Species composition
The study recorded 55 species of weedy nature in four locations of the upland d. s. rice in the chapori area of southern Majuli during 2021-2022. This enumeration included 39 broad leaved species belonging to 25 families, 3 sedges belonging to Cyperaceae family and 13 grasses belonging to Poaceae (12) and Typhaceae (1). It was interesting to note that, out of all these species, only few namely,
Centella asiatica,
Cynodon dactylon,
Echinochloa colona,
Eleusine indica,
Fimbristylis littoralis,
Hydrocotyle sibthorpioides,
Imperata cylindrica and
Paspalum notatum had common occurrence in all the four locations.
The area studied represented typical ecotone zone between the terrestrial agro-ecological vegetation of river bank areas and the hydrological vegetation of the river Brahmaputra, where the presence of
Ficus heterophylla,
Imperata cylindrica,
Paspalum notatum,
Tamarix dioica and
Typha latifolia represented the peculiarity of the ecotone vegetation of the river ecosystem. Out of that, presence of many other species like
Galeopsis tetrahit,
Ipomoea carnea,
Ricinus communis,
etc. was found to be quite unusual, as compared to the weed flora in d. s. rice of nearby villages with typical upland situations. That might be due to the migration of seeds to the chapori areas through various means including run-off and flood water. In such a situation, weed growth was quite enormous and the weed dry matter recorded during 20 to 65 days after sowing of d. s. rice varied from 105±07 g/m
2 (Location-3) to 123±0 g/m
2 (Location-1) with an average value of 117±12 g/m
2 (Table 3).
Weed spectrum
The data presented in Table 2 revealed that
Marchantia polymorpha, the colony forming Bryophyte, was the most dominant species in location-1 and location-3. However, the highest importance value index (IVI) in both the locations was due to higher values of basal area coverage, while the frequency (distribution) and density (population) were much lower and therefore, the species might be considered as falsely dominant one. Similarly,
Tamarix dioica in location-4 was also a falsely dominant weed. In contrary,
Cynodon dactylon-
Eleusine indica complex was the most populated, widely distributed and highly dominant group in all the four locations as reflected by quite higher values of relative frequency, relative density and IVI respectively as shown in Table 2.
Imperata cylindrica was found to be rather dominant than
Eleusine indica in location-1, 2 and 3, while,
Paspalum distichum and
Echinochloa colona in location-4.
Grasses are always found to be dominant in many crops including upland d. s. rice in high rainfall, subtropical environment. That was reflected in the location 4 which was separated by river canal where the cumulative IVI of grasses was 30.4 percent higher than that of non-grass species. On the other hand, the cumulative IVI of grass species were of 109, 49 and 75 per cent higher than the grass species in the locations-1, 2 and 3, respectively, which had land connectivity with the main land. That might be due to easy migration of weeds seed of board leaved annual and sedges in those locations (‘edges effect’).
The area under the study represented only 3 species of sedges namely
Cyperus rotundus,
Eleocharis congesta and Fimbristylis littoralis complex. In the river-bed atmosphere, the growing up weed vegetation under the heavy competition in the rice ecosystem is represented by the more or less lower values of relative frequency of the weeds in the locations-1, 2 and 3, while in the location-4, the values of relative densities which is greatly varied from minimum of 1.32 per cent in
Digitaria setigera, 6.58 per cent in
Echinocloa colona and to maximum of 7.89 per cent in
Paspalum distichum to 11.84 per cent in
Cynodon dactylon reflecting the early maturity of weed vegetation.
In ecosystem functioning in addition to environmental drivers, abundance of species (
i.
e. relative number of individuals) and frequency (
i.
e. occurrences across landscapes) always play significant role
(Poorter, et al., 2017; Sandoya et al., 2021). Species density or abundance depends on the long-term population dynamics,
i.
e. the balance between mortality and recruitment
(Lines et al., 2010). Hall et al. (1992) opined that numerically abundant species can be assumed to have a higher capacity of mobilizing resources and assimilating energy. Relatively higher magnitude of abundance was shown by
Cynodon dactylon - Digitaria setigera - Eleusine indica complex, often coupled with
Imperata cylindrica in the study area in direct seeded rice ecosystem along the bars of the river Brahmaputra (Fig 1). The number of species with abundance value of more than 4 was found to be higher in location-4 with altogether 18 species, out of which 12 species were perennial in habit. In contrary, high magnitude of species abundance (above 4) was recorded for 8 species in location 3 and 6 species each in location 1 and 2.
Species diversity
Amongst species diversity measures, the Shannon-Weiner Index or Simpson’s Diversity Index was the mostly used parameters
(Yeom and Kim, 2011), where the number of individuals of the species is given equal weightage to the taxonomic variation in the rank of species.
Giavelli et al., (1986) observed that all the indices were independent of the type of community considered, while for both absolute and comparative estimates, the Simpon’s Index was the most reliable. The data computed from the study of both the years revealed the highest Shannon-Weinner Diversity Index (‘H’) in location 3 (3.04) followed by location-1 (2.89), location 2 (2.86) and location 4 (2.36). Almost similar trend was depicted by Simpson Diversity Indices, which were 0.95, 0.93 and 0.86 in locations 3, 1, 2 and 4, respectively.
Species Richness and Species Evenness are two measurements that help to estimate the biological diversity of a particular ecosystem. Species richness describes the number of species in a particular geographical area and in contrast, species evenness describes how evenly the species are distributed in a particular area
(Lakna, 2022). Pielou’s Evenness Index (‘I’), which has used the number derived from Shannon-Weiner’s Diversity Index, was as high as 0.90 in location 3 and the least value (0.66) was recorded in location 4. In contrary, Margalef’s Species Richness Index was the highest in location 4, followed by location 1, 2 and then 3 (Table 3).
Data in Table 2 has revealed that the taxonomic diversity was also the highest in location 4, with as many as 38 weed species. That might be due to the migration of planting materials including seeds of the weeds through flood water, animals and human beings, as well as contaminants of crop seeds. As the site was separated from the shore area by river canal that annually inundated by rising river water at least during peak monsoon period, there existed least possibility of having a rich soil seed bank, compared to the other three sites which had land connectivity with shore areas.
Community dissimilarity
The result revealed that the Sorreson‘s Co-efficient was the highest (0.91) between the weed communities of locations 1 and 3. It was above 0.8 between location 1 and 2, as well as location 2 and 3, whereas, that was shared by 0.30 to 0.34 by the weed community of location 4 with other three locations (Table 4).
To measure the differences between plant communities, the taxonomic distances between them might not be enough, since simply sharing of any taxa (Species or Genus) without their population abundance or distribution pattern could not reflect the exquisite specificity of the site in certain space and time. Sorrenson’s dissimilarity Index or Coefficient of Communities, was one of the widely used parameters in depicting similarity or dissimilarity between communities, is based on species incidence (
i.
e. presence- absence) data, the number of species shared by two communities and number of species unique to each. The magnitude of dissimilarity between two communities is relatively of low significance, because of its focus on species incidence alone.
On the other hand,
Bray and Curtis (1957) dissimilarity Index between the communities was based on the compositional differences between two sides. It is bounded between ‘0’(extreme similar composition) and ‘1’ (total dissimilar composition), where the number of individuals counted at both communities is taken into account. Bray- Curtis dissimilarity Index amongst the weed communities of upland direct seeded rice of Brahmaputra chapories of Majuli District was as high as 0.92 between location 1 and location 2 and as minimum as 0.76 between locations 1 and 4 and locations 2 and 4 (Table 5).
Similarly, Simpson’s similarity Index is also a commonly used measure of the degree, which gives greater weightage to species common to the communities, than to those found in only one location.
Sreejit (2014) modified Simpson’s Similarity Index by adding equal weightage to the Importance Value Indices with the species incidence data. The results revealed the highest magnitude of similarity according to Simpson’s Modified Similarity Index between locations 1 and 3 (0.93) followed by locations 2 and 3 (0.87) and locations 1 and 2 (0.81). Weed community of location 4 shared 0.3, 0.35 and 0.38 similarity indices with locations 1, 3 and 2 respectively.
Isolation from mainland might be one of the major factors for developing uniqueness in composition of weed community in location 4. In addition, the presence of
Ficus heterophylla,
Phyla nodiflora,
Tamarix dioica and
Typha latifolia in this location with as much as 53.94, 33.06, 8.02 and 2.44 IVI have reflected the typical ecotone characteristic of this location along the bars of the river Brahmaputra, than the other locations. It is also seen that quite a good number of weeds present in the chapori areas were stubble borne which might be carried easily by river water during flood period. The most prominent stubble borne weeds dispersed by river water were
Acmella ciliata,
Ageratum houstorianum,
Alternanthera philoxeroides,
Chromolaena odorata,
Ipomea carnea,
Mikania micrantha and grasses belonging to the genera
Imperata,
Paspalum,
Typha,
etc.
All these findings could give an effective clue for adoption of management strategy of weeds as the developing or lag-phase is always been considered as the best phase for weed management
(Deka and Barua, 2023).
