Aboveground Biomass Stockpile and Carbon Sequestration Potential of Albizia saman in Chennai Metropolitan City, India

Albizia saman (Jacquin) F. Mueller belongs to the family Fabaceae (sub family: Mimosoideae) is a native to Northern South America. Commonly known as rain tree and locally known as Thoongu-moonchi maram (Tamil). The species’ introduced during Colonial period as an ornamental tree in Chennai metropolitan city (CMC). Though A. saman represent as a dominant tree species’ in CMC, there are voids in baseline data such as density, biomass stockpile, and annual C sequestration potential hence this study was conducted to fill these voids. A total of 2522 individuals which cover 1672.14 m basal area (mean = 9.61 ± 4.95 m ha; range = 0-24.96 m ha) was recorded from study plots. During study period A. saman stocked a sum of 6403.51 Mg aboveground biomass (AGB) (mean = 36.8 ± 18.9 Mg ha; range = 0-95.4 Mg ha) and 3201.76 Mg C (mean = 18.9 ± 9.45 Mg ha; range = 0-47.7 Mg ha). C storage of individual tree ranged from 3.74 to 4598.18 kg with a mean value of 1269.53 ± 1082.25 kg. On an average, each tree achieved 1.04 ± 0.27 cm horizontal growth yr. In a year A. saman population sequestered 111.23 Mg biomass in aboveground (in 174 ha). The mean C sequestration of study area was 319.62 ± 184.0 kg ha year. In total, the study area sequestered 55.62 Mg C year. Overall, in a year A. saman absorbed 204.13 Mg CO2 for C sequestration in study area. CO2 absorption ranged from 385.46 to 3009.29 kg ha -1 yr. The monetary value of C storage and annual sequestration of A. saman is also investigated. Though introduced from tropical Northern South America A. saman provides a considerable ecosystem services to CMC through C storage and sequestration. This study estimated monetary values of just two ecosystem services of A. saman, study that concentrates on all ecosystem services is essential to assess total actual ecosystem service values.


Introduction
Urban areas are home for about half of the global human population [1]. An estimation shows that urban human population will increase up to 5 billion by 2030. Approximately, 1.2 million km 2 area (three times larger compared to the year 2000) would come under cities in 2030, ultimately, this would lead to loss of biodiversity and forest cover around the world including India [2]. Thus, in-depth scientific studies are essential to understand the importance of urban forests and ecosystem services they provide. Tireless efforts and decades of continuous research work has advanced our understanding of urban forests and green spaces [3]. Urban forests and their biotic components play vital roles in reducing energy budgets of building and urban heat islands [4,5], augmenting water and air quality [6], decreasing the impacts of flooding [7], improving human health and reducing sound pollution [8]. Among lifeforms, trees are important constituent of urban ecosystems. Besides, urban trees do array of ecosystem services including biomass and carbon storage [9].
Urban forests are either rich in native species [10] or introduced species [11]. McKinney [12] named the introduced species as urban exploiters, found extensively from urban areas around the world. Introduced trees can also provide considerable quantum of ecosystem services [13]. Albizia saman (Jacquin) F. Mueller belongs to the family Fabaceae (sub family: Mimosoideae) is a native to Northern South America. Commonly known as rain tree and monkey pod, locally known as Thoongu-moonchi maram (Tamil). Now extensively grows throughout the tropics. It reaches up to 25 m tall and 30 m crown diameter, highly suitable for large homesteads, parks, roadsides and school play grounds [14]. The tree has good qualities, grows well at sea level to 300 m amsl, adapts to a broad array of soil types and pH ranges, growth rate is relatively high (2.5-5 ft yr -1 ), produces fodder and timber, generates 1700-4200 kg biomass in 5 years [14]. Besides that the tree also has economic importance as fuel wood [15], food and fodder [16], timber [17], gum and resin [18], nitrogen fixer and green manure [19], and medicine [20,21].
The species was introduced during Colonial period as an ornamental tree in Chennai metropolitan city (CMC) [22]. Now it grows extensively in parks, roadsides, playgrounds of academic institutions, and avenues in CMC [23]. The urban forest division of Chennai district prefers this tree for its fastgrowing nature, handsome dome-shaped crown and shade. Though A. saman represent as a dominant species in CMC there are voids in baseline data such as density, sequestered biomass, C stockpile and sequestration potential hence this study was conducted to fill these voids.

Study Area
Chennai Metropolitan city is 34th largest city in the world with the human population ~ 5 million [24]. CMC is one among the four mega-cities of the Indian subcontinent, and the capital city of Tamil Nadu state. The city is experiencing a tropical dissymmetric climate and receiving bulk of the rainfall during north-east monsoon (September-December). Mean temperature and rainfall were 30°C and 1300 mm [25]. East-side of the city is bounded with the Bay of Bengal and remaining three sides are bordered with Thiruvallur and Kanchipuram districts. CMC is endowed with rich plant diversity (1039 species) [24] which include both native as well as introduced species.

Field Survey
The entire geographical area of CMC (174 km 2 ) was divided in to the regular rectangular grids (1 km 2 × 1 km 2 ) by fishnet tool of ArcGIS software (version 9.3). The sample sites were selected randomly inside of the each grid. A total of 174 one-hectare sample plots were laid to record density and diameter at breast height (dbh) of Albizia saman (> 5cm dbh). Diameter of all trees >5cm dbh was measured at the height of 137 cm above the ground and recorded in field data sheet. In order to record DBH value for consecutive years, trees were tagged with consecutively numbered aluminium tags. Field survey was conducted during January-March on 2011 and 2012. Data on trees was recorded with the help of students of Botany departments across the Chennai city. Rainfall and temperature recorded in the year 2011 and 2012 were more or less equal to the mean rainfall and temperature of the study area, hence the study period represented Chennai's usual climatic and environmental conditions.

Estimation of Aboveground Biomass
A region-cum-species specific allometric formula developed by destructive sampling method was employed to estimate AGB of Albizia saman in study area [26,27]. AGB dry = exp (1.9724*LN (DBH) -1.0717); where, AGB dry is aboveground dry biomass of tree (kg); DBH is stem diameter at breast height (cm); LN is natural logarithm; 1.9724 and 1.0717 are constants. The allometric formula developed with the destructively sampled healthy individuals of A. saman (DBH range 4.45 to 178.7 cm). Due to hetero-scedasticity nature of field data, the error variance was not constant. The problem was dealt with the transformation of variables. But the de-transformed predicted values are biased [28]. To overcome those bias, the back transformed results from logarithmic unit was multiplied by a conversion factor (CF = 1.016) [29]. DBH of trees ranged from 5 to 176 cm in the present study. The coefficient of determination of allometric equation is high (r 2 ) i. e. 0.98. Standard error of the estimate is 0.76.

Assessment of Carbon Storage and Sequestration
To get carbon storage values of trees aboveground biomass multiplied by 0.50 [30]. The annual increase of stem diameter and biomass sequestration of trees were calculated by the difference in estimates of dbh and biomass stockpile between year x and x+1. Carbon storage and sequestration values were converted to CO 2 equivalent by multiplying with 3.67, the ratio of molecular weights of CO 2 to C [31].

Monetary Value of Ecosystem Services
The money value of ecosystem services provided by A. saman, namely C storage and sequestration was calculated based on international C price. International price for one tonne C is 41 US$ [32].

Tree Density and Basal Area
A total of 2522 individuals (>5 cm dbh) was recorded in 174 ha. Density of trees ranged from 0-30 ha -1 . The mean tree density of A. saman was 14.49 ± 8.52 ha -1 . Likewise, the basal area of trees varied from 0-24.96 m 2 ha -1 . The average basal area of A. saman was 9.61 ± 4.95 m 2 ha -1 . Few sample plots completely fell on water bodies where density, basal area and AGB were recorded as '0'. DBH of trees differed from 5-176 cm, while the mean dbh in study area recorded as 80.95 ± 43.53 cm (Table 1).

Aboveground Biomass
As on March 2011, A. saman stores a sum of 6403.51 Mg AGB in 174 ha study plots. The mean AGB of study area was 36.8 ± 18.9 Mg ha -1 (range, 0 to 95.4 Mg ha -1 ). The mean AGB of an individual tree was recorded as 2539.1 ± 2164.5 kg (range, 7.48 to 9196.35 kg) ( Table 1). DBH of tree is positively linked with AGB (r 2 =0.94, p < 0.01). The larger is the tree the more is the sequestered biomass. The largest tree holds nearly 1200 times more AGB than the smallest one in study area. AGB storage of diameter classes varied considerably in the study area, DBH class 135.

Annual Horizontal Stem Growth
On an average, each tree achieved 1.04 ± 0.27 cm horizontal growth yr -1 . There is a negative relationship exists between tree dbh and tree horizontal growth (r 2 =0.87, p < 0.01). The smaller is the tree the larger is the annual stem horizontal growth (Table 3).

Sequestration of Biomass and Carbon
In a year A. saman population sequestered 111.23 Mg biomass in aboveground parts (174 ha). AGB sequestration varied from 210.05 to 1639.94 kg ha -1 year -1 . The mean AGB sequestration of study area was estimated as 639.24 ± 367.99 kg ha -1 year -1 . Carbon sequestration of A. saman differed from 105.03 to 819.97 kg ha -1 year -1 among study plots. The mean C sequestration of study area was 319.62 ± 184.0 kg ha -1 year -1 . In total, the study area sequestered 55.62 Mg C year -1 .

Absorption of CO 2 for C Sequestration
Overall, in a year A. saman absorbed 204.13 Mg CO 2 for C sequestration in study area. CO 2 absorption ranged from 385.46 to 3009.29 kg ha -1 yr -1 . On an average, each onehectare plot absorbed 1173.0 ± 675.28 kg CO 2 to sequester C.

Monetary Values of Carbon Storage and Sequestration
The monetary value of C storage and sequestration of A. saman in study area (174 ha) is 131,272 and 2,280 US$, respectively. The money value of these kind of ecosystem services of A. saman for entire CMC (17400 ha) could be estimated as 13.12 and 0.23 million US$, respectively. On an average, the monetary values of C storage and sequestration of each hectare could be valued as 754 and 13.11 US$, correspondingly.

Tree Density and Basal Area
In an earlier tree diversity study conducted across different land uses of CMC A. saman constituted 6% tree community and topped in the list of important value index (IVI) among 45 species [10]. Likewise, A. saman constituted a considerable proportion of tree communities in urban forests of Bangalore and West Bengal, India [33,34]; Bangkok, Thailand [35,36]; Chittagong, Bangladesh [15] and USA [37].
The mean basal area recorded for A. saman (9.61 ± 4.95 m 2 ha) is not in agreement with previous study [23]. Earlier a study recorded 22.33 m 2 BA ha -1 for A. saman. However, the previous study concentrated only on a hectare area of CMC, present study concentrated on large area i.e. 174 ha. The mean basal area recorded for species' under study is larger than the mean stand basal area (of all species) of urban forests in Ohio, USA (4.8 m 2 BA ha -1 ) [38], and more or less equal to urban forests of Miami-Dade County, USA (10.0 m 2 BA ha -1 ) [39]. It is apparent that the occurrence of good proportion of well-grown large trees in the CMC (>60 DBH, 61.14%) contributed to larger tree stand basal' areas.

Population Structure
Albizia saman is showing a non-expanding population structure in CMC (Figure 2). Among 11 diameter classes, nine (except smallest and largest DBH classes) had more or less similar number (230-249) of individuals. Ongoing developmental activities such as construction of buildings and bridges, widening of roads etc. are contributing to the destruction of trees. Non-expanding population structure of A. saman indicates that the individuals of all girth classes are under disturbances.

Aboveground Biomass and Carbon Storage
The results obtained on mean AGB and C storage of A. saman (AGB=36.8 ± 18.9 Mg ha -1 ; C=18.4 ± 9.45 Mg ha -1 ) is higher than in urban forest of Tripura university campus, Northeast India (AGB=11.81 Mg ha -1 ; C=5.91 Mg ha -1 [40] [30]; and, Oakland, USA (22 Mg ha -1 ; 11 Mg ha -1 ) [46]. The population of A. saman composed of relatively larger trees (mean DBH=80.95 cm) hence stored good amount of biomass and C in its aboveground parts. Quantitative studies should be conducted to estimate biomass and C storage of all tree species in Chennai city. The absence of region-specific multi-species tree allometric models is the primary lacking for these studies hence research on these lines could be valuable.
Each tree stored 1269.53 ± 1082.25 kg C (range, 3.74-4598.18 kg) in study area. This value is higher than in Tshwane, South Africa (474.22 kg C) [47]; Beijing, China (98.87 kg C) [41]; Shenyang, China (58.51 kg C) [32]; and cities of USA (mean = 227.01 kg C; range = 91.81-638.95 kg C) [30]. On the other hand, the present study concentrated only on single species' thus studies that consider all tree species in CMC are essential to confirm the dominance.

Stem Horizontal Growth
The findings pertaining to mean stem horizontal growth  [48] who reported 1.1 cm stem horizontal growth tree-1 year-1 for urban trees of USA. While the result of current study is not in line with that of deVries [49], Nowak [50] and Smith and Shifley [51] recorded 0.61, 0.90 and 0.38 cm horizontal stem growth tree-1 yr-1 respectively for trees of central park, New Jersey, three USA cities, and Indiana and Illinois, USA. Fast growth nature of A. saman in CMC contributed to a high dbh growth tree-1 yr-1 in this study.
A negative relationship as obtained between stem horizontal growth yr-1 and diameter class is chiefly linked to the age of trees. Young trees show faster growth than larger trees. Age-related decreases in aboveground C sequestration were extensively recorded and reported around the world [52][53][54].
The per hectare C sequestration potential recorded for A. saman (319.62 ± 184.0 kg ha-1 year-1) is extremely lower than in urban forests of Pungol Eco-town, Singapore (3.61 Mg ha-1 yr-1) [43] and lower than in seven USA cities (458.57 kg ha-1 yr-1) [30]. Present study concentrated on single species', if all species taken in to consideration for C sequestration then the value may exceeds than in USA. Further studies are necessary to validate these lines. However, the per hectare C sequestration potential of A. saman is relatively higher than the cumulative C sequestration potential (individuals belongs to all species in a hectare) of urban trees in California (300 kg ha-1 year-1), Texas (300 kg ha-1 year-1), Arizona (300 kg ha-1 year-1), Rhode Island (300 kg ha-1 year-1), North Dakota (200 kg ha-1 year-1), and Wyoming (100 kg ha-1 year-1) of USA [47,48].

Monetary Value of two Ecosystem Services
The monetary value of two ecosystem services namely, C storage and sequestration of A. saman for entire CMC is 13.12 and 0.23 million US$, respectively ( Table 4). The monetary value estimated in this study is higher as well as lower than in urban forests elsewhere. Stoffberg et al. [44] reported 3 million US$ for Tshwane, South Africa; Liu and Li [32]

CO 2 Emission Reduction
In a day Chennai needs 1300 kilolitres petrol and 2000 kilolitres diesel. Use of petrol, diesel emits 3003, 5360 Mg CO 2 in to the atmosphere, respectively. In all, fossil fuel use in CMC releases about 8363 Mg CO 2 into the atmosphere per day. C stockpile and sequestration of A. saman is equal to 1175, 204.13 Mg CO 2 , correspondingly. The average effects of tree diameter classes listed in Table 5. In total, A. saman population in CMC provide C storage and sequestration equivalent to 16.49% of a day's CO 2 emissions by fossil fuels.

Conclusion
Though introduced from tropical Northern South America A. saman provides a considerable quantity of ecosystem services to CMC through C storage and sequestration. This study estimated monetary values of just two ecosystem services of A. saman, study that concentrates on all ecosystem services is essential to assess total actual ecosystem service values.