The forested area with 21-40% dieback severity,
Horton Plains was selected for the study and twenty-four permanent
plots were established. Standard compost, montane mycorrhizae,
standard compost with montane mycorrhizae, and a control were used
as treatments and the indicator plant used was Syzygium
0.015m diameter at the cotyledon scar existing in each plot. Soluble
soil Pb and soil organic matter (SOM) were compared using soil
samples collected at 0.20m depth. Foliar samples from the saplings
were tested for Pb. The health status of the saplings were duly
recorded during the experimental period. The results from soil and
foliar analysis revealed the status of Pb (p <0.001) contamination
which appears to have significantly linked with forest dieback.
Positive correlations between soil Pb and leaf Pb were significant (p=
0.001). Soil amendment with compost and montane mycorrhizae was
effective in reducing the Pb level significantly (p = <0.001) and the
amendment appears to be significantly effective (p <0.001) in
protecting saplings from dieback.
he upper montane forest called „Horton Plains‟, Sri Lanka
is a low, dense, slow-growing forest with a healthy and
vigorous appearance . It is one of the key catchment
areas of the country. Tributaries of the three major rivers
which are crucial for the agriculture in the country, originate
from the forest. The land area covered by the forest is
approximately 3,160 ha. There are 54 woody species, of which
27 (50%) are endemic to Sri Lanka. Belonging to different
sizes and age classes, tree species have been dying due to a yet
unknown factor. This phenomenon was first observed in
Horton Plains by  and  and about 654 ha, equivalent to
24.5% of the forest in the Horton Plains has been subjected to
dieback . One of the worst affected trees was Syzygium
HKSG Gunadasa (Ms) - M.Phil. (University of Peradeniya Sri Lanka),
B.Sc. (Sabaragamuwa University of Sri Lanka) Lecturer at Uva Wellassa
University, Sri Lanka (E mail: email@example.com)
PI Yapa (PhD) – PhD (Reading, UK), MSc. And BSc. ((University of
walkeri . Also, seedling establishment and forest
regeneration in the area is slow .
Healthy forest in the park amounts to about 2012 ha. The
extent of the damage to the forest from dieback appears to be
so severe that the stand structure in affected areas showed
dramatic changes. If this dieback continues with the current
rate, the majority of the large trees will disappear from the
forest soon and the forest will later converted to a savanna.
The vital functions offered by this precious forest will then be
subjected to significant changes most probably towards the
negative side. Work done by many researchers so far has
ended up with no significant clues about the causal agents and
remedial measures for the dieback. This study was based on
the hypothesis that the forest is polluted with Pb as a result of
increased vehicle emissions in the country and the
consequential soil pollution has strong links with forest
dieback and the sapling mortality of Syzygium rotundifolium.
Soil toxicity caused by Pb could effectively be neutralized by
enhancing soil organic matter level.
Horton plains, the highest plateau of Sri Lanka between
altitude of 1500 and 2524m and the geographical location is in
the Central Highlands of the Central Province, 6‟47 – 6‟50‟N,
80‟ 46‟- 80‟50‟E  was the area selected for the study. The
landscape characteristically consists of gently undulating
highland plateau at the southern end of the central mountain
massif of Sri Lanka and soil order Ultisol is characterized by a
thick, black, organic layer at the surface . Temperatures are
low, with an annual mean of 13°C, and ground frost is
common in February . Annual rainfall in the region is about
2540 mm .
Plot locations were selected to cover a 21 – 40 % of dieback
severity area and to maintain soil and topography as constant
as possible. Twenty-four permanent plots of 20 m
20 m were
was used with six replications. The experimental plots were
mapped using GPS (Global Positioning System) points with 20
cm accuracy. Five saplings of Syzygium rotundifolium with
approximately 1m in height and 0.015m in diameter at the
cotyledon scar of saplings were randomly selected from each
sampling plot. Syzygium rotundifolium was specifically
selected as the indicator tree because it is the worst affected
. Four treatments (a).compost - 2kg/sapling, (b).compost
and montane mycorrhizae - 4kg/sapling. (c). montane
mycorrhizae - 2kg/sapling including a control were used for
the study. The soil samples were collected from 0.20m depth
and 0.3m-0.5m away from each sapling representing four
Mitigation of Pb-Induced Forest Dieback in Sri
Lanka: Use of Soil Organic Matter
and Yapa PI
International Conference on Agricultural, Ecological and Medical Sciences (AEMS-2015) Feb. 10-11, 2015 Penang (Malaysia)
different time periods within 02 years. Soil Pb was measured
Atomic Absorption Spectrophotometry . 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. Standard GENSTAT statistical
software was used for the analysis of variance (ANOVA), t-
test and regression analysis of the results .
The results shown here are based on the work done during
the two-year study period within the 21-40% dieback severity
areas selected in Horton Plains.
Addition of compost has increased SOM content in the soil
(Figure 1). Also, the effect of the treatments on SOM content
was significant for all four stages of sampling – e.g. Stage-1 (p
= <0.001), Stage- 2 (p = <0.001), Stage-3 (p = <0.001) and
Stage-4 (<0.001) at the 0.20m depth.
(Mean comparison was done for different seasons separately
Fig.1. Status of SOM% among the treatments at four different
stages of sampling in 0.20m depth
3.2 Lead (Pb) in the soil
Results of soil and foliar analysis clearly indicated the status
of soil contamination with Pb in Horton Plains.
means appear with same letter were not significant at p<0.05)
Fig. 2. Status of Pb among treatments at four different seasons of
sampling in 0.2m depth.
Differences among the treatments were observed in terms of
(p=0.004) and -3 (p=0.004) but there was no significant
influence detected at Season-4 (p=0.79) (Figure 2). The
highest Pb content was detected in the control during Season-1
whereas, the lowest was observed under the treatment
Mycorrhizae, again during Season-1. However, the control
showed the highest soil Pb level during the Season-1 while the
treatments Compost, Compost with Mycorrhizae, and
Mycorrhizae showed significantly lower soil Pb levels
compared to the control.
Soil amendments with standard compost and mycorrhizae
are effective in controlling the death of Syzygium
rotundifolium saplings. Treatment effect on the death rate of
saplings was significant (p < 0.001) and the control showed the
highest death rate (Table 1).
The relationship between Pb concentration and the death
rate of Syzygium rotundifolium saplings was significant (p <
0.001) and the correlation showed that the death rate of
saplings has been largely affected by the Pb concentration in
the soil (Figure 4). Therefore, the death rate of the saplings
used for the experiment appeared to have increased with the
increasing availability of Pb in the soil. Results further
revealed that the crucial level of soil Pb in relation to the
survival of Syzygium rotundifolium saplings was around
60ppm in the area and beyond this level, even a slight increase
of available Pb in the soil may impose severe damages on
plant‟s metabolism leading to dieback. The results are in
agreement with the work done by .
International Conference on Agricultural, Ecological and Medical Sciences (AEMS-2015) Feb. 10-11, 2015 Penang (Malaysia)
Fig. 3 Pb concentrations in the soil Vs Death rate of saplings
3.5 Lead (Pb) concentrations in soils vs Pb concentrations
in foliage parts
Parallel to the increment of Pb levels in the soil, the Pb level in
the leaves of Syzygium rotundifolium saplings have also
increased. The relationship between soil Pb and the leaf Pb
was significant (p = 0.01) and the nature of the relationship is
linear – by –linear (hyperbola) (Figure 4).
Fig 4. Pb concentrations in soils Vs Pb concentrations in foliage.
The content of soil Pb is inversely proportional to the SOM
content and the relationship between them was statistically
significant (p = <0.001). The findings indicate that the
availability of Pb in the soil for plants in the study area could
be reduced by increasing SOM level. The nature of decline of
soil Pb with the increasing SOM level seems to be linear-by-
linear (Figure 5). Immobilization of soluble Pb by the humic
and fulvic acid molecules present in SOM has been
documented by several researchers (e.g., ).
Fig 5. Soil organic matter Vs Pb in the soil at four different stages
3.7 Soil organic matter content in the soil and dieback of
Results showed that the increase of SOM level helps to
reduce the death of saplings. The relationship between SOM
level and the death rate of saplings (Syzygium rotundifolium)
was significant (p = 0.05). The nature of the relationship
seems to be linear-by-linear and it further indicates that
maintenance of SOM level approximately above 4% will result
a significant reduction of the death rate of the saplings (see
S OM %
Fig 6. Soil organic matter content in the soil vs Death rate of
Deterioration of both quantity and the quality of soil organic
matter in terms of humic substances appears to have influenced
on the development of Pb toxicity on Syzygium rotundifolium.
Forest dieback may be linked with dozens of reasons which
include Pd toxicity as well. Effect of the treatments consisted
of SOM justified the argument that improvement of SOM will
be effective in controlling the dieback of Syzygium
P b (p p m ) s o i l
Y = 7.9 -6.2 / (1+0.1X)
P b (p p m)
D e a
Y= 24.14 - 0.001/ (1-0.02X)
Y = 20.3 – 1.8 / (1-0.3X)
Y = 7.9 -6.2 /
rotundifolium. One of the most important fractions of SOM,
the humic substances, are highly effective in neutralizing the
effects of toxic substances (e.g. Pb) in the soil .
Soil organic matter is often viewed as the thread that links
the biological, chemical and physical properties of a soil. It has
been associated with numerous soil functions such as nutrient
cycling, water retention and drainage, erosion control, disease
suppression and pollution remediation etc.
Just as soil organic matter buffers the soil from rapid changes
in soil pH, it also binds organic pollutants, keeping them out of
the soil solution where they would be taken up by plants or
leached into ground water. Soil Organic Matter (SOM) also
provides sites for microbes to colonize and decompose organic
The lower the level of SOM, the higher the level of
available soil Pb and therefore, the enrichment of forest soils
in the affected areas with quality organic matter with standard
levels of humic substances could be recommended as a control
measure of forest dieback. This argument is backed by the
death rate of the saplings where the results showed that the
lowest level of SOM represents the highest death rate.
The level of soil Pb has gone up to 106 ppm. However, it
should be noted that the maximum allowable limit for soil Pb
is about 100 ppm . Even the smallest amount of Pb may
impose severe damages on plant‟s metabolism leading to
dieback. Lead (Pb) at toxic levels has been identified as an
agent causing damages on plants‟ respiratory mechanism in
particular. . Horton Plains is an upper montane forest
consisting of specific montane forest vegetation which is
considered to be much more sensitive to the changes in the
environment . Therefore, together with other unidentified
causative agents, soil Pb at toxic level may have caused a
severe impact on the forest vegetation triggering forest
The main source of Pb to the soils in 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
rock types Khondalite and Charnokites do not contain Pb .
Status of air pollution in Kandy with vehicle emissions and
dust loaded with Pb and some other contaminants has been
documented by . Kandy is a city less than 50km away
from Horton Plains.  also has identified Pb as one of the
major air pollutants in Sri Lanka. As identified by ,
troposphere above another two cities in Sri Lanka, Colombo
and Kurunegala, is polluted with Pb and the researchers have
identified vehicle emission as the main source of Pb to the
troposphere. Therefore, during rainy seasons, continuous
addition of Pb to the soil with rain is anticipated. Rapid
industrialization in the neighboring India may also have some
links with the polluted airflow with Pb and many other
pollutants towards Horton Plains.
The soil samples collected during the rainy periods were all
found 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 these metals in higher concentrations for the soil
samples collected during rainy periods.
Parallel to the increase of soil Pb, leaf Pb has also been
increased. It means that the root absorption of Pb appears to be
enhanced by the increasing concentration of soil Pb.
Therefore, the development Pb toxicity in the forest appears to
create Pb toxicity in the vegetation.
One of the toxic heavy metals, Pb, may have exceeded the
Improvement of the quantity and the quality of SOM in terms
of humic matter content appears to mitigate the Pb toxicity on
forest vegetation. The level of SOM had better be maintained
roughly above 3.5% in order to help the saplings to escape
from untimely death.
This study was conducted with the financial support of the
Department of Wildlife Conservation and Sabaragamuwa
University of Sri Lanka. We are also grateful to the Park
Warden and the rest of the staff at Horton Plains National Park
for their unquestionable support given throughout the study.
Our very special appreciation should go to the Rubber
Research Institute of Sri Lanka for helping us to complete
classy laboratory analysis related to the research.
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