P.N. Ranasinghe et al. Tropical Montane Forests Dieback of Sri Lanka
30
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
31
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
32
Table 6: Total element concentrations in plant leaves
Species Dieback
Category
Slope
Leaf Pb/ppm
Leaf Al/ppm
Leaf
Mn/ppm
Leaf
Fe/ppm
Caw H
S 9.9 568.4 36.5
409.5
Caw M
S 32.1 385.1 22.7
588.0
Caw L
F 2.2 67.6 15.8
113.1
Caw L
F 11.8 53.9 12.7 96.6
Cov H
S 19.8 452.8
271.8
355.4
Cov L
S 10.0 125.2
357.0
124.5
Cov L
F 15.3 56.0 91.9
180.4
Cov M
F 16.9 745.4
241.9
160.3
Eu H S 21.7 62.3
110.6
380.7
Eu L S 22.1
20047.6
85.6
800.8
Eu H F 7.2 115.2
17.9
193.9
Eu L F 36.3 150.7
15.8
78.7
Mes H
S 8.7 72.0 25.7 66.9
Mes L
S 9.6 31.3 45.7
47.5
Mes H
F 12.6 55.1 32.8
310.6
Mes L
F 22.1 163.4 49.6
186.8
Sre H S 6.6 148.2
115.8
143.0
Sre M S 14.6 326.7 59.4
258.2
Sre H F 9.7 25.8 17.1
55.1
Sre L S 13.7 18.9 26.9
49.2
Sro L
F 4.2 60.6 53.9
112.1
Sro H
S 11.5 178.6
57.5
132.0
Sro H
S 24.9 101.6
112.4
201.0
Sro L
S 14.2 85.1
113.0
79.2
Syb H
S 10.1 7472.1
288.9
141.0
Syb L
S 8.8 18878.1
105.7
466.8
Syb L
S 28.8
18821.3
55.2
129.2
Nf L F 15.9 57.3
19.5
111.5
Nf L S 1.1 5.5 8.0
196.4
Nf H F 1.0 3.8 4.0
65.0
Caw - Calophyllum walkeri
Cov - Cinnamomum ovalifolium
Eu - Eugenia mabaeoides
Eu - Eugenia mabaeoides
Mes - Meliosma simplicifolia
Sre - Syzygium revolutum
Sro - Syzygium
rotundifolium
Syb - Symplocus bractealis
Nf - Nothapodytes foetida
H - High
M - Medium L - Low
S - Slope
F - Flat
Table 7: Pearson product moment correlation coefficient matrix for plant element levels.
Soil Pb/ppm
Soil
Al/ppm
Soil
Mn/ppm
Soil
Fe/ppm
Leaf
Pb/ppm
Leaf Al/ppm
Leaf
Mn/ppm
Leaf
Fe/ppm
0.17 0.14 0.11 0.14 1.00 0.29 0.12 0.30
0.38 0.47 0.57 0.47 n.d.
0.13 0.55 0.12
0.21
0.55
-0.33 -0.01 0.29 1.00 0.45 0.61
0.28 0.00 0.08 0.97 0.13
n.d. 0.02 0.00
-0.05 0.45 -0.23 0.05 0.12 0.45
1.00 0.35
0.82 0.02 0.23 0.79 0.55 0.02 n.d.
0.07
-0.04 0.54 -0.43 0.04 0.30 0.61
0.35 1.00
0.83 0.00 0.02 0.84 0.12 0.00 0.07 n.d.