Carob-tree as CO2 Sink in the Carbon Market

Abstract

Since the beginning of the XX century the median global temperature has raised more than 0.5 ºC, mostly due to the anthropogenic emission of carbon dioxide, methane and nitrous oxide. This rise will continue if emissions trend are not reversed and gases are allowed to remain in the atmosphere. Under Articles 3.3 and 3.4 of the Kyoto Protocol, developed countries are allowed the option to use net domestic changes in green house gases emissions through specific land use activities, including forest management to meet their reduction commitments. The present study contributes to this analysis by estimating the carbon assimilated by carob tree (Ceratonia siliqua) in the Algarve and the evaluation of its viability and generating potential in the carbon market. The method was based on mass balance and allometric relationships calculations for determining CO2 fixation, and inquiries to farmers for complementing information. CO2 eq fixation is 15.56 t CO2 eq/ha; with a total for the region 1 322 356 tCO2eq. The values are low when compared to other tree forests, but still important in a region where autochthonous species have naturally low densities. Carob tree has, however ecological advantages over other species in the region.

Full text

Carob-tree as CO
2
Sink in the Carbon Market
DANIEL GERALDO1, PEDRO JOSÉ CORREIA2, JOSÉ FILIPE3, LUÍS NUNES1
(1) Faculty of Sciences and Technology, University of Algarve, Faro,
(2) ICAAM/Pólo Algarve
(3) AIDA
PORTUGAL
e-mail: lnunes@ualg.pt
Abstract: - Since the beginning of the XX century the median global temperature has raised more than 0.5 ºC,
mostly due to the anthropogenic emission of carbon dioxide, methane and nitrous oxide. This rise will continue
if emissions trend are not reversed and gases are allowed to remain in the atmosphere. Under Articles 3.3 and
3.4 of the Kyoto Protocol, developed countries are allowed the option to use net domestic changes in green
house gases emissions through specific land use activities, including forest management to meet their reduction
commitments. The present study contributes to this analysis by estimating the carbon assimilated by carob tree
(Ceratonia siliqua) in the Algarve and the evaluation of its viability and generating potential in the carbon
market. The method was based on mass balance and allometric relationships calculations for determining CO2
fixation, and inquiries to farmers for complementing information. CO2 eq fixation is 15.56 t CO2 eq/ha; with a
total for the region 1 322 356 tCO2eq. The values are low when compared to other tree forests, but still
important in a region where autochthonous species have naturally low densities. Carob tree has, however
ecological advantages over other species in the region.
Keywords: - Carbon storage, carob tree, carbon market, carbon fixation, allometric relationships
1 Introduction
Climatic predictions have led to proposals for
conduct guidelines from various organizations who
share a common goal in minimizing green house
gases (GEE) emissions and reducing the possible
harmful side effects on the atmosphere, particularly
the effects on climate [1]. Specific GEE´s with a
rapidly growing concentration level include: carbon
dioxide (CO2), methane (CH4) and nitrous oxide
(N2O). Given the emission of such high quantities,
CO2 is becoming the main contributor to global
warming. If this concentration level continues to
rise, it can be predicted that the earth´s temperature
will rise, which in turn will cause various problems
in the atmosphere [2]. Since the beginning of the
XX century the median global temperature has risen
to 0,6 + 0,2º C [3]. In some regions it has risen even
more, as is the case in Europe, where the numbers
have reached 0,95º C [4].
Two different strategies for minimizing this
problem arose: the reduction of GEE emissions and
the discovery of alternatives for the absorption of
O2, through carbon fixation. The concept of carbon
fixation (sequestration) was ratified in 1997 at the
Kyoto Conference, setting a goal to contain and
revert CO2 in the atmosphere thus lowering the
greenhouse effect [5]. Historically, emerging
countries are not really held responsible for the
intensification of global warming. This fact, in
conjunction with the rise of GEE emissions
(especially in China and India) have contributed in
that these countries have gained significant power in
negotiating in the international climate regime [6].
Therefore, it is important to understand how
developing countries can act in an international
regimen, committing to reduce GEE emissions to
developing countries [7], alongside with measures
taken in, and by, developed countries in the same
direction. The GEE emissions in Portugal in 2000
reached 84,7 million tons of CO2eq, which
corresponds to a 30 % increase in relation to 1990.
In 2003, this increase corresponded to 38,6 % [8].
Portugal is accompanied in this increase by Spain,
Ireland and Italy [9]. National GEE projections for
emissions point to an increase in 2010 comparing to
the year 1990 between 46,5 % and 53,5 % [12]. In
accordance with these estimates, Portugal will have
to reduce its emissions between 12,6% and 17,5% in
order to comply with the PK [9].
Under Article 3.3 of the Kyoto Protocol, Parties
agreed in accounting certain activities for in meeting
the Kyoto Protocol’s emission targets, including
afforestation and reforestation since 1990. Activities
in land use, land-use change and forestry sector are
among the measures for cost-effective offsetting of
emissions. These include i) increasing the removal
of greenhouse gases from the atmosphere (e.g. by
planting trees or managing forests), or ii) reducing
emissions (e.g. by curbing deforestation).
Sustainable management of resources, plantations
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and the rehabilitation of forests can augment the
carbon storage (it is estimated that forests
worldwide hold 283 Gt C). Global rate of carbon
assimilation by forest sinks provided in 1999 is 1.59
Gt C / year [11].
Portuguese state set its objectives for
compliance with the PK is in the National Plan for
Climate Change ( PNAC). The PNAC promotes the
increase in the capacity for carbon fixation through
the bettering of management and the developing of
new forest populations, in a total of 492 thousand
ha. This measure permits the reduction of the
national liquid balance of GEE emissions to 0,8 Mt
CO2eq, by the beginning of 2011, which in turn
would value to about 60 million euros, according to
Kyoto arithmetic [10].
The present study contributes to the evaluation
of the potential of the Portuguese forest by
quantifying of the carbon assimilated by the carob-
tree in the Algarve and the evaluation of its viability
and generating potential in the carbon market. Carob
tree (Ceratonia siliqua) is found almost exclusively
in the Mediterranean basin with dry climate.
2 Material and Methods
2.1 Study area
The Algarve region occupies an area of 4 928 km2,
distributed throughout 16 councils which subdivide
into 84 sub councils of very diverse geographic
characteristics and distinct socio-economical
activities [13]. The areas with the most amount of
precipitation coincide with the mountainous areas,
particularly Monchique, which has a barrier effect in
relation to the Algarve [14]. Approximately 65 % of
the Algarve territory registers a median daily
temperature of 17,5º C, though there are variations
between mountain, coastline and Barrocal [15]. The
synoptic situations responsible for the occurrence of
rain in the Algarve region are due, in the winter, to
the passage of the polar front constituted by sea
currents, which originate masses of cold air, of
who´s influence is at times blocked by a warm
anticyclone, located in the Biscaia Gulf and the
British Islands. In the Autumn, the cyclonic
“families” from the Atlantic are preponderant,
reinforcing a tendency for unstable weather,
characterized by cloudy skies and rain, while the
summer is characterized by scarce precipitation, due
to the action of two stable anti cyclones [16]. The
distribution of carob trees in Portugal seems to
restrict itself to regions of marked
Euromediterranean characteristics (Algarve) [17].
Carob tree is a leguminous plant with evergreen,
noted for longevity, and cultivated extensively
through the Mediterranean basin [18]. By observing
their cultural distribution, it is noted that the trees
adapt to diverse edaphic conditions, but in the
Algarve its preponderance is observed on limestone
soils (karst topography) [19]. From a climatic point
of view, once dealing with a sub-tropical tree, the
most favorable conditions for growth are macro-
thermal aridity of the euromediterreanean zone [20].
In the Algarve it can be found in either pure
populations, disperse populations or, most often, in
conjunction with other species [17].
Fig. 1- Distribution of the carob tree in Algarve [17]
2.1.1 Biomass sampling
Five carob trees were cut down in three different
locations: three in Faro, one in Loulé and another in
São Brás de Alportel. Just before cutting, canopy
diameter (CD), height (H) and trunk diameter (Øt)
measurements were taken, to allow establishing
allometric relationships between biomass carbon
and the registered variables.
Table 1 – Characteristics of trees felled.
Proprieties Tree 1 Tree
2
Tree
3 Tree 4 Tree 5
Location
São
Brás de
Alportel
Faro Faro Loulé Faro
Annual
Precipitation
(mm)
653 600 600 653 600
Pruned No No No Yes Yes
Age (years) 5 1 12 55 37
Trunk
diameter
(cm)
9.87 2.85 5.16 56.69 27.37
CD (m) 2.20 – 1.60 8.50 7.00
Trunk girth
(cm) 31 0.91 16.2 178 86
Height (m) 4 0.48 1.90 8.5 7
Variety Bravo Bravo Bravo Mulata Mulata
Cut date Apr. May. Feb. May. Jul.
2.2 Data acquisition
2.2.1 Biomass carbon estimation
The trees were cut in various portions and the
respective wet weight was determined (OHAUS I-
10 model IS-15, 15±0,001 kg). After weighting
samples were dried in a ventilated greenhouse at
65º, until constant weight (dry weight). The value of
fixed biomass carbon (BC), kg, was determined by
Brown et al. [21]:
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BBC
⋅
=
5.0
Eq. 1
with B the dry weight (kg).
2.2.2. Tree allometric variables for carbon
fixation estimates
The following variables were determined: i) tree
height (H); ii) trunk diameter (Øt); and iii) canopy
diameter (CD). The first two variables were
determined by direct measurement, while canopy
diameter was obtained by fotointerpretation in
Arcview 3.2 validated by field measurements for 93
trees, of which 51 in Tavira, and 32 in Castro
Marim. Power law (quadratic) relationships between
these variables and biomass carbon were tested,
following conclusions from previous studies – see
[22] and references therein. Annual carbon fixation
in kg C/year per plant was estimated by averaging
the derivative of the equation that relates carbon
content per plant (kg) and plant age (year). The
estimate of carbon fixation in the region was
obtained by multiplying this estimate by the average
tree density and by total planted area.
2.2.3. Selected orchards and field extrapolation
Fotointerpretation of 106 orchards (17 in Tavira, 12
in Castro Marim, 64 in Loulé, 11 in Albufeira, 1 in
Faro and 1 in Olhão) was done in order to estimate
trunk diameter. Validation of interpretation was
made using data from field measurements for 93
trees. The 106 orchards corresponded to 240,8 ha
and 5 543 trees, which corresponds to an average
density of 23.02 trees per ha. Canopy area covered
an area of 5.23% of the total orchards plots (12.6
ha). Sampled area corresponds to 3.2% of the total
carob tree single species orchards area and to 0.28%
of the total area of orchard, including mix orchards.
2.2.4 Carbon conversion into CO2eq and CCE
Carbon content conversion into CO2eq is obtained by
a simple mass balance conversion, both in units of
mass:
BCCO eq
⋅
=
67.3
2
Eq. 2
3 Results and Discussion
3.1 Allometric models with biomass carbon
Allometric models with biomass carbon are
presented in Table 3, following data presented in
Table 2. With the exception of the relation between
trunk diameter and biomass carbon (Figure 2), all
the remaining models are only valid for plants older
than 15 years, which was considered as an important
limitation. Hence, consequent estimates of biomass
carbon estimates were based only on the allometric
relation with trunk diameter.
Table 2 – Tree measurements
Tree Ø t
(cm)
WW
(kg)
DW
(kg)
%
PH
BC
(kg)
H
(m)
CD
(m)
1 9,87 17,97 10,06 44 8,55 4,0 2,2
2 0,91 0,054 0,027 50,5 0,022 0,5 –
3 5,16 12,70 6,64 48,7 3,15 1,9 1,6
4 56,69 915,5 613,4 33 521,4 8,5 8,5
5 27,37 350,3 209,6 40,2 178,7 7,0 7
WW: wet weight; DW: dry weight
Table 3- Allometric relationships with biomass carbon
Relations Equations R2
BC vs H BC = 12.218 H2 – 48.496 H 0.944*
BC vs CD BC = 10.885 CD2 – 37.902 CD 0.929*
BC vs CD.H BC = 0.1388 (CD.H)2 – 2.8895 CD.H 0.995**
BC vs CD.H2 BC = 0.0012 (CD.H2)2 + 0.1086 (CD.H2) 0.999**
BC vs Øt BC = 0.1196 Øt 2 – 2.482 Øt 0.994**
* (ρ < 0.050); ** (ρ < 0.010)
0
100
200
300
400
500
600
0 10 20 30 40 50 60
Trunk diameter (m)
BC (kg)
Fig. 2 – Relationship between trunk diameter and
biomass carbon
3.2. Relationship between allometric
variables
Biomass carbon estimates for the region required
the determination of equations relating trunk
diameter and other easily measured spatial variables
such as densities of trees and canopy diameter.
Relationships between allometric variables are
shown in Table 4.
Table 4 – Equations for allometric relationships
Relations Equations R2
Øt vs CD Øt = 4x10-5CD2 + 0.0323
CD 0.783
Øt vs H Øt = 4.792 H 0.634
H vs CD H = 0.0096 CD 0.599
The equation relating trunk diameter and canopy
diameter (CD) showed a good correlation
coefficient, having been therefore used for
estimating total biomass carbon fixation in the
region.
3.3. Estimation of total biomass carbon in the
Algarve
Total biomass carbon in was estimated by
substituting the total area of canopy in the first
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equation of Table 4, and the result on the last
equation of Table 3. Results per orchard plot are
presented in Table 5.
Table 5 – Assimilated carbon for plots and in the
Algarve area
Location Nº of plots Area (ha) BC (kg) kg C/ha
Tavira 17 49.72 125363.56 2521.4
Castro
Marim 12 109.18 277350.53 2540.3
Loulé 64 66.15 515508.39 7793.0
Albufeira 11 9.15 73280.19 8008.8
Faro 1 6.22 25367.74 4078.4
Olhão 1 0.47 4305.94 9161.6
Total – 240.89 1 021 176.34 4239.2
Biomass carbon in the studied area amounted to
1021,2 tons (3.2% of the total), though BC changed
significantly between plots, by almost five times,
due to different tree densities. The average BC per
ha is 4239.2 kg/ha (15 557.8 kg CO2 eq/ha). For the
entire region BC estimate is of 31 912 tons (117 117
t CO2 eq) for single species orchards and of 360 315 t
C (1 322 356 t CO2 eq) for total orchard area. These
values indicate a low carbon fixation when
compared to other forests – see Table 6 for some
examples. It should be noted, however, that edaphic
and climatic conditions in Algarve are very
limitative for the development of dense forests.
Table 6 – Above ground carbon dioxide fixation
Reference Place CO2 eq/ha (t/ha)
Present study Carob tree - Algarve 15.56
NESFA [23] Northeast USA forest ~23.0 - ~36.0
Taki et al. [24] Japan – coastal black pine 171.4 – 320.0
Hsuan-Te et al [25] Camphor tree forest 120.5-256.4
Del Rio et al [26] Pine forest - Spain 182.3-529.3
3.4 Estimate of annual carbon assimilation
Data for carbon fixation per age considered only
plants taken from natural orchards, which excludes
plants 2 and 3. Though the resulting sample set is
very limited, an exponential relationship between
biomass carbon and age is well defined (Figure 3)
(r2 = 0.979).
Log [BC ]= 1.4911 Log [age]
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3
Log [age] (years)
Log [BC] (kg)
Fig. 3 – Relationship between biomass carbon and
plant age
The assimilation of carbon per year per plant may
be estimated by the derivative
δ
BC/
δ
age. The model
then becomes:
4911.0
4911.1/ ageageBC ⋅=∂∂
Eq. 3
Considering an average plant age of 30 years,
according to the results of the inquiries to the
farmers, the rate of assimilation is equal to 7.92
kg/year per plant, and the assimilation per ha equal
to 182.2 kg/ha/year. This value corresponds to 668.8
kg CO2 eq/ha/year, which is one order of magnitude
lower than that referenced for above ground pine
forest in a similar climate 17.4 kg/tree/year [26].
These results still need further verification with a
larger set, but seem consistent as the relationship
holds for an interval range between 5 and 55 years,
which includes the age range of most orchards.
4 Conclusions
Results presented in this article show that carob tree
has a low carbon fixation potential when compared
to other species, both due to a slow growth rate and
small densities. This crop has, however potential to
grow if with better cultural practices are
implemented. Moreover, the edaphic conditions in
Algarve limit the development of dense forests, with
the exception of pine in some the interior
mountainous areas. Carob tree orchards have at least
three competitive advantages over other forests in
Algarve: i) carob ranks first as agriculture product,
with very high annual revenues; ii) carob tree is
very resistant to water scarcity, frequent in
Mediterranean climates; iii) the plant is very
resistant to forest fires, in particular when compared
to pine forests. This latter issue may impose itself
one of the most relevant reasons in support of carob
tree as CO2 sink under the Kyoto Protocol’s Article
3.3, due to the lower risk of lost revenues in the
future, which has been keeping investors away from
the market (more on this may be found in Hamilton
et al [27] and Chenost et al [28].
Future works will include the collection of
more field data, both about allometric variables and
silvicultural practices, allowing the proposal of
alternative management practices.
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