Journal of Geological Society of Sri Lanka Vol. 13 (2009), 23-45
35
Manganese
Leaf Mn concentration of Hakgala plants varies from
4.0 to 357 ppm (Table 6). Mn contents in leaves are
higher in dying trees of Symplocos bractealis
Calophyllum walkeri, and Syzygium revolutum
(Figure 7). However, the variation of Mn levels in
soil and leaves of plant at different dieback stages
show that Mn in plants does not depend on the soil
level (Figure 8). Mn shows a significant positive
correlation with leaf Al content (spearman coefficient
0.45) (Table 7). Kruskal-Wallis test recognizes
differences between species based on leaf Mn
concentrations (Table 8). Critical toxicity levels of
Mn in leaves vary in a wide range due to the large
differences in Mn tolerance. Critical leaf Mn levels of
six crop species ranging from 160 to 7100 ppm have
been presented by El Jaoual and Cox (1998). Excess
available concentration of Mn can interfere with
absorption and utilization of other elements such as
Ca, Mg, P and Fe. However, no such significant
deficiency in these elements in leaf matter has been
observed in Hakgala plants by Jayasekara (1992).
Chandrajith et al. (2009) record Mn levels of 16.8
to70.5 ppm in
Calophyllum walkeri leaves, 19.6 to70
ppm in Syzigium rotundifolium leaves and 1.45 to
3.31 ppm in Cinnamomun ovalifolium leaves from
the HPNP. No significant deficiency in Ca, Mg, and
Fe levels in plant matter from dead and healthy trees
in the HPNP was reported in the results of Ranawana
et al. (2007). Mn toxicity displays initial symptoms
such as marginal chlorosis and necrosis on leaves,
and later, roots become brown after the shoots are
severely injured at critical stages. However, no such
symptoms were observed in dead or dying plants in
the Hakgala or dieback sites in HPNP during the
detailed plant pathological study related to forest
dieback (Adikaram and Mahaliyanage, 1999). Even
though high Mn levels could impose stresses on
certain plants, Mn toxicity cannot be recognized as
the root cause for unhealthy forests in Sri Lanka.
Lead
Mean Pb levels in leaves of the studied plant
species vary from 2.2 to 36.3 ppm (Table 6). Out of
the four Eugenia mabaeoides trees studied, three of
the leaf Pb levels exceed 20 ppm. Pb level in soil
seems to be controlled by the available Pb
concentration in soil (Figure 8). Pb levels do not
show a significant correlation with the dieback
intensity (Table 7). Also Kruskal-Wallis test could
not significantly recognize the differences between
dieback groups and species based on leaf Pb (Table
8). However, leaf Pb increases with dieback stage in
Calophyllum walkeri,
Cinnamomun ovalifolium and
Syzigium rotundifolium (Figure 8).
Chandrajith et al. (2009) recorded Pb levels of 2.06
to 5.73 ppm in
Calophyllum walkeri leaves, 1.52 to
4.74 ppm in Syzigium rotundifolium leaves and 1.45
to 3.31 ppm in Cinnamomun ovalifolium leaves from
the HPNP. These values are considerably lower than
those recorded in Hakgala for the same plants
species; Calophyllum walkeri (2.2 to 32.1 ppm),
Syzigium rotundifolium (4.2 to 24.9 ppm)
Cinnamomun ovalifolium (10 to 19.8 ppm). Bowen
(1979) reported Pb levels between 2 to 8 ppm in land
plants. Leaf Pb levels varying from 3 to 16 ppm have
been reported from Bavarian and Austrian Alps,
whereas values varying from 0.3 to18 ppm have been
reported from cereals and vegetables grown very
close to highways in Canada (Wedepohl, 1978).
Heliotis and Karandinos (1988) recorded leaf Pb
levels of 82.8 ppm in Pseudevernia furfuracea
grown on Mont Parnés, about 30 km outside the city
centre of Athens, Greece.
Table 8: Kruskal-Wallis test results for plant element data.
Leaf Pb
Leaf Al
Leaf Mn
Leaf Fe
Chi-Square
1.74 0.23 2.75 5.56
df
2.00 2.00 2.00 2.00
Asymp.
Sig.
0.42 0.89 0.25 0.06
Grouping Variable: Dieback intensity groups (High, Medium, Low)
Leaf Pb
Leaf Al
Leaf Mn
Leaf Fe
Chi-Square 2.94
9.71
15.21
4.29
df 6
6
6
6
Asymp.
Sig.
0.82 0.14 0.02 0.64
Grouping Variable: Species
P.N. Ranasinghe et al. Tropical Montane Forests Dieback of Sri Lanka
42
Chandrajith
et al. (2009) suggested that air pollution
could be contributing to the decline of forests in the
area. Aksoy and Şahin (1999) reported values of
180.21, 75.82, 50.56 and 16.81 ppm total leaf Pb
levels in unwashed Elaeagnus angustifolia from
industrial, roadside, urban and rural locations
respectively. Washed samples from the same sites
recorded values of 65.2, 35.25, 28.38 and 15.4 ppm.
Hauck et al. (2001) have reported a mean value of
179.02±80.6 mg/kg dry wt. from bark in the healthy
forest and 130.9±111.9 mg/kg dry wt. from those in
the dieback forest on the Harz Mountains in
Germany. Haiyan and Stuanes (2003) reported mean
total Pb contents ranging from 9.8 to 30.6 ppm dry
wt. in cabbage and 0.01 to1.5 ppm dry wt. in rice
grown on soils with total Pb contents of 209.7 to
262.2 ppm in an industrial area of the Hunan
province, China, during the period 1991-1997.
Concentrations of total leaf Pb levels at three selected
sites with different pollution levels were determined
for comparison. Results show that Pb concentrations
at Hakgala are almost similar to those of the
Homagama location, which is 250 m away from the
main traffic road and also near a large newspaper
printing press. Total Pb level in Syzygium sp. at
Dehiwala site, which is 50 m away from a heavy
traffic road, varied between 3.4 to 9.8 ppm and Pb
concentration of Ixora sp., ranged between 4.5 to 6.1
ppm. Concentration of total Pb levels of roots of the
same Syzigium plant at the Dehiwala site was 17
ppm. However, banning of use of leaded gasoline in
2002 in Sri Lanka must have a significant impact on
the leaf Pb content of the young trees tested at the
Dehiwala road side site. But Jayasekara and
Rossbach (1996) reported Pb levels of 0.29 to 1.1
ppm from about 1 kg plant material each from a
higher plant (Actinodaphne) a epiphytic orchid
(Bulbophyllum), of an epiphyitic fern and 3.19 to
4.26 ppm from a lichen (Usnea) and a bryophyte
(Pogonatum). However, the dieback status of the host
plants of these epiphytes is important because
epiphytic lichens are exposed to lower doses of air
pollutants in dieback affected forests than intact ones
due to lower intercepting surfaces and direct contact
with incident precipitation (Hauck and Raunge 2001).
Reduction of 40-50% of Pb levels in washed samples
from the Hakgala SNR (Jayasekara and Rossbach,
1996) is clear evidence for contribution of
atmospheric pollution to plant Pb levels in 1996, six
years before banning the use of unleaded gasoline.
The insignificant reduction of Pb levels in washed
plant samples of this study, which was carried out
four years after the banning of leaded gasoline,
further validates the occurrences of said
phenomenon. As such, high Pb recorded in plant
leaves during this study (carried out 4 years after the
banning of leaded gasoline) should have been
absorbed from the soil and subsequently accumulated
in leaves.
Even though Pb levels in some plants in Hakgala are
well above the normal Pb range of 2 to 8 ppm given
by Bowen (1979), they are not as high as some of the
polluted sites mentioned earlier. Pb uptake studies
have shown that roots have an ability to take up
significant quantities of Pb while greatly restricting
its translocation to above ground parts. In general, the
apparent concentration of Pb in aerial parts of the
plants decreases as the distance from the root
increases (Sharma and Dubey 2005). As such there
can be a higher Pb level in the root systems of these
plants, which were not evaluated during this study.
Pb toxicity in plants can cause a wide variation of
disorders such as stunted growth, inhibition of root
growth, chlorosis, inhibition of photosynthesis and
seed germination and upsetting the mineral nutrient
and water balance. However, as mentioned above,
healthy root systems were observed in dying and
healthy trees in the HPNP by Adikaram and
Mahaliyanage (1996). Nutrition experiments have
demonstrated that many plants responded to
increasing Pb availability to a very limited extent
unless aerial Pb was a major factor (Wedepohl,
1978). Even though no direct relationship between
dieback intensity and leaf Pb level could be
established, the presence of high leaf Pb levels in
certain individuals and species, increased dieback
intensity on slope areas.
Higher extractable soil Pb content on slope areas, all
further validate the hypothesis put forward by
Ranasinghe et al. (2007) of the possible Pb toxicity to
certain sensitive unique plants in montane forests of
Sri Lanka. Considering the fact that Pb accumulation
is highest in the root system and toxic effects and
tolerance levels depend on individual and species
level physiological factors, the absence of a direct
correlation between leaf Pb level and forest dieback
is not a valid argument to discard the possible toxic
effects on certain plants. As discussed earlier, the
ratio of acid leachable and total soil Pb contents in
Horton Plains as well as the ratio between total leaf
Pb contents in washed and unwashed samples clearly
proves an air borne pollution related Pb deposition in
the area prior to the banning of leaded gasoline use in
Sri Lanka.
Use of leaded petroleum as fuels until recently has
resulted in considerable Pb pollution in stream
sediments in Colombo and its suburbs (2 to 583 ppm)
(Ranasinghe et al,, 2007) as well as in cities close to
Hakgala (total Pb level of 65 to 92 ppm in stream
sediments) (Ranasinghe et al., 2007). Therefore the
emissions from vehicles are a likely major source of
Pb in the montane forest areas of the country. Even
though the literature recognizes Pb as an element
readily deposited close to highways, ability of strong
SW monsoonal winds blowing from the highly
populated area of Colombo and suburbs, situated at
about 100 km from Hakgala, to transport Pb cannot
be excluded without further investigations. Erel et al.