Bamboo in subtropical China: efficiency of solar conversion into biomass and C[O.sub.2] sequestration.
World oil and gas supplies are estimated to peak between 2010 and 2020 and will then fall below the level required to meet international demands within 10-20 years with net world carbon dioxide (C[O.sub.2]) emissions to the atmosphere projected to increase by 31.1 billion tonnes by 2010 (DOE, 2008). Although world deforestation has decreased over the last decade, this continues at an alarmingly high rate in many countries and adds to the problem (FAO, 2010). Clearly there is an urgent need to explore methodologies to capture more C[O.sub.2] from the atmosphere, to produce more renewable energy, and to reduce carbon emissions. Biofuel is one source of renewable and sustainable energy and if planted on marginal land it will not compete with staple food crops for limited fertile lands (Milliken et al., 2007; Shi, 2008).The USA is the main bioethanol producer, with 20 x [10.sup.9] L produced in 2006 (Heaton et al., 2008), compared with 1544 x [10.sup.4] tons in China in 2004 (Shi, 2008). The most productive perennial grasses used for this purpose in Europe and America have been switchgrass (Panicum virgatum L.), with average yields of 13.4 t/ha, and Miscanthus (Miscanthus x giganteus), with yields ranging from 10 to 40 t/ha in Europe (Heaton et al., 2008) and 27 to 44 t/ha in Illinois, USA (Heaton et al., 2004). The average annual conversion efficiency of solar energy into harvestable biomass is 1.0% in Europe (at 30 t/ha) and a maximum of 2.0% (61 t/ha) in Illinois.
Bamboo is a productive evergreen plant in the grass family (Gramineae) that is widespread in the tropics and sub-tropics, including China, with over 2 x [10.sup.7] ha (about 1% of total forest area in world) worldwide(Zhou, 1998). In China there is presently more than 5.2 Mha, with a 3% increase every year since 1980 (Zhou, 1998; Guo et al., 2005). A fully grown bamboo (Phyllostachy pubescenshas) plant has a diameter of 10 ~ 12 cm and a height of 15-20 m within 40 ~ 45 days after shoot emergence. Once established, a bamboo grove can be harvested every year for timber and shoots without detriment to existing stands. If well managed, it can be maintained for more than 100 years (Zhou, 1998). Currently, there are no available data that accurately describe solar energy fixation and carbon sequestration parameters for bamboo stands yet. The aim of the present study was to accurately estimate the energy conversion efficiency of bamboo forests in subtropical China.
Materials and Methods
A 15 m tall observation tower was set up in a bamboo forest at an altitude of 900 m in the Tianmu Mountain Natural Reserve (TMNR) in Linan County, Northwest Zhejiang Province, China (30[degrees]18'N, 119[degrees]23'E). Photosynthetic active radiation (PAR, [micro]mol/[m.sup.2]/s), and net photosynthetic rate (Pn, [micro]mol/[m.sup.2]/s or mol/ha/annum) were measured using LI-6400 (Li-COR, USA) within an illumination intensity range of 0-2000 [micro]mol/[m.sup.2]/s. Continuous measurements with 2000, 1500, 1000, 600, 300, 100, 80, 50, 20,0 [micro]mol/[m.sup.2]/s at the lower (from ground to the top: the 1st to 7th branches), middle (8th to 14th branches) and top (above 15th to top branches) layers of a 4 year old well grown plant's canopy from 9:00 am to 11:00 am of each measuring day (28th January, 5th March, 7th April, 12th May, 20th July, 11th October). The data were analyzed using Excel 2003 and SPSS 13.0 for statistical analysis and regression. The Pn-C[O.sub.2] response curve gave [R.sup.2] values of 0.9831, 0.9744, and 0.9809 for the upper, middle and lower layers of canopy, respectively. The ambient C[O.sub.2] concentration (Ca, [micro]mol/mol), photo respiration rate (Rp, [micro]mol/ [m.sup.2]/s), and related parameters were also obtained or calculated. Conversion efficiencies of solar energy into biomass were calculated for two locations of TMNR and Linfeng Bamboo Farm (LFBF) in a 150--200 m altitude hill area of Anji County (30[degrees]55'N, 119[degrees]54'E). LFBF has been a bamboo timber and shoots production farm for about 40 years and is located to the northwest of TMNR. It represents a mid-yield bamboo (Phyllostachy pubescens) production farm in the study area.
Total intercept solar radiation ranged from 3810-4450 MJ[m.sup.2]/aunum as measured by a local meteorological station Efficiencies of intercept radiation (total and PAR), converted into biomass, was estimated by the following equations (Zhou, 1998):
[E.sub.T](%) = [P.sub.T] x K x 100/Q (1)
Eh(%) = [P.sub.h] x K x 100/Q (2)
[E.sub.PAR](%) = [E.sub.T]%/0.45 (3)
[E.sub.T], and [E.sub.h] are the conversion efficiency of total intercept radiation per unit area per year into total biomass yield and harvestable biomass yield, respectively, while [E.sub.PAR] is the conversion efficiency of PAR. [P.sub.T] and [P.sub.h] are the total and harvestable biomass yields (dry weight) per unit area per year (kg/ha/annum). The combustion value (k) per unit biomass of bamboo varied for different parts of bamboo plants, recorded previously as 18.970 MJ/Kg for stem, 19.757 MJ/Kg for branches, 17.597 MJ/Kg for leaves and 17.944 MJ/Kg for underground stem and roots. Mean values for all parts were 18.567 and 18.775 MJ/Kg for harvestable parts (above ground organs: stem, branch, leaf) is (Zhou, 1998). Q is the total intercept radiation locally per unit area per year (MJ/ha/annum), with PAR generally of 45% total intercept radiation at this latitude (Zhou, 1998).
Results and Discussion
Data from TMNR indicated that Pn increased with increasing C[O.sub.2] concentration (Lin, 2007) and was highest in the lower bamboo canopy (Table 1). The maximum Pn response to C[O.sub.2] concentration and carboxylation efficiency (CE), C[O.sub.2] saturation point (CSP) as well as C[O.sub.2] compensation point (CCP) varied between the upper, middle and lower layers of the canopy. Pn-max was higher in the lower canopy where C[O.sub.2] concentration reaches to saturation point. Shi et al. (2005) reported a lower average annual Pn of 6.03 [micro]mol/[m.sup.2]/s in Jiangxi Province nearby this study area and Loretta Gratani et al. (2008) reported a Pn of 9.5[+ or -]4.5 [micro]mol/[m.sup.2]/s in at a bamboo grove at Botanic Garden of Rome.
The efficiency of conversion of intercept radiation to biomass was affected mainly by the density of plants, as well as by altitude, position in the canopy and the time and season of measurement. There was clear diurnal variation of light use efficiency (LUE,%) at different times of each measuring day in TMNR (Fig. 1); LUE was generally higher in the summer than in winter and in the afternoon than in the morning. LUE was affected positively by light intensity to a large degree, but there were exceptions (e.g. March 5th and July 19th) because LUE was also affected by other biotic and environmental factors.
The total radiation at TMNR was 38104 x [10.sup.3] MJ/ha/annum (Administration Authority for Tianmu Mountain Nature Reserve--AATMNR, 1992). Taking a bamboo leaf area index (LAI) of 7 (Zhou, 1998), the estimated amount of net C[O.sub.2] assimilation was 2238008 mol/ha/annum, equivalent to C[O.sub.2] sequestration of 98.47 t/ ha/annum (Lin, 2007) and 26.86 t/ha/annum of C. However, this was based on all of the sunny days within a year, although in fact, only 55% of the year had sunny days (AATMNR, 1992), providing a more realistic total C sequestration of 14.77 t/ha/ annum. Adjusting this figure to include the whole bamboo plant (underground stems and roots account for 50.4% of biomass, Zhou, 1998) provided a biomass dry weight yield of 29.30 t/ha/annum at TMNR. Conversion efficiencies of total intercept radiation and Photosynthetic Active Radiation were then calculated as:
[E.sub.T]% = 29.30 x [10.sup.3] x 18.567 x 100/38104 x [10.sup.3] = 1.43%
[E.sub.pAR]% = [E.sub.T]%/0.45 = 1.43/0.45 = 3.18%
[FIGURE 1 OMITTED]
TMNR effectively supports a natural bamboo forest, being located within a natural reservation area that does not allow disturbance; there has been no cutting, no harvest and no artificial inputs such as fertilizers or pesticides during the last 60 years. Therefore these estimates represent the fundamental conversion efficiencies of bamboo forests for subtropical China. In contrast, the LFBF bamboo grove with stand density of 4500 plants/ha and total bamboo biomass stock was about 100 t/ha/a (DW)(Zhou & Jiang, 2004). The annual harvest is done by select cutting of those 4-5 years old culms (in the last 10 years, an average of about 33.33 t/ha biomass was harvested) with which about 68% (22.33 t/ha/ annum) is economically usable (above ground stems and branches) biomass. The total intercept radiation was about 38000 x [10.sup.3] MJ/[hm.sup.2]/annum (AATMNR, 1992). Thus, the calculated [E.sub.T], [E.sub.PAR] and [E.sub.h] were 1.62%, 3.60% and 1.00%, respectively (Table 2).
Both energy conversion efficiency and C sequestration by bamboo forest in subtropical China (Table 2) were quite high. The calculated [E.sub.T] for the bamboo plant was close to that of C4 perennial grass Miscanthus x giganteus reported by Heaton et al. (2008). An [E.sub.h] value of 1.00% was also close to what was reported in Europe, but lower than the maximum of 2.0% reported in the USA for C4 perennial grass. For bamboo forest, however, the harvest consisted only part of the existing timbers (selected cutting of those 4-5 years old plants) while whole bamboo stands still remained, and even for those harvested culms there was a large amount of sequestered C in the underground stems and roots system and leaves in the soils. A bamboo forest can maintain active production for more than a hundred years (Zhou, 1998) whereas comparable lifecycles of C4 grass are not yet known. Furthermore, it is expected that efficiency and the potential C sequestration capability of bamboo forests could be further increased by expanding the bamboo plantation area to those marginal land. Presently, there is a total of 12.62 million ha of marginal land for afforestation in China (China Energy Net, 2009, 02, 16, www.china5e.com). If one third of this marginal land is suitable (climate and land topography conditions) for bamboo plantation, then there will be a new increment of 4.21 million ha which will be more than half of the total bamboo forests area (7.2 million ha) currently present in China. Cultivation management can be further improved for better biomass production such as via practices of scientific fertilization, disease control, topping, density and forest structure etc. (Zhou et al., 2006). Genetic process to release better species or varieties for higher biomass yields and phytolith Carbon sequestration is also suggested (Parr et al., 2010).
1. This study revealed that average maximum Pn of the bamboo (Phyllostachy pubescens) canopy at TMNR was 16.7-17.8 [micro]mol/[m.sup.2]/s, at C[O.sub.2] saturation point, varying in lower, middle and upper layer leaves of the canopy.
2. Annual radiation conversion efficiency into total biomass ([E.sup.T]) was 1.43% and 1.62%, and into harvestable biomass ([E.sub.h]) was 0-1.0% at the two locations.
3. Based on the average total biomass production, the carbon sequestration was 14.8-16.7 t/ha/annum at low and mid-yield levels in subtropical China.
4. We argue there is a great potential and real option to counter climate change if (i) bamboo forests are expanded into the vast marginal land of tropical and subtropical China/worldwide; and (ii) variety breeding and cultivation management can be further improved.
Acknowledgement We would like to acknowledge the financial support from Zhejiang A & F University, China, and also express our appreciation to Miss Q.Y Lin, Mr. J.S. Wu and Mr. X.B. Qian for their assistance in field measurements and laboratory analysis. We would also like to express our sincerely thanks to two reviewers and Dr. Anna O.W. Leung for their valuable comments in improving the quality of this paper in scientific point of view and for English editing.
Published online: 10 June 2011
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Zhihong Cao (1,2,3,5,6,7) * Guomo Zhou (1,2,3) * Guosheng Wen (4) Peikun Jiang * (1,2,3) * Shunyao Zhuang (1,2,5,6) * Hua Qin (1,2,3) * Minghung Wong (5,6)
(1) Joint Laboratory for Forest Soil and the Environment, Institute of Soil Science, CAS, Nanjing 210008, China
(2) School of Environment Science and Technology, Zhejiang A & F University, Hangzhou 311300, China
(3) Zhejiang Provincial Key Laboratory of Carbon Cycling in Forest Ecosystem and Carbon Sequestration, Zhejiang A & F University, Hangzhou 311300, China
(4) School of Forestry and Bio- Technology, Zhejiang A & F University, Hangzhou 311300, China
(5) Joint Laboratory for Soil and Environment, Institute of Soil Science, CAS, Nanjing 210008, China
(6) Hong Kong Baptist University Kowloon Tong, Hong Kong SAR, People's Republic of China
(7) Author for Correspondence; e-mail: firstname.lastname@example.org
Table 1 Response of Pn-C[O.sub.2] and Related Parameters at Various Layer of Bamboo Forest Canopy (a) Layer (b) Pn-max([micro]mol/ CE% [m.sup.2]s) Upper 16.77 0.39 Middle 16.66 0.43 Lower 17.76 0.49 Layer (b) Rp([micro]mol/ CSP([micro]mol/ [m.sup.2]/s) [m.sup.2]/s) Upper 1.25 462.13 Middle 1.32 416.31 Lower 1.42 389.74 Layer (b) CCP([micro]mol/ [m.sup.2]/s) Upper 32.09 Middle 30.55 Lower 28.79 (a) all of the data were calculated from Pn-C[O.sub.2] regression equation for lower. Middle and upper layers of canopy (b) Lower, Middle and Upper levels of Canopy are of 1st -7th, 8th -14th, and above 15th to top branches respectively Table 2 Energy Conversion Efficiency and Carbon Sequestered by Bamboo Forests in West Zhejiang, Subtropical China Site Location Total DM Carbon (a) Fixed (a) TMNR 30[degrees]18'N, 29.36 14.78 119[degrees]181 LFBF 30[degrees]55'N, 33.34 16.67 119[degrees]54'E Site Location Harvested [E.sub.T] DM (a) % TMNR 30[degrees]18'N, -- 1.45 119[degrees]181 LFBF 30[degrees]55'N, 22.33 1.62 119[degrees]54'E Site Location [E.sub.par]% [E.sub.h]% TMNR 30[degrees]18'N, 3.32 -- 119[degrees]181 LFBF 30[degrees]55'N, 3.60 1.00 119[degrees]54'E (a) unit of DM (dry matte) and Carbon fixed: t/ha/annum
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|Author:||Cao, Zhihong; Zhou, Guomo; Wen, Guosheng; Jiang, Peikun; Zhuang, Shunyao; Qin, Hua; Wong, Minghung|
|Publication:||The Botanical Review|
|Date:||Sep 1, 2011|
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