Dieback in tropical montane forests of sri lanka

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Table 4

Table 2: Results of the preliminary study - mean extractable element concentration in soils (first stage)

Depth

/cm

Dieback

stage

Slope pH Conduc

/us

Moisture

(%)

Organic

C(%)

(ppm)

Healthy

F 4.31 152.75 20.03

0.33

240.8 41.7 46.8 152.0 78.9

0.7

0.9

11.4 285.8 0.3

62.86

30 Healthy F

4.47

43.25 18.38 0.48 125.0

29.3 15.5

14.9

14.4

0.6 0.6 1.0

515.1

0.3

40.07

75 Healthy F

4.65

24.33 17.50 0.37 42.0

26.4 45.8

3.6 3.6 0.5 0.5 0.3

319.8

0.1

36.70

High

F 5.70 247.50 33.83

350.5 55.1 202.8 1586.0 288.6 0.9

1.6

33.2

87.5

0.1

30 High F

5.24

50.00

24.00

92.4

12.8 50.7

69.2

High

S 5.21 167.50 23.88

4.34

303.5 26.7 141.6 445.1 230.6 1.1

1.1

16.3 164.1 0.1

39.46

30 High S

4.68

33.00

24.75

0.78

217.7

8.9

23.7

51.2

15.3

0.3

0.7

107.9

0.1

61.23

75 High S

4.71

23.50

18.13

1.02

142.8

54.8 10.4

29.9

0.2

25.8

0.1

122.67

ND - Not Detected

F – Flat

S – Slope

Journal of Geological Society of Sri Lanka Vol. 13 (2009), 23-45

Table 3: Extractable soil element levels at different plant species (Second Stage)

Species Dieback

Category

Slope

Soil Pb/ppm

Soil Al/ppm

Soil Mn/ppm

Soil Fe/ppm

Caw H

S

1.0

152.3

4.3

190.1

Caw M

S

1.4

285.4

3.1

173.8

Caw L

F

0.7

72.5

2.3

103.1

Caw L

F

1.0

14.8

17.0

220.1

Cov H

S

1.3

94.1

8.3

149.7

Cov L

S

0.4

390.8

1.7

113.5

Cov L

F

0.5

84.8

4.9

48.1

Cov M

F

0.4

122.6

2.3

223.1

Eu H  S  1.1

51.1

4.0

171.6

Eu L  S  0.6

33.3

2.5

61.5

Eu H  F  0.7

72.5

2.3

103.2

Eu L  F  1.8

28.1

25.2

98.7

Mes H

S

1.7

23.1

4.7

252.6

Mes L

S

1.4

4.4

18.1

141.9

Mes H

F

1.3

4.5

7.8

166.0

Mes L

F

1.1

56.0

49.2

172.5

Sre H  S

1.0

141.6

4.8

144.6

Sre M  S

1.3

311.0

3.9

133.2

Sre H  F  1.1

14.4

57.2

89.1

Sre L  S  1.0

17.4

40.2

171.2

Sro L  F  1.2

0.7

21.8

50.5

Sro H  S

1.4

7.4

6.8

110.1

Sro H  S

1.5

134.6

4.5

150.5

Sro L  S  1.2

15.3

25.0

372.1

Syb H  S

1.4

249.2

2.8

102.6

Syb L  S

1.7

88.1

6.9

102.7

Syb L  S

2.4

22.5

27.5

121.7

Nf L  F  0.6

39.4

3.5

64.9

Nf L  S  1.1

5.5

8.0

196.4

Nf H  F  1.0

3.8

4.0

65.0

Caw - Calophyllum walker                Cov - Cinnamomum ovalifolium

Eu - Eugenia mabaeoides

Eu - Eugenia mabaeoides                Mes - Meliosma simplicifolia

Sre - Syzygium revolutum

Sro - Syzygium rotundifolium

Syb - Symplocos bractealis

Nf - Nothapodytes foetida

         H – High              M – Medium

L – Low

S – Slope

F - Flat

Table 4: Results of the Mann-Whitney test for soil element data

Soil Pb/ppm

Soil Al/ppm

Soil Mn/ppm

Soil Fe/ppm

Mann-Whitney U

59.5

100.5

Wilcoxon

137.5 147 271.5 144

Z -2.06

-1.65

-0.32

-1.78

Asymp.

Sig.

(2-tailed) 0.04 0.10 0.75 0.08

Exact Sig. [2*(1-tailed Sig.)]

0.039(a)

0.104(a)

0.755(a)

0.079(a)

a. Not corrected for ties.

b. Grouping Variable: slope

P.N. Ranasinghe et al. Tropical Montane Forests Dieback of Sri Lanka

Also when the soil is under continuously poor

aeration, high Fe concentration may limit the

absorption capacity of available N. Therefore, even

though no direct relationship could be observed

amongst extractable soil Fe level, leaf Fe level and

dieback intensity, impact of Fe toxicity on forest

health cannot be excluded.

Manganese

First stage sites report 11.4 - 33.2 ppm extractable

Mn concentrations for soil depths varying from 0-75

cm (Table 2). There is a clear decrease of Mn levels

with depth.

Sites of the Second stage report DTPA extractable

Mn levels varying from 1.7 ppm to 57.2 ppm (Table

3). Jayasekara (1992) reported 126.5 ppm, an average

Mn level from the top 60 cm soil layer of the Hakgala

SNR. Chandrajith et al. (2009) recorded 19-197 ppm

acid extractable Mn and 0.026 – 0.45 % total MnO

concentration from HPNP. Unlike Pb, Al and Fe, soil

Mn level does not show a significant correlation with

slope (Table 5). Soil acidification enhances the

solubility of Mn (Fernandez, 1989, Kitao et al.,

2001). Mn toxicity generally occurs at pH values less

than 5 (Foy et al., 1978 and 1988). Toxicity threshold

of Mn varies for different plant species. For seedlings

of Agathis australis, this threshold is as low as 2.5

ppm (Peterson, 1962).

Lead

First Stage study recorded DTPA extractable Pb

values of 1.7 - 3.2 ppm from the top soils. Sites of the

second stage report DTPA extractable Pb

concentrations varying between 0.6 - 2.4 ppm (Table

3). Extractable soil Pb levels show a significant

correlation with slope (Spearman coefficient 0.38)

indicating high Pb contents on the slope areas (Table

5). A similar situation was observed in the HPNP

where higher concentrations (>1.5 ppm) of soil Pb

were reported from the slopes of Thotupolakanda and

Kirigalpotta ridges (Ranasinghe et al., 2007). Mann-

Whitney test shows no evidence to suggest that soil

Pb values on slope and flat areas belong to the same

population (Table 4). Chandrajith et al. (2009)

reported 14-29 ppm acid extractable Fe concentration

from HPNP. Ranawana et al. (2007) reported total Pb

values ranging from 16.5 - 29 ppm from dieback and

healthy forests sites in the HPNP. It was also reported

that most of the Pb in soils were in acid leachable

form. Hence, these authors concluded that most of

the Pb in soils was loosely bound, and accordingly,

an anthropogenic origin was proposed. As no specific

rock can be considered as the source of Pb in the

area, strong monsoonal winds are believed to have

brought Pb from the industrialized SW area and

deposited the same on slopes of the ridges.

Extractable soil Pb values in several relatively

polluted and unpolluted sites around the country

show that high values represent polluted sites. DTPA

extractable Pb levels in Ohio farm soils range from

1.5 to 7.7 ppm whereas total values range from 9 to

39 ppm (Logan and Miller, 2007). Average

concentration values in the world vary from 2 to 200

ppm (Baker and Chesnin, 1975). Impact of elevated

extractable exotic Pb levels on the healthy forests

cannot be assessed directly, as the toxicity thresholds

for delicate endemic montane forest plant species are

not known.

Element Concentrations in Plant Matter

Due to practical considerations, concentrations of Al,

Pb, Fe and Mn in plants were assessed using plant

leaves, even though certain elements tend to

concentrate in the root. Both plant samples in

selected trees and soil samples in the immediate

vicinity were studied in order to study the

relationship between element concentrations in plants

and soil.

Journal of Geological Society of Sri Lanka Vol. 13 (2009), 23-45

Aluminium

Leaf aluminium levels in plants studied vary widely,

ranging between 18.9 to 20047 ppm (Table 6). No

significant relationship between soil Al level and leaf

Al level could be observed (Figure 5a). Syzygium

rotundifolium, Calophyllum walkeri and,

Cinnamomum ovalifolium which

are highly

susceptible to dieback, show increasing concentration

of Al has a relationship with dieback intensity. It is

noteworthy that regardless of the dieback extent, all

Symplocos bractealis trees have abnormally high Al

values exceeding 7000 ppm (Figure 5). One Eugenia

mabaeoides tree also showed 20047 ppm on total leaf

Al concentration. Jayasekara (1992) reported Al

concentrations lying between 54 to 148 ppm in six

species including Eugenia mabaeoides which had a

mean value of 148 ppm.

Reported mean Al level from the undergrowth at the

Hakgala SNR is 12800 ppm (Jayasekara 1992). Leaf

Al levels show a significant positive correlations with

soil Al levels (Spearman rank coefficient 0.54). Also

leaf Al levels shows significant positive correlation

with leaf Fe and Mn levels (sig. <0.05). But they do

not show a linear relationship with dieback intensity

(Table 7). Kruskal Wallis test could not recognize a

significant difference between dieback intensity

groups and species with respect to leaf Al level

(Table 8). Truman et al. (1986) specified 800 ppm for

Pinus radiate, and for more sensitive species, 32 ppm

value has been proposed by Steinen and Bauch

(1988). Álvarez et al. (2005) found that leaf Al level

varied greatly within the same species. They reported

foliage Al levels higher than 800 ppm in all species

growing on granodiorite soils indicating possible

existence of Al stress.

The most prominent symptoms of Al toxicity in

plants are the inhibition of root growth and unhealthy

roots which generally lead to deficiencies of nutrients

such as P, K, Ca and Mg (Haug and Vitorello, 1996).

Symptoms manifested in the shoots are usually

regarded as a consequence of injuries to the root

system (Vitorello et al., 2005). However, Adikaram

and Mahaliyanage (1999) reported healthy root

systems of dying trees in the HPNP, where similar

extractable Al levels and pH levels are reported

(Ranasinghe et al., 2007). Jayasekara (1992) also

reported the absence of such significant deficiency of

nutrients in plants in the Hakgala SNR.

It has been known for a long time that many plant

species show wide variability with respect to their

resistance to Al toxicity (Vitorello et al. 2005). As

such, there is a strong possibility for developing a

resistance in these plants to high Al levels having a

geological origin, unless there is a sudden increase of

dissolution due to soil acidification caused by acid

rains.

Iron

Iron concentration in leaves of the studied plants

varies between 47.5 to 800 ppm (Table 6). Figures

6(a) and 6(b) clearly show that Fe levels in plant

leaves do not depend on the levels in soil. Dying

plants of Syzygium rotundifolium, Calophyllum

walkeri and Cinnamomum ovalifolium have high Fe

contents (Figure 6). It shows a significant correlation

with leaf Al contents (Spearman coefficient is 0.61).

The Spearman correlation coefficient between

dieback intensity and leaf Fe content is 0.36 (sig.

0.06) (Table 7). Kruskal – Wallis test does not

recognize differences between dieback groups or

species based on leaf Fe contents (Table 8).

Jayasekara (1992) reported mean leaf Fe

concentrations from 94 to 169 ppm on 6 different

plant species in Hakgala. Chandrajith et al. (2009)

recorded Fe levels of 52.9 to 157 ppm in

Calophyllum walkeri leaves, 63 to 426 ppm in

Syzigium rotundifolium leaves and 65 to 270 ppm in

Cinnamomun ovalifolium leaves in HPNP. The

toxicity threshold of Fe in plants varies widely

depending on the species. Glyceria fluitans having

100.5 ppm Fe in shoots does not show toxicity

symptoms whereas affected plants have Fe levels of

1131 ppm (Lucassen et al., 2000). Fe uptake in

plants is highly regulated to prevent excess

accumulation (Kim and Guerinot 2007). Lack of

significant correlation between soil Fe and leaf Fe

levels can be due to the regulating of Fe uptake. As

such, dieback caused by Fe toxicity is unlikely.

P.N. Ranasinghe et al. Tropical Montane Forests Dieback of Sri Lanka

Table 6: Total element concentrations in plant leaves

Species Dieback