Population status of Heritiera fomes Buch .-Ham . , a threatened species from Mahanadi Mangrove Wetland , India

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.


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 km 2 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 (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.

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 ( 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

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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 (R 2 = 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,

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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  Taxa | www.threatenedtaxa.org | 26 October 2021 | 13(12): 19791-19798 19797 J TT (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.

Journal of Threatened
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.