Carbon Dioxide Absorption of Common Trees in Chulalongkorn University

This paper studies the relevance between carbon dioxide (CO2) absorption rates of common trees in Chulalongkorn University (Thailand), and environmental factors -light intensity, air temperature, leaf temperature, and CO2 concentration in air -by forming non-linear models. The common tree species are Pterocarpus indicus, Samanea saman, Peltophorum pterocarpum, and Terminalia catappa. Measuring CO2 absorption was done by chamber analysis approach. The experiment was carried out by gauging 10 leaves, 7 hours per day, and 2 days per species. According to the models, it is obvious that light intensity is the most influential factor to CO2 absorption for all studied species. Peltophorum pterocarpum and Samanea saman reach their maximum CO2 uptake rates of 24.5 and 20.9 CO2 μmol·ms, when photosynthetically active radiation is 1100 and 1500 μmol·ms respectively. The other two do not reach their maximum rate within model data range. The regressions were best fitted with Gaussian function and Sigmoidal function. It is also suggested that Peltophorum pterocarpum and Samanea saman are good carbon sink and they should be planted more in the city for optimal CO2 absorption.


Introduction
Environmental problems are such a threat that people all over the world should have concerned.While high technology keeps on in a fast pace, the nature has been contaminated seriously in past decades.Environmental scientists have classified carbon dioxide (CO 2 ) as the most significant anthropogenic greenhouse gases (Stangeland, 2007;Smith et al., 2003).It has been risen by 39 percent (from 280 parts per million: ppm, to 388 ppm) since the beginning of Industrial Revolution (Wright & Boorse, 2011).There is a possibility of CO 2 effecting on human health if inhaling air with carbon dioxide 426 ppm or more for a long term; and the body may immediately effects from 600 ppm (Robertson, 2006).Carbon dioxide concentration in urban area is likely to be more than 426 ppm because of dense population and human activities (Janagkakan, Chavalparit, & Kanchanapiya, 2012).In order to impede the increase of ambient CO 2 concentration, CO 2 -causing activities must be decreased.Or else, trees must be grown more as trees have capacity in carbon dioxide absorption (McPherson, 1998).For the utmost benefit, it is necessary to do research in many tree species to find suitable ones for absorbing CO 2 in standardized and reliable way.
Chulalongkorn University is located on the heart of Bangkok, the most crowded urban area of Thailand.The trees grown in the campus, especially abundant ones, are proved to survive well (Anitha, Joseph, Chandran, Ramasamy, & Prasad, 2010) in polluted air.Therefore, the campus is chosen as the study area.The specific objectives of this research are to find the most influential environmental factor on CO 2 absorption of common tree species in Chulalongkorn University; and to compare the CO 2 absorption capacity among the common species.

Carbon Dioxide Absorption Experiment
This research uses Li-6400 Portable Photosynthesis System (LI-COR Inc., USA) to measure the CO 2 assimilation.The four most common species in the campus based on abundance, are Pterocarpus indicus (generally called Burmese rosewood), Samanea saman (Rain tree), Peltophorum pterocarpum (Copper pod), and Terminalia catappa (Indian almond).As preliminary to the experiment, measuring CO 2 absorption from one leaf was conducted from 8 a.m. to 5 p.m. per species.Respectively for the main experiment, carbon dioxide absorption rates of each common species were measured from 10 leaves of different trees for 7 hours a day, 2 days for one species.Apart from carbon dioxide absorption (CO 2 µmol•m -2 s -1 ), the instrument also simultaneously measures the actual environmental conditions considered in this research --photosynthetically active radiation or PAR (photon µmol•m -2 s -1 ), ambient air temperature ( o C), leaf temperature ( o C), and CO 2 concentration in air (ppm).To be representative of a species, the selected leaves must meet 4 qualifications: a) The leaves are at canopy level.
b) The leaves directly get the sunlight without any shadow.
c) The leaves are in good shape and perfect, no hole on the leaves.
d) The leaf age is mature; besides, they should be of the same size on average to others.

Analyzing Experiment Result
In accordance with plant physiological theory (Taiz & Zeiger, 1991), relations between environmental factors, especially PAR, and CO 2 absorption are not likely to be linear (Baligar, Bunce, Elson, & Fageria, 2012;Utsugi, Okuda, Luna, & Gascon, 2009).In this paper, non-linear regressions are formed to find a one-way relation between the environmental variables, and the net carbon assimilation of each species.The followings are 5 choices of regression functions: b) 3-parameter Logistic: e) 3-parameter Gaussian: The rise-to-max exponential, Logistic, and Sigmoidal function shows a level-off maximum, that is the CO 2 absorption saturation point to the external factors; while hill and Gaussian function shows a peak, which means increasing external factors after peak negatively affect the absorption.
Coefficients of 5 choice forms subject to each external factor per species are estimated.Regression analysis techniques such as Analysis of Variance (ANOVA), Coefficient of Determination (R 2 ) are used to explain the best correlation between each variable and the CO 2 absorption of a species.Then, rank influential factors of each species by F value and determine the most suitable regression form for every factor.The soundest regression subject to the most influential external factor is determined as the carbon dioxide absorption model of each species.Combined in one graph, models of all species are compared how much different trends they have.

Preliminary Results
The diurnal carbon dioxide absorption of the 4 species is shown in Table 1.As can be seen from Table 1, the CO 2 assimilation of all species was fluctuated all day differently.That was due to unstable weather in the rainy season and measuring in different days.Light intensity (PAR) fluctuated dramatically that were not the same shape as in the research of Sunakorn and Kasemsap (2010), or of Dou, Zhang, Feng, and Liu (2005).It shows that PAR was very low from 8 to 10 a.m. on the day that Pterocarpus indicus and Terminalia catappa were collected, whereas there were much lighter on the day of Peltophorum pterocarpum and Samanea saman.Very low PAR data, below 200 photon µmol•m -2 s -1 , refers to overcast sky (McDonald & Norton, 1992).It is suggested that comparing CO 2 absorption capacity among species needs many more observations with wider range of data from various trees.The main experiment can deal with this comparison.

Forming Models Using Data from the Main Experiment
Five different non-linear regression functions were applied to each independent variable, and the most suitable function for that variable, one with highest R 2 , was selected for each species.Table 2 shows that among 5 choice functions, only 3-parameter Gaussian function and 3-parameter Sigmoidal function were the most appropriate regression forms.Testing Analysis of Variance, all of the regressions were tested as follows: H 0 : α = β = X 0 = 0 (null hypothesis) H A : not all parameters are zero (alternative hypothesis) Given Significant level (α) = 0.001, critical F 0.001 value is 11.6 when residual degree of freedom is around 200; therefore, the null hypothesis is rejected if calculated F value outnumber the critical F 0.001 .All calculated F value of the regressions shown in the table (except those not applicable) passed the critical F value and accepted the alternative hypothesis.
Taking F values into account, light intensity (PAR variable) was the most influential factor for all plant species.
The R 2 values, 0.79 to 0.89, were very high despite the experiment with uncontrolled environment.Ambient CO 2 concentration, in opposite, was little related to carbon dioxide absorption rate.The CO 2 concentration data of Pterocarpus indicus, Peltophorum pterocarpum and Terminalia catappa cannot be regressed by any function due to their extremely scattering data.Only the data of Samanea saman had a relation in some level to absorption rate (R 2 was 0.358).According to F and R 2 value, another factor affecting CO 2 assimilation next to PAR is leaf temperature.Air temperature was also an explainable factor at some level, but less than leaf temperature.It is suggested that temperature at the leaf surface directly affects photosynthetic enzyme activities (Hikosaka, Ishikawa, Borjigidai, Muller, & Onoda, 2006;Sage & Kubien, 2007), while air temperature does not.Determined that photosynthetically active radiation was the most important environmental factors affecting CO 2 absorption, the results of estimating regression coefficients of each species with highest R 2 are shown in Table 3.
It is commonly known that regression is able to predict the dependent variable only in data range used.The model of Pterocarpus indicus, Samanea saman, Peltophorum pterocarpum, and Terminalia catappa should be valid from PAR 30 to 1700 µmol•m -2 s -1 , 80 to 1700 µmol•m -2 s -1 , 30 to 1700 µmol•m -2 s -1 , and from PAR 15 to 1500 µmol•m -2 s -1 respectively.Nevertheless, Janjai and Wattan (2011) found that light intensities higher than 1650 µmol•m -2 s -1 should occur in Bangkok only around 11 a.m. to 1 p.m. in March and April, typically warmest months of the year.These carbon dioxide assimilation models should be feasible in most of the time.

Comparing Carbon Dioxide Absorption Capacity
Figure 1 presents photosynthetic response graphs of 4 common trees plotted from models in Table 3.According to the graph, Samanea saman and Peltophorum pterocarpum have higher CO 2 assimilation than Pterocarpus indicus and Terminalia catappa through the photosynthetically active radiation range from 100 to 1800 µmol•m -2 s -1 .In the range of 300 to 1400 µmol•m -2 s -1 of light intensity, Peltophorum pterocarpum's CO 2 assimilation is higher than Samanea saman.Peltophorum pterocarpum shows maximum CO 2 assimilation of 24.5 CO 2 µmol•m -2 s -1 at light intensity of 1100 µmol•m -2 s -1 , while Samanea saman reaches its CO 2 assimilation plateau of 20.9 CO 2 µmol•m -2 s -1 at 1500 µmol•m -2 s -1 light intensity.There are few other pieces of research conducted in Bangkok that gauge carbon dioxide absorption of the 4 studied species.It can be seen that at PAR 800 µmol•m -2 s -1 , the CO 2 absorption rate by Puangchit and Royampaeng (1994) was 6.51, while the absorption rate by Samphantharak (2005) was around 4.1.Applying the same PAR (800 µmol•m -2 s -1 ) to the Gaussian model ( 6) of Pterocarpus indicus in this study, the estimated absorption rate was 6.35 µmol•m -2 s -1 .It can be seen that the estimated CO 2 absorption from this research were comparable to the other studies.

Conclusion
According to the data, it can be concluded that the carbon dioxide absorption rate in the same environmental condition depends on the species of trees.Photosynthetically active radiation (PAR) clearly affects the CO 2 absorption of trees.The non-linear models derived in this study are well fitted and useful to compare carbon dioxide absorption with other plants.It is implied that carbon dioxide absorption characteristics of these plant species general to the city.In addition, it is recommended that Samanea saman and Peltophorum pterocarpum should be planted more in the city in order to reduce ambient CO 2 during day time.Good management of green area in the campus could make better air quality not only for students and staffs, but also for city people.

Figure 1 .
Figure 1.CO 2 photosynthetic response curves of common species to light intensity

Table 1 .
Diurnal CO 2 absorption and photosynthetically active radiation in preliminary measurement

Table 2 .
Regression forms of CO 2 absorption subject to external factors with best R 2 in ( ), and F value in [ ] *N/A = not applicable with any function.

Table 3 .
Estimated CO 2 uptake regressions subject to light intensity (X)

Table 4 .
Table 4 presents maximum CO 2 absorption of Pterocarpus indicus from 2 previous studies compared with this paper's estimation.Comparing CO 2 absorption of Pterocarpus indicus from other studies and this paper