Biological invasions of alien species are widely recognized as a significant component of human-caused global environmental changes, associated with significant thrashing in the economic value, biodiversity, and the physiochemical structure of invaded ecosystem [1] - [9] . Studies on alien plants provide us with unique opportunities for research into ecological relationship, where the topic is apparent as having serious practical importance [10] [11] .

Although, many alien plants are economically beneficial and often cultivated, and may provide food, medicines, fuel and fodder to the local communities [12] , their invasion is considered as second leading threat to the plant diversity after the habitat destruction [13] . These species often produce enormous seeds that are dispersed widely and remain viable in the soil for a long period of time [14] . Faster rate of growth and higher reproductive efficiencies of the alien species make them be well adopted on new habitat [15] [16] and noxious to the environment, habitats, ecosystem, native biodiversity, economics and even human health [17] . They can change the hydrology and ecosystem function [18] and also can show all allopathic inhibition to the different abiotic conditions [19] . Nonetheless, both the biotic and abiotic properties of the target habitat are likely to be as important as the autecological attributes for the influencing of invasive success [20] .

Due to drastic breaching of the biogeographical barriers, earth’s biota is trans- forming local and regional floras and faunas, which were isolated continentally for millions of years [21] . Such biotic humanization is a result of increasing global trade and transport; consequently, the number of flora and faunas is trans-located by the peoples, either deliberately or accidentally [1] [22] [23] [24] [25] . Peoples also introduce the alien species into the new habitat [26] for their economic benefit over the native species [27] , whereas, neutralized species are introduced species that consistently reproduce without direct interference of publics [28] [29] . Country introduces many alien species to fulfill their over demand as ornamental uses, which were primarily neutralized to the new habitat but, sometimes a number of species become invasive to that habitat and harshly disturb the native ecosystem.

Therefore, it is an apparent need for a region or country to develop an authentic database of flora on invasive alien species for monitoring their spread, their application by local inhabitants, their ecological impacts on native ecosystem and their invasion status in the region. In view of the above facts, the present study was aimed to compile the first authentic catalogue of the alien flora of foot Himalayan hill in the state Tripura, NE India, along with supplementation of some information of each species viz. origin, life form, habit, habitat, use, invasion status, mode/purpose of introduction and their diversity assessment. Such base line study would be the foundation of further advance studies in the invasion ecology and for the future assessment of invasive species in this biodiversity-rich zone.

The present investigation reveals that the alien flora of north eastern hilly state, Tripura, is comprised of 216 plant species (~14.3% of the state flora) belonging to 158 genera within 58 families. Among all the alien plant species dicotyledons comprises maximum 187 species with 136 genus and 49 families while, monocots are represented only 24 species with 18 genus and 7 families followed by five species of gymnosperm belonging to 4 genera and 2 families which include Araucariaceae and Cupressaceae only (Table 1).

Maximum numbers of alien species belonging to the families are Asteraceae (23), Caesalpiniaceae (14), Solanaceae (13), Papilionaceae (10), Malvaceae (10), Convolvulaceae and Euphorbiaceae (nine species of each), Poaceae and Cyperaceae (seven species of each) and Mimosaceae (six species). The 10 families represent 50% of the total alien flora. Relative to their total genera of the families Caesalpiniaceae represents maximum (75%) and Poaceae shows minimum (12%) alien genus in the top 10 families (Table 2). The genus Cassia and Ipomoea represents highest numbers of alien species (8 species in each) followed by

Cyperus (6), Solanum (5), Hibiscus and Brassica (4 species in each); Acacia, Alternanthera and Euphorbia (3 species in each). In terms of diversity of the species, the genus Alternanthera showed 100% invasiveness, followed by Ipomoea (88.88%), Cassia (87.5%) and Brassica (80%) (Table 3). Among the all alien plant species one hundred twenty six or 58.33% genera represents only one species (Table 4).

While estimating the contribution of source flora according to their distribution we compiled them in Tabular form (Table 5). In continental basis it is estimated that 61.57% of the alien species are of American origin followed by Asia (15.28%), Africa (12.96%), Europe (7.41%) and Oceania (3.70%).

Habit wise plants are categorised as annual (40.28%) and perennial (59.72%). Estimating the life forms of all the alien species, herbs shows maximum (42.6%), followed by shrubs (23.61%), tree (22.22%), climber (5.09%), grass and sedges (3.24% each). About 23.61% of the species are abundant in various ornamental garden, followed by cultivated field, waste lands, road side and in forest ecosystem are 18.05%, 11.11%, 8.33% and 5.55% respectively. Seven species are found solely from aquatic habitat, nine from crop field, four from moist place, two each from river bed and home garden and one species Hevea brasiliensis is cultivating vigorously in the state. Remaining all the species are distributed in various habitat. A total of 66 (30.55%) species have been introduced intentionally for various economic, medicinal and ornamental purposes and remaining 150 (69.44%) species were introduced unintentionally by way of various unmannered trades (Figure 2).

On the profile of economic uses of the alien plant species, 63 (29.17%) species are ornamental in the state. The second most frequent economic uses are medicinal plant species (15.70%), followed by food (9.72%), fruit species (6.02%) while, 14.81% of the species has no use value. Other various uses like timber, as vegetables, fibre, hut making, latex collection of the alien species are documented through the consultation with local people in the state (Table 4 and Figure 3).

IE, Invasive elsewhere; NIE, Not invasive elsewhere, NAS, Number of alien species, AS, Alien species.

Abbreviations: *= Newly introduced alien plant species (not included in the state flora). OR, Origin: NAM-North America; CH-China; TAM-Tropical America; AU-Australia; EU-Europe; SAM-South America; BUR-Burma; SETA-South East Tropical Asia; POL-Polynesia; MD-Madagascar; TAF-Tropical Africa; WI-West Indies; TWA-Tropical west Asia; BZ-Brazil; MDT-Mediterranean; MY-Myanmar; AM-America; MEX-Mexico; TEAF-Tropical East Africa; YU-Yugoslavia; SAF-South Africa; SY-Syria; ML-Malaysia. LF, (Life form): H-Herb; C-Climber; US-Undershrub; S-Shrub; SE-Sedges; B- Bamboo; T-Tree; G-grass; HA, (Habit): A-Annual; P- Perennial. HT, (Habitat): W-Wasteland; CF-Cultivated field; F-Forest; AR-Along roadside; A-Aquatic; P-Parasites; C-Crop field; RB-River beds; G-Garden; MP- Moist place; RF-Rice field; PF- Plantation Forest; HG-Home Garden; U, (Used): AD-Adulteration; B-Basket making; BF-Biomass fuel in rural area; BR-Bio-fertilizer; TM-Timber; M-Medicinal; V-Vegetable; FI-Fibre; FO-Fodder; HP-Hedge plant; SM-Smoking; SB-Soli binder; FT-Fruit; LC-Latex Collection; NK-Not Known; HB-House building; FD-Food; O-Ornamental; HU-Hut; DR-Drink; D-Dye; SP-Spices; MI, (Mode of Introduction): AF-Agro forestry; UI- Unintentional; I-Intentional; Invasion status: CL = Cultivated alien; CS-Casual alien; NT-Naturalized alien; IN-Invasive alien.

Invasion status of the total alien flora (216) are categories as casual 51 (23.61%), cultivated 34 (15.74%) naturalized 79 (36.57%) and invasive alien 52 (24.07%) which are the 3.4%, 2.67%, 5.27% and 3.47% of the state flora respectively (Table 5).

The present study also reports five newly introduced plant species for the first time from the state. These are includes: Acacia mangium Willd. (Mimosaceae), Tridex procumbens L. (Asteraceae), Alternanthera brasiliana (L.) Kuntze (Amaranthaceae), Caladium bicolor (Ait. ex Dryand.) Vent. (Araceae), Peltophorum pterocarpum DC. (Caesalpiniaceae).

The present work also analysed the diversity of 52 invasive alien species from this alien flora. For evaluating their phytosociological attributes the data pertaining to frequency, density, abundance and their relative values and Important Value Index were recorded from the various habitats (Table 6). The most frequent invasive species in the study area is Chromolaena odorata (71%), followed by Ageratum conyzoides (60.5%), Alternanthera philoxeroides (50.5%) and Mikania micrantha (49%). The most densely species in the study area is Chromolaena odorata (6.11) followed by Ageratum conyzoides (5.02), while Ipomoea alba (0.10) shows least. Chromolaena odorata (8.60) is the most abundant species in the study area followed by Parthenium hysterophorus (7.76). IVI calculated for the individual invasive species in the study area, Chromolaena odorata shows highest (18.82) followed by Ageratum conyzoides (15.74), Hyptis suaveolens (12.16) and lowest in Ipomoea alba (1.40).

Abbreviation: F, Frequency; RF, Relative frequency; D, Density; RD, Relative density; A, Abundance; RA, Relative abundance; IVI, Important Value Index.

Although, some of the alien species are invasive and a major threat to the local ecosystem when they are introduced unintentionally outside their natural habitat, but many alien species are supporting our farming and forestry structure in an immense manner [55] .

A total of 216 alien species are distributed in various ecosystems in the state which belongs to 58 families, from which only 10 families accounts the 50 % of the total alien flora. Asteraceae is the dominant alien family having 10.64% of total alien flora in the state followed by Caesalpiniaceae, Solanaceae, Papilionaceae, Malvaceae, Convolvulaceae, Euphorbiaceae, Poaceae, Cyperaceae and Mimosaceae. Our finding about the distribution of families of alien species in Tripura is in accordance with the findings of earlier workers in other parts of India [19] [50] [56] [57] . In this catalogue of alien species 18 families are represented by single species while 13 families have only two species (Table 4).

From the newly introduced five species those are not yet included in the state flora have some ecological impacts. Peltophorum pterocarpum (Caesalpiniaceae) is naturalized in terrestrial habitat and geographically originated from South East Tropical Asia. Alternanthera brasiliana (Amaranthaceae), Caladium bicolor (Araceae), and Tridex procumbens (Asteraceae) are invasive to the native ecosystem in the state. Alternanthera brasiliana, Caladium bicolor were originally introduced as ornamental aliens but at present they have invaded the natural flora and become establish in the local ecosystem replacing the original population thus becomes detrimental. This kind of invasiveness of the ornamental plant was also reported from Kashmir valley [58] . Most of the invasive species are distributed along the road side of the state which matched earlier report [59] . Only one species Acacia mangium (Mimosaceae) was introduced in 1990s for timber production, which is geographically Australian origin.

In earlier reports [45] [60] estimated that, the contributions of alien flora in India were 40% and 18% respectively while in the state it is nearly 14.5%, which is less than the respective data to our country. Recently few studies have been conducted by few workers on alien flora of the different region of the country [38] [40] [42] , the main purpose of such studies were to provide a list of “alien” within their respective region without keeping in mind any kind of invasion status of the species. On comparison with these studies we compiled here the whole alien flora in the state with the proper observation on their invasion status within the state.

The American alien plant species in the state (61.57%) exceed the total American contribution (55%) alien plant species in India [45] . The probable reasons for such higher proportions of successful introduction of the alien species in the state flora of Tripura could be the (i) more or less climatic similarities with the tropical America and/or (ii) due to increase in temperature in the region which invite the Tropical American flora. Although two decade ago the state had favourable environment and highest proportion of native forest, but now due to mono-culturing practices of Hevea brasiliensis, temperature is increasing and supports the adoption of the Tropical American flora. If such climate change will continue, the contribution of American flora to the country will increase and most of the species those are now naturalized in condition they could be invasive to the country in future. While, less proportion of the alien species are contributed by Oceania and Europe, which support the earlier findings [45] .

Herbs are the dominant alien species (92) in the state followed by shrubs and tree which supports the findings of previous work [40] on invasive species diversity of Western Ghats, India. The 52 invasive species are creating a pressure on ecology and socio-economic structure of the state but some of them are playing a beneficial role in ecological restoration, soil conservation and healthier economy of the state. Commercial use of some invasive alien plants (e.g. Hevea brasiliensis) has resulted in elevated economic condition of the rural community in the state. Like earlier studies [29] [50] [61] [62] [63] , the present catalogue of the state also include 34 (15.74%) cultivated alien species. Most of them are intentionally introduced from European Country.

The present study also shows the relatively higher percentage (31.02%) of ornamental alien species introduced in the state are in accordance with the observations of the earlier workers in the flora of Azores Archipelago and Taiwan [61] [62] [63] . Field survey and investigation reveal that deliberately introduce alien tree species like Eucalyptus maculata, Acacia auriculiformis, Leucaena leucocephala, and unintentionally introduced some woody shrubs like Lantana camara, Chromolaena odorata, cassia tora are supplying the rural fuel wood and timber demand. The species like Delonix regia, Lagerstromia speciosa and Peltophorum pterocarpum are the colour in the state forest and they are also used as ornamental tree in the garden.

The invasive alien species can rapidly adapt to a wide range of environmental condition which may assist their dispersal to a far distance from their place of origin. Some herbaceous and shrubby alien plants like Chromolaena odorata, Cassia tora, Lantana camara, Ageratum conyzoides, Amaranthus spinosus, Eichornia crassipes and Alternanthera sessilis are harmful to the mankind. Their invasion and rapid propagation has triggered ecological imbalance within the native floristic composition. The studies on historical and present perspective of Lantana camera showed that it was introduced in Indian Botanical Garden, Shivpur as an ornamental hedge at 1809 [38] , but at present it is a serious invader and now invasive to the state of Tripura and it is also attacking the forest ecosystem of the state. Chromolaena odorata first arrived in Kerala [64] [65] , later it is invaded in all hot places in the country. In the state it inhabits almost in all ecosystem through its rapid propagation which creates a serious menace to the native elements. Croton bonplandianum was first introduced in 1897 in East Pakistan from South America by ship [38] . The species furthered propagated along railway track and spread and invaded in various parts of India [66] . At present it is a hazardous weed in all the agricultural field of the study area. Parthenium hysterophorus arrived in the state just one decade ago and adversely affect the urban ecosystem as well as various river catches of the state. It was first reported from India in 1956 from western part of peninsular India [67] . Mimosa pudica another invasive species was introduced from Brazil and presently it is expanding at an alarming rate in the country. Now the species is vigorously attacking the grass land and creating a major threat to the grazing to livestock.

In this investigation, we have compiled all the alien flora of the state with their invasion status. Such careful compilation and categorization of regional alien flora will provide vital clues for revelation of the problem resulting from species invasion. Most of the invasive species in the catalogue are noxious to the agro- ecosystem as well as to the indigenous flora in the state. In this perspective this catalogue could be effective for the prediction of invasive aliens, their monitoring and management practices. This catalogue will obviously help in the agricultural sector in the state as well as in the whole country. It also gives a clue for further introduction of any species in the country. It may help the Department of Forest and Agriculture for further advancement of research in their related field. The present work, hopefully, will help the prologue of such aliens to any new place of Indian sub-continent, Indo barman region as well as other parts of the world.

Authors are thankful to the CSIR for providing financial support which helps us to carry out this work. We are thankfull to BSI, Shillong for their kind help to confirm our identification of some species. We are highly gratitude to Smt. Bina Sinha, retired professor of MBB College, Agartala Tripura for her support and useful suggestions in this work. Authors are also thanks full to the farmers and agriculturist of the state for sharing their valuable information about the alien weeds and their infestation. We also admire those, without whose helps the compilation of the catalogue was not possible.

Debnath, A. and Debnath, B. (2017) Diversity, Invasion Status and Usages of Alien Plant Species in Northeastern Hilly State of Tripura: A Confluence of Indo-Barman Hotspot. American Journal of Plant Sciences, 8, 212-235. http://dx.doi.org/10.4236/ajps.2017.82017