Evaluation of carbon accumulation in the Kandelia obovata Sheue, Liu & Yong plantation in northern Vietnam

This study was conducted to evaluate carbon accumulation in a mangrove plantation containing one species of Mangrove tree (Kandelia obovata Sheue, Liu & Yong) in Giao Thuy District, Nam Dinh Province, Vietnam. Sixteen sample plots were established and surveyed from December 2008 to May 2010 in order to determine the amount of carbon present in the trees and forest soil.

This paper is available online at http://stdb.hnue.edu.vn

EVALUATION OF CARBON ACCUMULATION

IN THEKandelia obovata SHEUE, LIU & YONG PLANTATION

IN NORTHERN VIETNAM

Mai Sy Tuan

Faculty of Biology, Ha Noi National Universuty of Education

Abstract. This study was conducted to evaluate carbon accumulation in a

mangrove plantation containing one species of Mangrove tree (Kandelia obovata

Sheue, Liu & Yong) in Giao Thuy District, Nam Dinh Province, Vietnam Sixteen

sample plots were established and surveyed from December 2008 to May 2010 in

order to determine the amount of carbon present in the trees and forest soil The

amount of CO2 released from the forest soil was estimated once a month using a

KIMOTO-HS7 gas absorption machine The amount of carbon accumulation was

calculated from estimated variables The results show that the amount of carbon

accumulated in trees and forest soil is higher than that of emitted CO2 The annual

amount of carbon that is accumulated by K obovata plantations is equivalent to the

amount of CO2emitted, and this amount increases as the forest ages, reaching the

highest value in the 9 year old forest (27.138 ton/ha/year) in this present study (a

CO2 equivalent of 99.596 ton/ha/year (39.35%) The least amount of CO2 emitted

was observed in the 1-year old forest (2.207 ton/ha/year), a CO2equivalent of 8.099

ton/ha/year (3.20%) Therefore, a K obovata plantation can accumulate a large

amount of carbon and function as a carbon sink and reduce the amount of CO2 in

the atmosphere This study presents scientific data that could be used to justify the

implementation of mangrove plantation projects under CDM and PES programs

along the Vietnamese coastline that could protect the environment, inhibit climate

change, improve living standards, alleviate hunger and reduce poverty among local

populations

Keywords : Mangrove plantation, Kandelia obovata, carbon accumulation,

Northern Vietnam

Received September 10, 2012 Accepted November 2, 2012.

Contact Mai Sy Tuan, e-mail address: tuanmaisy@yahoo.com

1 Introduction

Mangrove forests were found along the coastlines in most countries in the tropics and subtropics and many still remain or have been rather recently planted Tropical forests

in general are a disproportionately important component in the global carbon cycle and are thought to represent 30 - 40% of the terrestrial net primary production and storage

of carbon [4, 10] Although, mangrove forests represent only a small fraction of total tropical forest area, the amount of carbon accumulated in the biomass of mangrove forest trees and/or sediments is significant In addition to this accumulation, the mangrove ecosystem also includes a carbon release process that involves the aqueous environment and particularly the sediment due to the decomposition of microorganisms

Although a large number of studies have been done which have examined carbon accumulation in natural mangrove ecosystems around the world [9], there is little information available about carbon accumulation in man-made mangrove plantations Vietnam’s 3260-km long coastline and dense river system with abundant alluvial effluent has the potential to support a substantial area of mangrove trees Eleven years ago, in 2001, the Forest Inventory and Planning Institute carried out a national forest inventory [5] and at that time it was estimated that there were 156,608 ha of mangrove trees in Vietnam, about two thirds of that being plantation forest trees This current study has been carried out in order to evaluate carbon accumulation in mangrove plantations in northern Vietnam, the specific objectives being: 1) to evaluate the efficacy of mangrove plantations as carbon sinks, 2) to propose an effective methodology for measuring carbon accumulation, 3) to contribute to the means of valuating carbon credits based on the Kyoto protocol approved in 2002 regarding Clean Development Mechanism (CDM) and Payment for Environmental Services (PES) carbon options and 4) to help policy makers and managers formulate an integrated management plan regarding mangrove plantation areas that would result in increased carbon accumulation

2.1 Materials and methods

2.1.1 Study site

This study was conducted from December 2008 to May 2010 in plantations of K obovata Sheue, Liu & Yong mangrove trees (other mangrove/associated species were not found) The trees are now 1, 5, 6, 8, 9, and 12 years old and were planted in the Giao Lac Commune, Giao Thuy District, Nam Dinh Province, in Northern Vietnam Giao Lac Commune is situated between 20◦13’- 20◦15’ latitude and 106◦15’- 106◦30’ longitude The commune is bordered by the Giao An Commune to the north, the Giao Xuan Commune to the south and the sea to the east (Figure 1)

Figure 1 Study site in the coast of northern Vietnam

Giao Lac Commune is in the Xuan Thuy Ramsar Site which is a buffer zone At this time in the Giao Lac commune, approximately 407.7 ha have been planted in mangrove trees The mangrove ecosystem here receives a large amount of alluvial material from the Red River and the mangrove habitat soil is basically mud mixed with clay and sand In this site, plantations of 8, 9 and 12 years old mangrove trees are situated in relatively high elevation areas with a long non-tidal inundation time (4,776 hours/year) and have tree density of 17,900, 18,200 and 18,050 trees/ha respectively Mangrove plantations in mid elevation areas (4358 hours/year) are 5 and 6 years old with tree density of 17,300 and 17,500 trees/ha and the plantations in the lower elevation areas (3,960 hours/year) are 1 year old with the tree density of 15,400 trees/ha

According to the Vietnam National Hydro-meteorological Service and Marine Hydrological Center, the tide is diurnal at this site Low tide is 0.1 m and high tide is 3.9 m The mean annual temperature is 23 - 24◦C Mean rainfall ranges from 1,056 to 1,470 mm/year, maximum rainfall is normally in July and August (227 - 315 mm/month) and the least amount of rain falls in January (9 mm/month) Average humidity is about 82%

2.1.2 Data collection

Sixteen sample plots (10 m × 10 m) of trees of 6 age classes were surveyed; for each age class (1, 5, 6, 8, 9 years) we measured the diameter of the trees growing in 3 randomly selected plots but in the 12-year old plantation we measured the trees in only 1 plot and was left out of our analysis due to its small area In each sample plot, 3 mangrove trees, of the big, medium and small size, were cut and categorized in regards to roots,

stems, branches, leaves and reproductive organs The tree parts were weighted at that time Samples obtained from these parts were then dried at 80◦C and the dry weight was recorded Thirty-six soil samples were obtained from plantations of each age of tree (1, 5,

6, 8, 9 years), collected in a 20 cm × 20 cm × 20 cm frame ranging in depth from surface soil to 100 cm in depth at low tide The samples were brought to the Environment and Soil Analysis Laboratory for treatment and analysis

2.1.3 Determining the amount of carbon in the trees

The amount of organic carbon (%) in the trees was determined using the L.O.I (loss on ignition) method Basing on the accumulated carbon, the amount of CO2 absorbed by photosynthesis and converted to mangrove tree biomass is 3.67 times the total accumulated carbon (ton/ha) Therefore, 3.67 can be used as a conversion constant

to convert organic carbon to CO2, this being determined from the molecular weight of

CO2 and carbon

2.1.4 Determining the amount of carbon in the soil

The amount of organic carbon in the soil was determined using the Walkley-Black method [13] This method makes use of the following equation

3C + 2K2Cr2O7+ 8H2SO4 →3CO2+ 2K2SO4+ 2Cr2(SO4)3+ 8H2O

The redundant amount of K2Cr2O7was titrated by 0.5N FeSO4

2.1.5 Determining the amount of CO2emitted from soil

The gas sampling method was used to measure the amount of CO2 emitted from the soil The gas absorption machine, a KIMOTO-HS7, was placed in a 1 m3 box tightly covered in transparent white nylon that had an absorption speed of 1 litre/minute (Fig 2)

Figure 2 KIMOTO-HS7 equipment and sampling of gas emitted

from soil in 1-year old forest

The amount of carbon dioxide was determined referring to the absorption rate of barite The formula used is as follows

CO2 + Ba(OH)2 →BaCO3+ H2O

Air with CO2 was added to a given amount of barium hydroxide and the redundant amount of barium hydroxide was titrated by acid oxalic The colored indicator is 0.1% phenolftalein

Ba(OH)2 + HOOC-COOH → Ba(COO)2 + 2H2O

The added amount of barium hydroxide is calculated based on the amount of redundant barium hydroxide The amount of CO2in the air can then be calculated

In this study, the amount of CO2 being emitted from forest soil was estimated once

a month at low tide

2.2 Results and discussion

2.2.1 Carbon accumulation inK obovata plantation forest biomass

* Amount of carbon accumulated in K obovata forest

Amount of carbon accumulated in K obovata plantation forest trees increased with

the rising age of the trees (Table 1) and is in direct proportion with tree biomass

Table 1 Amount of carbon (C) accumulation (ton/ha) (n = 36)

Forest

age

Planting

year

Density

Carbon accumulated in forest trees

0.226

0.954 ± 0.728

0.022 ±

1.253

2.856 ± 2.142

2.244 ±

1.873

3.895 ± 0.784

2.327 ±

2.658

6.800 ± 2.162

3.276 ±

1.475

12.793 ± 2.769

2.621 ±

Among mangrove forest trees which are 1, 5, 6, 8 and nine years old, the greatest

amount of carbon accumulated in 9-year old K obovata plantation trees (48.02 ton C/ha),

followed by 8-year old plantation trees (40.005 ton/ha), 6-year old plantation trees (29.077 ton/ha), 5-year old plantation trees (27.234 ton/ha), and 1-year old plantation trees (1.015 ton/ha) On each tree in the 1-year old plantation was found to be growing from 14 to 50 leaves; therefore, the ability of these trees to fixate CO2 for synthesis of organic carbon

is low In addition, the 1-year old plantation of this study is situated in a relatively low elevation area which is inundated by sea water for 10 - 14 hours/day This would also lower the trees’ photosynthetic ability Despite the low amount of accumulated carbon in newly planted mangrove trees and their poor growth performance, their survival rate is greater

than 70%, showing that K obovata trees are quite resilient with regards to relatively high

concentrations of salinity in its environment

* Absorption of CO2in K obovata plantations

For the study on absorption of CO2in mangrove plantation forests, it is essential to determine the forest biomass From an estimated total forest biomass, we can determine the amount of carbon accumulated in each tree and then calculate the amount of CO2 absorbed during photosynthesis The amount of CO2 absorbed in forest trees was found

to be highest in the 9-year old forest (176.263 ton/ha), followed by the 8-year old forest (146.818 ton/ha), 6-year old forest (106.713 ton/ha), 5-year old forest (99.949 ton/ha), and the lowest in the 1-year old forest (3.725 ton/ha) (Table 2)

Table 2 Amount of CO2 absorbed by K obovata forest (ton/ha)

carbon

Absorbed

CO2 (trees/ha) (ton dw/ha) (ton C/ha) (ton CO2/ha)

The annual average carbon accumulation of mangrove trees in a 1-year old plantation forest is 0.839 ton/ha/year, in a 5-year old forest 7.31 tons of C /ha/year, in

a 6-year old forest 8.03 tons of C/ha/year, in an 8-year old forest 13.42 tons of C/ha/year and in a 9-year old forest its 15.09 tons of C/ha/year, which is the equivalent of 3.08, 26.82, 29.49, 49.26 and 55.38 tons of CO2/ha/year, respectively (Table 3)

Table 3 Annual average amount of carbon accumulation of K obovata forest (ton/ha/year)

Forest age Annual increase

in biomass

% carbon in biomass

Annual carbon accumulation rate

Annual CO2 absorption rate (ton dw/ha/year) (ton C/ha/year) (ton CO2/ha/year)

It can be said that the absorption of CO2 by mangrove trees in general and by

K.obovata trees in particular is fairly large However, to evaluate the role of plantation

mangrove forests in greenhouse gas reduction, we need to consider the whole process

of carbon accumulation in the soil and the emission of CO2 through soil respiration in the forest ecosystem Through the process of CO2 absorption (photosynthesis), carbon accumulates in forest trees, and when forest leaves and branches fall to the ground, carbon is returned to the forest soil Carbon returns to the air in the form of CO2 through soil respiration and decomposition Thus, only if CO2 absorption exceeds CO2 emission should the planting of mangrove forests under CDM and PES carbon options be considered effective and feasible

2.2.2 Carbon accumulation in forest soil

The amount of carbon in forest soil varies at different depths in the soil; the amount

of carbon is higher at a surface layer and becomes less at increasing depth in the soil (Table 4) It was found that most of the carbon accumulates at a depth of 0 - 40 cm

Table 4 Amount of carbon (ton C/ha) accumulation at different soil depths (n = 36)

Soil

0 cm 14.04 ± 0.26 16.49 ± 1.95 15.55 ± 1.06 18.62 ± 0.30 18.68 ± 0.50

20 cm 13.46 ± 0.26 14.74 ± 0.69 18.48 ± 3.94 17.86 ± 0.32 17.46 ± 0.06

40 cm 12.76 ± 1.22 13.66 ± 3.29 13.26 ± 0.22 15.52 ± 1.68 16.82 ± 0.30

60 cm 11.77 ± 1.73 11.00 ± 1.91 10.26 ± 1.03 12.96 ± 0.70 15.33 ± 0.71

80 cm 9.00 ± 1.11 8.43 ± 0.64 10.65 ± 2.35 11.45 ± 0.19 13.54 ± 0.67 100

cm 7.35 ± 0.76 7.99 ± 0.16 8.62 ± 0.34 9.73 ± 0.25 10.35 ± 0.64

Table 4 shows carbon accumulation in forest soil The amount of carbon accumulated at depths of 0 - 40 cm is greater than the amount of carbon accumulated

at depths of 40 - 100 cm The amount of carbon accumulation at a depth of 0 - 20 cm

in 8 and 9-year old forests is less than that accumulated in the soil of 5 and 6-year old forests An explanation for this could be that the 5 and 6-year-old forests are located a mediumelevations and are inundated by tidal water for a longer period of time Increased tidal inundation means increased amount of organic matter in sea sediment and river alluvia

The amount of carbon in forest soil decreased corresponding to increased soil depth

The research results are in accordance with those of Fujimoto and et al [6] whose study

presented the amount of carbon accumulated in Ca Mau and Can Gio forest soils in southern Vietnam The author stated that most of the carbon accumulated in soil is at

a depth of 0 - 60 cm with the amount of carbon decreasing at deeper soil layers [12] Carbon accumulation in mangrove forest soil rises with increasing forest age [12]

In this study, the amount of carbon accumulated at depths of 0 - 100 cm in the soil of

K obovataplantations ranges from 68.373 to 92.183 ton/ha The highest value is found

in 9-year old forest at 92.183 ton/ha, followed by an 8-year old forest at 86.140 ton/ha, a 6-year old forest 76.820 ton/ha, a 5-year old forest at 72.397 ton/ha, and the lowest is seen

in 1-year old forest at 68.373 ton/ha (Figure 3)

Figure 3 Carbon dynamics in K obovata plantations

in Giao Thuy District, northern Vietnam

Carbon accumulates in the soil of mature forests at a higher rate than in the soil of newly planted forests Soil in a 9-year old forest accumulates 12.41 ton of carbon/ha/year,

in an 8-year old forest 4.82 ton/ha/year, in a 6-year old forest 3.57 ton/ha/year, in a 5-year old forest 3.02 ton/ha/year, and in a 1-year old forest 1.46 ton/ha/year Although the rate

of carbon accumulation in a 1-year old forest is not high, it can be seen that forestation

is significant factor in carbon accumulation, making a significant contribution to the reduction of greenhouse gas

Carbon accumulation in soil increases with forest age and biomass increases with the age of forest trees, especially root biomass It was found that forest tree roots contribute more to total organic material in soil than leaf litter However, when there is a large amount

of carbon accumulated in mangrove soil, decomposition of organic matter in the soil (mainly the roots) proceeds very slowly Albright L J [1] states that 90% of mangrove tree leaf litter decomposes within 7 months At the same time, 50 - 88% of root tissue is intact after one year and when roots are buried in the soil, decompositionis even slower Studies on root decomposition in boreal peat bogs have also shown that under normal conditions, roots decompose more slowly than above-ground components [14] Leaf litter breaks down very rapidly or is carried away by tidal water In contrast, roots decompose slowly and accumulate for a long time and therefore roots play a crucial role in carbon accumulation in mangrove soil [11]

2.2.3 Emission of CO2from mangrove soil

The amount of CO2 emitted from mangrove soil rises with increasing forest age (Table 5) The amount of CO2 emitted from a 1-year old plantation is 0.338 ton

CO2/ha/year, from a 5-year old plantation its 0.51 ton CO2 /ha/year, from a 6-year old plantation 0.59 ton CO2/ha/year, from an 8-year old plantation 0.89 ton CO2/ha/year, from

a 9-year old plantation 1.32 ton CO2/ha/year

Table 5 The amount of CO2emitted from the soil at different forest ages (ton CO2/ha/year)

Forest age (year)

Amount of CO2

emitted from soil

(ton/ha/year)

In this study, CO2 emissions were found to increase with increasing forest age due to 1) decreased bed elevation; 2) litterfall (leaves, stems and branches) and organic matter brought in by tides and deposited on forest floors, an important factor influencing emission of CO It was found that litterfall increases as forests age and the older the forest plantation the more the litterfall there will be A high amount of organic matter

in forest soil means potential increased decomposition and therefore an increase in CO2 emissions and 3) older plantations have more root biomass than younger plantations and

CO2 emission occurs due to root respiration The decomposition of dead roots also causes

an increased amount of CO2to be emitted from the soil

Our findings are consistent with those of Alongi [2] about the contribution of mangrove forests to global carbon cycle and greenhouse gas emission The rate of CO2 emission from mangrove soil varies from 2 to 373 mmol/m2/day or 0.088 to 16.412 g/m2/day, this wide variation in rate being dependent on both the amount of organic matter and soil temperature

2.2.4 Carbon balance in forest plantations of different ages

Through photosynthesis, trees absorb and convert atmospheric CO2 to organic matter, and a part of this organic matter is added to forest soil as litterfall With soil respiration, CO2 is released back into the atmosphere This process was summarized in Figure 4

The amount of carbon that has accumulated in the forest after soil respiration is the

‘credit’ CO2of the mangrove forest The carbon accumulation of the mangrove plantations could be evaluated by using the linkages/relations illustrated in Figure 4 and the following equation:

A = (CCt + CDt) - CRt where:

A [ton/ha/year] : Amount of carbon accumulation in a forest;

CCt [ton/ha/year]: Amount of carbon accumulated in the trees at time t;

CDt [ton/ha/year]: Amount of carbon accumulated in the soil at time t;

CRt [ton/ha/year]: Amount of carbon emitted from soil respiration at time t

Figure 4 The carbon cycle in a Kandelia obovata plantation

The amount of carbon accumulated by K obovata plantations in the study site was

determined using the above procedure The result is presented in Table 6

Table 6 Carbon accumulation of K obovata plantations (ton C/ha/year)

Forest

age

Density

(trees/ha)

Carbon accumulation

in trees

Carbon accumulation

in soil

Total carbon accumulation

in trees and soil

CO2 emitted from soil respiration

Carbon accumulation of the mangrove forest

By comparing the amount of carbon that has accumulated in trees and in the soil with the amount of CO2 that is emitted due to soil respiration, it can be seen that the amount of CO2 emitted from soil respiration is much lower than that which has accumulated (Figure 5)