Article

Introduction

Riparian forests form transitional zones between terrestrial and aquatic ecosystems and are among the most productive and species-rich habitats, providing essential ecological functions such as nutrient retention, pollutant filtration, and bank stabilization [1,2,3]. Within these ecosystems, the herbaceous layer constitutes the largest proportion of plant diversity and responds rapidly to environmental change, making it a reliable indicator of ecosystem condition and disturbance [4,5,6].

Herbaceous vegetation contributes substantially to primary productivity, nutrient cycling, soil stabilization, and energy flow through the production and turnover of biomass [7,8,9,10]. Owing to their rapid regeneration and short life cycles, herb species respond more quickly to environmental disturbances than woody vegetation, making herbaceous biomass an effective measure of vegetation dynamics and ecosystem recovery [11,12,13]. Variations in above-ground and below-ground biomass also reflect changes in soil moisture, nutrient availability, and seasonal resource allocation [5,14]. Within Nagaland, the importance of below-ground edaphic conditions has also been demonstrated through studies of rhizospheric soils, where soil physicochemical characteristics and fungal communities were examined in relation to soil health [15].

Riparian habitats are increasingly affected by land-use change, grazing, and other anthropogenic disturbances, which alter species composition and biomass production [6]. Disturbance often favours fast-growing, opportunistic species that dominate community biomass and influence ecosystem functions, including nutrient cycling and phytoremediation potential [12,16].

The present investigation originated from field observations along the Tsurang River under Changki village, Mokokchung district, where riparian stretches occur under markedly contrasting conditions. One stretch is exposed to grazing, periodic flooding, and proximity to drainages originating from coal-mining areas, whereas another retains relatively natural forest vegetation. This contrast provided the basis for proposing a comparative assessment of herbaceous biomass to determine whether disturbance is reflected in above-ground and below-ground biomass production.

Although several studies have examined herbaceous biomass dynamics in riparian ecosystems [7,8,12,13,14,17], information on biomass variation under contrasting disturbance conditions remains limited. Therefore, this study compares above-ground and below-ground herbaceous biomass between an anthropogenically impacted riparian site and a relatively undisturbed reference site along the Tsurang River, Nagaland, North-East India.

Materials and methods

Study area

To compare the influence of site conditions on changes in monthly herb biomass production, two sites were selected along Tsurang river under Changki village in Mokokchung district of Nagaland. The first site selected was labelled as “Impacted Site” (IS): Latitude: 26⁰48′47.8″N and Longitude 94⁰34′24.8″E, where the riparian area was exposed to anthropogenic disturbances such as the frequency of animal grazing, periodic flooding and the site’s proximity to drainages sourced from coal mine sites. A second site labelled as “Reference Site” (RS) of natural forest:  Latitude 26⁰47′82.4″N and Longitude 94⁰33′16.1″E, was selected to compare with the IS biomass and to estimate the differences in above-ground (AG) and below-ground (BG) biomass production. Four seasons were demarcated for the study, viz Spring (Feb, 2023 to Apr, 2023), Summer (May, 2023 to Jul, 2023), Autumn (Aug, 2023 to Oct, 2023) and Winter (Nov, 2023 to Jan, 2024).

Sampling design

A 50m x 50m area was demarcated on both sites IS and RS, stretching from the edge of the riparian vegetation. For harvesting the AG and BG herbaceous biomass, fifteen 1m x 1m quadrats were sampled at random for 13 months [7] on both sites from January 2023 to January 2024 to complete one growing cycle. Monthly collection was done to identify the peak of herb biomass.

Biomass collection and dry weight estimation

Biomass was harvested on site, segregated into AG and BG, and brought back to the laboratory, where samples were thoroughly washed to remove all soil debris. AG and BG biomass samples were then oven-dried until constant weight was achieved. All sample dry weights were recorded using a weighing balance and calculated as “g/m²”.

Statistical Analysis

To determine whether biomass differed significantly among seasons, a one-way Analysis of Variance (ANOVA) was performed. Statistical significance was assessed at p < 0.05. All statistical analyses were performed using IBM SPSS Statistics.

Results and Discussion

Herbaceous Species Composition

A total of 59 herbaceous species belonging to several taxonomic families were recorded from the two riparian sites during the study period (Table 1). The Reference Site (RS) supported a richer herbaceous community, with a greater abundance of moisture-loving ferns and broad-leaved herbs, whereas the Impacted Site (IS) was dominated by grasses and disturbance-tolerant species, reflecting the influence of grazing and other anthropogenic disturbances. Members of the Poaceae and Cyperaceae were well represented at both sites; however, species such as Paspalum conjugatum, Chromolaena odorata, Ageratum conyzoides, and Mimosa pudica predominated at the IS, while Diplazium esculentum, Persicaria chinensis, Colocasia esculenta, and Boehmeria macrophylla were more common at the RS. These compositional differences indicate that disturbance favours opportunistic, fast-growing species, whereas relatively undisturbed conditions support moisture-dependent herbs that contribute to greater ecosystem stability and biomass production [5,17,18].

IS biomass vs RS biomass

Variations in AG biomass

The monthly pattern of above-ground (AG) biomass differed between the Impacted Site (IS) and the Reference Site (RS) (Figure 1). Overall, the RS produced greater AG biomass than the IS throughout most of the study period. AG biomass at the IS ranged from 1198.56 g m⁻² in February to 1792.30 g m⁻² in November, while the RS showed distinct peaks in May (2036 g m⁻²) and December (2311.00 g m⁻²). Total AG biomass in the RS was found to be greater than at IS; however, during the months from March-April and July-September, IS AG biomass was slightly higher, as during these months’ opportunistic species from Poaceae were found predominating the site. The higher biomass at the RS is likely associated with more favourable soil moisture and nutrient conditions supporting a diverse herbaceous community [8,17].

Variations in BG biomass

The RS consistently recorded greater BG biomass than the IS throughout the study period (Figure 2). BG biomass at the IS ranged from 1033.92 g m⁻² to 1811.30 g m⁻², whereas the RS attained peak values of 2078 g m⁻² in May and 2298.50 g m⁻² in December. The lower BG biomass at the IS can be attributed to the dominance of shallow-rooted grasses, whose roots rarely exceeded 15 cm in depth but formed dense surface mats that tightly bound soil particles. In contrast, the RS was dominated by moisture-loving species such as Colocasia esculenta, Diplazium esculentum, Persicaria chinensis, Boehmeria macrophylla, and Thelypteris dentata , which possess more extensive underground systems and are indicative of nutrient-rich, moist soils. These favourable site conditions contributed to greater BG biomass throughout the year. Both sites exhibited relatively higher BG biomass during winter, suggesting increased allocation of assimilates to underground organs for nutrient storage and subsequent regeneration during the following growing season [11].

Statistical Comparison of Biomass

Mann-Whitney Test was performed to examine the difference in AG and BG between the two sites and to define the magnitude of biomass difference between the two sites. Mann-Whitney test did not find significance between the AG between IS (mean rank = 11.31) and RS (mean rank = 15.69), p = 0.153. Significant difference was found at BG biomass, p = 0.026. The BG biomass at RS was significantly higher (mean rank = 16.85) than IS BG biomass (mean rank = 10.15).

One-way ANOVA was performed to find seasonal significance between AG and BG biomass.  AG biomass did not show seasonal significance at IS, (F (3, 9) = 1.329, p = 0.325). The same was observed for AG at RS (F (3, 9) = 1.468, p = 0.288). In contrast, BG biomass varied significantly among seasons at both the IS (F (3, 9) = 4.627, p = 0.032). Spring – Summer season showed significant seasonal difference (p = 0.030) in BG biomass generation, p = 0.030. The same was observed at RS (F (3, 9) = 4.004, p = 0.043), with a distinct Spring-Summer BG biomass difference (p = 0.046). These results suggest that seasonal changes had a greater influence on below-ground biomass allocation than on above-ground biomass, indicating that root growth and nutrient storage were more responsive to seasonal environmental conditions than shoot production.

Biomass and Rainfall Pattern

Monthly herbaceous biomass generally followed the seasonal rainfall pattern, with biomass increasing monsoon periods and reaching its peak value during the post-monsoon, particularly at the RS. This delayed response suggests that biomass accumulation depended not only on rainfall but also on sustained soil moisture and nutrient availability [7,8,14]. The IS also followed this trend but showed comparatively smaller seasonal fluctuations, likely due to anthropogenic disturbance and the dominance of disturbance-tolerant grasses. Similar rainfall-driven patterns of herbaceous productivity have been reported in other riparian ecosystems [12,17,19].

Conclusion

Herbaceous biomass varied between disturbed and undisturbed riparian conditions, demonstrating the strong influence of anthropogenic activities on vegetation productivity and biomass allocation. The RS consistently supported higher AG and BG biomass, reflecting favourable soil moisture and nutrient conditions that promoted the growth of diverse herbaceous communities and extensive root systems. In contrast, the IS was dominated by disturbance-tolerant grasses and opportunistic species that maintained relatively high AG biomass but lower BG investment. Although AG biomass did not differ significantly among seasons, BG biomass exhibited significant seasonal variation at both sites, highlighting the importance of seasonal resource allocation to underground organs. The observed increase in BG biomass during winter suggests enhanced nutrient storage and preparation for subsequent growth.

The monthly assessment of herbaceous biomass provided greater temporal resolution than conventional seasonal studies, enabling the identification of peak biomass production under contrasting riparian conditions. Overall, herbaceous biomass proved to be a sensitive indicator of riparian ecosystem condition and disturbance. These findings provide baseline ecological information for riparian vegetation monitoring and support the use of herbaceous communities in restoration, conservation, and phytoremediation programmes in North-East India.

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