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International Journal of Marine Science 2013, Vol.3, No.9, 72-78
http://ijms.sophiapublisher.com
76
Table 3 Difference between actual and estimated TOC per kg of
plant biomass for each plant component and total plant of
Bruguiera gymnorrhiza
and
Lumnitzera racemosa
3 Discussions
3.1 Distribution of TOC among plant components
Half the biomass of
B. gymnorrhiza
(0.52 kg/kg
biomass) as well as of
L. racemosa
(0.54 kg/kg
biomass) is composed of organic carbon and the
values are compatible with that for other mangrove
species reported by Suratman (2008).
L. racemosa,
however, contains more carbon (96.5%) in stems,
large branches and roots than
B. gymnorrhiza
(78.7%),
hence is a species more capable in sequestering carbon.
Percentage carbon content in mangroves is also
similar to that of tropical and sub-tropical woody
plants (0.47~0.49 kg/kg biomass) (Hughes et al., 2000;
McGroddy et al., 2004; Aalde et al., 2006) and it
signifies that the carbon sequestration capacity of
mangrove plants equals that of tropical and
subtropical terrestrial plants.
Nearly 25% of the carbon in both the species is
accumulated in the aerial and underground roots and
thus the above ground carbon accumulation of these
species is three times greater than that of the roots.
Reported data on biomass partitioning of some
mangrove species indicate that above:below ground
biomass is low, indicating relatively high biomass in
the roots relative to above-ground parts (Komiyama
et al
.
, 2008). The present study reveals that
approximately half the biomass is accounted by
carbon and thus the above and below-ground carbon
partitioning represents a ratio of 3.0. This may slightly
change when the biomass of aerial roots are excluded
from the root fraction. An contrary observation (i.e.
above:below ground biomass ratio of 2.0), has been
reported by Hoque et al (2010) from Okinawa island,
Japan, where the tidal amplitude is 1.5~2.0 m. Comley
and McGuinness (2005) report above:below ground
biomass ratio of 0.75~2.0 for four mangrove species
in Darwin Harbour, Australia, under macro-tidal
conditions where the amplitude ranges up to 8 m. In
upland forests below ground fraction become less
compared with above ground portion and the ratio
ranges between 3.9 and 4.5 (Cairns et al., 1977). The
two mangrove species subjected to present study that
occupy an intertidal habitat with an average tidal
range which does not exceed 50 cm (relatively low
inundation depth, duration and frequency) results in
plants with a above:below ground biomass/TOC that
resembles plants living under terrestrial conditions,
(i.e. relatively low below ground than the above
ground biomass and hence carbon content).
Besides, inundation and consequent anaerobic edaphic
conditions, above:below ground biomass appears to
vary with salinity. According to the observations
documented by Saintilan (1997) with four mangrove
species in south-eastern Queensland, under low
salinity (upstream) conditions above:below ground
biomass lies greater than 2 and this ratio decreases to
1.1~0.65 in saline conditions at the estuary and further
declines to 0.3 in hyper-saline environments. These
results reveal that increasing salinity reduces
above-ground growth, may be as a result of energy
expenditure for salt exclusion and thus leads to a
lower above:below biomass as well as TOC ratio.
Furthermore, greater number of knee roots was
observed in
L. racemosa
plants that were found to
occur in inundated areas than in others, indicating that
inundation also may induce biomass diversion to
below-ground parts to expand air passage and storage
volume, thus contributing to declining above:below
biomass/carbon ratio.
Results reveal that stems and branches contributed the
most components to TOC in both
B. gymnorrhiza
and
L. racemosa
(Figure 1). Carbon retention of larger
trees of
L. racemosa
was found mostly in branches.
Inherently profuse branching pattern and tree
architecture of
L. racemosa
may contribute to
Total organic carbon
(kg TOC /kg plant biomass)
Component
Actual Estimated Difference
Stem
5.74
5.75
+0.01
Leaves
2.70
2.34
-0.36
Root
4.71
4.61
-0.10
Above-gr
ound
12.70
12.58
-0.12
Bruguiera
gymnorrhiza
Total tree 19.02
19.36
+0.34
Stem
8.31
8.78
+0.47
Leaves
1.48
1.47
-0.01
Root
6.10
6.14
+0.04
Above-gr
ound
19.04
18.89
-0.15
Lumnitzera
racemosa
Total tree 25.89
26.83
+0.94