al. (1999) observed only modest
(approximately 10–15%) declines in assimilation per unit
leaf area for these evergreen species during the dry season
relative to the wet, although photosynthesis of semi- and
brevi-deciduous species declined by 25–75%. Fully
deciduous species were leafless for some period of the
dry season. Despite the continuation of photosynthesis by
evergreen species during the dry season, albeit at reduced
rates compared to the wet season, carbon assimilated is
apparently not utilised for shoot growth or significant leaf
production. Below-ground storage, especially in lignotu-
bers, and dry season flowering and fruiting (Setterfield
and Williams 1996) is likely to be a significant sink for
this carbon. Mucha (1979) observed a similar confine-
ment of stem increment in E. tetrodonta to the wet season,
with growth increment of 3 mm per month occurring
during January and February, higher than rates of
increment observed in this study. Hoffmann (2002) also
observed strongly seasonal stem growth, despite relatively
aseasonal patterns of gas exchange in evergreen trees of
cerrado savannas of south-central Brazil and stored
carbon is likely to be used to initiate fine root growth
and leaf production prior to the onset of wet season rains.
In woodlands and savannas, SOC tends to be more
than three-quarters of the total ecosystem carbon stock
(Scholes and Hall 1996). However, in comparison with
tropical forests or temperate grasslands, savannas gener-
ally have a low SOC content due to high soil respiration
rates (Chen et al. 2002) and soil carbon losses occur due
to frequent burning (Kalpage 1974; Montgomery and
Askew 1983; Bird et al. 2000). SOC content of savannas
generally increases with increasing soil clay content,
rainfall, tree cover and decreasing temperature (Scholes
and Hall 1996). The SOC density in these Eucalypt open
forest savannas (151€33 ton C ha
or 15.1€3.3 kg C m
was significantly higher than the savanna mean
(5.65€4.60 kg C m
) and was similar to the mean for
tropical woodlands (11.8€5.43 kg C m
) as given by
Scholes and Hall (1996). The high level of SOC of these
savannas is likely to be high below-ground carbon
allocation and fine root productivity in the wet season.
Savanna productivity
All productivity indices (GPP, NPP, NEP) describe a
savanna ecosystem where carbon fluxes are tightly
coupled to seasonal patterns of rainfall and resultant
changes to soil water content. This feature has been
observed at leaf (Prior et al. 1997a, 1997b; Eamus et al.
1999) and canopy scale (Hutley et al. 2000, 2001; Eamus
et al. 2001) and is evident at the ecosystem scale (this
study). The production efficiency of this savanna (NPP/
GPP) was approximately 53% (Table 3), close to the often
assumed ratio of NPP/GPP of 0.5. In a review of carbon
balance of contrasting ecosystems, Malhi et al. (1999)
obtained similar production efficiencies for tropical
(51%), temperate (55%) and boreal (54%) ecosystems.
Murphy and Lugo (1995) reported the range of total or
ecosystem NPP for tropical dry forests and savanna as 8–
21 ton DM ha
, with 6–16 ton DM ha
ANPP. For total NPP, this is approximately 4–10 ton C
, and 3–8 ton C ha
ANPP. At the
Howard Springs/Humpty Doo sites, total NPP is at the top
of this range, yet ANPP is near the bottom, at 3 ton C ha
(Table 4). This pattern of average to high NPP for
these Northern Territory savannas, but low ANPP is also
seen in Table 4, with NPP of this study comparable to
other savannas and drought-deciduous woodlands, al-
though ANPP is significantly lower. This further indicates
that savannas of northern Australia have relatively high
below-ground carbon allocation (see ratio of ANPP/NPP,
Table 4). BNPP accounted for 70% of NPP and fine root
net primary production (NPP
) accounted for 87% of the
total BNPP. Therefore, fine root production is the largest
single component contributing to NPP for these savannas.
Fine root systems of these savannas are essentially
deciduous (D. Bowman, personal communication), with
little production during the dry season. This seasonal
cycle of root production coincides with rapid growth of
annual grasses (Sorghum spp. and Heteropogon spp.)
of the understorey, leaf flushing of overstorey woody
species (Williams et al 1997) and large increases of soil
efflux (Chen at al. 2002). Grass biomass produced
during the wet season can be consumed by fire the
following dry season or, if unburnt, is decomposed over
subsequent wet seasons with some amount entering the
SOC pool.
Using annual incident solar radiation, energy conver-
sion efficiency and energy content of woody biomass,
Linder (1985) calculated the potential biomass production
for Darwin to be 111 ton DM ha
, the highest
value of any region in Australia. Using these simple
parameters, tropical savannas of Northern Australia
should have higher NPP and AGB relative to temperate
Australian woodlands, given the high year-round radia-
tion loads and non-limiting temperatures for growth,
coupled with high annual rainfall. This potential NPP is
well in excess of that measured for these savannas
(22.2 ton DM ha
) and is an overestimate as it
does not consider the seasonal distribution of rainfall and
the occurrence of an annual drought, low soil nutrient
status and the effects of frequent fires, all of which limit
Low intensity dry season fires of the mesic savannas of
northern Australia are widespread and account for 50–
70% of all fires of the Australia continent, consuming up
to 23.6 Mt of biomass per annum (Russell-Smith et al.
2002). Theses fires combust understorey fuels, resulting
in significant leaf death of overstorey tree and shrub
canopies (up to 80–90% for hot fires, Beringer et al. 2003)
and at frequently burnt sites, can result in significant tree
mortality (Williams et al 1999). When subjected to an
experimental fire regime of annual, late-dry season
burning (fire intensity of ~8,000 kW m
), Williams et
al. (1999) reported a 27% decrease in live-tree basal area
in open-forest savannas of Kakadu National Park. These
measurements were conducted over a 4-year period. A
single, high intensity fire (~20,000 kW m
) resulted in a
live stem basal area reduction of 41%. Unburnt plots
showed a 3.5% increase in live-stem basal area over the
same period. Williams et al. (1999) conducted their study
in savanna communities that are floristically and struc-
turally similar to sites used here, although the fire regime
of our sites is closer to biannual burning as opposed to the
annual treatment imposed by Williams et al. (1999). Fire
also had significant impacts on the survivorship of large
(>30 cm DBH) trees and we conclude that fire would
limit AGB and productivity in these savannas. A further
limit to production is due to termite damage and
hollowing of tree boles, a common occurrence in the
dominant tree species of these savannas (Andersen and
Lonsdale 1990). As trees age, termite damage can become
extensive and is further compounded by fire, as flames
penetrate boles via cavities formed from the action of
Table 4 Comparison of ANPP (ton C ha
), NPP (ton C ha
) and ratio of ANPP/NPP for savanna ecosystems, other
Australian Eucalypt communities and tropical forests
Forest type ANPP NPP ANPP/NPP Rainfall (mm) Reference
Eucalypt open-forest savanna, NT 3.0 11.0 0.27 1,750 This study
Drought-deciduous woodland 9.7 12.7 0.76 Menaut and Cesar (1979)
Tropical savanna (global mean) 5 300 Scholes and Hall (1996)
Drought-deciduous woodland (S. America) 2.1 4.4 0.48 Scholes and Hall (1996)
Sahelian shrub savanna 2.2 450 Hanan et al. (1998)
Trachypogon savanna (S. America) 4.6 1,300 San Jose and Montes (1989)
Drought-deciduous woodland 7.5 15.7 0.48 Menaut and Cesar (1979)
Drought-deciduous woodland 8.6 13.2 0.65 Menaut and Cesar (1979)
Drought-deciduous woodland 6.5 12.3 0.53 Menaut and Cesar (1979)
Tropical grasslands (Thailand) 7.2 10.0 0.72 Long et al. (1992)
Tropical grasslands (Australia) 2.3 3.6 0.64 Christie (1978)
Australian Eucalyptus forests
Eucalypt woodlands, Box-ironbark forest 0.5–2.0 350–500 Grierson et al. (1992)
E. regnans (young) 9 1,000 Grierson et al. (1992)
E. regnans (mature) 6.5 1,000 Grierson et al. (1992)
E. regnans (regenerating) 36 Attiwill (1991)
E. obliqua (45 years old) 14 Attiwill (1979)
E. pauciflora (mature) 12 17 0.71 1,200 Keith et al. (1997)
Tropical forest
Tropical rainforest (global mean) 8.7 15.6 0.56 2,200 Malhi et al. (1999)
ANPP is above-ground NPP and NPP includes both above and below ground
Cited in Scholes and Hall (1996)
termites. These dual processes also constrain the produc-
tion of AGB.
A striking feature of these savannas is the high rate of
NPP relative to the biomass and total carbon storage, i.e.
the short residence time (Table 5). The mean residence
time for biomass carbon in savannas is between 5 and
9 years (Table 5), while the biomass residence time for
temperate, boreal and tropical forest biomass is over
10 years (Malhi et al. 1999). Using data given in Scholes
and Hall (1996), the average residence time for savanna is
3.4, similar to that estimated in this study (Table 5).
Clearly, carbon within the biomass of savannas is quickly
turned over and returned to the atmosphere. While
savannas have relative low carbon stocks, they have
great potential to influence carbon cycling at regional and
global scales because of their extensive area, short
residence time and concomitant high rate of cycling.
This short residence time is attributed to the distinct wet-
dry seasons and with frequent atmospheric emission of
carbon via biomass burning followed by re-growth the
following wet season. Turnover of soil carbon is slower
and is of the order of 20 years, similar to soil carbon
turnover of tropical forests (Malhi et al. 1999).
Seasonal patterns carbon sink strength
In this study, NEP was positive (3.8 ton C ha
indicating that the sites studied are a carbon sink,
sequestering approximately 14 CO
. Using
eddy covariance measurements, Miranda et al. (1997)
provide seasonal estimates of net carbon exchange from
Brazilian cerrado savanna vegetation; extrapolating these
data provides an NEP estimate of approximately 3.1 ton C
. Cerrado sites used by Miranda et al. (1997)
received annual rainfall of 1,550 mm per annum, with a
6–8 month wet season and LAI (1.4–0.62 wet to dry
season), similar to sites of the current study and the values
of NEP are comparable for these two savannas. These
values of NEP also comparable to Sahelian fallow
savanna (0.32 ton C ha
, Hanan et al. 1998),
Amazonian rainforest (1 ton C ha
, Grace et al.
1995 and 5.9 ton C ha
, Malhi et al. 1999),
European temperate deciduous forest (2–5 ton C ha
, Goulden et al. 1996; Greco and Baldocchi 1996,
5.9 ton C ha
Malhi et al 1999), eastern North
American deciduous forests (0.7 to 3.2 ton C ha
Curtis et al. 2002) and Siberian Scots pine forests (0.19–
1.36 ton C ha
, Wirth et al 2002). Given an LAI
that ranges from 0.8 to 2.5, comparative studies cited
above suggest that an NEP of approximately 3 ton C ha
as reported for this study is relatively high. It is
possible that sink strength is maintained by frequent and
re-occurring disturbance and that this savanna ecosystem
is in recovery from previous disturbance events, both
long-term (cyclonic damage, 25–50 year cycle) and short-
term (fire, 1–3 year cycle). From an analyses of tree size
class distributions of these coastal open forests of the
Darwin region, Wilson and Bowman (1987) and O’Grady
et al. (2000) concluded that these forests are dominated by
small trees (DBH <20 cm) and are young and actively
growing following the major disturbance of Cyclone
Tracey in 1974. Consequently, a sink strength in the order
of 2–4 ton C ha
is possible despite a low LAI
(0.6–2.5, dry to wet season), and would represent a
maximal value of NEP for these mesic, coastal savannas.
Net ecosystem productivity was negative (0.2 ton C
) during the dry season (May to October)
and was positive (4 ton C ha
) during the wet
season (November–April), indicating seasonal shifts in
net carbon exchange with the atmosphere. These seasonal
patterns are similar to cerrado vegetation of Brazil, which
was a strong sink for carbon during the wet season months
(November–April, 0.5–1.8 g C m
) but became a
source at the height of their dry season (August–
September, Miranda et al. 1997). At the Howard Springs
and Humpty Doo sites, there is little understorey photo-
synthesis during the dry season, as the C4 grasses have
largely senesced and the evergreen trees are the most
significant component of the ecosystem that are actively
photosynthesising. Mean daily temperatures are not
strongly seasonal and maintenance respiration rates of
evergreen trees and shrubs continue over the dry season at
rates similar to those of the wet season. Other dry season
sources of carbon would include respiration from slowly
decomposing grasses and continuing soil CO
(microbial plus root respiration), although dry season F
was 60% lower than wet season rates. However, our data
suggest that the dry season reduction in canopy assimi-
lation (overstorey and understorey) is larger than the
reduction in ecosystem-scale respiration wet to dry
season, and as a result the site becomes a weak carbon
source during the dry season months. Eddy covariance
measurements of ecosystem-scale CO
fluxes confirm
these large seasonal changes in carbon exchange at this
site, as we observe a dry season decline in canopy-scale
Table 5 Ratio of total biomass
carbon to NPP (carbon resi-
dence time) for a range of
Ecosystem Residence time
Savanna 5 This study
Savanna (global mean) 3.4 Scholes and Hall (1996)
Savannas (global mean) 4.4 Whittaker and Likens (1973)**
Woodland and shrubland (global mean) 8.6 Whittaker and Likens (1973)**
Tropical rainforest 16 Malhi et al. (1999)
Temperate forest 10 Malhi et al. (1999)
Cited from Scholes and Hall (1996)
flux of 75% relative to wet season rates (Eamus et al.
2001). Eamus et al. (2001) also used these daily CO
fluxes measured over the wet-dry seasonal cycle at the
Howard Springs site to derive an NEP of 2.8 ton C ha
, in reasonable agreement with the estimate pro-
vided by this study using a biomass-inventory approach.
Although this study was conducted at savanna sites
subjected to burning, our NEP estimate does not specif-
ically include impacts of fire, which represent a non-
respiratory carbon loss to the atmosphere (Wirth et al
2003). A more accurate assessment of savanna carbon
sink strength of this region should include some estimate
of this loss, which would represent the longer-term carbon
sink strength, or the Net Biome Productivity (NBP, after
Schulze et al. 2000). For these savannas, net biome
production (NBP) can be estimated as NEP minus carbon
losses to the atmosphere due to fire. Using data of
Russell-Smith et al. (2002), typical fuel loads for these
savannas is approximately 4 ton ha
(4.6 ton ha
study). Assuming a burning efficiency of 0.72 (ratio of
fuel pyrolised to fuel load within areas over which flames
have passed, Russell-Smith et al. 2002) gives an annual
mass of fuel burnt of 2.9 ton DM ha
(fuel load
multiplied by burning efficiency factor) or approximately
1.5 ton C ha
. Assuming an NEP of 3.8 ton C ha
, NBP is estimated at 2.3 ton C ha
for this
site. This suggests that of the 2–4 ton C ha
sequestered, 40–50% is lost to fire per annum, a
significant fraction. This value of NBP is close to the
2.6 ton C ha
woody stem and course root
increment observed, which suggests that above-ground
understorey productivity (wet season grass production and
understorey shrub growth) may be close to carbon
’neutral’ due to fire (and respiration) losses. Long-term
carbon sequestration may therefore be a function of
woody growth and inputs to soil carbon pools and fire
impacts (Burrows et al. 2002). Burrows et al. (2002)
measured a net carbon sink in the grazed, semi-arid
savanna woodlands of north eastern Queensland (annual
rainfall 450–600 mm) and attributed this sink to reduced
fine fuel accumulation due to grazing resulting in a
reduced fire frequency coupled with active fire suppres-
sion. Long-term (18 year study) woody biomass incre-
ment averaged for 52 sites gave a sink strength of 0.53 ton
. This compares with 1.6 ton C ha
from this study, although these ecosystems are significant
different in terms of rainfall and grazing pressure and our
measurements are short-term only. Despite uncertainty
inherent in our calculations relating to fire emissions and
carbon (e.g. area averaged fuel loads, burning efficien-
cies), more precise spatial estimates of CO
flux and fire
emission are clearly required to provide a more precise
estimate of net productivity of Australia’s tropical
This study describes a tropical savanna with generally low
AGB (32 ton C ha
) and below-ground biomass (19.3 ton
). Soil carbon storage was large at 151 ton C ha
Carbon flux was highly seasonal with 75% occurring in
the wet season, which was dominated by fine root growth
(5.7 ton C ha
). GPP was 20 ton C year
, high for
an ecosystem with a low LAI (0.8–2.5). Productivity is
likely to be constrained by the wet-dry climate, poor soils
and constant disturbance from frequent fires. This results
in a short residence time (5 year) for carbon stored in
savanna biomass.
Acknowledgements The project was support by the Cooperative
Research Centre for the Sustainable Development of Tropical
Savannas (TS-CRC) at the Northern Territory University (NTU).
Dr. Tony O’Grady and Ms. Georgina Kelley participated in field
work and Ms. Megan Langerveld provided help with the chemical
analysis of soil carbon. Dr. Dick Williams of CSIRO, Darwin also
provided useful comments and suggestions on the manuscript and
Dr David Bowman of the NTU provided useful insights and ideas
for site selection. Comparative data on stem respiration was kindly
provided by Chloe Tame and Dr. Jason Beringer of Monash
University. X.C. is grateful to the TS-CRC and NTU for
postgraduate scholarships.
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... For example, eddy covariance methods provide continuous measurements of NEE, but localized coverage, differences in sensors, data collection and analysis techniques, and systematic and random errors can increase uncertainty in estimates of ecosystem productivity (Goulden et al., 1996;Miller et al., 2004;Teets et al., 2018). Similarly, rates of net primary production (NPP) and ecosystem production (NEP) can be estimated from field plots using well-known inventory methods (Chen et al., 2003;Clark et al., 2013;Grace et al., 2006), but many components, such as herbivory and below ground production processes, are poorly known and/or rarely measured (Clark et al., 2001a). Furthermore, inventory measurements are time and labor intensive, which severely limits their widespread and/or prolonged use (Teets et al., 2018). ...
... 11.0 Eucalypt forest-savanna I Australia Chen et al. (2003) 10.2 ± 5.9 a Forest/woodland I Various Grace et al. (2006) (n = 10) 5.9 ± 3.6 a Savanna I Various Grace et al. (2006) (n = 12) ...
... Carbon use efficiency was on average 0.58 for the Cerrado forest and 0.28 for the mixed grassland, compared to CUE values reported for tropical forests (0.30-0.51; Chambers et al., 2004;Malhi et al., 1999Malhi et al., , 2009) and savanna (0.53; Chen et al., 2003). The lower value for the mixed grassland presumably reflects the relatively low growth rates of the C 4 grasses and dominant tree species (Curatella americana), which has a lower RGR than many other tree species observed the Cuiabá Basin and Pantanal . ...
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Estimates of net primary (NPP) and ecosystem production (NEP) are needed for tropical savanna, which is structurally diverse but understudied compared to tropical rainforest. We used eddy covariance and inventory methods to estimate carbon (C) fluxes for an upland mixed grassland and a seasonally flooded forest to determine the correspondence between these methods and assess the contribution of C cycling components to the total NPP. Both techniques provided similar estimates of net ecosystem CO2 exchange (−3.0 ‒ 2.3 MgC ha⁻¹ y⁻¹ for eddy covariance vs. −2.0 ‒ 4.3 MgC ha⁻¹ y⁻¹ for inventory), gross primary production (7.5–16.3 MgC ha⁻¹ y⁻¹ for eddy covariance vs. 8.7–18.4 MgC ha⁻¹ y⁻¹ for inventory), and total NPP (0.9–7.5 MgC ha⁻¹ y⁻¹ for eddy covariance vs. 2.0–9.5 MgC ha⁻¹ y⁻¹ for inventory). Belowground NPP accounted for 49%–53% of the total NPP for both ecosystems, followed by aboveground litter (26%–27%) and wood (16%–17%) production. Increases in water availability increased the potential for C storage, but the mechanism was different in the savanna types. Compared to other savanna ecosystems, the mixed grassland had a lower productivity and C use efficiency (CUE = NPP/GPP = 0.28), while the Cerrado forest had a high CUE (0.58) and similar C flux rates to other tropical savanna forests. The agreement in the C cycle estimates derived from the eddy covariance and inventory methods increases our confidence in the productivity estimates for these tropical savanna ecosystems.
... Other African savanna patches are found in East and West Africa [13]. The second largest global fraction of savannahs are in northern Australia, where they cover about 2 million km 2 [14]. Australia's savannahs comprise 12% of the global savannah biome, and they have some of the world's most extensive and intact eucalyptus stands. ...
... Australia's savannahs comprise 12% of the global savannah biome, and they have some of the world's most extensive and intact eucalyptus stands. Nearly 75% of Australia's total burnt land area occurs in the savanna ecosystem [14]. South America has the most diverse types of savanna ecosystems, covering approximately 2.7 million km 2 , which is about 8-10% of the global savanna biome [15][16][17][18]. ...
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In this paper, the patterns of the occurrences of fire incidents over sub-Saharan Africa are studied on the basis of satellite data. Patterns for the whole sub-Saharan Africa are contrasted with those for northern sub-Saharan Africa and southern-hemisphere Africa. This paper attempts to unravel linear trends and overriding oscillations using regression and spectral techniques. It compares fire patterns for aggregated vegetation with those for specific types, which are savannahs, grasslands, shrublands, croplands, and forests, to identify key trend drivers. The underlying cyclic trends are interpreted in light of climate change and model projections. Considering sub-Saharan Africa, northern sub-Saharan Africa, and southern-hemisphere Africa, we found declining linear trends of wildfires with overriding cyclic patterns that have a period of ∼5 years, seemingly largely driven by savannahs, grasslands, and croplands.
... Details of instrument setup, measurements, and data processing are provided by Galvagno et al. (2017Galvagno et al. ( , 2013. To estimate daily aNPP, GPP was converted to NPP (g C m -2 day -1 ) by multiplying it by a fixed conversion factor of 0.53 (Chen et al., 2003;Zhang et al., 2009) and then by 2 to convert g C to g DM (White et al., 2000). ...
... One source of error in model evaluation may have been the simplification of deriving NPP from GPP, which is not actually observed, but derived from the flux tower with the eddy covariance method. A fixed coefficient is usually used in the literature to convert GPP to NPP (Chen et al., 2003;Zhang et al., 2009), but this approach is theoretically less correct than methods based on estimating autotrophic respiration . Concerning the model initialization, a spin-up process based on the first biomass sample of the season is required to properly initialize VISTOCK at the beginning of each year. ...
This article presents the structure and results of a simplified model (VISTOCK) for simulating grass growth and water dynamics of grassland systems. The model, based on a process-based approach coupled with proximal (SKR 1800 2-Channel Light Sensor) and remote (Sentinel-2) NDVI-derived data for estimating LAI, simulates aboveground biomass (AGB), net primary production (NPP), evapotranspiration (ET), and the fraction of transpirable water in soil (FTSW). VISTOCK simulated a grassland system with few meteorological data (i.e., minimum and maximum daily temperatures, precipitation, global solar radiation), considering limitations to vegetation growth due to thermal and water stresses. It was calibrated for a natural alpine grassland in Italy (site T) during the most contrasting meteorological seasons of the dataset (2012, 2017, and 2018). It was then evaluated for the remaining years at site T (2013, 2014, 2015, and 2016) and for other two sites in Italy (sites B1, B2 and M) with different soil and climate conditions and diverse management strategies (2020 and 2021). VISTOCK accurately predicted AGB during the growing season (RMSE = 445, 240, 219, 365 kg DM ha⁻¹ for T, M, B1, and B2, respectively) as well as for NPP, ET, and FSTW at site T. Simulation results suggest the ability of the model to simulate grassland in diverse environments with few inputs and parameters to be calibrated. The model’s simplified structure, combined with easy-to obtain input data and easy applicability, encourages its wider use for out- and/or upscaling and decision making.
... So total carbon stocks are significantly impacted by the conversion of forest to grassland and cultivated land. The study is consistent with the studies of [20,21]. ...
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The stocking and sequestering of increasing atmospheric carbon dioxide (CO 2) and the reduction of greenhouse gas (GHG) emissions that result from improving the carbon sink are two important ways that forested land contributes to the fight against global warming. The purpose of the study is to estimate the rate of carbon sequestration (CS) in Edo State, Nigeria, as well as the volume of deforestation and its impact on CS. To gauge the changes in carbon stock, stock-difference and gain-loss methods were employed. The gain-loss method predicts gains and losses based on off-take and growth rates, while the stock-difference approach uses actual measurements of carbon stocks over a given period of time. These two methods presuppose that changes in carbon stock and CO 2 flows to or from the atmosphere are equal. To quantify the decline of the forest, geographical studies and satellite imagery were used. Comparing the area covered by forest in the same region at two distinct eras allowed researchers to determine the annual rate of change. The outcome showed that tree cover loss (TCL, kg/ha) was decreased in 18 local government regions (LGAs). As a result, throughout the baseline consideration period of 2010 to 2022, Etasko East (EE) and Estako West (ES), Ovia South East (OSW), and Ovia North have had the least loss in tree cover. The increased demand on human survival brought on by the expanding population may provide an explanation for this observation and discovery. As a result of this development, forests underwent transformation and were used to produce food, build cities and homes, and generate energy. The region with the highest rates of tree cover loss and deforestation was associated with the highest CS, which was calculated at 2700 tC/ha at OSW, and the lowest CS value point at 22.2 tC/ha at Oredo Edo (OE). As a result, OSW showed that dense forests had higher biomass carbon storage than grazing land and open forests. In conclusion, the study showed that Edo State has a significant potential for raising the level of carbon sequestration in order for the state to generate a profit from the sale of carbon stock and enhance climate change mitigation efforts.
... Savannas are a mixture of trees and grasses, spatially and temporally heterogeneous ecosystems shaped by highly seasonal rainfall and local disturbance processes. In the Eucalypt-dominated north Australian tropical savannas, the main disturbance processes include frequent fire, convective storms, and cyclones, which in combination with termite impacts (Hill and Hanan, 2010;Williams and Douglas, 1995) drive rapid turnover of vegetation (Chen et al., 2003). Exacerbated by climate and land use change, these interactions result in high spatial and temporal heterogeneity in stand structure and above-ground biomass (AGB). ...
... The conversion of savanna woodlands to maize fields represents a high impact on the fine root C stocks as it entails a 70% decrease of the fine root biomass. Our fine root biomass values for savanna were at the lower end of the range of values reported for other tropical savannas and dry forests (0.4-11.86 Mg ha −1 ) (Roy and Singh 1994;Chen et al. 2003;February and Higgins 2010;Moore et al. 2018). Differences might be due to the lower mean annual precipitation, the lower number of trees at our savannas and to its semi-natural condition, as they are subject to logging and burning pressure that has been intensifying during the last decades (Agrawala et al. 2003;Hemp and Hemp 2018). ...
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Tropical forests are threatened by anthropogenic activities such as conversion into agricultural land, logging and fires. Land-use change and disturbance affect ecosystems not only aboveground, but also belowground including the ecosystems' carbon and nitrogen cycle. We studied the impact of different types of land-use change (intensive and traditional agroforestry, logging) and disturbance by fire on fine root biomass, dynamics, morphology, and related C and N fluxes to the soil via fine root litter across different ecosystems at different elevational zones at Mt. Kilimanjaro (Tanzania). We found a decrease in fine root biomass (80-90%), production (50%), and C and N fluxes to the soil via fine root litter (60-80%) at all elevation zones. The traditional agroforestry 'Chagga homegardens' (lower montane zone) showed enhanced fine root turnover rates, higher values of acquisitive root morphological traits, but similar stand fine root production, C and N fluxes compared to the natural forest. The decrease of C and N fluxes with forest disturbance was particularly strong at the upper montane zone (60 and 80% decrease, respectively), where several patches of Podocarpus forest had been disturbed by fire in the previous years. We conclude that changes on species composition, stand structure and land management practices resulting from land-use change and disturbance have a strong impact on the fine root system, modifying fine root biomass, production and the C and N supply to the soil from fine root litter, which strongly affects the ecosystems' C and N cycle in those East African tropical forest ecosystems.
... While this increase in carbon storage is small relative to the total stock of organic carbon in the ecosystem (e.g. 204 t C ha -1 in a high-rainfall savanna near Darwin: Chen et al. 2003), it still has the potential to earn significant income for fire managers, if the carbon can be It is important to place our findings in the context of work to quantify the magnitude of carbon fluxes in Australian savannas. Beringer et al. (2015) provide a recent, comprehensive review of the interplay between fire, tree biomass and net biome productivity (NBP)-net primary productivity minus respiration losses minus losses due to disturbance-in this region. ...
Tropical savannas are characterised by high primary productivity and high fire frequency, such that much of the carbon captured by vegetation is rapidly returned to the atmosphere. Hence, there have been suggestions that management‐driven reductions in savanna fire frequency and/or severity could significantly reduce greenhouse gas emissions and sequester carbon in tree biomass. However, a key knowledge gap is the extent to which savanna tree biomass will respond to modest shifts in fire regimes due to plausible, large‐scale management interventions. Here, we: (1) characterise relationships between the frequency and severity of fires and key demographic rates of savanna trees, based on long‐term observations in vegetation monitoring plots across northern Australia; (2) use these relationships to develop a process‐explicit demographic model describing the effects of fire on savanna tree populations; and (3) use the demographic model to address the question: to what extent is it feasible, through the strategic application of prescribed burning, to increase tree biomass in Australian tropical savannas? Our long‐term tree monitoring dataset included observations of 12,344 tagged trees in 236 plots, monitored for between 3 and 24 years. Analysis of this dataset showed that frequent high‐severity fires significantly reduced savanna tree recruitment, survival and growth. Our demographic model suggested that: (1) despite the negative effects of frequent high‐severity fires on demographic rates, savanna tree biomass appears to be suppressed by only a relatively small amount by contemporary fire regimes, characterised by a mix of low‐ to high‐severity fires; and (2) plausible, management‐driven reductions in the frequency of high‐severity fires are likely to lead to increases in tree biomass of about 11.0 t DM ha–1 (95% confidence interval: ‐1.2–20.8) over a century. Accounting for this increase in carbon storage could generate significant carbon credits, worth on average three times those generated annually by current greenhouse gas (methane and nitrous oxide) abatement projects, and has the potential to significantly increase the economic viability of fire/carbon projects, thereby promoting ecologically sustainable management of tropical savannas in Australia and elsewhere. This growing industry has the potential to bring much‐needed economic activity to savanna landscapes, without compromising important natural and cultural values. This article is protected by copyright. All rights reserved.
... For other restored vegetation types, such as open eucalypt woodland at Gove in Australia's Northern Territory, the excess P added may be even greater since 25 kg P ha −1 is applied (Spain et al., 2015) while the aboveground biomass removed is only ca. 108 tonnes ha −1 (Chen et al., 2003), although P concentrations in wood are unknown for this site. Bauters et al. (2022) demonstrated that total above-ground P stocks in several neotropical forests range from approx. ...
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Despite nutrient enrichment having widely reported negative impacts on biodiversity, fertilizer is routinely applied in post mining restoration to enhance plant growth and establishment. Focusing on surface mine restoration (predominately bauxite and mineral sands), we outline the long-term negative impacts of fertilizer, particularly phosphorus fertilizer, on plant community composition, species richness, fire fuel loads, and belowground impacts on nutrient-cycling. We draw from extensive research in south-western Australia and further afield, noting the geographical coincidence of surface mining, phosphorus impoverished soil and high plant biodiversity. We highlight the trade-offs between rapid plant-growth under fertilisation and the longer-term effects on plant communities and diversity. We note that the initial growth benefits of fertilisation may not persist in water-limited environments: growth of unfertilised forests can eventually match that of fertilised forest, throwing doubt on the premise that fertilisation is necessary at all.
Forest conservation entails both halting destruction and protecting the state of the forest's vegetation. Monitoring forest cover and restoring degraded forests are critical ecological aspects for India's forestry sector's long‐term growth. Cropland expansion and intensification are the primary approaches for increasing agricultural productivity in response to increased biomass demand, but they are also major drivers of biodiversity loss. We evaluated the ecosystem carbon
Despite the scientific consensus that vegetation modifies erosion/deposition processes, traditional erosion and landform evolution models often include a simplified representation of vegetation dynamics, which does not consider the influence of the various biomass pools on erosion mechanisms. In this paper, we present a new modelling framework that couples dynamic modules of hydrology, vegetation, biomass pools and landform evolution/erosion processes. We analyse the effect of above and belowground biomass pools (leaves, roots, litter and soil carbon) changes in erosion rates by considering: (1) the effect of root biomass on soil erodibility, (2) the effect of leaf cover on soil diffusivity, (3) the effect of litter on flow resistance, and (4) the effect of soil carbon on soil water retention. We implement the model using daily data for an open-forest vegetation and ran different idealized experiments for a period of 100 years. The objectives of the experiments are to understand the combined effects of biomass pool dynamics and climatic seasonality on erosion patterns, and to compare results including the effects of various biomass pools on erosion versus those obtained using simplified formulations (i.e., bare soil, constant vegetation). Our results indicate that the erosion protection effects of the individual biomass pools are not simultaneous, with different biomass pools providing protection at different times of the year. We also find that simplified vegetation formulations (e.g. constant vegetation) can significantly under/overestimate erosion when compared with the fully dynamic biomass pools formulation. These findings highlight the importance of including a detailed representation of dynamic vegetation and biomass pools in models, to capture the feedbacks between erosion and vegetation processes and better understand and predict erosion/deposition processes.
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Gross carbon budgets for vegetation in forest ecosystems are difficult to construct because of problems in scaling flux measurements made on small samples over short periods of time and in determining belowground carbon allocation. Recently, empirical relationships have been developed to estimate total belowground carbon allocation from litterfall, and maintenance respiration from tissue N content. The author outlines a method for estimating gross carbon budgets using these empirical relationships together with data readily available from ecosystem studies (above ground wood and canopy production, aboveground wood and canopy biomass, litterfall, and tissue nitrogen contents). Estimates generated with this method are compared with annual carbon fixation estimates from the Forest-BGC model for a lodgepole pine Pinus contorta and a Pacific silver fir Abies amabilis chronosequence. -Author
The savanna biome is diverse, including formations ranging from almost treeless grasslands to more or less closed-canopy woodlands with considerable variation in plant composition, biomass, and net primary productivity (NPP). Savannas cover an extensive area in the tropics, inhabited by a fifth of the human population, and supporting the majority of the world's livestock and large mammals. Savanna ecosystems and dynamics are currently poorly understood because little attention has been paid to these areas compared to tropical forests or temperate grasslands. Yet it is emerging that savannas have higher biodiversity, greater productivity, and larger impact on global carbon cycles than previously realized. Most research carried out in savanna ecosystems has been in the form of short-term site studies of one or two ecological aspects. This will have large impacts on global NPP predictions and climate change, thus it is important to improve the understanding of them. The proportion of trees and grasses is highly variable in space and time, yet very little is understood about the complex dynamic processes and interactions controlling them. There is a great paucity of even basic data for these highly heterogeneous ecosystems because relatively few studies have been carried out in this biome in the past. Additionally, there is a need for large-scale syntheses of existing data to improve knowledge and understanding.
Aims, goals and general methods, S.P.Long and M.B.Jones the primary productivity and photosynthesis of savanna grasses in Kenya - studies at Nairobi National Park, J.I.Kinyamario and S.K.Imbamba saline grassland near Mexico City, E.Garcie-Moya and P.Montanez Castro moist savannas of Thailand, A.Kamnalrut and J.Evenson the productivity of echinochloa polystachya, a semiaquatic grass in the Amazonian floodplain, M.T.F.Piedade et al the productivity and photosynthesis of bamboo with reference to phyllostachya pubescens forest in subtropical China, C-X.Qiu et al remote sensing of primary producation in natural tropical grasslands and articifial mixed-species canopies, J.M.O.Scurlock synthesis and conclusions, M.B.Jones and S.P.Long UNEP epilogue, R.J.Olembo.
Soil respiration is an important component of the net carbon dioxide exchange between agricultural ecosystems and the atmosphere, and reliable estimates of soil respiration are required in carbon balance studies. Most of the field measurements of soil respiration reported in the literature have been made using alkali traps. The use of portable CO 2 analysers in dynamic closed chamber systems is recent. The introduction of this new technique requires its evaluation against existing methods in order to compare new information with older data. Nine intercomparisons between dynamic systems and alkali traps were made. Measurements of F c,s obtained by both chambers showed a good agreement in all but two comparisons in which alkali trap measurements were lower than the dynamic chamber by about 22%. This first report of agreement between both techniques suggests that many measurements made in the past using alkali traps may be comparable to the measurements made more recently using the dynamic chambers. Analysis of the soil temperature and CO 2 concentration inside the alkali traps failed to explain why the alkali traps occasionally underestimated the fluxes. Soil respiration measured with a dynamic closed chamber were also compared to eddy-correlation measurements. The results did not reveal any consistent bias between techniques but the scattering was large. This dispersion is likely the result of the difference between the areas measured by the two techniques. Key words: Carbon dioxide, greenhouse gases, CO 2 flux, soil carbon
Lamto savannas (Ivory Coast) are characterized by the heterogeneity of their structure and by their dynamic evolution towards forest. Life-forms and phenological cycles of herbs, shrubs, and trees reflect the constraining factors of the environment. Biomass and productivity are largely dependent on soil and climate. The specific cycles of above- and belowground biomass allow an estimate of the primary productivity. Production of shrubs and trees, obtained from size-biomass correlations and growth measures, is compared with herb production to give an insight into the ecological balance of the savanna communities.
Stem girth measurements taken monthly for two years on trees of Eucalyptus tetrodonta, E. miniata and E. nesophila growing in open forests of northern Australia, in a tropical monsoon climate, showed that diameter growth in these species was regularly confined to the annual wet season. The stem wood of these trees exhibited growth rings, and, although the identification of the annual growth was not easy, it was concluded that a tentative assessment of approximate tree ages from ring counts is feasible, at least for trees protected from fire.
Herbivorous insects are undoubtedly important in savanna ecosystems, but have been largery ignored in studies of herbivory in favour of native ungulates and domestic cattle. In Australia. where the native mammalian herbivore fauna is depauperate, attention has strongly focused on cattle production. In this review we consider three major classes of herbivorous insects, namely grazers, folivores and seed predators, and synthesize information on (1) their composition, diversity and abundance, (2) their ecological effects as herbivores, (3) their importance relative to that of herbivorous mammals, and (4) insect herbivory in Australia compared with that in savannas elsewhere in the world. The most important grazing insects are grasshoppers and harvester termites, although the latter are probably mostly detritivorous. Consumption rates by grasshoppers in African savannas can be comparable to those by large populations of ungulates and cattle: in Australia they are probably the major grazing animals. Folivory and pre-dispersal seed predatin by insects are extremely poorly known in Australian savannas, although the results of work on southern species of Eucalyptus and Acacia (the dominat genera of woody plants throughout Australia) are likely to be at least partly relevant. Harvester ants are the most important post-dispersal seed predators: they consist primarily of omnivorous species of Monomorium and Pheidole, but also include an endemic radiation of granivorous Meranoplus. The over all composition and abundance of herbivorous insects in Australian savannas appears similar to that in other savannas. However, their ecological effects as herbivores are almost totally unknown, and ought to be a priority area for future savanna research.
Dimension analysis was used to estimate biomass and annual net primary production for a post oak-blackjack oak (Quercus stellata-Q. marilandica) forest in central Oklahoma. Concentrations of six mineral elements in various plant tissues were determined and used with biomass and production estimates to calculate the annual cycle of N, P, K, Ca, Mg, and Mn in the forest. Total organic material in the forest is 245,000 kg/ha, of which 1.6% is leaves, 26.4% live branches, 5.9% dead branches, 44.8% trunks, 15.9% roots, 9.6% understory, and 4.4% litter. Annual net primary production is 14,900 kg/ha, distributed as follows: 32.0% leaves, 28.0% twigs and branches, 24.9% trunks, 15.1% roots, and 2.0% understory. Maximum leaf area index was 4.8. Yearly mean litterfall is 5,400 kg/ha and is distinctly biomodal, with peaks in November and March. The biomass contains 1,157 kg/ha N, 101 kg/ha P, 1,258 kg/ha K, 4,549 kg/ha Ca, 311 kg/ha Mg, and 124 kg/ha Mn. Yearly mineral budgets were determined for the six elements. Unusually high values for Ca in the biomass and in the mineral cycle were due to high concentrations of Ca in post oak bark (90,200 @mg/g). High annual values for increment of biomass and for retention of mineral elements indicate that the stand has not reached a steady state, a conclusion that is confirmed by observations of stand structure.