Total Biomass Carbon Sequestration Ability under the Changing Climatic Condition by Paulownia tomentosa Steud

The concentration of carbon dioxide (CO2) has risen continuously in atmosphere due to human induced activities, and has been considered the predominant cause of global climate change. Paulowina tomentosa Steud. (P. tomentosa), a multipurpose tree is popular around global market for its timber and its potential role in CO2 sequestration. In this study, the total biomass carbon of five years old and newly planted P. tomentosa has been estimated. The results indicated that the average total biomass carbon of five years old plant was found to be 4.52±0.53Kg C Year per tree i.e. 9.04±1.06-ton C ha Year (assuming 2000 plants per hector). Likewise, the average total biomass carbon of newly planted P. tomentosa within 4 months was found to be 6.07±0.38 Kg in remote village area in Nepal. The estimated biomass carbon in one year of newly planted plants was found to be 18.21±1.14 Kg Year i.e. 0.36-ton C ha Year. These findings reveled that short rotational trees like P. tomentosa can be implemented in agroforestry system to reduce the green house emission in cities and emphasizes the carbon storage potential of agroforestry. In vitro micro propagation technique could be implemented to produce genetically uniform clone of P. tomentosa and can be applied in agroforestry system for the adaptation and to mitigate global climate change.


Introduction
Most important greenhouse gas (GHG), carbon dioxide (CO2) is essential for photosynthesis, which sustains the life of plants (Rogers et al., 1994).Human induced activities have escalated the concentration of CO2 in atmosphere.Its from fossil fuels combustion is 34% by sector (Edenhofer et al., 2014).Kumar et al., 2011 have suggested the positive effect of increased atmospheric CO2 such as improvement of soil physical and chemical properties by soil organic matter build up and nutrient stocks in soil resulting plant productivity.However, increased of atmospheric CO2 is considered the predominant cause of global climate change (Jose and Bardan, 2012).
Green trees and woodland play a vital role in the sequestration of carbon dioxide from the atmosphere.Trees utilize carbon dioxide and store as carbon in the trunk, branches, roots and leaves through the process of photosynthesis.CO2 sequestration potential mainly depends upon the biological productivity, climatic condition, topography, management practices and varies from place to place on different regions on the basis of different species (House et al., 2002).Altitude in Nepal ranges from 60 -8,848 m height with diverse climatic and ecological zone.Geographically, it is divided into Himalaya region, Hilly region and Terai region.Government of Nepal have recognized Churia region (between Hilly and Terai regions) as vulnerable landscape.
One of the mitigating methods advocated by The Kyoto Protocol (Protocol, 1997) on reduction of carbon emission is to trade off carbon emission by sequestration in natural sink like trees (Sedjo and Sohagen, 2012).This strategy is relatively cost effective as it requires little new technology, readily themselves to local adoption and diffusion to the rural poor (Brown, 2002).Ever since, mitigation of increased atmospheric CO2 by agroforestry sink strategy is being recognized widely.Many studies on agroforestry as a strategy for carbon sequestration have been carried out (Nair et al., 2009;Jose and Bardan, 2012).Others have studied potential soil organic carbon sequestration in agroforestry system (Lorenz and Lal, 2014).Nonetheless, limited details are available regarding carbon sequestration value and information about P. tomentosa and its implemented in agroforestry for climate change mitigation.Therefore, the main objective of this study was to measure the total biomass carbon storage potential of previously and newly planted P. tomentosa and to promote the plant as opportunity in agroforestry system to deal with CO2 related mitigation option.

Agroforestry System and P. tomentosa
The awareness of agroforestry's potential for climate change adaptation and mitigation systems is growing worldwide (Cubbage et al.,2012Nair et al., 2009;Schoeneberger et al., 2012) offering the greatest CO2 sequestration potential among different land use system by 2040 (Noble et al., 2000).Agroforestry system combines trees and shrubs (perennial) with agronomic crops (annual or perennial).Therefore, they have both forest and grassland sequestration and storage patterns active (Schroeder et al., 1993).Globally, an estimated 700, 100, 300, 450, and 50-ton ha -1 of land are used for tree intercropping, multistrata systems, protective systems, silvopasture, and tree woodlots respectively (Nair et al., 2012).These practices have greater potential to increase carbon sequestration (Lee and Jose 2003;Nair et al., 2009;Schoeneberger et al., 2009;Morgan et al., 2010).Potentially accumulated carbon stored in above and belowground compartment of tree (Mosquera-Losada et al., 2011).Thus, management of agricultural systems to sequester carbon has been accepted as a partial solution to climate change (Morgan et al., 2010).Sustainably managing agroforestry system, carbon can be retained in these systems for centuries (Dixon, 1995).Additionally, agroforestry systems have been recommended to reduce soil erosion and improve water quality (WBCSD, 2010).It is also purposeful for a variety of benefits and services such as increasing crop yields, reducing food insecurity, enhancing environmental services, and resilience of agroecosystems (Ajayi et al., 2011).
Natural forest system is declining due to urbanization, expanding farm land and increasing the demand of timber (Chakravarty et al., 2011).The loss of these systems causes severe damage to regions ecosystem (Pimentel, 2006) .These alterations cascade through the ecosystem, resulting in increased temperature altered rainfall patterns and degraded soil profiles.Hence, apart from fossil fuel combustion, decimation of forest and its products could also be a factor that aids in changing global climate.
One of the ideal options to abate global climate change is implementing Paulownia trees in agroforestry system.It is fast growing multipurpose bio-energy tree which is readily sustainable and suitable for agroforestry system (Mosquera-Losada et al., 2011).Genus Paulownia belonging to family Paulowniaceae are divided into different species according to their flower and fruit morphology (HU, 1995).Poulowina is popular around global market for its timber due to its characteristics of rot resistance, dimensional stability and very high ignition point (Bergman and Whetten, 1998;Silvestere, 2005).Plantation of Poulowina can be done with about 2000 trees per hector and it can be harvested in 15 years for its valuable timber (Akyidis, 2010).Under natural conditions a 10-year-old Paulownia tree measures 0.3 -0.4m in diameter at breast height (dbh), and yields timer volume of 0.3 -0.5 m 3 (Flynn and Holder 2001).
Deep root system of tree in agroforestry system has been receiving increased attention for climate change adaptation and mitigation (Nair, 2012).Paulownia is deep rooted tree with a well-developed root system (El-showk & El-showk, 2003).Contrary to several others large, dichotomously branched roots growing downwards up to length of 8 meters (El-showk and El-showk, 2003), extensive root system of Paulownia grows deep in the earth and its crown develops a loose structure (Ayan, 2006) that significantly enhance the microclimate for growing crops (Wang and Shogen, 1992).Thus, Paulownia can be intercropped in crop field in agroforestry system.Paulownia intercropped with wheat enhanced the production rate when with no canopy (Zhao, 1986).Similarly, the production rate of maize and beans was decreased whereas the ginger production when intercropped with Paulownia increased greatly (Zhaohua, 1988).Nevertheless, Paulownia is intercropped mostly with winter crops and vegetables as trees have a period of rest during winter, meaning that there is no major competition for water and other nutrients at the period of rest (Wang and Shogren, 1992).However, the reason why a specific agroforestry practice contributed to carbon sequestration at a specific site where as another practices is not well understood (Jose and Bardhan 2012).
Natures of leaves of some species of Paulowina indicate border leaves of P. tomentosa among others (Rao 1986;Stankovic et al., 2009).Standard heart shaped leaves of Paulowina is large arranged in alternate pairs on steam ranging from 20.8 to 30.6 cm in longitudinal diameter (Kobayashi, 2008) can support CO2 sequestration largely by effective photosynthetic activity.Leaves of Paulownia are good source of fat, sugar and protein and utilized as fodder for pigs, sheep and rabbits (Chaires, 2016).Furthermore, vegetation nitrogen concentration is linearly related with photosynthesis (Boegh et al., 2002).The nitrogen content (2.67% dry weight) in P. tomentosa leaves can be compared with some leguminous plants (Hui-jun and Ingestad, 1984).Photosynthetic rate increases substantially with increase CO2 influx (Sigurdsson et al., 2002).Apart from these, the polyphenolic compounds produced by Paulownia shows high antioxidant property so research on antioxidant properties and medicinal value of secondary metabolites of Paulownia is gaining importance (Vaidhya et al, 2013).P. tomentosa is mainly grown in hilly loess areas and in sandy, clay or salinized soil (Ipekci and Gozukirmizi, 2003).It can grow well without artificial irrigation as it has an ability to withstand drought (Xu et al., 2014).Furthermore, it exhibits strong transpiration rate and real tolerance to high metal concentration in both hydroponics and field (Doumett et al., 2010).
Propagation of P. tomentosa can be achieved through seed, cut steam and root and in vitro propagation technique (Burger et al., 1985).However, in vitro propagation technique is predominant than conventional methods while later is unreliable because of disease and pest problem, low germination (59%) and slow growth (Bergman and Moon, 1997).Different parts of plant can be used in in vitro propagation technique such as leaf, node, shoot tip (Song et al., 1989;Rahman et al., 2013) and via organogenesis (Corredoira et al., 2008).Our recent study on tissue culture of P. tomentosa (Magar et al., 2016) with hormone combination of BAP and NAA (1.0 mg/l BAP and 0.1 mg/l NAA) gave optimum results with disease free plants and well developed roots were found to be 80%.Thus, in vitro propagation, a powerful technique to generate lager number of healthy, genetically uniform plants can be applied in forest biotechnology and implement P. tomentosa as ideal species in agroforestry system.

Study Site
Study site was selected in previously planted P. tomentosa garden at Godavari Botanical Garden, Nepal.The site is located at 27.59 o N to 85.39 o E in elevation of 1500 meters above sea level.Garden lies 14 Kilometer southeast of Kathmandu in Lalitpur district of Nepal.Thirty plants (P.tomentosa) were planted on June, 2011 in a row without uniform morphology.Similarly, CO2 sequestration was also measured for newly planted P. tomentosa (2 years old) in another site Bhimesthan-5, Sindhuli, Nepal located at 27.25 o N to 85.97 o E and elevation of 1077 meters above sea level.Plants were planted by individual farmer where 100 plants were taken for this study.Forest type in both sites includes moist coniferous evergreen and deciduous mixed forest.We also conducted field survey at other location including Ramechhap and Bhaktapur district of Nepal.

Above Ground Tree Biomass (AGTB) Estimation
Trees with Diameter at Brest Height (DBH) 5cm were considered for this study.DBH was measured at 1.3 m from ground level using diameter tape.Clinometer (Sunato, Japan PQ9) was used to measure the tangent angle of tree top from the observer's eye for estimation of tree height.Measuring tape was used to measure base distance from tree to the observer.Appropriate allometric equation was selected for estimation of above ground biomass.

Allometric Equation
The guideline published by ICIMOD suggested the allometric equation (modal) in estimating AGTB.It was developed by (Chave et al., 2005) on the basis of climate and forest stand types.According to the guideline following equation was used to calculate AGTB (Above ground tree biomass).AGTB= 0.0509*ρD 2 H, Where, ρ= wood specific density (gmcm -3 ), D= tree diameter at breast height (cm) and H = tree height (m)

Below Ground Biomass (BGB)
The relationship between root (below ground) to shoot (above ground) biomass is root to shoot ratio.(MacDicken, 1997) recommended that the root to shoot ratio is 1:5; that is, below ground biomass is estimated as 20% of above ground biomass.

Total Biomass Carbon
The biomass carbon is obtained by multiplying total biomass of tree with default value of carbon fraction 0.47 (Eggleston et al., 2006)

Results and Discussion
Observation of the plantation inside Godavari Botanical Garden revealed that there are number of other woody plant species including P. tomentosa, shrubs, grass and weeds.Some of the plants include Cinnamomum camphora, Taxus mairei, Magnolia grandiflora, Cycas revoluta, Prunus cerasioiles, Rhododendron arboreum, Thuja orientalis etc.
In this study total biomass carbon of previously planted P. tomentosa was estimated for a year (August 2015 to July 2016).The average biomass carbon within 12 months was found to be 135.68 ± 16.1 kg.The total biomass carbon for single tree per year was estimated to be 4.52 ± 0.53 Kg.Details carbon biomass for each month is mentioned on Table 1.
Similarly, 100 plants were planted by single farmer's crop field but, it was not intercropped with any crops.Average total biomass carbon within 4 months was found to be 6.07 ± 0.38 Kg.Estimated average total biomass carbon in one year was calculated as 18.21 ± 1.14 Kg i.e. 0.36-ton C ha -1 Year -1 .Detail carbon biomass for different months is indicated in Table 2.
Our field study in four districts found that plantation of P. tomentosa was done in hilly region of Nepal ranging from 700-2500 m.Our findings on total biomass carbon indicated that 5 years old P. tomentosa in Godavari sequestered 4.52 ± 0.53 Kg C Year -1 per tree i.e. 9.04-ton C ha -1 .In contrast, lower carbon biomass was yield by newly planted plants in Sindhuli which showed 6.07 ± 0.38 Kg of total biomass carbon within 4 months.Estimated biomass carbon in a year of newly planted plants was found to be 18.21 ± 1.14 Kg Year -1 i.e. 0.36-ton C ha -1 Year -1 .The reason for this is that the plants planted in Sindhuli were obtained through seed culture and early seedling growth by seed culture is slower than vegetative propagated plants from in vitro culture (Bergman and Moon, 1997).Site analysis in Sindhuli also indicates that early care to the plantation was neglected.Nevertheless, efficient growth, deep root system and large leaves of P. tomentosa contributes to generate large amount of C-rich biomass in above ground and below ground making it an ideal tree in agroforestry system.Characteristic of root system is an important factor for success of agroforestry system as it contributes to maintain soil fertility and lowers the competition with crops (Schroth, 1995).Deep root system forms safety nets against nutrients leaching below the crop rooting zone (Van Noordwijk and Heinen, 1991) and it is a reasonable strategy for above ground tree functioning.In this study we have highlighted P. tomentosa a deep rooted woody tree (Ayan et al., 2006) as a model plant which can be implemented in agroforestry system.Moreover, large leaves of P. tomentosa with efficient photosynthesis activity support CO2 sequestration at greater amount.It could be alluded from (Zeng et al.,2013) that quantifying the carbon sequestration potential of the sampled four forest types, shrub, pine (Pinus massoniana) forest, pine and broadleaf mixed forest and evergreen broadleaf forest.Carbon storage in biomass increased significantly from the early succession stage shrubs (6.21-ton ha -1 ) to the late stage broad leaf forest (134.87-tonha -1 ) this evidence support that the P. tomentosa with broad leaves can support CO2 sequestration at greater rate.However, changes in the steam wood density in response to elevated CO2 not only have clear consequence on carbon sequestration but also for wood quality for timber and paper production (Druart et al., 2006).Furthermore, establishment of large scale short rotational woody crop plantation has been advocated as an effective method of CO2 sequestration and mitigating increased CO2 level (House et al., 2002).

Conclusion
Intercropping of P. tomentosa, a fast growing commercial woody plant in agroforestry system might be helpful in the context of GHGs mitigation strategy.The planted P. tomentosa have higher total biomass that might be highly supportive elements in carbon fixation and biomass production.Indeed, it is important to consider all the comportments of tree while determining accurate total carbon pool of tree.Further research is needed on forest trial to estimate realistic value of total carbon biomass of P. tomentosa.

Table 1 :
CO2 sequestration ability of previously planted P. tomentosa

Table 2 :
CO2 sequestration ability of newly planted P.tomentosa