Plant diversity in terai grassland vegetation : a landscape level assessment with special reference to north-eastern uttar pradesh , India

The landscape level plant diversity was measured in Terai grassland of north-eastern Uttar Pradesh. The field assessment, based on random quadrats, was undertaken at 11 sites, sampling an area of 333.75 ha. The grassland vegetation was comprised of a greater species richness compared to old-field vegetation. The grassy landscape as a mosaic of grassland patches contained 287 species in the sampled quadrats, which represented 177 genera from 53 families. The species composition of sites varied significantly with an increase in species richness corresponding with landscape heterogeneity. The species richness of a few sites was significantly high due to mesic condition and topographic heterogeneity. The fully exposed sites had average moisture with moderate disturbance and showed greater diversity and lower dominance. However, the partially shaded locations had high moisture and low disturbance resulting in minimum diversity and maximum dominance. The presence of high levels of disturbance in the form of severe grazing, trampling and/or cutting favoured prostrate, perennial, herbaceous-grasses such as Cynodon dactylon and Imperata cylindrica as well as the forbs such as Desmodium triflorum, Evolvulus nummularis, and Rungia repens. The marked differences among grassland habitats and the intensity of various biotic processes within the grassy landscape of the adjoining forest result in an array of grassland patches with marked differences in their diversity. Due to changes in agricultural practices and grazing intensification, the Terai grassland has been significantly reduced and highly fragmented resulting into the rarefaction of species that were once common and widespread. The shrinkage of specialized habitat owing to intense cultural activities, severe grazing and recurrent trampling has pushed several medicinally important and other rare plant species towards local extinction. Therefore, we suggest that appropriate management guidelines be implemented in order to conserve at risk species from extinction on the Terai grassland.


INTRODUCTION
Grassland is one of the most widespread ecosystem types worldwide.It is closely related to agro-ecosystem *Corresponding author.E-mail: srivastava980@gmail.com.
Author(s) agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License and forms over 24% of the world's vegetation (Shantz, 1954).Species richness is currently the most widely used measure and easily interpretable indicator of biological diversity (Whittaker, 1977;Stirling and Wilsey, 2001).The number of species reflects the quantum of gene pool and adaptation potential of the plant community (Odum, 1963) and has long been used to characterize its taxonomic structure (Simpson, 1949;Margalef, 1958).Diversity is regarded as the result of species interaction or community adaptation to its environment over its evolutionary time (Whittaker, 1972;Rice and Westboy, 1982).Disturbance is a common and widespread phenomenon in nature.It is a distinct event that modifies population structure, community, ecosystem and landscape beside the passage of time and therefore, the role of disturbance in the dynamics of ecological system can be seen as one of the basic ideas in modern ecology (Pickett and Whyte, 1985).Disturbance contributes to long-term maintenance of ecological diversity (Huston, 1994).An understanding of the response of communities to both repeated and new disturbance regimes is, therefore, necessary.
In India, grasslands constitute one of the major biomes and form an important component of terrestrial vegetation.Indian grasslands are seral in nature but they tend to be stable under the constant influence of biotic disturbances (Pandeya, 1953).Grassland vegetation of different parts of India has been managed by a number of organisations [grassland vegetation of Bombay (Bharucha and Dave, 1944), Varanasi (Misra, 1972;Ambasht et al., 1972), Jodhpur (Gupta and Sharma, 1973) and Kurukshetra (Singh and Yadava, 1974)].The grass-cover types recognized by Dabadghao and Shankarnarayan (1973) are not homogeneous.
There are two distinct eco-climatic zones parallel to sub-Himalayan in north India viz.Bhabhar and Terai.Bhabhar is a narrow, dry ecosystem in comparison to the Terai which is significantly larger and wetter ecosystem.The grassy landscape lies between the Sarju River and the foothills of the Himalayas, covering 11 districts of eastern Uttar Pradesh.A major part of this landscape is adjacent to the managed Sal forests of the eastern Terai and the region is known for its unique biodiversity and high productivity (Ansari et al., 2006;Krishnan et al., 2012).These grasslands consist of patches of natural herbaceous vegetation interspersed among agricultural fields.The region has undergone severe alterations due to the expansion of agriculture coinciding with urbanization, resulting in fragmentation and degradation of natural habitats (Johnsingh et al., 2002) which have adversely affected the Terai flora and fauna (Javed and Rahmani, 1998).Tripathi and Shukla (2007) undertook a comparison of species richness between the vegetation found in a natural grassland with those found in managed grassland.Shukla (2009) discussed the abundance and diversity patterns of plant species across different physiognomic units over a considerably large Terai landscape and found that natural sal and mixed forests had higher species diversity than plantations and other forest types.Most of the rare and threatened species were confined to forest edges and scrub vegetation. .Srivastava et al., (2014) observed changes in indigenous community composition due to invasive plant species.
The present study aimed, to create baseline data on Terai grassland vegetation with special reference to north-eastern Uttar Pradesh (U.P.), ecologically the least explored region of India.We measured plant diversity and analyzed the grassland floristic community of the region on a landscape scale.Our study is in contrast to other phytosociological studies which work out the distribution of species abundance in ecological assemblages of various sites as determined by natural as well as anthropogenic disturbances.The results found in this study will provide critical baseline data for management decisions.

Study sites
The plains of north-eastern U.P. cover 16 districts of eastern U.P. The study was conducted in 11 of these districts (Figure .1).The climax vegetation is forest.The natural forest has largely been converted into plantation forests and grassy landscapes; the major part is agricultural fields.The abandoned arable land has developed into grassland through secondary succession and may remain stable under the influence of biotic disturbances such as fire, grazing and cutting practices.The study area is bordered by Nepal in the north and Bihar state of India in the east.The regional plain slopes gently from north-west to south-east and is transversed by many rivers, rivulets, nullahs, lakes and ponds.

Climate
The climate of the region is typically tropical monsoonal with three distinct seasons, viz., summer (March to mid-June), monsoon (mid-June to mid-October) and winter (mid-October to February).Average annual rainfall is 1814mm with 87% occurring during the wet summer and monsoon seasons.The number of rainy days per annum is 51±3.2 with average relative humidity ranging between 74 to 87%.The north -eastern plains of Uttar Pradesh receive more rainfall over a longer period and therefore, possess much richer plant biodiversity than western and southern districts of the state.Mean maximum and minimum temperature range during wet summer 26.2 to 35.2°C, winter 12.1 to 27°C and dry summer season 24.2 to 9.3°C respectively (Indian Metrological Department, 2000to 2005).

Soil
The soil of this region is part of the trans-Sarju Plain and comprises Gangetic alluvium brought down by the rivers Ghaghara, Rapti, Rohin and Gandak from the Himalayas in the north.The texture is sandy loam and pH is near neutral.In the northern area there are a few elevated mounds, locally called Dhus, which range in size from a few hundred meters 4 to 5 km consisting of brown sandy soil.

Vegetation
The grassland vegetation presents a mosaic of plant communities.These grasslands, developed and maintained by various cultural practices represent compositions which vary primarily according to the type of soil and available moisture within the upper layer.The growing season extends from mid-June to mid-September when most species flower and set seed.Many annuals are ephemerals and complete their life cycle before the end of October, while perennial species dry off in winter.

Sampling
This study commenced in June 2011.A general survey was conducted over a vast stretch of grassland vegetation of northeastern Uttar Pradesh covering an approximate area of about 35, 48,000 ha encompassing 11 districts with marked differences in habitat conditions.Thirtyone sites were selected and sampled     (Rabinowitz 1981;Rabinowitz et al., 1986).(Whitford, 1948) and Family Importance Value (FIV) were derived.FIV was taken as the sum of relative density, relative diversity, and relative vegetal cover.The relative diversity of a family was evaluated as the number of species within the family expressed as percentage of total number of species within all the families represented in the community (Mori et al., 1983).The degree of similarity among communities at different study sites was computed through Jaccard's methods for Coefficient of Similarity (Jaccard, 1908).The Rabinowitz classification scheme was used to categorize each of threatened and endangered taxa into one of the seven types.They were defined on the basis of geographic range size (wide vs. narrow), habitat specificity (broad vs. restricted), and population size (large, dominant vs. small, scattered) (Table 2a).From the combination of these traits, eight categories were formed to decide commonness vs. rarity of a given species (Rabinowitz, 1981 andRabinowitz et al., 1986) (Table 2b).The causes of threats to plant species may be natural or anthropogenic.All species were assigned to one or more threats as per the observations (Srivastava et al., 2015).All the data was analysed by using PAST (Paleontological Statistics software) Version 2.17.

Species richness and diversity
The landscape-level plant diversity assessment was made within 333.75ha of the grassy landscape.The details of each study site, sampled area and quadrats laid are shown in Table 3.A total of 287 plant species were encountered within the sampled quadrats, which represented 177 genera under 53 families.Dicotyledons made up 94.5% of the total families, 74.1% of genera and 69.7% of total species.The remaining species were monocots.The species: genus ratio was 1.62 and species: family ratio was 5.42.The site of Gorakhpur district which represented mostly intermediate upland topography and mesic habitat showed maximum species richness (147) of plant species and shared 51.2% of total species richness of the grassy landscape.The minimum species richness occurred at Sant Kabir Nagar (45 species).All other sites showed some intermediate value of species richness (Table 4).While Cynodon dactylon commonly occurred at all sites, species like Aneilema nudiflorum, Desmodium triflorum, Launaea asplenifolia, Lindernia deccusata, Oldenlandia corymbosa, Rungia Table 2b.Seven form of rarity (2-8) of regional rare taxa.Each plant species was assigned to a rarity category based on their trait combinations.Category 1 is for common species.Our categorization has been compared with that of Rabinowitz's (1981).The analysis of various phytosociological attributes shows that the erect annual herbs shared maximum values of various analytic indices (Table 5).The number of individuals/m 2 was also maximum for erect annuals (490.97)followed by prostrate perennials (39.87).The annual twiner climbers showed much less values (0.17).The erect annual herbs were much more abundant (1081.87)as compared to prostrate annuals (236.93).Shrubs were least abundant (6.12 to 6.52).Furthermore, erect annual herbs also showed much more basal area cover (30.27) as compared to twiner annuals.Overall, the landscape vegetation of Terai showed fairly high species diversity dominated by annual herbs (H=3.74) and quite low dominance (Cd = 0.058).The total vegetation density, however, was 811.5 individuals m -2

Trait combinations
).The life-span data of different grassland species showed that about 90% of species were annuals and the remainder perennials.There were a variety of different growth habits and dominant families within each study sites across the grassy landscape vegetation of Terai (Table 6).The maximum number of herbaceous plant species was encountered in Gorakhpur district (129) followed by Deoria (102) and Balrampur (103), with Sant Kabir Nagar having the minimum number of herbaceous plant species (40).Further, the number of shrubs and climbers was also much greater in Gorakhpur district as compared to Siddharth Nagar, Maharajganj, Basti and Gonda.No climbers were observed in Gonda district.At the landscape level, 250 species of herbs, 28 climbers and nine shrubs were found.Poaceae and Cyperaceae were the most speciose and ubiquitous families of the region.

Species richness and Importance value of different families
Six dominant families accounted for 56% of total species richness.Poaceae was the most common (17.2%), followed by Cyperaceae (11.2%),Papilionaceae (10.5%),Asteraceae (9.8%), Scrophulariceae (4.9%) and Euphorbiaceae (3.5%).The remaining 44% of species represented 47 families.Six families were represented by more than 10 species.Poaceae, Asteraceae and Papilionaceae showed maximum genera richness in that order with 38, 19 and 16 genera, respectively.In terms of individuals, two families, Scrophulariaceae and Poaceae were dominant with a total > 25,000 individuals within the sampled quadrats of the grassy landscape (Table 7).
The actual importance of a family within its community comes from FIV, an index which heavily depends upon the vegetal cover of component species in addition to the density values.Family Poaceae accounted for 17.4% of total species and 19.9% of total individuals of the landscape.They shared 18.49% of the total vegetal cover and contributed 18.6% to its FIV.Further, the other dominant families like Cyperaceae, Papilionaceae, Asteraceae, Scrophulariaceae and Euphorbiaceae shared 39.72% of total species richness and 52.37% of total individuals.These families together shared 47.49% of total vegetal cover and 52.37% of density (individual/m 2 ) sum and also contributed 46.52% to sum of FIV.The families, Scrophulariaceae, Poaceae, Papilionaceae and Acanthaceae accounted for >10,000 individuals within the sampled area.The 31 families were poorly represented in terms of the number of individuals.Of these families, Martyniaceae, Sphenocleaceae and Zygophyllaceae were monotypic and shared only 0.40; 0.38 and 0.36% FIV respectively across the Terai grassy landscape (Table 8).

Abundance distribution
The value of sum of frequency, density and abundance at 11 different sites across the grassy landscape were compared (Figure 2).The fluctuations in the sum of density were more conspicuous as compared to that of abundance and frequency.The sum of density was    9).The A/F ratio for these species was very low (< 1) with exception of Imperata cylindrica.Analysis of Jaccard coefficient of similarity showed that the Deoria and Kushinagar districts shared maximum similarity of 0.50 followed by Deoria and Maharajganj (0.44).The least similarity (0.09) was observed between sites at Siddharth Nagar and Sant Kabir Nagar.Over 70% of species were found to be common to all sites within the grassy landscape of north-eastern region (Figure 3).

Dominance-diversity relationship
The resource utilization and niche occupancy of the species within a community is frequently expressed by dominance-diversity curve.Only a few species preempted most of the niches at 11 different study sites (Figure 4).The pattern of niche pre-emption by Lindernia decussata was quite comparable for Bahraich and Kushinagar.The topography and soil type of the two sites were quite similar but the disturbance regime was different at Bahraich, the grazing intensity was higher as compared to Kushinagar.Further, the Gorakhpur and Shrawasti sites were similar in habitat conditions and disturbance regime.They showed quite similar niche occupancy pattern.The species such as Desmodium triflorum, Evolvulus nummularis, Rungia repens and Cynodon dactylon occupied maximum niche space of all other species in total.Evolvulus nummularis showed maximum number of individuals and occupied maximum niche space at sites of Balrampur, Gonda and Basti.Balrampur and Gonda sites were similar in light and disturbance regimes, grazing pressure and moisture regimes except for some marginal soil differences.Sant Kabir Nagar sites showed unique habitat conditions with fully open, high moisture and complex disturbance conditions and showed maximum niche occupancy by

Medicago polymorpha.
The Siddharth Nagar site was quite similar to the former site except some degree of fire as disturbance factor.Imperata cylindrica occupied its maximum niche space.The fire, grazing and trampling together increased its occurrence and density.The expansion of the species is facilitated by its rhizomatous growth.Deoria and Maharajganj sites were quite similar in habitat condition with some difference in moisture regimes.The two moisture-loving species Rungia repens and Phyllanthus urinaria occupied their maximum niche spaces.

Rare species and its population
The total number of individuals or density, habit, posture categories, distribution, population status and the specific habitat conditions of rare species revealed that at the landscape level, only 31 species showed rare occurrence and occupied specific habitats (Table 10).They represented 27 genera under 18 families.The common species with large distribution range showed wide habitat specificity.The intense agricultural practices, mining, livestock grazing and trampling were found to be the major threats causing rarity of a great number of grassland herbs.A few species such as Chyranthellum indicum and Spermacoce pusilla, however, were encountered only at one of the eleven sites (Bahraich) but in the localized spots the individuals were much greater in number.These species occupied a habitat possessing sandy to sandy loam soils with average moisture and exposed condition.A few highly medicinal plant species such as Astercantha longifolia, Bacopa monnieri, Centella asiatica, Chyranthellum indicum, Ionidium suffructicosum, Evolvulus alsinoides, Leucas aspera, L. cephalotus, Vernonia adscendens and Tribulus terrestris have suffered seriously due to voracious harvesting for medicinal purpose from the wild.The very rare and poor occurrences of some lowland species such as Cyperus niveus, Lobelia alsinoides and Sphenoclea zeylanica may be attributed to severe grazing just before their full bloom or seed-setting stage.

DISCUSSION
The Terai grassland vegetation has considerable diversity represented by plant species of different habits.A landscape level approach addresses the changes in biodiversity due to anthropogenic activities and the effect of such changes in biodiversity on the functioning of biological system (Franklin 2001).Since humans play a key role in altering ecosystem level processes, its integration as part of the ecosystem is quite obvious.Habitat modifications and management practices that change functional diversity and functional composition are likely to have large impacts on ecosystem processes (Tilman et al., 1997).Societies, by their activities also affect landscape function (Ramakrishnan, 1992).The developmental stage of community is complicated by disturbance types and their intensity.Disturbance may increase species richness by lowering the dominance of a few species, freeing resources for early successional plants, and providing opportunities for herbaceous species to spread rapidly (Tripathi, 1999;Sood et al., 2011).The landscape of north-eastern Uttar Pradesh forms an interrupted regional community of grassland which is quite heterogeneous due to small-scale changes in topography, soil and light conditions and, therefore, may be easily recognized on the basis of species composition, habitat characters and patch size.The different patch type characterise different habitats and species composition, and the total number of species in a landscape increases with increase in heterogeneity of landscape (Sluis et al., 2014).Our result showed clear difference in species richness of patch sizes at different study sites.The total number of species of two sites such as Gorakhpur and Balrampur were fairly high.At Gorakhpur, 147 species were encountered within 40.88 ha area; with 120 species recorded within 13.34 ha at Balrampur.Further, Sant Kabir Nagar recorded only 45 species within 11.33ha area.These results indicated that the sizes of patches are significantly related neither to species richness nor species composition as also evident from the Bisteau and Mahy (2005).Other studies have however, reported that size is the main factor affecting species richness in fragmented grasslands (Krauss et al., 2004) and isolation may also be a significant factor (Piessens et al., 2005).Low species richness may result from the dominance of a few competitors and ruderals.Both, a reduction in species richness and increased dominance of a few species are common phenomenon in highly fertilized grasslands (Berendse and Elberse, 1990).
The density and abundance distribution of individual species are measurable indicators of plant diversity (Wattenberg and Breckle, 1995).The importance of natural and anthropogenic disturbance in maintaining species richness and diversity have been pointed out by many authors including Grime (1979), Pickett et al., (1989) and van der Maarel (1993).Variation in disturbance intensity exerts profound influence on the pattern of diversity and species richness; the everincreasing biotic pressure often leads to a mosaic of patches with various degrees of ecological maturity (Rescia et al., 1994).Increased diversity and reduced dominance has been found in case of fully exposed locations having average moisture and facing moderate disturbance.It was minimum in case of partially shaded locations having high moisture and facing low disturbance.It has been shown to be associated with increased stability (McNaughton, 1967).Bahraich and Gonda showed low diversity and high dominance in comparison to other sites.This may be due to the presence of severe disturbances in the form of grazing, trampling and cutting.The disturbance in the form of periodic clipping caused greater dominance and low diversity, and the species of prostrate habit dominated the localities.It has already been established that diversity is lower in absence of disturbance as well as in presence of too much of disturbance (Srivastava et al., 2015).The moderate level of anthropogenic disturbance, therefore, are compatible with maintenance of high biodiversity of landscape (Gentry, 1991).
The annual herbaceous species have been reported to respond more positively to disturbance (Belsky, 1986).The prostrate perennial herbaceous grasses like Cynodon dactylon and Imperata cylindrica and few forbs such as Desmodium triflorum, Evolvulus nummularis, and Rungia repens had maximum dominance due to multiple reproductive strategies (Patrica et al., 2002) and rapidly occupied the horizontal space created by disturbance especially by severe trampling and grazing.Annual and perennial species showed maximum seed and ramet proliferation even in regularly trampled habitat.The abundance of a species in an area depends on the ability of its propagule and tolerates features of the environment (Harper, 1967).Lindernia decussata exhibited the greatest density, abundance and A/F ratio of all other species.The species is ephemeral, moisture loving and produces numerous seeds where discrimination is favoured by grazing.The species such as Cynodon dactylon, Desmodium triflorum, Evolvulus nummularis and Rungia repens, showed overall maximum occurrence in the region.R. repens also exhibited maximum density and abundance but its A/F ratio was low in comparison to annual species.The high importance value of these species may be attributed to efficient mode of seed production and ramet growth.Relatively, privileged number of species can be attributed to moderate grazing and grazing may develop mutualism between the grazers and grazed species.The saliva and dung of grazers may also promote the growth of some species (Grace and Jutila, 1999).
The sum of frequency value changed notably from one to other study sites which may be attributed to the sudden increase in the number and abundance of a few more opportunistic weeds in response to increased exposure.Low value of both abundance and frequency showed rare occurrence of species while very high value indicated their dominance quite often in the form of extensive patches.Hubbell and Foster (1986) found that most rare species were specialist either in habitat utilization (topography or edaphic condition) or in regeneration niche (regenerative condition in gap openings) which implies that a high heterogeneity of environmental condition is necessary to accommodate so many specialist species.The sum of frequency may be much lower in highly disturbed sites because of much greater abundance of a few adaptive species (Stachurska, 1994, Pandey and Shukla, 2003, 2005).
The ability of large, fast-growing perennial herbs suppressed the growth of smaller species.A nearly nonrandom species distribution within the community in the form of widely distributed dominant species and locally distributed subordinate species is quite common (Kolasa et al., 1989).The pattern of species abundance may be related to habitat factors and growth pattern of the species which specialize the niche (Brown et al., 1995).
Our results are consistent with the prediction that, as generalists use various habitat types in the landscape, they should be less affected by habitat fragmentation than specialists, which are more dependent on one or few habitat types (Brouat et al., 2004).Specialist's plant species are also expected to be adversely affected by landscape disturbance (Kithahara et al., 2000, Kassen, 2002).Our observations are also consistent with the empirical findings that the human induced landscape degradation cause decline of specialist species (Krauss et al., 2003, Devictor et al., 2007a,).We found little support for the hypothesis that common species are competitive dominants and rare species are weak competitors (Whittaker 1965;McNaughton and Wolf 1970).Our results indicate that response to competition may be important for coexistence.We found that rare species were relatively tolerant of competition which allowed them to persist at low abundances and coexist with common species (Rabinowitz et al., 1984).The rare forbs suffered weak competition from common species because they started growing and flowering earlier in the spring allowing them to get a head-start before competition from other species was established.
Severe change in habitat conditions resulted in fall of specialist species and rise of opportunist species.The population of some grassland species such as Chrysanthellum indicum and Spermacoce pusilla showed least tolerance to these disturbances and were found limited to one site having sandy to loamy soils.On the other hand, species such as Baccopa monnieri, Cassia absus, Cyperus niveus, Eragrostis capensis, Eragrostis cilianensis, Heliotropium ovalifolium, Leucas cephalotus, Leucas aspera, Lobalia alsinoides, Martynia annua, Psoralea corylifolia, Sphenoclea zeylanica, Atylosia scarabaeoides, Teramnus labialis, Tribulus terrestris, Vernonia adscendens and Zephyranthes citrina generally occupied quite specialized habitats and had a small population size within a narrow distributional range.Due to their medicinal importance they have been overexploited, their natural habitat is almost destroyed and they are at the verge of local extinction.Voracious and unmanaged harvesting is another major threat to the existence of several minor but valuable plant species of the region (Shukla, 2009).
The recent urbanisation and associated disturbances have caused a much greater loss to specialized niches of the region.For example, Crotolaria calycina, and Ionidium suffruticosum happen to be rare and their population size has reduced significantly during the last decade.Considerably shaded and damp habitats within village forests or orchards are preferred.The species have suffered seriously, due to the fast reduction of such habitats mainly by increased urbanization and agricultural expansion.It has been established that the shade tolerant species are more sensitive to habitat fragmentation than shade intolerant species (Metzger, 2000).Species like Astercantha longifolia, Bacopa monnieri, Centella asiatica and Heliotropium ovalifolium mostly grew in areas facing some degree of water-logging on clayey soil and most often show aggregation.
Currently, they have become infrequent across waterlogged or lowland regions despite their two-pronged regeneration strategy that is, through seeds as well as through sprout and ramets.Thus, not only upland but also the marginally lowland habitats of the region are severely disturbed.The highly specialized species in nature have significant positive associations and they cannot survive outside of their natural habitat (Hubbell and Foster, 1986).Bruun (2000), identified numerous functional groups including specialist grassland species which were positively correlated with habitat area.With the small population size and restricted habitats more species will be prone to localized extinction (Menges, 1998;Butaye et al., 2005).
The resource sharing and occupancy of niche space are frequently expressed by dominance-diversity curve (Whittaker, 1975).As evident from these curves, generally fewer species in the landscape preempted most of the niches.The conditions like moderate grazing and lesser clipping and trampling allowed relatively greater number of species to share community resources, thus reducing the degree of dominance at community level, as evident from the less steeper and more flattened curve (Raizada et al., 1998).Disturbance has positive effects on the grassland vegetation as reported earlier by some workers (Sundriyal et al., 1987).Further, disturbance caused by herbivores may reduce the negative effect of competition (Nautiyal et al., 2004).In comparison to other species of exposed community, erect herbaceous component was dominant especially in condition of low moisture and moderate disturbance.Lindernia decussata was more frequent, highly dominant and occupied maximum niche space (Bahraich and Kushinagar).Desmodium triflorum, Evolvulus nummularis, Rungia repens and Cynodon dactylon showed high dominance and occupied maximum niche space at two other study sites (Gorakhpur and Shrawasti).The latter two sites showed slight differences in habitat conditions but their disturbance regime was quite similar.On the contrary, almost uniform and periodic clipping inhibited the establishment of most of the upper strata species and promoted dominance of only a few prostrate species like Rungia repens and Desmodium triflorum which occurred most frequently within low disturbance zone (Dwivedi, 1978).
Habitat modification or fragmentation due to persisting disturbance (grazing and other anthropogenic pressure), creates a platform for the onset of biological invasion by species such as Hyptis suaveolens, Parthenium hysterophorus and Solanum sisymbriifolium (Srivastava et al., 2015).The replacement of many endemic species by a few widespread species could, by itself, promote large-scale homogenization especially in open habitats with reduced competition, often created by grazing and man-made disturbance (Wu et al., 2004;Huang et al., 2009).The survival and growth of small population into large population of several plant species indicated that the habitat quality was not always worse for all species especially invaders.They can easily colonize and fruitfully outcompete the native species.So, the invasive alien species form one of the major threats after fragmentation, against the survival and growth of several native species (CBD, 2005).They have potential to damage or eliminate already small populations of rare taxa (Menges, 1991a).
The anthropogenic disturbance is an important factor in determining the vegetation pattern across different habitat and often more dominating as compared to other (Angassa & Oba 2010;Dargie & Demerdash 1991;Gunaga et al. 2013, Körner, 1995).The human induced changes have caused, major changes in vegetation composition of Terai and a vast area of grasslands is converted into agro-ecosystem.Currently anthropogenic disturbance has affected the structural variability of these grasslands at local scale.

Conclusion
The present study explores the species composition and diversity pattern of Terai grassland vegetation with special reference to north-eastern Uttar Pradesh.This analysis emphasizes that for conservation of species, their habitat priority should be given to the maintenance of large grassland patches.The common species often show intraspecific competition and rare species show interspecific competition.The strong competitive effects for common species and weak effects for rare species suggest that competition promotes, rather than inhibits diversity.The land use change and agricultural intensification have led to the loss of habitat heterogeneity.Our analysis showed that generally, the moderate level of disturbance specially, grazing, trampling and cutting play a major role in maintaining optimum species diversity of the landscape but habitat condition such as marginal differences in soil, light condition and moisture regime also affect the level of species richness and overall biodiversity.
During the first phase of restoration it is important to focus on improvement of immigration.Connectivity between different sites is better as the main management tool because animals transport propagules and thus increase the speed of formation of field layer.

Figure 1 .
Figure 1.The red rectangle within the inset map shows the position of grassy landscape (A) and the sampled location (B) within different districts (study sites) are numbered from 1to 31.

1 .
The combination of major factors determining habitat condition at different study sites, Light: o = open, ps = Partial shade; Soil moisture: Mh = High moisture, Ml = Low moisture, Ma = Average moisture;Textural type: S = Sandy, G = Gravel, C = Clay, L = loam and Disturbance: Dh = High disturbance, Dm = Moderate disturbance, Dl = Low disturbance.

Table 2a .
Rare species categories based on geographic distribution, habitat specificity, and local population size

According Rabinowitz's (1981) Our observation (Srivastava et al., 2015)
Large geographic range, narrow habitat specificity, large population size Rare Rare 4. Large geographic range, narrow habitat specificity, small population size Rare Rare 5. Small geographic range, wide habitat specificity, large population size Rare Rare 6.Small geographic range, wide habitat specificity, small population size Rare Rare 7. Small geographic range, narrow habitat specificity, large population size Rare Rare 8. Small geographic range, narrow habitat specificity, small population size Rare Rare repens, Setaria glauca, Sida cordata and Sida rhombifolia were absent from quadrats sampled especially at sites in Sant Kabir Nagar.

Table 3 .
Details of study sites and sampling of grassy landscape.

Table 4 .
Richness of various plant taxa viz., family, genera and species across grassy landscape.
maximum in Gonda and Bahraich.Conversely it was low for both Sant Kabir Nagar and Kushinagar.The sum of abundance was higher only for Bahraich, Gorakhpur and Maharajganj as compared to other sites.Increase in abundance of species is a quantitative indication of its patchiness or hyper-dispersion.Several common herbaceous species showed hyper-dispersion across the landscape as evident from the

Table 6 .
Species of different habit forms and dominant plant families (represented by >5 species) at different study sites.

Table 7 .
The number of plant genera, species and their individuals under different families as recorded within the sampled area.

Table 8 .
Species richness and Family Importance Value (FIV) of constituent families across grassy landscape of Terai.

Table 9 .
Species showing most frequent and most prolific distribution across grassy landscape.

Table 10 .
The distribution and population status of rare species with their habit and posture categories.Abbreviation: Light: O= open, PS= Partial shade; Soil moisture: HM= High Moisture, LM= Low moisture, AS= Average moisture and Textural types: SS=Sandy soil, GS= Gravel soil, CL= Clayey soil, L= loam.

Table 11 .
The number of rare species (decreasing order) and their % share against total species within different families.