Increased Vehicle Emissions, Soil Pollution and Forest Dieback: Has Soil Pb Played a Key Role in Deteriorating Montane Forests of Sri Lanka?

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Increased Vehicle Emissions, Soil Pollution and Forest Dieback: Has 

Soil Pb Played a Key Role in Deteriorating Montane Forests of Sri 


Gunadasa HKSG 


, Yapa PI 


, Nissanka SP 

and Perera SP 




 Postgraduate Institute of Agriculture, University of Peradeniya, Sri Lanka 


Faculty of Agricultural Sciences, Sabaragamuwa University of Sri Lanka, Sri Lanka. 


Faculty of Agriculture, University of Peradeniya, Sri Lanka. 


Rubber Research Institute, Agalawatta, Sri Lanka




 Soil pollution and forest dieback appears to be a major threat to the most important montane 

forest in Sri Lanka, named “Horton Plains”. This study focused on tracing key causes for the problem. The 

experiment consisted of twenty-four permanent plots within an area of 61-80%  severity of dieback and three 

soil amendments through the addition of compost, montane mycorrhizae, and compost + montane 

mycorrhizae, alongside the control that made up four treatments. Treatments were applied to Syzygium 

rotundifolium saplings of approximate height of 1m and 0.015m diameter breast height (DBH) residing in 

each plot. Soil Pb content was compared using soil samples collected at 0.2m and 0.5m depths. These 

comparisons were done for the samples collected during a dry period and three rainy periods. Foliar analysis 

was also done for Pb. During the experiment, saplings were closely monitored and changing health status was 

duly recorded. The results show the contamination of soil and leaves with Pb which may impair plant 

metabolism leading to dieback.


ompost and montane mycorrhizae significantly reduce the death rate of 

saplings (= <0.001). Soil Pb has dropped significantly (p = <0.001) during the dry period and soil Pb and 

leaf Pb was significantly correlated (p= 0.001). 


 Vehicle emission,  Air pollution,  Soil Pollution,  Forest dieback.  

1.  Introduction  

An identical tropical montane forest, the Horton Plains, occupies the eastern boundaries of the anchor 

shaped central hills of Sri Lanka, which lies between 1500 and 2524m ASL [1]. Geographical location is 

about 32 km south of Nuwara Eliya in the Central Highlands of Central Province, 6’47 – 6’50’N, 80’ 46’-

80’50’E. The land area covered by this montane rain forest is approximately 3,160 ha. Rocks are of Achaean 

age, belonging to the highland series of the pre-Cambrian, and include khondalites and charnokites [2] and 

soil order Ultisols is characterized by a thick, black, organic layer at the surface and is acidic (pH



Park receives rainfall from both northeast and southwest monsoons as well as inter-monsoonal rains. 

Frequently occurring mist and clouds are one main source of precipitation. Annual rainfall in the region is 

about 2540 mm [3], but for Horton Plains may exceed 5000 mm [4]. Rain covers throughout most of the year 

but there is a dry season from January to March. Temperatures are low, with an annual mean of 13°C, and 

ground frost is common in February [5]. Strong winds at gale scales are common during the south west 

monsoons period [6]. There are 54 woody species, of which 27  (50%) are endemic to Sri Lanka, 21 (39%) 

are restricted to the forest of south India and Sri Lanka, and the remaining 6 species (11%) are ubiquitous to 

the forests of south east Asia. [7] and [8] observed patches of dead and dying forest trees on the slopes of 

Thotupolakanda, 100m above the plains. Dieback in the Horton Plains involved a large number of species. 





  Gunadasa HKSG. Tel.: + 94 0718196520 

   E-mail address: 



2012 International Conference on Environmental Science and Technology 

IPCBEE vol.30 (2012) © (2012) IACSIT Press, Singapore 

Thirty-seven of the fifty species have been identified, belonging to 19 families were susceptible to dieback. 

One of the worst affected trees is Syzygium rotundifolium. Recent evaluations discovered that about 654 ha, 

equivalent to 24.5% of the forest in the park has been subjected to dieback [9].  The work done so far by 

many researchers on tracing the causes for the forest dieback have ended up with little or no success. 

Therefore, the main objective of this study was to identify the key causes for the forest dieback in this 

montane forest under the hypothesis that soil contamination with Pb as one of the key causes.  

2.  Methodology 

The location of the experiment was in Horton Plains, the highest plateau of Sri Lanka between altitudes 

of 1,500 and 2,524m [1]. Twenty-four permanent plots of 20 m 


 20 m were established to represent an 

affected area in the Horton Plain National Park. Randomized Complete Block Design (RCBD) was used with 

six replications. Plot locations were selected to cover a 61 – 80 % dieback of trees and to maintain soil and 

topography as constant as possible. The area is generally exposed to the wind since it has been reported that 

wind accelerates dieback [9]. A sketch of the area and the experimental plots mapped using GPS (Global 

Positioning System) points with 20 cm accuracy. Five saplings of Syzygium rotundifolium (approximately 

1m in height and 1.5cm in Diameter of Breast Height (DBH)) were randomly selected from each sampling 

plot. The most important reason for the selection of the tree species Syzygium rotundifolium was due to the 

fact that of all species that have been affected, this specie was the worst affected. Four soil amendments (a). 

compost-2kg/sapling, (b). compost and montane mycorrhizae-4kg/sapling. (c). montane mycorrhizae-

2kg/sapling including a control were used for the study while taking Syzygium rotundifolium as the indicator 

plant. An Investigation Pb in the soil samples was measured by wet ash method [10] and the extractants were 

analyzed for the above elements by Atomic Absorption Spectrophotometry [11]. The soil samples were 

collected from 0.2m and 0.5m depths and 0.3m-0.5m away from each sapling representing three different 

time periods. Furthermore, Death rates of the saplings were calculated by keeping records of the selected 

saplings throughout the experimental period and counting the deaths at the end of the trial.  

3.  Results and Discussion 

Two sets of soil samples taken during contrasting rainy and dry periods were compared. The results 

shown for stages 1, 2 and 4 indicate the Pb content in the forest soils during the rainy season while the results 

for stage 3 exhibits the levels of soil Pb during the dry season. Rainfall, temperature, wind speed/direction 

data for the area under investigation are shown in table 1.   

Table 1: Weather conditions prevailed during the study period (Meteorological Station - Nuwara Eliya - Latitude: 6° 58' 

11 N, Longitude: 80° 46' 12 E). 

Sampling Stage 






Monthly Temperature (



          Wind Data 

Max Min 

Mean  Speed 


Direction (


From North 

Stage 1 


Stage 2 


Stage 3 


Stage 4 





February/ 2010 


June/ 2010 












































Lead in the soil 

Results from both soil and foliar analysis clearly indicated the contamination of soil and vegetation in the 

Horton Plains from this trace element. Treatment effects on soil Pb at 0.2m depth were significant at 

sampling stages -1 (p=0.01),-2 (p=0.004) and -3 (p=0.004) but was not significant at the stage-4 (p=0.79) 

(Fig 1(a)). The highest Pb content was observed in the control. The results for the soils collected at 0.5m 

depth also showed significant differences among the treatments  at the sampling stage -1 (p=0.03), -3 

(p=0.03) and -4 (0.04) but not at the stage-2 (Fig 1 (b)). The level of soil Pb in the samples collected at 0.2m 


depth has gone up to 106 ppm in the control at the stage-1 sampling. The maximum allowable limit of Pb is 

100 ppm [12] however, depending on the situation, even a tinny amount may impose severe damages on 

plant’s metabolism leading to dieback [13]. The level of soil Pb at 0.2m and 0.5m depths was not 

significantly different. Gradual downward movement of Pb with rainfall may have distributed Pb almost 

evenly throughout the profile.  It was clearly evident that the levels of Pb in the soil were significantly higher 

(p=0.001) during the rainy period when compared to the dry period. The main source of Pb to the soils of 

Horton Plains must be the rain for several reasons.  For example, external addition of soil amendments are 

not taken place within this well-protected reserve and also the underlying bed rock mainly consists of 

Khondalite and Charnokites groups which are not considered to be rich with Pb [14]. Status of air pollution 

in Kandy, a city that is less than 50 km away from Horton Plains has been documented by [15]. Vehicle 

emissions loaded with Pb and many other toxic elements and compounds have been blamed for this air 

pollution menace [16].  Therefore, during rainy periods, continuous addition of Pb to the soil with rain water 

may be unavoidable. The soil samples collected during the rainy periods were all in moist condition with rain 

water soaked into the soil. Air-drying the samples only removes water from the samples leaving Pb behind. 

Hence, the laboratory analysis would have reflected this metal in higher concentrations for the soil samples 

collected during rainy periods.  Burning diesel, gasoline and lubricants releases Pb to the atmosphere. 

Additionally, the friction by brake pads, clutch liners and tires release these elements to the atmosphere. 

Strong monsoon winds seem to be the most possible transportation source of Pb from the polluted south 

western part of the country and following pioneer studies, Pb is subjected to long-range atmospheric 

transportation to a greater extent [17] where Pb can be transported for a distance greater than 120km [18]. 

Moreover, with increasing visitors to the Horton Plains, motor traffic within the Horton Plains itself has 

increased. Therefore, contamination of atmosphere may have been increased to an alarming level so that it is 

very unlikely the rain falling onto the area is free from Pb. According to past studies, [19] have reported 

several –fold increase of Pb concentration in the moss, Hypnum cupressiforme in Horton Plains during the 

period 1860 -1970.  A fraction of Pb may leach out from the top soil while another fraction may be absorbed 

by the vegetation. Results from foliar analysis indicate the entry of Pb into the plant bodies (see table 2). 

When the levels of Pb in the soil during the dry period are considered, plots treated with mycorrhizae showed 

lower values when compared to the values observed in the other plots. Even though this decline is not 

statistically significant, the results cannot be ignored. Mycorrhizae significantly increase the absorption of 

various elements from the soil including Pb [20]. Therefore, it could be assumed that the mycorrhizae are 

responsible for the reduction of Pb in the soil treated with mycorrhizae. 



Stage 1= Rainy season; Stage 2= Rainy season; 3= Dry season; Stage 4= Rainy season (Means appear with same letter are not significant at p<0.05).  

Fig. 1: (a) Status of Pb among  treatments at four different stages of sampling in 0.2m depth; (b) Status of Pb among  

treatments at four different stages of sampling in 0.5m depth.   

Table 2: Variation of Pb in the leaves from different treatments 

Treatments Control  Compost Comp+ 

Myco Mycorrhizae 

Pb (ppm) 


4.133 2.1  4.217 


(0.04) (0.0)  (0.05) 


Standard error for the respective mean is given within brackets 






 of Syzygium rotundifolium saplings 

It was clearly evident that the addition of standard compost and mycorrhizae has significantly controlled 

the death of Syzygium rotundifolium saplings. Treatment effect on the death of saplings was significant (p=

0.001) since the control clearly showed the highest death rate (Table 3). The standard compost consists of 

humic acid and fulvic acid formed during the microbial decomposition of organic materials. These specific 

molecules, known as humic substances, possess extraordinary capability of immobilizing soil contaminants 

such as Pb. Additionally, dozens of fractions in compost help the plants to withstand stressful conditions 

such as drought, nutrient imbalances, acidity and so on [20]. In addition, standard compost is a good 

reservoir of all forms of essential plant nutrients and growth factors of plants [20].  

Table 3:  Variation of death rate of Syzygium rotundifolium saplings 

Treatment Control 


Compost  Comp+Myco  Mycorrhizae 

Death rate (%) 










Standard error for the respective mean is given within brackets 

P b (pp m)






















P b (p p m ) s o i l
















f o


i a





Fig. 2: (a) Pb concentrations in the soil Vs Death rate of saplings; (b) Pb concentrations in soils Vs Pb concentrations in 

foliage parts 


Lead in the soil and dieback of plants


The relationship between Pb concentration and the death rate of Syzygium rotundifolium saplings was 

significant (p = <0.001) while the correlation showed the death rate of saplings has been largely affected by 

the Pb concentration in the soil (Fig 2(a)). Therefore, the death rate of the saplings used for the experiment 

has appeared to be increased with the increasing availability of Pb in the soil. Results further revealed that 

the crucial level of Pb in relation to the survival of Syzygium rotundifolium saplings was around 60ppm in 

the Horton Plains soil, beyond this level, even a slight increase of available Pb in the soil may impose severe 

damages on plant’s metabolism leading to dieback [13]. 


Lead concentrations in soils vs Pb concentrations in foliage parts


Results showed that the increase of Pb level in the soil results in an increase of the level of Pb in leaves 

of  Syzygium rotundifolium saplings. The relationship between soil Pb level and the Pb in leaves was 

significant (p = 0.01) and the nature of the relationship is linear – by –linear (hyperbola) (Fig 2(b)).  

4.  Conclusions 

Soils of the montane forest have been contaminated with Pb. The key source of the contaminant appears 

to be the contaminated rain that may have been a result of air pollution. Increased vehicle emissions in 

congested cities nearby and rapid industrialization of India may have some links with this air pollution. 

Saplings of Syzygium rotundifolium are severely affected when the concentration of soil Pb exceeds ~60ppm. 

Increment of soil Pb increases the entry of Pb into the saplings.       




Y = 7.9 -6.2 / (1+0.1X) 

P = 0.01 




Y= 24.14 - 0.001/ (1-0.02X) 

P = < 0.001 



 = 55% 



This study was conducted with the financial support of Sabaragamuwa University of Sri Lanka and the 

Department of Wildlife Conservation. We are also grateful to the Park Warden and the rest of the staff at 

Horton Plains National Park for their support given throughout the study. Our very special appreciation 

should go to the Rubber Research Institute of Sri Lanka for helping us to complete all the sophisticated 

laboratory analysis related to the research.  

6.  References 

[1]  T.C. Whitmore. Tropical Rain Forests of the Far East. Claredon Press, Oxford. 1984. 

[2]  R.D.B. Jayasekera. Horton Plains. Loris.1970, 12: 79-81. 

[3]  R.A. Wijewansa. Horton Plains: a plea for preservation. Loris. 1983, 16: 188-191. 

[4]  S. Ratnayeke and S. Balasubramaniam. The structure and floristic composition of the montane forest in the Horton 

Plains, Sri Lanka. Institute of Fundamental Studies, Colombo, Sri Lanka. Unpublished report: 14.1989. 

[5]  K.H.G. Silva de. Aspects of the ecology and conservation of Sri Lanka's endemic freshwater shrimp Caridina 

singhalensis. Biol  Conserv.1982,  24: 219-231. 

[6]  S. Balasubramaniam,  S.A. Rathnayake, R.White. The montane forests of the Horton Plains nature reserve. 

In:Ecology and landscape management in Sri Lanka (eds. W Erdelen, C Preu, N Ishwaran, CM Maddumabandara). 

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[7]  W.R.H. Perera. Thotupolakanda - an environmental disaster? Sri Lanka Forester.1978, 13: 53-55. 

[8]  W.L. Werner. The Upper Montane forests of Sri Lanka. The Sri Lanka Forester. 1982, 15: 119-135. 

[9]  N.K.B. Adikaram, K.B. Ranawana, A .Weerasuriya. Forest dieback in the Horton Plains National Park.  Sri Lanka 

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[13]  A.B. Pahlsson. Toxicity of heavy metals (Zn, Cu, Cd, Pb) to Vascular Plants. Water Air Soil Poll.1989, 47: 287-


[14]  V.M. Goldschmidt. The principles of distribution of chemical elements in minerals and rocks. Journal of the 

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[16] O.A. Illeperuma. Kandy air most polluted, Daily News,Tuesday, 13 July 2010 paper.2010. 

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