Dieback in tropical montane forests of sri lanka
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Figure 5b: Plot of Al concentration (in ppm) in the plant vs. its dieback stage 25
25 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.
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
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Figure 6(a): Plot of Fe concentration (in ppm) in the plant vs. its dieback stage
Fe (p pm) Fe (p pm ) Fe (p pm ) Fe (p pm ) Fe (p pm) Fe (p pm) Fe (p pm ) Fe (p pm) 30
30 30
30 15
15 Journal of Geological Society of Sri Lanka Vol. 13 (2009), 23-45 37
(p pm ) Fe (p pm ) Fe (p pm ) Fe (p pm ) Fe (p pm ) Fe (p pm ) Fe (p pm ) Fe (p pm )
30 30 P.N. Ranasinghe et al. Tropical Montane Forests Dieback of Sri Lanka
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Figure 7a: Plot of Mn concentration (in ppm) in the plant vs. its dieback stage
Mn ( ppm ) Mn (p p m ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) 30
30 30
30 15
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( ppm ) Figure 7b: Plot of Mn concentration (in ppm) in the plant vs. its dieback stage. Continued..
Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) Mn ( ppm ) 30
30 P.N. Ranasinghe et al. Tropical Montane Forests Dieback of Sri Lanka
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Figure 8a: Plot of Pb concentration (in ppm) in the plant vs. its dieback stage
Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) 30
30 30
30 15
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(p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) Pb (p p m ) 30
30 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.
Journal of Geological Society of Sri Lanka Vol. 13 (2009), 23-45 43
(2002) have found that the load of foreign atmospheric Pb in Jerusalem, which was brought there by transboundary migration of atmospheric Pb from Egypt, Turkey, and Eastern Europe, is similar to the average local atmospheric Pb in the area. The possible impact of atmospheric Pb transport to the central highlands of Sri Lanka from industrialized south India has also to be investigated. As discussed by Hauck and Runge, (1999), if air pollution is the main source of Pb in plants, modification of Pb levels could be expected with the death of plant due to reduction of the intercepting area and exposure to direct precipitation, which aids removal of Pb from the plant surface. Leaf morphology would play a key role in retaining such air borne Pb, and therefore it may also be responsible for the irregular high and low Pb concentrations recorded from different species.
Pot experiments at similar pH levels and different Pb and Al concentrations to recognize the tolerance and sensitivity of highly susceptible species to forest dieback would be very important to accept or exclude the possible contribution of these elements to the forest dieback in Sri Lanka.
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