Journal of Threatened Taxa |
www.threatenedtaxa.org | 26 October 2021 | 13(12): 19791–19798
ISSN 0974-7907 (Online) | ISSN 0974-7893
(Print)
https://doi.org/10.11609/jott.7018.13.12.19791-19798
#7018 | Received 22 December 2020 | Final
received 14 February 2021 | Finally accepted 15 September 2021
Population status of Heritiera fomes Buch.-Ham.,
a threatened species from Mahanadi Mangrove Wetland, India
Sudam Charan
Sahu 1, Manas
Ranjan Mohanta 2 & N.H. Ravindranath 3
1,2 Maharaja Sriram Chandra Bhanjadeo University (Erstwhile North Orissa University), Baripada, Odisha 757003, India.
3 Centre for Sustainable
Technologies, Indian Institute of Science, Bengaluru, Karnataka 560012, India.
1 sudamsahu.bdk@gmail.com
(corresponding author), 2 manasranjan.mrm@gmail.com, 3 ravi@ces.iisc.ernet.in
Editor: Anonymity
requested. Date of publication:
26 October 2021 (online & print)
Citation: Sahu,
S.C., M.R. Mohanta & N.H. Ravindranath
(2021). Population status of Heritiera fomes
Buch.-Ham., a threatened species from Mahanadi Mangrove Wetland, India. Journal of Threatened Taxa 13(12): 19791–19798. https://doi.org/10.11609/jott.7018.13.12.19791-19798
Copyright: © Sahu
et al 2021. Creative Commons Attribution
4.0 International License. JoTT allows unrestricted use, reproduction, and
distribution of this article in any medium by providing adequate credit to the
author(s) and the source of publication.
Funding: SERB, Department of Science and
Technology, Government of India [Project file No. SB/FT/LS- 122/2012].
Competing interests: The authors
declare no competing interests.
Author details: Sudam Charan Sahu, MSc, PhD, F.I.A.T. is working
as an assistant professor in the Department of Botany, Maharaja Sriram Chandra Bhanjadeo University, Baripada
(Odisha), India. His specialization and research fields include plant taxonomy,
ethnobotany, forest ecology, climate change and biodiversity conservation. He
is recognized as Fellow of Indian Association for Angiosperm Taxonomy. Manas Ranjan Mohanta (MSc, MPhil in Botany) is doing his PhD in
botany at Maharaja Sriram Chandra Bhanjadeo
University, Odisha, since 2017. He is an active researcher in the field of
biodiversity, ecology and climate change. Prof.
N.H. Ravindranath is working as professor (Retd.) in the Centre for Sustainable Technologies, Indian
Institute of Science Bangalore, India. He focused his research, advocacy and
publications on various dimensions of climate change -mitigation assessment,
impact of climate change and vulnerability assessment in forest and agro-ecosystems, adaptation and climate resilience, forest
ecology, CDM and REDD+ Projects.
Author contribution: Study design: NHR, SCS.
Fieldwork: SCS, MRM. Data analysis: SCS Writing: SCS, MRM Feedback: NHR, SCS.
Acknowledgements: We are thankful to the SERB,
Department of Science and Technology, Government of India for necessary funding
and the forest department, Government of Odisha, for allowing to work inside the
forest.
Abstract: Heritiera fomes Buch.-Ham. is assessed as
an endangered mangrove species by IUCN, and information on population status is
lacking. The present study assesses the status of H. fomes
in Mahanadi Mangrove Wetland on the east coast of India. Three forest
blocks were selected and sampled for this study. Among these, the mean girth at
breast height (GBH) of H. fomes was the
highest in Hetamundia (HD) forest block. GBH of H.
fomes was inversely proportional to the
cumulative disturbance index (R2= 0.7244, p value <0.005). The
relative density was maximum for H. fomes
(56%) at Bhitarkharnasi (BK), and for Excoecaria agallocha at
Hetamundia (HD; 35%) & Kansaridia
(KD; 54%), respectively. Excoecaria agallocha is a dominant species possibly impacting
natural populations of H. fomes. Climate
change and rising sea levels may also negatively affect the existence of this
species. Therefore, appropriate strategies should be taken for conservation of
this globally threatened mangrove species prior to its extinction.
Keywords: Conservation, disturbance index,
diversity indices, East Coast of India, relative density, threatened mangroves.
Introduction
Mangrove ecosystems are important
with respect to their contribution towards biodiversity, carbon storage,
ecosystem balance, prevention of soil erosion, economic development, health
care and protection against natural calamities (Ellison 2008). Mangrove flora
are major reservoirs of biological carbon that contribute significantly towards
mitigation of climate change (Mohanta et al. 2020).
The social economy in coastal areas is highly dependent on vegetation directly
or indirectly through fishing activity, tourism, and medication (Alongi 2008).
Lack of conservation and
protection of mangrove habitats in recent decades has resulted in sparse
distribution of species and regional extinctions; 35% of mangrove area was lost
between 1980–1990 alone worldwide (Valiela et al.
2001). However, an increase in land
cover by mangroves has been observed over the last few years in some Indian
states, including Maharashtra, Gujrat, Odisha, and Andhra Pradesh (ISFR 2017; Khare & Shah 2019). Odisha has 243 km2 under
mangrove vegetation in the coastal districts of Balasore,
Bhadrak, Jagatsinghpr, Kendrapara, and Puri (ISFR 2017).
Information on threatened species
is needed for formulation of conservation policies helpful in defining marine
protected areas and resource utilization for coastal development (Polidoro et al. 2010). Studies of distribution, ecology,
adaptation, and threat assessment are all prerequisites for effective
management of resources in general, and threatened species in particular (Lewis
et al. 2016; George et al. 2019). Recent global assessment reveals that 16% of
total species of mangrove (70) are under threat of extinction.
In India, two species, Heritiera fomes
Buch.-Ham. (Endangered) and Sonneratia
griffithii Kurz
(Critically Endangered) are under the IUCN category of threatened species (Polidoro et al. 2010). H. fomes
(Sterculiaceae) is native to India, Bangladesh,
Malaysia, Myanmar, and Thailand. In India, it is found only in the Sundarbans
in West Bengal, and Bhitarkanika (abundant) and
Mahanadi Mangrove Wetland (MMW; rare) in Odisha. H. fomes
is locally called ‘Bada Sundari’ in Odisha. It
mostly grows towards landward in low saline (5–15 PSU) habitats with fresh
water association in upstream estuarine zones and in high intertidal regions.
Climate change, sea level rise and salinification of coastal habitats have had
adverse effects on sustainability of this species in different habitats. Hence,
it can be taken as an indicator of global climate change and sea level rise.
H. fomes is an important traditional
medicinal plant, with reported activity to treat infections and diseases
including goiter, skin diseases, gastrointestinal
disorders, diabetes, and cancer (Mahmud et al. 2014; Islam et al. 2019).
Timbers of the plant have high utility due to their hard and elastic nature.
The timbers are used as constructive material for bridges, houses, boats, and
hard boards (Ghosh et al. 2004). Locally, the timber is used as fire wood as
well. However, the species is disappearing due to absence of fresh water and
low seed viability (Kathiresan 2010). There is chance
of local extinction of H. fomes in India as
populations are declining rapidly due to anthropogenic and natural pressures (Kathiresan 2010). In Bangladesh, the species is facing the
problem of dieback causing a severe loss of mangroves (Hussain & Acharya
1994). Due to its threat of extinction and lack of data regarding its
population structure, it is essential to assess the population status of H. fomes for further planning to conserve and manage this
species.
Materials
and methods
Study area
The Mahanadi mangrove wetland is
located on the eastern coast of India in Kendrapara
district of Odisha, which lies between 20.30–20.53 N and 86.66–86.80 E. The area is covered with dense mangrove forest
which extends from Hukitola Bay (North) to Paradip Port (South). The climate of the area is generally
tropical monsoon in nature, with about 2,000 mm of rainfall annually. The area
faces severe cyclonic storms each year peaking during May–July and
October–November. The tidal amplitude ranges as high as 6 m during monsoon and
as low as 1.2 m. in dry seasons. The combination of fresh water streams and
tidal water in the inter-tidal regions of the Mahanadi river mouth provide
luxuriant habitats for mangrove flora. However, there is variation in the
salinity level at different seasons, and areas based on the precipitation and
distance from sea to river and creeks, respectively. The salinity level of
water becomes higher as 11.5 to 19.9 pptv near the
sea and becomes lower at interior mangrove regions (landward) as 0.3 to 0.7 pptv (Ravishankar et al. 2004). The wet land is divided
into eight forest blocks, i.e., Kansaridia (KD), Bhahar Kharnasi (BK), Bhitar Kharnasi (BK), Hukitola, Jambu, Kantilo, Kendrapatia, and Hatamundia. In these forest blocks two important species Heriteira fomes and
Sonneratia griffithii
are found which are considered as globally threatened plant species.
Data collection
Field surveys were carried out to
study the population status of H. fomes in MMW
from 2013 to 2016. After searching all the forest blocks of MMW, this species
was found only in three reserve forests: Bhitarkharnasi
(BK), Hetamundia (HD), and Kansaridia
(KD). We laid nine quadrats (20 × 20 m) in total, three in each forest block.
Woody trees with GBH ≥10 cm were considered for the study. The size class wise
distribution of plants was estimated considering three levels, i.e., <10 cm
(lower GBH class), 10–20 cm (mid GBH class), and ≥ 20 cm (higher GBH class).
Distributions of plants in these classes were compared to understand the future
trend of distribution of the species. To study the regeneration status of H.
fomes, stems <10 cm girth were counted and
recorded under each plot. For studying the stem size class distribution, plants
with <10 cm GBH were treated as seedlings and those of ≥10 cm GBH as trees
(Pascal 1988). The global positioning system (GPS-Garmin Oregon-600) was
used to record spatial location (latitude, longitude, and altitude) of each
quadrat.
The data were analyzed
through different diversity indices (Shannon-Weiner index and Simpson’s index)
and other diversity parameters (Menhinick’s species
richness and evenness) following Magurran (2004).
Relative density was also
calculated following the formula,
Relative Density = 100* (Density
of one species/Density of all species)
where, density was calculated as
number of individuals of the species/ha.
Each forest block was observed to
determine the type and level of disturbance following Tadwalkar
et al. (2012). Observations were based on disease infection, cut stumps, and
salinity of tidal water. These three features were measured through four
levels, i.e., 0= no impact, 1= low impact, 2= moderate impact, and 3= high
impact (Patwardhan et al. 2016). Cut stumps were taken as a sign of active
anthropogenic disturbance whereas salinity of tidal water and disease
infections were as natural disturbances. The salinity of the water was
determined by following standard methodology given by APHA (2005). A cumulative
disturbance index (CDI) was estimated for different forest blocks by adding
these three scores. The estimated CDI were compared against other diversity
parameters and mean GBH values of H. fomes in
different forest blocks to evaluate the correlation between them.
Results
and Discussion
Among eight studied forest blocks, H. fomes was found only in three: Bhitarkharnasi
(BK), Hetamundia (HD), and Kansaridia
(KD) reserve forests. These forest blocks comparatively represented with low
salinity conditions due to absence of continuous tidal water. The cumulative
species richness of all the studied plots was 10, including a single herb
species named Acanthus ilicifolius L. The
other woody species were H. fomes, Excoecaria agallocha L.,
Cynometra iripa Kostel., Xylocarpus granatum J.Koenig., Avicennia officinalis L., Phoenix palludosa Roxb., Ceriops tagal (Perr.) C.B.Rob., Pongamia pinnata (L.) Pierre, and Rhizophora
apiculata Blume. H. fomes
was found to occur at an altitude of 6–25 m. The detailed ecological
information of sampling sites is given in Table 1. The Shannon diversity index
varied among the three forest blocks as HD having the highest diversity index
(1.32 ± 0.18) followed by KD (1.26 ± 0.01) and BK (1.16 ± 0.20) (Table 2). The
overall Shannon diversity index was 1.25 ± 0.15. Simpson’s Index was highest at
HD (0.70 ± 0.04) followed by KD (0.64 ± 0.01) and BK (0.60 ± 0.09). Here, the
HD seems to have better diversity than BK but closer to the KD. The visible
variation in the diversity indices values is may be due to the differences in
potential threat effects or soil nutrient status or both. However, the lower
diversity indices of mangrove forest compared to other tropical forests
ecosystem is quite common due to lower species richness (Gevana
& Pampolina 2009; Stanley & Lewis 2009; Joshi & Ghose 2014).
The total number of individuals
of H. fomes recorded in nine plots (0.36 ha)
of three forest blocks was 482, including 398 trees and 84 regenerating
individuals (Seedlings). BK possessed 231 individuals followed by HD (133) and
KD (118). The values of tree number in the region is quite satisfactory but
number of seedlings was very low, indicated towards reduction of population
size in future because the number of juveniles must exceed the number of trees
to ensure population expansion (Upadhyay & Mishra 2014). Similarly, for a
stable population size these numbers should be equal or nearly equal to satisfy
one to one replacement condition (Upadhyay & Mishra 2014). But the cumulative result of these nine
studied plots put the ratio near 5 : 1 (Tree : Seedlings), which indicates poor
regeneration status of the species in the area and urgent need for
conservation.
The distribution of GBH class
revealed that in BK, 25 individuals had stems less than 10 cm GBH (seedlings),
78 had stems between 10–20 cm, and 128 had stems greater than 20 cm (mature
trees), indicating a healthy population structure with good representation of individuals
of all size classes. In HD, stems less than 10 cm GBH (seedlings) was 39
individuals followed by stems between 10–20 cm at 78 individuals and stems
greater than 20 cm (mature trees) at 16 individuals. In KD, stems less than 10
cm GBH (seedlings) was 20 individuals followed by stems between 10–20 cm at 64
individuals and stems greater than 20 cm (mature trees) at 34 individuals
(Figure 2). Among these three forest blocks, there was a visible difference
among of matured tree numbers. BK was bearing the highest number whereas it was
lowest in HD, indicating the difference in the level of potential threat among
two blocks. The reason evaluated for less number of mature trees in HD was the
more anthropogenic activities in the region (Highest CDI, Table 1). KD showed
less number of seedlings than other forest blocks indicating poor regeneration
status of H. fomes in this block. It may be
due to more salinity (6.8 ± 1.2 pptv) and disease
infection. The lowest salinity level (4.7 ± 1.5 pptv)
and moderate anthropogenic disturbance in BK supposed to support highest tree
density and seedling density among blocks. Different parameters like viable
seed number, germination, establishment, and growth are the indicators of
regeneration of plant community and also structurally reforms the community
based on the age group distribution to the habitat (Cunningham 2001). The
relative density was highest for H. fomes
(56%) at Bhitarkharnasi and Excoecaria
agallocha at HD (35%) and KD (54%), respectively
(Figure 3). This indicates that Excoecaria agallocha is invading and dominating in a very fast
rate due to its more ecological adaptations to the existing environmental
conditions.
The mean CDI for the three forest
blocks were: 3.36 (BK), 6 (HD), and 5 (KD). In the analysis, the CDI was found
to be negatively correlated with mean GBH of each plot (R2= 0.7244,
p value <0.005) (Figure 4). However, there was a clear indication cumulative
disturbances in the distribution of higher GBH class individuals in HD.
Similarly, the lowest mean CDI value in BK supported the maximum distribution
of trees with higher GBH class. The CDI value did not show any significant
correlation with other variables like density and altitude. However, it was
found that CDI value was highest in the forest block at lower altitude and
similarly it was less in higher altitude. This was clearly showing the tidal
effect on habitat modification and distribution of flora. The density among
study plots showed high variability, ranging from 925 to 2,475 individuals/hectare.
Variation was also observed among plots within forest blocks, being highest in
BK and lowest in KD. These variations are common due to uneven distribution of
plants due to tidal effects and variations in soil nutrient status (Xin et al.
2013). The role of human disturbances cannot also not be ignored.
Threats
H. fomes grows well in low salinity
condition about 2–5 PSU (Mitra et al. 2004;
Ravishankar et al. 2004). Growth of mangroves is impacted by salinity level (Mitra et al. 2004). The higher salinity level in water
affects the concentration of chlorophyll pigments a and b in leaves that
decreases chances of sustainability of low salt tolerant plants in those
regions (Clough 1985). Hence, salinity level can be a cause for stunted and
rare distribution of flora or even to their regional extinction. In our study,
lowest mean GBH was found in HD forest block indicating highest Cumulative
Disturbance Index (CDI) due to high salinity (5.4 ± 1.3 pptv)
and disease infection. In Bangladesh Sundarban, about
20% of the entire forests have been affected due to top dying disease in H. fomes (Kathiresan 2010).
Natural conditions like excessive flooding, increased soil salinity,
sedimentation, imbalance in soil nutrients, and cyclone induced threats are the
known factors for the disease. In the present study we observed some H. fomes trees were infected with gall cankers (Image 1).
Biological invasion is a major
threat to biodiversity (Biswas 2003; IUCN 2003). In the present study no known
invasive species was reported, but the domination of some true mangrove species
and their associates was observed which might be giving inter-specific
competition to H. fomes in different aspects.
These species of MMW might have impacts on the natural population of H. fomes in competing for light and nutrients to suppress
natural regeneration and cause physical damage. We recorded seven such species
belonging to seven families and seven genera. Out of seven species, Excoecaria agallocha
L., Acanthus ilicifolius L., and Ceriops decandra (Griff.)
W.Theob. are pre-dominating the study area.
The distribution of E. agallocha and C. decandra in the region may also have negative impact
over the H. fomes. It is because a previous
study at Sundarban mangrove reported a negative
association between H. fomes with E. agallocha and C. decandra (Ellison
et al. 2000). Further, we assumed that the future environmental
conditions may support extensive growth of Excoecaria
agallocha in the region because the species is
highly salt tolerant, faster growing, and have high ability to colonize in
degraded habitats (Harun-or-Rashid et al.
2009). The other important species were Derris trifoliate Lour.,
Clerodendru minerme (L.)
Gaertn., Eichhornia crassipes (Mart.) Solms, and Saccharum spontaneum L..
Although, the relationship H. fomes with these
species is not clear but there must be a negative interaction in them that
supposed to affect the regeneration of the species due to inter specific
competitions. Hence, the nature of relationship between H. fomes and other invasive species is the part of future
research.
The unscientific utilization and
management of resources is a major threat to biodiversity. Globally, there is
not been a single policy that ensures sustainable use and conservation of
mangrove resources (Romanach et al. 2018). Repeated
logging and unscientific exploitation of plant species force distributions to
be sparse. However, the present condition in the study areas are much organized
and restricted for logging and other activities due to active involvement of
forest department. But extensive logging of large trees for construction and
firewood in these areas was quite frequent in the past. The interaction with
local people revealed that the information about the threat status of the
species was completely unknown by the local communities. There was complete
absence of awareness programs regarding environmental education and threatened
species in the area among tribes. This was the one among the valid reasons for
disperse distribution of the species.
The study site is located in the
eastern coast of India where tropical cyclonic storms are very common,
specifically the coast of Odisha state. Disastrous tsunamis with high tidal
force and above 150 km/hr wind speed severely affect coastal mangrove
vegetation almost each year. Recent examples of such natural calamities are
cyclone Phailin (2013), cyclone Hudhud
(2014), and cyclone Fani (2019). The disastrous
effect of the super cyclone in 1999 to the coastal areas of Odisha is well
known. Apart from immediate damage, post-cyclonic changes in habitat create
issues such as increased infections and temperature reduction (Shengyan et al. 2019). Thus it is a major challenge for
forest managers to protect mangrove diversity and effectively implement
conservation strategies. In these conditions, the regeneration and
sustainability of H. fomes is highly affected
due increase in habitat salinity levels and disease after cyclones.
Conservation measures
Effective conservation measures
are essential for the sustainability of H. fomes
in the region. This can be achieved by supply of fresh water to high salinity
areas, for better growth and regeneration. Preparation of specialized habitats
and plantation areas with optimal salinity condition for H. fomes may aid conservation in the area. Further,
plantation areas could be established at a safe distance from the sea so that
required amounts of fresh and saline water can be channeled,
and arrival of high tidal water during cyclones and other natural calamities
prevented. The present policies of plantation of mangrove flora are quite
successful in the area, but the active and effective participation of local
people is lacking. Here, it can be suggested that the population strength of H.
fomes in the area can be achieved through active
participation of local people and taking the species at high priority.
H. fomes has been reported to be infected
by different pests and diseases that directly affect regeneration. Hence,
effective research to the problem is required to overcome this issue in an
eco-friendly way. Use of artificial pesticides may be an option, but use of bio
pesticides will be better for healthy ecosystem development. Germination of
this species is low, and seedlings are few. Further study of germination and
seed viability of this species in different environmental conditions is
required.
Inter-specific competition is
common in natural ecosystems, and it is a major determinant of population
structures. Further study is needed to observe the impact of Excoecaria agallocha
and other species on the life cycle of H. fomes.
The awareness programs regarding importance of biodiversity and sustainable
utilization of the resources to be conducted in regular intervals in the
coastal areas. Government and local community should involve in plantation of H.
fomes in suitable areas where the salinity level
is low and sufficient supply of fresh water is available. Tree cutting should
be completely avoided providing alternative livelihood for the local
communities. The Coconut plantation, oil extraction, tourism promotion, and
small scale industry development can be seen as alternatives for livelihood
development for the associate communities. Further, climate change and sea
level rise may negatively affect existence of this species (increase in salinity
level) and appropriate adaption strategies may be taken for conservation of the
globally threatened mangrove species H. fomes prior
to its extinction in the region.
In summary, well-organized and
coordinated efforts of researchers, forest managers, and administrators are
needed to achieve the goal of H. fomes
conservation. Fruitful investment of funds, effective implication of policies,
continuous supervision, and evaluation are key to effective conservation
strategies.
Table 1. Ecological information
of study sites.
Forest block/ Plot no. |
Latitude (degree, minute) |
Longitude (degree minute) |
Altitude (meter) |
Density (ha-1) |
Mean GBH |
CDI |
BK-1 |
20.393 |
86.719 |
21 |
2475 |
19.49 |
4 |
BK-2 |
20.364 |
86.719 |
25 |
1200 |
31.02 |
3 |
BK-3 |
20.371 |
86.726 |
23 |
2100 |
22.96 |
4 |
HD-1 |
20.352 |
86.767 |
13 |
1000 |
14.55 |
6 |
HD-2 |
20.355 |
86.768 |
9 |
1275 |
10.45 |
7 |
HD-3 |
20.35 |
86.765 |
6 |
1050 |
16.26 |
5 |
KD-1 |
20.368 |
86.763 |
17 |
1040 |
18.90 |
5 |
KD-2 |
20.369 |
86.719 |
16 |
925 |
15.40 |
6 |
KD-3 |
20.37 |
86.719 |
15 |
975 |
15.79 |
4 |
BK—BhitarKharnasi
| HD—Hetamundia | KD—Kansaridia
(1, 2, 3 represents sampling plots).
For
figures & image - - click here
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