Diversity and inventorization of angiospermic flora in Dibrugarh district, Assam, Northeast India

The floristic composition of the Dibrugarh District of Assam has remained unexplored and the present study on Angiosperms addresses this fact. A total of 462 species of flowering plants belonging to 334 genera under 106 families recorded as per the APG IV system of classification and 116 families were recorded according to Bentham and Hooker system. 39% of native species were included in superorder Superrosids, 29% from superorder Superasterids, 18% from superorder Monocots and 7% from superorder Magnoliids. The orders with the highest number of native species were Lamiales (35), Malpighiales (29), Rosales (26), Gentianales (23), Sapindales (18), Fagales (19), Alismatales (17), Poales (14), Zingiberales (13) and Laurales (11) which accounted for 60.8% of the species. A total of 129 nonnative or invasive species were represented in 12 superorders. Habit analysis showed that herbs were represented by the highest proportion followed by trees, shrubs and climbers. Twenty species are listed under various IUCN threat categories and one species under Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). Traditional uses of some reported species were also documented. The present study was conducted to understand the diversity of local flora, which can be helpful in the conservation of natural vegetation and sustainable utilisation of resources in this region.


Introduction
Biodiversity functions in all kinds of the ecosystem, which harbours the formation of different habitats (1). These habitats are home to different plant resources, which provide services through natural vegetation and play an important role in regulating the function and monitoring the stability of the ecosystem. Due to the increasing rate of land degradation and overexploitation of natural resources, there is an immense loss of habitats and vegetation. Forests are degraded due to industrialisation, mining and other commercial purposes. The destruction of tropical forests and habitats causes global biodiversity degradation (2). Floristic diversity study helps us to evaluate the floristic wealth and its prospects of an area. Floristic inventories help us perceive biological aspects such as endemism, evolution, speciation and isolation (3). It helps to assess the country's plant wealth, distribution and status (4). There are 15000 species of flowering plants in India belonging to 2250 genera and 315 families, contributing 6% of World's flowering plants (5). The present number of flowering plants in India is found to be 18666 species under 2991 genera and 251 families (6). If we compare the data regarding endemism, it is clear that in the year 2011, the number of genera endemic to India was 49 (7), but in the year 2019, the number increased to 58 genera and 4303 taxa (8). This may be due to extensive exploration. There is a long history of floristic diversity studies being carried out in different parts of Assam for the past two centuries. Understanding the flora of a region is so complex that its conservation cannot be measured without botanical investigation and scientific analysis. Many workers had earlier contributed to the flora of Assam (9)(10)(11)(12)(13)(14)(15)(16)(17), including a detailed account of the Flora of Assam (18). Contribution on a comprehensive checklist of both the angiosperms and gymnosperms of the State was carried out (19). Many authors who carried out floristic works in different districts and protected areas of Assam (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33). There are many areas of the State where a detailed floristic study is yet to be carried out. No account of the flora of the Dibrugarh district has been carried out. Hence, the present work can be considered as the right step to understanding the flora, which will help assess the regional biodiversity. Despite the richness of flora and high endemism, no attempt has been made for the systemic investigation of angiosperm flora of this region. The main objectives of the present study are: (i) to make an inventory of the angiosperms of Dibrugarh District (ii) classification of the angiosperms according to APG IV (Angiosperm Phylogeny Group IV) and Bentham and Hooker system (iii) to document the various traditional uses of plant species by local people. Quantitative inventories help provide valuable information on the structure, diversity of a place, biodiversity conservation and sustainable use of rich diversified plant resources (34,35).

Study area
Assam is one of the seven states of Northeast India, Arunachal Pradesh and Bhutan surround in the North, Manipur, Nagaland and Arunachal Pradesh in the East, Mizoram and Meghalaya in the South and Tripura, West Bengal and Bangladesh in the West. The geographical area of Assam is 78438 sq km accounting for about 2.39% of the country (36).
Dibrugarh is the easternmost part of the State covering an area of about 3381 sq km extending from 27° 5'-38"-N to 27° 42'-30"-N latitude and 94° 33'-46"-E to 95° 29'-8"-E longitude (37). In the North it is surrounded by the Dhemaji district and Brahmaputra River. In the east it is surrounded by Tinsukia district, Arunachal Pradesh to the South-east and Sivsagar district to the South-west (Fig. 1). During heavy rain and flood, the mineral and silts deposited in the Brahmaputra river come down to the lowland area making the soil fertile for cultivation. Other environmental factors such as temperature, atmospheric pressure, rainfall, humidity enhance the luxuriant vegetation and diversity of the region (38)(39)(40).

Data Collection
Plant specimens were collected from different habitats of the district of Dibrugarh through several field trips approximately, 150 days covering all seasons during 2018-2020. The habitats from where the collection was carried out included forests, wetlands, river banks, roadsides as well as private gardens. With the help of local inhabitants, the plants were collected from the respective sites and the vernacular names of the plants were also recorded in a structured questionnaire, comprising of the botanical name, family, habit of the plant, local names and application. The plants were collected mainly during the flowering stage for proper identification. Data collection was carried out based on standard procedures (41)(42)(43).
During the field survey, collection of specimen for floristic study, plant specimen were collected, photograph were taken, field data were also recorded in the field notebook, emphasising those characteristics which may not be easily observed in dried specimens. Herbarium specimens of the collected plants were made by following the standard herbarium technique (44). For proper taxonomic identification, the plant specimens were examined by consulting Flora of Assam (11); A checklist of angiosperms and gymnosperms (19). Other literature like eflora of China, online databases like The Plant List (www.theplantlist.org), The International Plant Name Index (www.ipni.org). The plants were enumerated and the arrangement was made according to Angiosperm Phylogeny Group IV Classification (45). The voucher specimens of the collected plants were deposited in the Gauhati University Botanical Herbarium (GUBH) of the Department of Botany, Gauhati University, Assam.
According to APG IV classification the highest number of native species were found in the family Fabaceae [16], Lamiaceae [15], Araceae and Moraceae [13] each, Rubiaceae [12], Acanthaceae, Lauraceae, Poaceae [11] each, Achariaceae and Apocynaceae [10] each. The genus Ficus represented the highest number of species (9) (Fig. 6). Out of the 462 species, 236 species were found to have economic importance. These are used as medicine, food, cosmetics, dye, fodder, furniture, rituals, biofencing and artefacts by the people in the district (Fig. 7). Out of the 236 economically important plants, 88 species were claimed to have medicinal properties, which the people used to cure different ailments such as gastrointestinal disorders, external injuries, urinogenital, hormonal, lymphatic, infections etc. Medicinal plants accounted for the highest proportion of species representing 37.28 %, followed by wild edible plants (57,24.15%). The plants used as vegetables and fruits are eaten cooked or raw based on dietary habits. Many plants are also used as fodder (25, 11%). Timber yielding plants are used as wood (17,7.20%) to construct houses, bridges and furniture. Plants like Jatropha curcas L., Ricinus communis L. are also found to grow luxuriantly which has tremendous potential to be used as biofuel. Plants used in ritual (15, 6.35%) plays a social role in festivals and ceremonies. Different species of Bamboo are used as biofencing (14, 5.9%).    The bark, leaf and seeds of some plants are a good source of dye (13, 5.5%) used for colouring clothes, fabrics and utensils like earthenwares. Plants with antibacterial property and yielding essential oil are used as cosmetics (7, 2.9%) to treat hair and skin (Fig. 7)   divaricata (L.) R.Br. ex Roem. and Schult., Scoparia dulcis L. (69,70).
Wild edible plants are consumed as food and used in the preparation of traditional recipes. Some of these plants are cultivated in the home gardens and some are collected from the wild. These are also found to be sold in the local markets by village people, thus making them accessible to other people. It was found that, 24% out The local people collect fuelwood from the nearby forests. Both main and sub-branches of trees are chopped as a source of fuel. Apart from fuelwood, some trees are found to be used for the construction of houses, furniture, equipment for farming, fishing, boats etc. In the study, 7% of plant species were recommended by people as a source of wood. The most commonly used wood tree species are Magnolia griffithii Hook.f. and Thomson. Carallia brachiata (Lour.) Merr., Salix tetrasperma Roxb., Albizia lebbeck (L.) Benth, Lagerstroemia speciosa (L.) Pers., Chukrasia tabularis A. Juss., Dysoxylum binectariferum (Roxb.) Hoof. and f. ex Bedd, Toona ciliata M. Roem., Alstonia scholaris (L.) R.Br., Gmelina arborea Roxb. (49,(76)(77)(78). Dipterocarpus retusus Blume is the state tree of Assam that naturally grows in forests in Upper Assam. It is best known for use in plywood industries. Bischofia javanica Blume, Shorea assamica Dyer, Dalbergia sissoo Roxb., Tectona grandis L.f. are used by the local people for construction purposes and artefacts. These trees yield a quality wood product that is favourable for the manufacture of various household items.
The use of plants in bio-fencing has been an old practice among the people of the State for protecting the land and sustainable use of natural resources (81) from grazing of animals and encroachment (82). Because of grazing, crop cultivation is hampered to a great extent. Biofencing is therefore essential for the protection of land from massive destruction. It was found that 6% of plant species were used by the local inhabitants for bio-fencing purposes. These include Duranta repens L., Lantana camara L., Chromolaena odorata (L.) R. King (84,85). The natural dyes obtained from these plants are used to colour different clothes by the weaver (86,87). The utilisation of natural dyes over synthetic dyes and its application in textile industries is gaining more interest, providing employment and a source of income to the weaker section in the rural areas (88). The plant species used for natural dye extraction by the local people in the district are Aegle marmelos (L.) Corr., Mallotus phillipensis Muell. -Arg, Trichosanthes tricuspidata Lour., Bauhinia purpurea L., Abrus precatorius L., Acacia farnesiana (L.) Willd., Adenanthera pavonina L., Butea monosperma (Lamk) Taub., Cassia fistula L., Cassia tora L., Punica granatum L., Bixa orellana L. and Chukrasia tabularis A. Juss. (89)(90)(91)(92)(93)(94)(95)(96).
Information on seven plants used to treat different skin problems like burns, rashes, prickly heat, pimples and cosmetics were collected from the people of Dibrugarh. The herbal formulas were prepared from different parts of the plant along with water, milk or honey and were administered either orally or externally. The highly recommended plants are Sapindus mukorossi Gaertn., Bacopa monnieri (L.) Wettst., Eclipta alba (L.) L., Centella asiatica (L.) Urb., Magnolia hodgsonii (Hook.f. and Thomson) H.Keng, Aloe vera (L.) Burm.f. and Rosa rugosa Thunb (97,99). A study showed that 6.3% of the patients undergo treatment for skin ailment in the medical centres (98). The humid tropical climate enhances the formation of many bacterial and fungal infections on the skin causing irritation and pain. The rural people are mostly associated with agriculture and farming due to which they have to stay outside throughout the day being exposed to dermatological infections (99).

Conclusion
The present study on angiosperm diversity of the Dibrugarh district is an initiative for monitoring the species composition and checking the changes in diversity caused by several ongoing disturbances. In the present work, Angiosperms of the Dibrugarh district are classified according to APG and Bentham and Hooker system. Asteraceae was found to be the largest family in dicots. On the other hand, among monocotyledons, Poaceae was found to be the largest family. B and H system is not showing any remarkable difference with the families of APG system. APG IV is followed here because it is one of the most relevant molecular-based systems of classification which is having tremendous attraction all over the world. Along with this system, we adapt Bentham and Hooker system also because till today this system is in use all over India. This work may be helpful for the researchers to carry out floristic study in different regions. The present work focuses on inventorization of angiosperms inorder to establish a comprehensive checklist of the floristic wealth of the district along with traditional knowledge based information which will definitely be helpful in providing sustainable utilization of resources, conservation strategies and management of plant wealth.