Increased Vehicle Emissions, Soil Pollution and Forest Dieback: Has
Soil Pb Played a Key Role in Deteriorating Montane Forests of Sri
, 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
= <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.
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 . 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  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 , but for Horton Plains may exceed 5000 mm . 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 . Strong winds at gale scales are common during the south west
monsoons period . 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.  and  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.
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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 . 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.
The location of the experiment was in Horton Plains, the highest plateau of Sri Lanka between altitudes
of 1,500 and 2,524m . 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 . 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  and the extractants were
analyzed for the above elements by Atomic Absorption Spectrophotometry . 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).
Monthly Temperature (
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  however, depending on the situation, even a tinny amount may impose severe damages on
plant’s metabolism leading to dieback . 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 . Status of air pollution
in Kandy, a city that is less than 50 km away from Horton Plains has been documented by . Vehicle
emissions loaded with Pb and many other toxic elements and compounds have been blamed for this air
pollution menace . 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  where Pb can be transported for a distance greater than 120km .
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,  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 . 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+
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 . In addition, standard compost is a good
reservoir of all forms of essential plant nutrients and growth factors of plants .
Table 3: Variation of death rate of Syzygium rotundifolium saplings
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
Fig. 2: (a) Pb concentrations in the soil Vs Death rate of saplings; (b) Pb concentrations in soils Vs Pb concentrations in
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 .
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)).
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)
Y= 24.14 - 0.001/ (1-0.02X)
= < 0.001
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.
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