Carbon Sequestration Potential in Aboveground Biomass of Hybrid Eucalyptus Plantation Forest

Forests are a significant part of the global carbon cycle. Forests sequester carbon by conducting photosynthesis, which is the process of converting light energy to chemical energy and storing it in the chemical bonds of sugar. Carbon sequestration through forestry has the potential to play a significant role in ameliorating global environmental problems such as atmospheric accumulation of GHG's and climate change. The present investigation was carried out to determine carbon sequestration potential of hybrid Eucalyptus. This study was conducted primarily to develop a prediction model of carbon storage capacity for plantation forest of hybrid Eucalyptus in Aek Nauli, Simalungun District, North Sumatera. Models were tested and assessed for statistical 2 validity and accuracy in predicting biomass and carbon, based on determination coefficient (R ) and correlation coefficient (r), aggregative deviation percentage (AgD), and the average deviation percentage (AvD). The best 0,876 (0,094 ) general model to estimate the biomass of hybrid Eucalyptus was Y = 1351,09x .e . Results showed that hybrid Eucalyptus had an average above-ground biomass in year 0 (the land without the eucalyptus trees) up to year 3 as -1 large as 1.36, 11.56, 43.18, and 63.84 t ha . The carbon content of hybrid Eucalyptus were 0.61, 5.2, 19,43 t, and -1 -1 28,73 t C ha while the carbon sequestration potential were 2.23, 19.08, 71.31,and 105.43 t CO ha respectively. 2


Introduction
Global warming, basically consitutes a phenomenon of increasing earth temperature which occurs globally from year to year.Global warming is caused by increasing emission of greenhouse gasses such as carbon dioxide (CO ), 2 methane (CH ), dinitrooxide (N O), and (CFC).This causes 4 2 solar energy being trapped within earth atmosphere (Cruz et al. 2007).This change in global climate would affect forestry and agricultural sector (Anderson et al. 2011).
In relation with forest ability to absorb carbon, emission trade or carbon trade constitutes a new paradigm in forestry sector and could become opportunity for Indonesia which is a developing country, to obtain foreign exchange from this sector.Global initiative, such as Kyoto Protocol has created new challenge to view forest from a different angle.Forest is not only stacks of wood which could generate foreign exchange, but also constitutes global commodity which possesses non wood potency.
One of the efforts to minimize impacts from climate change is stabilizing the CO concentration in atmosphere.
2 This is related with forest ability to absorb CO from 2 atmosphere, and then store it in forest stand in the form of organic matter or plant biomass.Therefore, potency of forest in carbon absorption could be estimated through calculation of plant biomass, because half of the biomass comprise carbon (IPCC 2001).
Eucalypt is one of the plant species which has potency in development of industrial plantation forest (Mindawati 2010), and has opportunity to produce carbon within rotation period (cutting cycle) which is considerably short (7-10 years) from growth process of the the planted trees (Aswandi 2007).Estimated volume of Eucalyptus urograndis stand at rotation 1 and rotation 2 at the ages of 3 -1 5-5.5 years could reach around 176-197 m ha .Eucalyptus is one of the fast growing plant species which could be developed in many places (Bison et al. 2007).Eucalyptus spp., such as the species E. urophyllz, E. pellita, and E. grandis constitute the species which are developed as pulp industry raw materials, in large scale in PT.Toba Pulp Lestari with cutting cycle (rotation) of 7-8 years.Hybrid species of Eucalyptus, such as E. pellitagrandis (E.pellita × E. grandis) and E. urograndis (E.uropyhlla × E. grandis) have been succesfully developed widely in HTI (Industrial Plantation Forest) in North Sumatra with better growth performance as compared to their parents (Latifah 2009).Considering the forest function as carbon sequestering medium, information on the amount of carbon which is stored by a forest territory (carbon stock) becomes important.
Research on biomass and plant carbon of hybrid eucalypt is important to be conducted because this will be useful for plantation forest management plan, and constitutes one of the keys to support the success of HTI development in Indonesia in sustainable manner in the future.Sustainable forest would have potential of environmental service to sequester carbon in increasing amount and duration in accordance with the determined rotation time.
Efforts to quantify the amount of carbon which could be sequestered and stored (C-stock) by forest is needed very much for utilizing forest as carbon sequestering medium through a framework of carbon trade.Research results in the form of techniques and method of carbon storage estimation could be socialized to forest managers.There are also acknowledgements from international world toward efforts being conducted by Indonesian people to mitigate and adapt with the global warming.Indonesia possesses large area of HTI of Eucalyptus with large potential of biomass storage.Research on the biomass estimation of HTI of Eucalyptus needs to be conducted to provide data for estimating environmental service of carbon sequestration of a forest territory, particularly the plantation forest of hybrid eucalypt.

Methods
Research was conducted in April-May 2011 in HTI of PT Toba Pulp Lestari in Aek Nauli sector, Medan, which is geographically situated between 02°40'00"-02°50'00" North Latitude and 98°50'00"-99°10'00" West Longitude, and Laboratory of Forest Inventory, Forestry Study Program, Faculty of Agriculture, University of North Sumatera.Research activities were focused on obtaining reliable model to estimate above ground biomass in hybrid eucalypt stand at 1-3 years of age.
Equipments being used in this research were measurement tape, hypsometer, and chainsaw.Materials being used in this research were stems, branches, leaves, and shoots which were taken from stand which were 1-3 years old.
Measurement plot (PU) being used in this study had the size of 30 × 20 m ( 6 PU for each age class) and distance between PU were 10 × 10 m ( JICA 2002).Placement of PU was conducted by systematic sampling with random start.The collected data comprise diameter at breast height (dbh), branch free height, area size of crown section, and total tree height.Measurement of undergrowth vegetation was conducted at sub plots, measuring 2 × 2 m, within the PU.
Harvesting of biomass carbon stock is generally derived from the quantity of above ground biomass by assuming that 45% of the biomass value is composed of carbon (Onrizal 2004).Method which is most accurate is through destructive approach by cutting down trees and weighing all parts of the tree.Such destructive approach is frequently used to validate other method which tend to be less disturbing and requires low cost, such as carbon stock estimation by in situ non destructive and remote sensing measurement (Clark et al. 2001;Hua et al. 2007).
Selected sample trees (3 trees for each age class) were cut down and their diameter measured at points of 0, 0.3, 1.3, 3.3, 5.3, 7.3, and 9,3 m above ground surface, and afterwards there was separation of tree parts into stems, branches, and leaves.Weighing was conducted for all tree parts.In stem part, there were data collection on length and fresh weight per section, whereas in branches, leaves, and shoots there were data collection on fresh weight.Litters and undergrowth vegetation were collected and classified into particular parts, and their fresh weight were measured.
Characteristics of sample trees Wood test samples were in the form of cube measuring 2 × 2 × 2 cm, comprising 3 replications for each lower end, middle and upper end of stem.All wood test samples were weighed for determining their fresh weight, and were subsequently dried in oven at temperature of 103 C ± 2 C for determining their dry weight.Percentage of water content (WC) was calculated with the following formula: [1] note: % WC = percentage of water content ODW = oven dry weight of test sample (g) FW = fresh weight of test sample (g) Plant tissues and litter were weighed as much as ± 100 g with 3 replications and dried in oven with temperature of ± 80 °C until reaching constant weight (dry weight).Density of wood test samples need to be known to calculate biomass of stem parts through approach of wood density with the following formula (Haygreen & Bowyer 1986): [2] note: -

Biomass of stem and undergrowth vegetation
Development of biomass model by using the approach of wood density is based on equations which have been studied by biomass expert researchers in the past.Biomass calculation of stem segments on the basis of water content data (Haygreen & Bowyer 1986) is as follows: [3] note: ODW = oven dry weight of test sample (g) FW = fresh weight of segment of eucalypt tree (g) % WC = percentage of water content Biomass of undergrowth vegetation and litter was calculated with the following formula: Forest biomass could be used to estimate carbon content within forest vegetation because 45% of the biomass is composed of carbon.Carbon content could be estimated with the following formula (Onrizal 2004): on the largest determination coefficient (R ) and correlation coefficient (r), aggregative deviation percentage (AgD) < 1%, and average deviation percentage (AvD) > -1% and < 10%.The quantity of those values are as follows: [8] [9] note: Ba = actual tree biomass Be = estimated tree biomass N = number of tree sample Selection of model also considers the simplicity of model and their practicability to be used in the field (Catahan 2008).

Results and Discussion
Water content Laboratory results showed variation of water content (Figure 1).Descriptively, water content for each age class tended to decrease with increasing age.This was caused by phenomenon that the older the plant, the greater would be the wood constituent elements.Stems in young plants still had little wood element constituent, so that their water contents were greater.Stems in older tree have possessed maximum amount of wood element constituent, so that water content at older age become smaller (Haygreen & Bowyer 1986).Research results by Pambudi (2011) showed the same trend, where the water content of Rhizhophora apiculata tended to decrease with increasing tree age.biomass could use one of the variables.Independent variable is diameter or total height, whereas dependent variable is tree biomass.According to Baker et al. (2002), choice of variable diameter at breast height (dbh) would improve the efficiency and reduce uncertainty of measurement result on the basis of equation which had been established, whereas choice of variable tree height tended to reduce measurement efficiency because variable tree height is more difficult to measure than that of diameter at breast height.

Models of biomass estimator
Model of biomass estimator of hybrid eucalypt used data of stem dry weight, branch dry weight, twig dry weight, and leaf dry weight in the biomass measurement.On the basis of measurement of sample trees, there was construction of model which described the relationship between biomass and several variables.Several allometric models of biomass estimator were tried to obtain the best model for biomass estimation.Reliability test of the model was conducted to measure the model ability to estimate a group of new data which possessed relatively similar condition with condition of data which were used to establish the model.
In terms of model application, alometric equation is specific for a particular species and location, so that allometric equations could not be compared across different species and locations.However, in terms of composition of variables and forms of equation, various allometric equation could be compared to obtain the best model (Wenbo 2007).Basically a good model is that which is simple enough, easy to be analyzed and easy to be applied.Apart from those, models should have considerably high accuracy of estimation (Latifah 2000).In general, the tested allometric model were grouped into 2 categories, namely general and specific model.General model constitutes model which is used to estimate biomass by combining several age classes.In this research, general models were tested, and could be seen in Table 1.
General model for biomass estimation by using 1, 2, and 3 variables could be seen in Table 1.In general, the selected model for each variable being tested, had good performance.On the basis of statistical analysis, it was obtained that 0.876 (0.094x) allometric model Y = 1,351.09x.e had the best performance.This allometric model possessed R² value as large as 98,29%, r as large as 0.9849, AgD as large as 0,08%, and AvD as large as -0.28%.Therefore, this model fulfilled the requirement as a reliable model.Catahan (2008) stated that a model is categorized as reliable if the aggregative deviation percentage (AgD) < 1% and -1 % < AvD < 10%.Besides statistical consideration, according to Catahan (2008), the choice of best equation model should also consider the factor of practicability, efficiency, and ease in the collection of independent variable data in the equation.
Average deviation and aggregate deviation were used to measure the accuracy of a model.The smaller the values of AgD and AvD the more accurate would be the model.The value of AgD as large as 0.08% showed that the percentage of difference between value of estimation result and value of biomass measurement result of hybrid eucalypt was 0.08.On the other hand, value of AvD as large as -0.28% showed that average value of biomass estimation of hybrid eucalypt deviated as large as 0.28 from average value of their measurement result.Allometric 0.876 (0.094x) model Y = 1351,09x .e had possessed good value, while its AgD and AvD were in conformity with the determined criteria.

Potency of biomass and carbon storage
Biomass constitutes the amount of organic matter being produced by organism per unit area at a certain particular time.Biomass could be classified into 2 categories, namely above ground biomass and below ground biomass.This research was focused on above ground biomass estimation.Biomass was The general allometric model which had been selected was used to estimate carbon storage per tree by using data of stem diameter measured in the measurement plot.From 0.876 0.876(0.094x)results of calculation, using model Y = 1351,09x .e which constituted the best model by using plant diameter data from measurement results, eucalypt carbon storage were obtained and shown in Table 2. Estimation of stand biomass at various systems of land unit in Indonesia have been frequently conducted.Research results showed that biomass of hybrid eucalypt stand at ages of 1, 2, and 3 years were -1 consecutively 7.8, 31.9, and 40,4 ton ha .On the other hand, carbon potency at hybrid eucalypt stand at ages of 1, 2, and 3 -1 years were consecutively 3.5, 14.3, and 18.2 ton ha (Table 2).Biomass and carbon potency of hybrid eucalypt stand tended to increase with increasing plant age.Research results by Baker et al. ( 2002), Pambudi (2011) and Andre et al. (2005) showed similar trend where plant biomass tended to increase together with increase in tree age.
Research results showed that potency of biomass and carbon of 3 years old hybrid eucalypt was 40.4 and l8.18 -1 ton ha (Tabel 2) which were much higher as compared to biomass and carbon potency of acacia plant which was -1 29.53 and 18.04 ton ha (Heriansyah 2005).Istomo (2002), in his research in swamp forest of Rokan Hilir district, Riau, showed that total biomass of that swamp forest was -1 329.18 ton ha , whereas Hairiah (2007) showed that natural forest possessed the highest amount of stored C (around -1 497Mg ha ) as compared to other land use system.Disturbance which convert natural forest into secondary  13, 52.7, and 66.7 ton CO ha .In Table 3, it could be seen 2 that increase in plant age is linearly related with increase in carbon storage and sequestration of CO by the stand.This 2 phenomenon was in agreement with that in Aswandi (2007) research which stated that with increasing plant age, the number of plant composition was also larger.Biomass and carbon in a particular area describe the ability of that area to fix CO from the atmosphere, and also describe 2 the potential energy possessed by the area.The denser the vegetation of an area, the higher would be the ability to fix CO from the air, and the greater also would be the energy On the basis of Table 3, it could be seen that the amount of stand biomass was linearly related with carbon potency and carbon absorption by the stand.Biomass potential energy which is utilized properly can be beneficial for human life, while uncontrolled use of the biomass, such as for burning, will increase carbon emission to the air (one of the cause of greenhouse gas effect and global climate change).On the other hand, disposal of biomass to water bodies will pollute the water itself.
The amount of carbon content in forest stand is affected largely by the ability of vegetation in absorbing carbon from the environment through photosynthesis.Products of that photosynthesis process are accumulated in plant biomass, so that the total carbon of hybrid eucalypt stand is positively correlated with volume and weight of the biomass.

Potency of biomass and carbon of undergrowth vegetation and litter
Undergrowth vegetation and litter constitute the medium for carbon storage, besides the forest stand.Sundarapandian (2009) explained that undergrowth vegetation constitutes non tree vegetation, such as grasses, herbs, bush and scrub, and liana which grow in the forest floor.
In relation with age classes, average biomass of -1 undergrowth vegetation was as large as 1,35 t ha , carbon -1 storage was 0.61 t C ha , and absorption of CO was 2.23 t 2 -1 ha .The amount of biomass, carbon storage, and absorption of CO increased progressively at the the age of 1 year, and 2 -1 were consecutively 3.32, 1.49, and 5.48 t ha .At the age of 2 years, the value of biomass, carbon, and absorption of -1 CO were consecutively 3.12, 1.41, and 5.1612 t ha .On the 2 other hand, at the age of 3 years, the value of biomass, carbon, and carbon absorption were consecutively 7.71, -1 3.47, and 12.73 t ha (Table 4).
Analysis results which could be seen in Table 4 59  25.97 t ha .In general, it could be suggested that with the increasing age of the stand, the potency of biomass, carbon, and carbon absorption of the litter would become higher.This phenomenon could be explained because the ability of leaves, twig, branch and stem to regenerate, would become greater.The older the age of the stand, the greater would be the need for the trees to regenerate and therefore the greater would be the shedding of tree parts, such as leaves, twig, branches, and stem.Therefore, the amount of litter would become greater with increasing age of the stand.

Potency of biomass and carbon above ground surface
Potency of carbon storage above ground surface could be estimated from the amount of biomass of the upper part of the hybrid eucalypt stand.Such biomass constituted the summation of biomass of undergrowth vegetation and litter in the hybrid eucalypt stand (Table 2, Table 3, and Table 4).
Table 5 shows the amount of the carbon storage.Biomass at 0 year old constituted the summation of litter and undergrowth biomass on land without hybrid eucalypt stand (because 0 year old plantation implied that there was no hybrid eucalypt plants).Biomass, carbon storage, and absorption of CO 2 above ground surface of hybrid eucalypt stand exhibited increase with increasing age of the plants (Table 5).Physical properties or environmental condition constituted the important factor for supporting plant growth and development.Accumulation of biomass in tropical region is greater than that in temperate area, because photosynthesis rate in tropical region is greater (Indriyanto 2006).According to Mulyana et al. (2011) variable of environmental condition, namely temperature, air humidity, soil temperature, and litter thickness showed differences among various types of land cover.Existence of large difference of biomass in forest stand was due to difference in soil fertility, site, and the species of eucalyptus (Wei 2000).This could be explained by the phenomenon that acceptance of CO is greater in site with lower elevation (CO is heavier 2 2 than air and tends to go down during low turbulence).Forest land possesses more fertile site, and this will affect the level of photosynthesis in producing carbohydrate in stem, so that forest stand biomass will increase (Medhurst et al. 2001).
Besides that, soil and water management constitute the important factor in affecting the success of plant growth, and this will afterwards affect the volume and weight of plant biomass (Datta 2007).Research result by Hopmans et al. (2005) showed that controlled land burning could substantially increase the content of coarse charcoal in soil, and hence will also increase the total carbon content.Also, the management of harvesting would affect forest biomass and soil carbon content in eucalypt forest stand.Soil carbon content ranges between 2-9%, and was lower than result of simulation of harvesting of the whole tree.On the other hand, soil carbon revealed from harvesting simulation was greater (in the range between 5-17% ) than that in land where there was no harvesting activity (Ranatunga et al. 2008).
According to Latifah (2009), hybrid eucalypt possessed better growth than that of species E. grandis, E. pellita, and E. uropyhla so that biomass of hybrid eucalypt tended to be greater as compared with that of other species of Eucaliptus.Research result by Yue et al. (2007) also showed that height growth of E. globulus was smaller than that of hybrid eucalypt.
Research result by Baker et al. (2002) showed that above ground biomass of 45 months old (almost 4 years) and C (100 Mg ha ) ( Means et al. 1992).Widhiastuti (2009) reported that carbon potency stored in trees, poles, and saplings which were studied non destructively (biomass in seedlings and necromass was not yet measured) in forest territory of TWA Sicikeh-cikeh was -1 as large as 113.96 t ha .Amount of carbon stored in each ecosystem varied.This was due to difference in diversity and complexity of components which compose the ecosystem.Research by Aide ( 2001) showed that above ground biomass and species richness of secondary forest site were nearly similar with those of old forest growth (> 80 years) after undergoing succession of around 40 years.Complexity of ecosystem would affect the rate of carbon cycle which pass through each of its component (Indriyanto 2006).
According to Suhendang (2002), one of the environmental services which could be produced from forest is the very big role of the forest in ecological cycle (such as controlling the cycles of carbon, oxygen, nutrient element, water and world climate cycle).Countries which are able to absorb the excess of green house gas emission produced by other countries which exceed their quality standard, would have the right to get incentives for this service.Absorption of excessive emission of green house gases emission by forest vegetation will occur only if there are tree growths in new forest or in young forest.The amount of total emission of green house gases all over the world which exceed the allowable emission threshold constitute a world demand in international market.This total excessive carbon emission is being traded, and this is known with the term carbon trade.In the carbon trade, actually the item being traded is forest services in absorbing the excessive (as compared to the allowable emission) carbon emission in the world.
One of the factors which could reduce the accumulation CO in the atmosphere is absorption by vegetation.Industrial 2 plantation forest, besides its ability to produce harvested timber in the end of its production cycle, can also act as air cleaner by utilizing CO in the atmosphere in photosynthesis 2 process.Plantation forest of hybrid eucalypt could provide very relevant contribution in mitigating green house gases in certain period of time.

Conclusion
The potency of biomass, carbon, and carbon absorption above ground surface in industrial plantation forest of hybrid eucalypt increased in line with increasing plant age.Indonesia has great potential to become a country which absorb carbon emission due to its possession of large areas of tropical forest.Even, such potential could be increased by efforts of planting and rehabilitating forests which are heavily damaged and widely distributed, including the efforts of HTI program.At present, Indonesia should be able to look at its forest potential through different paradigm which implies that forest is not identical with wood anymore.Forests could provide services in other forms, namely playing a great role in protecting the earth atmosphere through absorption of carbon emission.

3 D
= density of wood test sample (kg m ) M = oven dry mass of wood test sample (kg) 3 V = air dry volume of wood test sample (m ) dry weight (kg m ) FWtot = total fresh weight (kg) DWts = dry weight of test sample (g) FWts = fresh weight of test sample (g) 2 Sarea = area size of sample (m )

Table 2
Carbon storage of hybrid eucalypt stand

Table 3
Potency of carbon absorption in hybrid eucalypt stand

Table 4
Potency of biomass, carbon, and absorption of CO in undergrowth vegetation and litter in hybrid eucalypt stand

Table 5
Biomass, potency, and carbon absorption in hybrid eucalypt stand