Reconstructing mid–late Holocene ecosystem dynamics in the Upper Brahmaputra Valley of Assam,Northeast India,using non-pollen palynomorphs (NPPs)

Abstract

This study presents the first detailed investigation of non-pollen palynomorphs (NPPs) from a 1.5 m-deep sediment profile of an endangered wetland in the Upper Brahmaputra Valley, Assam. A total of 34 NPP types were identified, dominated by fungal spores with notable amount of algal, zoological and biostructured phytoclast remains. The assemblages delineate five distinct zones, reflecting past climatic shifts, vegetation dynamics and faunal interactions. NPP analysis emerges as a valuable proxy for reconstructing ecological and anthropogenic changes beyond the resolution of pollen data. The first NPP zone, spanning 4,040–2,260 cal yrs, BP indicates a relatively warm and humid climate that supported woody vegetation, as evidenced by marker fungal spores such as Gelasinospora, Cookeina, Geastrum and Tetraploa, as well as the presence of degraded brown phytoclasts. Subsequently, from 2,260 to 1,460 cal yrs BP, relatively warmer conditions and a rich diversity of NPPs, was recorded, indicating favourable ecological conditions. In contrast, Zone 3 (1,460–1,100 cal yrs BP) reflects a shift toward colder and drier conditions, marked by a notable decline in fungal spores and algal remains. However, taxa such as Diplodia, Alternaria and Telitia, known for their resilience in drier climates, showed relatively high abundance during this phase. During 1,100–500 cal yrs BP, the retrieval of warmer, moderately humid climatic conditions was observed, likely corresponding to the Medieval Climatic Anomaly, and saw a resurgence of vegetation and wood-associated fungi such as Nigrospora, Curvularia, Valsaria and Glomus, indicating increased soil erosion. Over the past 500 cal yrs BP onwards, warmer but less humid conditions have persisted, with a rise in coprophilous spores, such as Sordaria, Cercophora, Arnium and Delitschia, suggesting intensified human activity, grazing and nearby agriculture, as evidenced by the abundance of Helminthosporium. The overall study highlights the growing utility of NPPs as sensitive indicators of past environments, vegetation dynamics, grazing pressure and anthropogenic influence.

INTRODUCTION

Non-pollen palynomorphs (NPPs) are a diverse group of microscopic organic remains found in quaternary sediments that are not derived from pollen grains (Cook et al., 2011; Pandey et al., 2023). These include fungal spores, algal remains, zoospores, testate amoebae, other biological structures such as plant organic remains and fragments of insect cuticles, and microspore mother cells of vascular cryptogams (trilete and monolete). These are primarily studied through microremains found in Quaternary sedimentary and archaeological archives (Leal et al., 2021; Pandey et al., 2023; Zimny, 2024). Among biotic proxy records, unlike pollen, which is primarily used to reconstruct past vegetation and climate, NPPs capture local environmental conditions, such as those of wetlands, floodplains or dry soils. They also provide insights into hydrological regimes and biotic interactions, including herbivory, dung deposition and microbial or fungal activity, which are often obscured in pollen records. Consequently, NPPs are now emerging as a strong biotic proxy and integrated into multi-proxy analyses, offering more profound insights into long-term ecological dynamics (Ejarque et al., 2011; Revelles & van Geel, 2016).

Over the past few decades, research on NPPs has grown significantly, driven by increasing recognition of their potential in complementing pollen data. NPPs offer unique insights into complex interactions between climate, vegetation and ecological responses (Barhoumi et al., 2024; Basumatary et al., 2020; Enevold et al., 2018; Pandey et al., 2023). Moreover, their analysis yields valuable information on depositional habitats (Montoya et al., 2010), local effective moisture conditions (van Geel, 2001) and anthropogenic influences, such as livestock grazing (Cugny et al., 2010; Ejarque et al., 2011). In many cases, due to uncertainties in taxonomic identification, NPP morphotypes are assigned alphanumeric codes and are still widely used in palaeoecological reconstructions (Miola, 2012). To enhance the consistency and utility of NPP data, an international nomenclature system was developed at the University of Amsterdam (Hooghiemstra & van Geel, 1998, and references therein). Nevertheless, significant gaps remain in our understanding of the taxonomy, ecology and distribution of many NPPs (Blackford et al., 2006).

Owing to their occurrence across a variety of depositional environments and associations with specific biological hosts or substrates, NPPs can demonstrate ecological processes, hydrological regimes, herbivore activity, decomposition pathways and anthropogenic landscape modifications across a broad range (Basumatary et al., 2020; Ejarque et al., 2011; Pandey et al., 2023). Globally, numerous studies have investigated NPPs in both surface deposits and sedimentary layers, demonstrating their effectiveness in revealing Quaternary palaeoenvironment and human influences on the landscape. The presence and abundance of NPPs can shed light on key aspects of ecosystem functioning. Each category of NPP plays a distinct ecological role, whether as decomposers, symbionts, parasites or indicators of specific environmental conditions, making them powerful tools for reconstructing past environments and understanding ecological change. In India and beyond, significant research has focused on various types of NPPs. For example, coprophilous fungal spores are well-established indicators of herbivore presence and grazing intensity, often linked to pastoral practices (Ghosh et al., 2017; Quamar et al., 2024). In addition to that, extensive research on dung samples from various regions of India has also been executed to provide valuable insights into the dietary habits of herbivorous animals and to contribute significantly to the reconstruction of past herbivory patterns and palaeoecological conditions (Basumatary et al., 2017, 2020, 2021; Tripathi et al., 2019). On the other hand, saprophytic and parasitic NPPs depend on the availability of specific host plants or organic substrates, and their abundance may signal the prevalence of particular plant communities or decomposition levels (Barthelmes et al., 2012; Quamar, 2014). Furthermore, some NPPs are associated with freshwater systems, such as aquatic algae and testate amoebae, serving as sensitive indicators of hydrological regimes, water availability and climatic conditions favourable for aquatic ecosystems (Basumatary et al., 2018; Danesh et al., 2013, 2024; Mandaokar et al., 2008; Stivrins et al., 2022). Conversely, specific cosmopolitan taxa exhibit broad ecological tolerances, making them markers for detecting broader climatic shifts and environmental stability (Menozzi et al., 2010; Pandey et al., 2023; Quamar et al., 2024). While NPP studies have been conducted globally, research in Northeast India remains scarce. In the few cases where such studies exist, they are largely limited to modern surface sediments (Basumatary et al., 2017, 2020), with no detailed investigation of sedimentary profiles. Although some NPP analyses from sediment cores have been carried out in other parts of India, many of them lack a well-defined sequential chronology. This study represents the first attempt to generate an NPP data set from a sediment core spanning the mid to late Holocene in the eastern region of India.

Recognising this diverse potential and the need to fill this research gap in the northeast region of India, we applied NPP analysis to the Sakali wetland on Majuli Island, the world’s largest river island in Assam, northeast India. Located in the island’s central part, the wetland is highly vulnerable due to continuous exposure to the Brahmaputra River’s discharge, leading to recurrent flooding, severe soil erosion, widespread deforestation and increasing anthropogenic encroachment, all of which threaten its ecological stability. To examine these dynamics, we analysed a 1.5 m sediment core from the wetland, identifying 34 distinct NPP types. Due to its geomorphological setting, this wetland shows poor pollen preservation, as recurrent flooding disrupts the vegetation cover and sedimentation processes. These disturbances reflect broader ecological instability across Majuli Island. To address this limitation, we employed NPPs analysis as an auxiliary tool, providing valuable insights into past habitation patterns, vegetation succession, climatic variability, erosional dynamics and human impacts during the mid-to-late Holocene. A previous study by Basumatary et al. (2018) used pollen and NPP data from surface samples collected from two wetlands, Sakali and Duboi, located at different distances from the Brahmaputra River on Majuli Island, to compare their vegetation and geohydrological conditions. Their analysis showed that Sakali was heavily influenced by flooding, as indicated by the presence of extra-regional and allochthonous taxa. In contrast, Duboi exhibited ecological stability, dominated by local arboreal taxa and experiencing minimal external fluvial disturbance. These temporal changes in the composition, abundance or complete disappearance of specific NPPs often reflect environmental stressors, ecological disturbances or shifts in land-use patterns. Consequently, the presence, absence and relative abundance of NPPs in sedimentary archives are crucial for reconstructing detailed palaeoecological and palaeoclimatic narratives and can serve as an alternative biotic proxy to pollen (Zimny et al., 2025). When integrated with other proxies, such as pollen, palynofacies and charcoal records, NPPs greatly enhance our understanding of past environmental dynamics, including vegetation succession, climatic variability, domestication processes and anthropogenic influences on ecosystems (Cugny et al., 2010; Enevold et al., 2018; Romero et al., 2021).

As a geomorphologically active and environmentally sensitive landscape, Majuli provides an ideal natural archive for tracing Holocene environmental changes. This study aims to decode palaeoecological changes during the mid to late Holocene from a sedimentary profile retrieved from the Sakali wetland on Majuli Island. Through the analysis of 34 identified NPP types, with a predominance of fungal spores, we reconstruct past climatic regimes, vegetation dynamics, faunal presence and anthropogenic influences over the last ~4,000 calibrated years. By integrating key indicator taxa, such as Cookeina, Entophlyctis lobata and coprophilous fungi including Sordaria and Podospora, we highlight temporal shifts in moisture conditions, forest cover, grazing intensity and land use. The findings underscore the growing relevance of NPPs as auxiliary tools for interpreting the intricate ecological narratives preserved in wetland sediments of the Brahmaputra Valley, Upper Assam, Northeast India.

The primary objectives of this study are threefold. First, to generate a comprehensive NPPs data set spanning the mid- to late Holocene from Majuli Island, northeast India, thereby contributing to the limited palynological records available for this region. Second, to reconstruct past vegetation dynamics and landscape changes in response to climatic fluctuations, as inferred from variations in the NPP assemblage. Finally, the study aims to identify signatures of major global climatic events, such as the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA), within the local NPP record and evaluate their correspondence with regional and global climate shifts.

STUDY SITE

Between the towns of Dibrugarh and Lakhimpur in Assam, the Brahmaputra River bifurcates into two channels: the northern Kherkutia channel and the southern Brahmaputra channel (Figures 1 and 2). The two channels converge again approximately 100 km downstream, enclosing Majuli Island, recognised as the world’s largest river island by the Guinness Book of World Records. Majuli is located in northeast India and represents a unique ecological zone shaped by a dynamic interplay of monsoonal climate, fluvial processes and cultural evolution (Mahanta et al., 2024). The island was formed during a river capture event in the sixteenth century, when the Brahmaputra shifted its course from north to south. The island is not a sandbar sensu stricto, but is a part of the mainland surrounded by rivers, which have come into existence due to river reorganisation processes (Sharma & Phukan, 2004). The island is widely regarded as the cultural capital of Assam and is currently included in the tentative list of UNESCO World Heritage Sites. It holds immense historical and spiritual significance, as the revered saint and reformer Srimanta Sankardeva, a pioneer of the medieval Neo-Vaishnavite movement, established several hermitages and monasteries, known as Satras, across Majuli. There are approximately 65 such Satras on the island, though ongoing riverine erosion has forced many to be relocated to other parts of Assam (Lahiri & Sinha, 2014). This island also comprises several small wetlands, among which Sakali (located at 26°51′36″ N, 94°10′12″ E) is notable as an endangered site, having suffered severe soil erosion due to annual flooding over an extended period. In general, upper Assam tends to receive around 2,500–3,000 mm of rainfall per year, with variation by district and topography. Most rainfall comes with the south-west monsoon, roughly June to September. Pre-monsoon (April–May) also sees showers. Post monsoon (October–November) brings some rain, but much less. Winter is mainly dry. Its importance lies in what it reveals about Majuli’s dynamic geological and ecological history, particularly its recurring flooding. Like other wetlands on Majuli, Sakali likely holds cultural significance and provides recreation opportunities, contributing to the local economy and lifestyle. This region is known for its extensive network of water bodies that support diverse flora and fauna, including over 250 bird species. In recent years, extensive deforestation caused by human activities such as urbanisation, road construction and the impact of recurring floods has made this island increasingly vulnerable. Its land area is steadily shrinking, along with the surrounding vegetation. This ongoing degradation poses a serious concern, highlighting the urgent need to protect and preserve the island’s natural habitat for our present and future survival.

Figure 1.

DEM map of Assam state showing the study site Majuli Island, upper Brahmaputra Valley of Assam, Northeast India.

Figure 2.

Graphical representation of study area, i.e., Sakali wetland on Majuli Island, Assam.

Figure 3.

Frequency diagram of non-pollen palynomorph (NPP) assemblages from 31 sediment samples collected from the Sakali Wetland, Majuli Island, Assam.

MATERIALS AND METHODS

NPP extraction and chronology of the sediment samples

Sediment samples for NPP analysis were collected at regular intervals from a 150-cm-deep sedimentary profile obtained from a wetland site on Majuli Island, Assam. A standard volume of each sample was processed using conventional palynological techniques optimised for both pollen and NPPs recovery (Faegri & Iversen, 1989; Moore et al., 1991). Each sediment sample was first treated with 10% hydrochloric acid (HCl) to remove carbonates, followed by 10% potassium hydroxide (KOH) to eliminate humic acids and deflocculate the sediments. The residue was then thoroughly washed with distilled water after each treatment step. To dissolve siliceous material, the samples were treated with 40% hydrofluoric acid (HF) and left to react for approximately 24 hours under a fume hood. This step was followed by repeated rinsing with distilled water to remove any remaining acid. The cleaned residue was subjected to acetolysis with a 9:1 mixture of acetic anhydride and concentrated sulfuric acid to remove cellulose and other organic matter. The samples were heated in a water bath at 70°C–80°C for 5–10 minutes, then rapidly cooled and washed multiple times with glacial acetic acid followed by distilled water. To concentrate the palynomorphs and remove large debris, the samples were passed through a 10 µm mesh sieve using gentle pressure. The resulting suspension containing pollen, spores and NPPs was centrifuged, and the supernatant was discarded. The final residue was mounted on microscope slides using polyvinyl and DPX as a mounting medium for microscopic examination. A minimum of 200–400 NNP and spore grains were counted per slide under a light microscope at 40× magnification (Figures 4, 5, 6 and 7). NPPs were identified alongside pollen grains and counted separately. NPP and other spores were determined using several papers by van Geel (2001), Miao et al. (2015), Leal et al. (2021), Pandey et al. (2023) and published atlases. NPPs were categorised based on morphological characteristics and existing NPP reference literature (van Geel, 1978; van Geel et al., 2003). This extraction protocol ensures the preservation of both pollen and a wide variety of NPPs, enabling integrated palaeoenvironmental interpretation at each sedimentary level.

Figure 4.

Light microscopic photographs illustrating diverse morphotypes of non-pollen palynomorphs (NPPs) recovered from sediment samples of the Sakali Wetland. 1. Coniochaeta, 2. Alternaria, 3. Diporotheca, 4 & 5. Ascospore of Cookeina, 6, 7 & 29. Arnium Type, 8. Entophlyctis lobata, 9. Ascodesmis, 10. Nigrospora, 11 & 12. Geastrum, 13. Telitia, 14. Tetraploa, 15. Sordaria, 16. Delitschia, 17. Diplodia, 18. Gelasinospora, 19. Podospora, 20. Byssothecium, 21 & 22. Valsaria, 23. Unidentified, 24 & 25. Glomus, 26. Unidentified, 27. Plant cuticle, 28. Zopfiella lundqvistii, 30. Curvularia.

Figure 5.

1. Botryococcus, 2, 3. Pseudoschizaea, 4. Thecamoebian, 5. Zygnema, 6. Spirogyra, 7. Biostructured Phytoclast, 8. plant remain, 9. Neorhabdocoela, 10. Monolete, 11 & 12. Trilete.

Figure 6.

PCA plots showing the distribution of NPPs across the 31 sediment samples based on their abundance. (SW 1-SW31 are the sediment samples retrieved from the 1.5 m deep sedimentary profile).

Figure 7.

Note: CF: Coprophilous Fungi, NCF: Non-coprophilous Fungi, ZM: Zoomorphs, AR: Algal Remains, BP: Biostructured Phytoclast, PTERIDO: Pteridophytes.

Five bulk sediment samples rich in carbon from the Sakali sedimentary sequence were dated using ¹⁴C dating at the BSIP laboratory. Radiocarbon ages ranged from 3,740±30 yrs BP at 140 cm to 430±30 yrs BP at 25 cm, corresponding to calibrated ages of 4,038, 2,255, 1,460, 1,098 and 493 cal yrs BP, respectively. A Bayesian age-depth model was constructed using the ‘Bacon’ package in R (Blaauw & Christen, 2011) with the IntCal20 calibration curve (Reimer et al., 2020), bracketing the sequence from ~4,100 cal yrs BP to the present.

Statistical methods

Principal component analysis (PCA)

The statistical examination of NPP frequencies obtained from a 1.5m deep sedimentary section collected from the Sakali wetland on Majuli Island has been analysed using PCA with the Canoco Model 5 software (Smilauer & Leps, 2014) (Figure 6). The p value of ≤.05 was used to determine the significance of the results. The value was imported into CorelDRAW-12 for minor data-label editing.

Box-Whiskers plot

To compare and visualise the main NPP groups, such as coprophilous fungi, non-coprophilous fungi, zoomorphs, algal remains, biostructured phytoclast and pteridophytes, 31 sediment profile samples were analysed palynologically and represented through six box-plot graphs, in 5 different zones, generated with PAST 4.03 software (Ø. Hammer, 2001) (Figure 7). The NPP data set’s presentation was standardised using this statistical technique, which was based on a five-number summary (Tripathi et al., 2023). The outliers are also represented in the plot by a black dot (•), which indicates the values that are beyond the minimum and maximum limits. The interquartile range defines the wealth of taxa across different substrates, along with their maximum and minimum limits and outliers. The box-plot graphs show the variation in abundance and dearth of pollen groups separately across all the studied regions.

NPP results of the Sedimentary core

For NPPs’ assemblage in the sedimentary section, a frequency diagram was plotted through the TILIA programme, and zones were defined using the CONISS function (Grimm, 1987, 1991; Figure 3).

Description of NPP assemblage in each zone of the sedimentary core

The NPP sequence generated from the Sakali wetland is categorised into five zones, based on fluctuating trends of various NPPs. The non-pollen zones are demarcated from bottom to top, namely SW I, SW II, SW III, SW IV and SW V, which are described below (Figures 3 and 7).

NPP Zone I (150–100 cm, 4,040–2,260 cal yrs BP)

Interestingly, in this zone, all the groups were present in a fair amount, among which non-coprophilous fungi were predominant (33.32%), followed by zoomorphs (21.65%), biostructured phytoclast (18.52%), pteridophytes (11.31%), algal remains (10.16%) and coprophilous fungi (5.04%). The non-coprophilous fungal taxa like Gelasinospora (7.89%) were present in the highest frequency in this zone, followed by Cookeina (3.75%), Tetraploa (2.00%) and Byssothecium (3.62%). The other taxa, like Diplodia, Curvularia, Geastrum, Telitia and Nigrospora, were recorded within 2%. Some other non-coprophilous taxa, like Glomus (1.93%) and Entophlyctis lobate (1.19%), were observed in trace values, whereas the taxa like Meliola, Alternaria and Helminthosporium were present in scarce amounts. After non-coprophilous fungal taxa, the zoomorphs and biostructured phytoclast were present in an adequate amount. The zoomorphs like, Neorhabdocoela (8.18%) was dominant, followed by flatworm residue (7.40%) and Thecamoebian (6.07%), whereas among biostructured phytoclasts, degraded brown organic matter (6.94%) was dominant, followed by opaque (5.96%) and structured phytoclast (5.62%). The pteridophytes, like monolete (7.67%) and trilete (3.64%), were also found in sufficient amounts. The algal remains and coprophilous fungal taxa were present in small amounts in this zone. Among algal taxa, Botryococcus (7.80%) was present in the highest frequency, followed by Spirogyra (1.24%) and Zygnema (1.11%). Whereas the coprophilous taxa like Sordaria (1.79%) and Cercophora (1.23%) were present in fair amounts, and the rest of the taxa, like Arnium type (0.58%), Delitschia (0.46%), Ascodesmis (0.24%), Coniochaeta (0.35%), Podospora (0.30%) and Sporomiella (0.08%) were found in scarce amounts.

NPP Zone II (100–80 cm, 2,260–1,460 cal yrs BP)

In this zone, the non-coprophilous fungi were predominant (43.60%), followed by pteridophytes (16.07%), coprophilous fungi (15.94%) and biostructured phytoclast (11.35%). The other two groups, like zoomorphs (5.91%) and algal remains (4.28%), were present in relatively lesser amounts compared to the previous zone. Among the non-coprophilous taxa like Diplodia (18.64%), Entophlyctis lobata (8.90%) and Telitia (6.13%) were present in the highest frequency in this zone, followed by Cookeina (2.83%) and Gelasinospora (2.01%). The other taxa, like Alternaria (1.41%) and Byssothecium (1.22%), were present in scarce amounts. After non-coprophilous fungal taxa, the pteridophytes, coprophilous fungi and biostructured phytoclast were present in an adequate amount. Among pteridophytes, monolete (11.91%) was dominant, followed by trilete (4.17%), whereas among coprophilous fungal taxa, Delitschia (3.80%) and Sordaria (3.44%) were dominant, followed by Ascodesmis (2.47%), Podospora and Cercophora (2.41%), while other taxa like Coniochaeta and Arnium type were present in scarce frequencies, less than 1%. The biostructure phytoclast was dominated by degraded brown organic matter (11.35%), followed by opaque phytoclast (2.84%). Among the zoomorphs, Thecamoebians (3.23%) were found in abundance, followed by Neorhabdocoela (2.68%), whereas Botryococcus (3.07%) was found in abundance, followed by Zygnema and Spirogyra in algal remains.

NPP Zone III (80–55 cm, 1,460–1,100 cal yrs BP)

In this zone, the non-coprophilous fungi were the most predominant (74.86%) as compared to the other zones, followed by coprophilous fungi (17.82%), biostructure phytoclast (3.46%) and Pteridophytes (3.26%). Among the other two groups, zoomorphs (0.60%) were present in very scarce amounts, whereas algal remains were absent in this zone. Among non-coprophilous fungi, Diplodia (21.62%) was present in rich frequency, followed by Telitia (14.95%), Nigrospora (7.27%) and Glomus (7.13%), whereas Alternaria (5.59%) and Microthyriaceae (3.46%) were recorded in a fair amount. The other forms, like Cookeina, Byssothecium and Meliola, were present between 1.67% and 1.07% while Gelasinospora, Tetraploa and Helminthosporium were present in very small amounts, ranging between 0.87% and 0.30%. Among coprophilous fungi, Sordaria is most dominant (7.07%), followed by Delitschia (4.58%) and Coniochaeta (3.26%). Sporomiella (1.00%) was recorded in less amount, whereas Podospora, Cercophora (0.77%) and Arnium type (0.37%) were recorded in very scarce frequencies. Among biostructured phytoclast degraded brown organic matter (2.05%) was found in abundance, followed by structured phytoclast (1.41%), whereas among the pteridophytes, monolete (3.26%) was found in abundance and trilete were absent. Among the zoomorphs, only Thecamoebians (0.60%) were found in scarce frequencies, and the other morphs were totally absent.

NPP Zone IV (55–25 cm, 1,100–500 cal yrs BP)

In this zone, the non-coprophilous fungi were dominant (60.51%), followed by coprophilous fungi (20.85%), zoomorphs (9.64%) and biostructure phytoclast (6.41%), while other groups like pteridophytes (1.90%) and algal remains (0.69%) were present in very scarce amounts. Among non-coprophilous fungi, Telitia (13.76%), Glomus (10.59%), Nigrospora (7.15%), Curvularia (6.49%), Geastrum (6.30%), Tetraploa (5.21%) and Diplodia (3.84%) were found in rich frequencies whereas Byssothecium (2.45%) and Entophlyctis lobata (2.28%) were found in a little lesser amount. The other taxa, like Cookeina (0.92%), Gelasinospora (0.62%), Microthyriaceae (0.37%), Meliola (0.29%), Alternaria (0.14%) and Helminthosporium (0.10%), were found in very scarce amounts. Among the zoomorphs, Thecamoebians (5.26%) were abundant, followed by flatworm residue (2.89%) and Neorhabdocoela (1.49%). Among biostructured phytoclasts, the opaque phytoclast (3.00%) was found in abundance, followed by the structured phytoclast (2.77%) and degraded brown organic matter (0.63%). Among the pteridophytes, trilete (1.69%) were abundant, followed by monolete (0.21%). Among the algal remains, Botryococcus (0.69%) was only evident in a scarce amount.

NPP Zone V (25–0 cm, 500 Onwards)

In this zone, the non-coprophilous fungi were predominant (40.32%), followed by biostructured phytoclast (17.37%), coprophilous fungi (17.23%), zoomorphs (11.82%), pteridophytes (8.79%) and algal remains (4.47%). Among the non-coprophilous taxa, Glomus (6.97%) was the most abundant, followed by Telitia (4.30%), Nigrospora (3.89%), Geastrum (3.72%) and Cookeina (3.39%). The other taxa, like Tetraploa (2.95%), Gelasinospora (2.85%), Curvularia (2.84%), Byssothecium (2.23%), Diplodia (2.16%) and Helminthosporium (2.13%) were present in fair amounts, whereas Meliola (1.56%) and Microthyriaceae (1.32%) were present in trace amounts. After non-coprophilous fungal taxa, the biostructured phytoclast and coprophilous fungal spores were found in significant frequencies. Among biostructured phytoclasts, the opaque phytoclast (7.69%) was dominant, followed by structured phytoclast (6.80%) and degraded brown organic matter (2.88%). Among coprophilous fungi, Sordaria (3.89%) and Delitschia (3.23%) were abundant, Podospora (2.62%) and Cercophora (2.29%) were found in fair amounts, whereas Coniochaeta and Sporomiella were present in very scarce amounts between 0.90% and 0.74%. Zoomorphs were dominated by Thecamoebians (4.74%), followed by flatworm residue (3.63%) and Neorhabdocoela (3.45%). Among the pteridophytes, trilete (5.06%) were abundant, followed by monolete (3.74%). The algal remains were dominated by Botryococcus (3.23%), followed by Spirogyra (0.78%) and Zygnema (0.47%).

Result of PCA analysis

The PCA score plot shows eigenvalues of 0.56 for Axis 1 and 0.1534 for Axis 2. Axis 1 explains 57% of the variance, while the cumulative variance increases to 72% with Axis 2. Overall, 83% of the total variance in the SW samples (SW1–SW31) is explained by the distribution of NPP taxa, including coprophilous fungi, non-coprophilous fungi, zoomorphs, algal remains, biostructured phytoclasts, and pteridophytes. The PCA clearly differentiates ecological groupings. Moisture-loving NPPs such as monolete spores, Zygnema, Botryococcus, Neorhabdocoela, Spirogyra, Byssothecium, Tetraploa, and Cookeina are predominantly associated with the lowermost samples (SW31–SW17), indicating relatively wetter environmental conditions during this phase (Zone 1 & 2). In contrast, taxa such as Entophlyctis, Diploidia, and Alternaria are more prominent in samples SW16–SW12, suggesting comparatively warmer and drier conditions (Zone 3).

The uppermost samples (SW11–SW1) are characterized by an increased abundance of coprophilous fungal spores along with other spores, including Arnium type, Sordaria, Nigrospora, Ascodesmis, Glomus and Telitia. These taxa are commonly associated with soil erosion, grazing and agricultural activities, reflecting enhanced anthropogenic influence during this period (Zone 4 & 5).

Result of box-plot analysis

To assess the range and diversity of the six major NPP groups across the five designated zones (Zone I to Zone V), a visual representation was created using the NPP frequency data set (Figure 7). Box plots were employed to compare the frequency and abundance of these groups across the zones. The analysis reveals that non-coprophilous fungal taxa are the most dominant and widely distributed group across all zones. Their frequency and diversity are consistently higher compared to other NPP groups. Notably, in Zone III, these taxa exhibit the highest frequency, including an outlier, which suggests a significant expansion of open landscapes, likely driven by a shift towards cold and dry climatic conditions. In contrast, the other NPP groups, such as coprophilous fungal spores, zoomorphs, algal remains, biostructured phytoclasts and pteridophytes, are generally present in moderate frequencies across the zones. However, there are notable exceptions. Zoomorphs and algal remains are almost absent in Zone III, indicating that this zone may possess ecological constraints, such as dry and cold climatic conditions, that hinder the proliferation of these groups. Coprophilous fungal spores show relatively higher frequencies in Zones II, IV and V, which likely reflects increased human and livestock activity in these areas. Zoomorphs and biostructured phytoclasts are prominently represented in Zones I, II and IV, respectively. This pattern suggests regions of intense monsoonal precipitation and improved water availability, fostering ecological conditions favourable for these NPPs. Additionally, the presence of structured phytoclasts points to substantial forest cover, further supporting the idea of a relatively humid and vegetated environment in these zones.

DISCUSSION

The analysis of NPPs studied sedimentary core reveals a high diversity of fungal spores, with notable correlations to major global climatic events for the last 4,000 years, such as the Roman Warm Period (RWP), Medieval Warm Period (MWP) and LIA. Among the identified NPPs, non-coprophilous fungal spores were consistently abundant throughout the sedimentary core, although their frequencies varied across different stratigraphic zones and depths. In contrast, the diversity and abundance of coprophilous fungal spores increased during the recent time span, likely reflecting intensified anthropogenic activity. The extensive presence of biostructured phytoclasts suggests a densely forested environment. Additionally, the recovery of zoomorphs, algal remains and pteridophytes spores indicates warm and humid climatic conditions, with favourable environmental factors such as good water quality, climatic resurgence and robust surrounding vegetation. These significant variations in NPP assemblages provide valuable insights into the ecological shifts and anthropogenic influences in the region over time, as discussed in detail below.

Strong evidence of consolidated forests and favourable climatic conditions during 4,040 to 2,260 cal yrs BP

The NPP assemblage from this zone is predominantly composed of non-coprophilous fungal spores such as Cookeina, Byssothecium and Tetraploa, which are known plant pathogens and water-loving fungal spores, respectively. These fungi typically thrive in warm and humid climates, supporting the inference of a moist environment conducive to fungal proliferation and organic decay (Sieber et al., 1991; Zimny et al., 2025). Moderate frequencies of zoomorphs and biostructured phytoclasts suggest the presence of stable wetland conditions and a well-developed woody forest cover, respectively. The occurrence of degraded brown phytoclasts further indicates active decomposition of organic matter, consistent with warm and forested conditions (Sebag et al., 2006; Tiwari et al., 2024). Coprophilous fungal spores such as Sordaria and Cercophora are present in fair frequencies (5.14%), suggesting moderate herbivore activity and limited anthropogenic influence during this phase. These findings contrast with studies by Leipe et al. (2014) in the Trans-Himalayan region and Tarasov et al. (2019) in NW China, who inferred increased aridity between 4,000 and 2,000 cal yrs BP, based on higher abundances of charred plant remains, expansion of open landscapes and evidence of grazing. In contrast, our results are more consistent with Kramer et al. (2010), who reported prolonged warm and humid conditions during the mid-Holocene, reflected in the dominance of aquatic NPPs. The moderate occurrence of degraded brown organic matter and opaque phytoclasts may be associated with relatively high temperatures and precipitation levels (Medeanic & Silva, 2010). Moreover, the notable presence of zoomorphs, particularly Neorhabdocoela and Thecamoebians, suggests stable, slow-moving freshwater systems. These organisms are typically associated with stagnant or sluggish water bodies, as also observed in lake sediments from East Africa (Gelorini et al., 2011), indicating prolonged humid conditions. Overall, the NPP assemblage indicates a slightly warm and humid climate, characterised by abundant parasitic and saprophytic fungal activity within a predominantly woody, forested landscape. This interpretation is further supported by the presence of pteridophyte spores (11.93%) and algal remains (10.13%), including Botryococcus, Spirogyra and Zygnema, suggesting a warm, humid and mesotrophic aquatic environment.

Climatic fluctuation and vegetation alteration during 2,260–1,460 cal yrs BP

The NPP assemblage from this zone reveals a significant ecological and climatic shift from the preceding phase. The dominance of non-coprophilous fungal spores (43.60%) was observed, along with notable occurrences of pteridophyte spores, coprophilous fungi and biostructured phytoclasts, indicates warm and humid conditions, consistent with the RWP, a global climatic event. This phase coincides with the southward migration of the ITCZ, influencing the regional monsoon patterns (Fleitmann et al., 2003). The abundance of coprophilous fungal taxa such as Delitschia, Sordaria, Podospora and Cercophora suggests increased grazing activity and the expansion of open vegetation, indicating significant anthropogenic influence during this phase of early civilisation development. This interpretation is further supported by the findings of Quamar et al. (2024), who reported that the mid-Holocene period is well-represented by NPPs indicative of pastoralism and sustained human pressure. Together, both studies highlight a consistent pattern of land-use change and human-induced ecological impact during the mid to late Holocene, marked by intensified grazing and vegetation modification. This zone also records the highest mean annual precipitation (up to 3,000 mm), as reported by Pandey et al. (2026), creating favourable conditions for moisture-loving fungal taxa like Entophlyctis lobata, Cookeina and Gelasinospora (Ghosh et al., 2017). However, these findings align with Tarasov et al. (2019), who observed a shift toward wetter conditions, increased aquatic NPPs and intensified human activity in NW China between 2,000 and 1,500 cal yrs BP. In contrast, Leipe et al. (2014) reported a drier phase during this time, indicated by grass epidermal fragments and reduced Poaceae pollen. Also, during this phase, some forest canopy may have transitioned to shrubby taxa and open vegetation expanded, the substantial presence of pteridophytes suggests sustained soil moisture and a humid environment.

Vegetation regression and Climatic deterioration during 1,460–1,100 cal yrs BP

The NPP assemblage from this phase indicated a shift towards colder and drier climatic conditions following the prolonged warm and humid climatic phase. This transition is strongly supported by the overwhelming dominance of non-coprophilous fungal spores (74.86%), particularly Diplodia and Telitia, which are known to tolerate and persist under environmental stress and unfavourable climatic regimes. The increased presence of parasitic and symbiotic fungi such as Glomus, and Alternaria further reflects adaptation to a drier and possibly more open landscape, where herbaceous vegetation likely became more prominent. These fungi often associate with the roots or surfaces of stress-tolerant plants, suggesting a shift in vegetation composition in response to declining moisture and temperature. The coprophilous fungal spores were found in scarce frequencies, indicating a reduction in grazing activity and possibly a decline in human or livestock presence (Pandey et al., 2023). Additionally, the scarcity of biostructured phytoclasts limited mostly to degraded organic matter points to reduced forest cover and slower rates of organic matter turnover, consistent with less productive and more arid environmental conditions (Ghosh et al., 2017). The near absence of algal remains and zoomorphs suggests that aquatic ecosystems were minimal or under stress, likely due to reduced precipitation and declining surface water availability. Our findings are consistent with the conclusions of Leipe et al. (2014), who identified a dry phase around 1,800 cal yrs BP, characterised by an expansion of grassland and limited anthropogenic activity. Similarly, Cugny et al. (2010) also interpreted this period as a drier phase, based on a reduction in arboreal pollen percentages. However, unlike our results, their study reported relatively high percentages of coprophilous fungal spores, which typically indicate the presence of grazing animals and, by extension, human activity. This discrepancy suggests regional variability or differing environmental conditions that may have influenced NPP spore deposition and preservation. Collectively, the NPP data from this phase reflect significant ecological regression, driven by colder and drier climatic conditions, leading to simplified vegetation structures dominated by hardy, stress-tolerant taxa and a general decline in biodiversity and productivity.

Monsoonal resurgence and anthropogenic involvement during 1,100 to 500 cal yrs BP

This zone reflects a distinct shift in ecological and anthropogenic dynamics, aligning with the MWP. The high dominance of non-coprophilous fungal spores (60.51%), followed by zoomorphs and biostructured phytoclasts, suggests a warm climate with relatively stable vegetation cover. The presence of diverse fungal taxa such as Tetraploa, Cookeina, Byssothecium and Curvularia points to a heterogeneous landscape comprising both woody and herbaceous vegetation, likely shaped by mixed land-use practices including farming, settlement and secondary vegetation growth. The abundance of Tetraploa, indicating tropical to subtropical environments with freshwater habitats and local humid conditions (Romero et al., 2021), reflects high water availability during this phase. This is further supported by the high frequency of Glomus, a mycorrhizal and erosion-indicator fungus (Ghosh et al., 2017; Pandey et al., 2023). Coprophilous fungal spores such as Delitschia, Sordaria, Arnium, Cercophora, Sporomiella and Podospora suggest intensified grazing and domestic animal presence, indicating sustained human settlement near wetlands. The moderate occurrence of Helminthosporium, a known paddy crop pathogen, points toward agricultural activity in the vicinity. The zoomorphs (12.68%) especially Neorhabdocoela, flatworm residues and Thecamoebians indicate the persistence of aquatic or semi-aquatic microhabitats, potentially maintained through irrigation or localised wetland conservation. The presence of degraded brown phytoclasts (7.18%) and other structured organic particles suggests ongoing vegetation litter decomposition and partial ecological stabilisation. Pteridophyte spores (7.85%) and algal remains (4.15%) imply periodic moisture and wetland presence, though at lower levels than in earlier wetter phases, indicating a shift in seasonal hydrology. The palaeoclimatic interpretation is consistent with MCA, indicating warm and humid climatic conditions, which further reflects a landscape shaped by human influence. These findings align with Kramer et al. (2010), who reported changes in NPP composition across the eastern Tibetan Plateau during the late Holocene, likely tied to intensified human activity, including grazing. Similarly, Leipe et al. (2014) observed increased coprophilous fungi and wild grass remains after 1,800 cal yrs BP, indicating expanding pastoralism. In contrast, Cugny et al. (2010) found this zone challenging to interpret due to the very low abundance of NPPs. While pollen data from their study suggest relatively wet local conditions which could have suppressed fungal development and limited animal presence (and thus dung deposition) the site’s sloping mire topography rules out significant water accumulation. However, surface runoff remains a plausible factor for reduced pollen and NPP deposition in this region. This interpretation is supported by the presence of exogenous Glomus chlamydospores, which are commonly associated with intense soil erosion, likely resulting from increased surface flow. Hence, the NPP assemblage from this zone captures a transitional period marked by moderate vegetation cover, localised anthropogenic activity and a mosaic of aquatic and terrestrial features, reflective of a modernising landscape during the MWP.

Relatively reduced precipitation with increased anthropic influence for the last 500 cal yrs

The NPP assemblage from this zone reflects ecological conditions consistent with the LIA, a period characterised by cooler temperatures, reduced seasonality and diminished human activity compared to preceding phases. The dominance of non-coprophilous fungal spores, such as Telitia, Nigrospora, Geastrum and Cookeina, suggests a cooler yet relatively moist environment, with stress-tolerant fungi adapted to lower temperatures and moderate organic input. As this zone is followed by a warm and humid climatic phase, some influence of these conditions is evident, marked by the high dominance of non-coprophilous fungal spores along with the dominance of biostructured phytoclasts, especially opaque (7.69%) and structured types (6.80%), indicating continued but moderate organic matter decomposition, possibly reflecting slower litter turnover in cooler, less biologically active conditions. The reduced presence of degraded brown matter (2.88%) further supports this interpretation. The high presence of Glomus, often associated with soil disturbance and herbaceous vegetation, may point to localised erosion or natural vegetation turnover under subdued climatic conditions. Coprophilous fungal spores are present but not dominant, suggesting a decline in grazing pressure and reduced human or livestock activity during this time. While Sordaria (3.89%) and Delitschia (3.23%) occur in notable amounts, the lower frequencies of Podospora, Cercophora, Coniochaeta and Sporormiella indicate less intensive pastoralism, aligning with historical accounts of decreased agricultural productivity during the LIA (Quamar et al., 2024). Zoomorphs led by Thecamoebians, flatworm residues and Neorhabdocoela, suggest persistent, though possibly reduced, aquatic microhabitats, likely supported by stable moisture regimes under cooler temperatures. Pteridophyte spores especially trilete, further indicate the retention of some forest and wetland elements, while the modest presence of algal remains dominated by Botryococcus supports the idea of less productive but stable aquatic systems. Overall, the NPP data from this zone point to a slightly cooler and less humid climate, reduced seasonal variation and lower anthropogenic impact. The ecological signals are consistent with the broader environmental trends of the LIA, marked by stabilised landscapes, subdued disturbance and a balance between natural vegetation processes and limited human intervention.

CONCLUSIONS

The NPP analysis from the Sakali sedimentary profile on Majuli Island reveals a detailed and dynamic palaeoecological history spanning the last ~4,100 years. Examination of each stratigraphic zone reflects distinct climatic conditions and varying degrees of anthropogenic influence, highlighting the evolving interaction between natural processes and human activity in this endangered wetland ecosystem. Importantly, NPP records allow for pronounced ecological signals to be decoded, sometimes overcoming the biases inherent in pollen preservation. During the earliest phase (Zone I, 4,040–2,260 cal yrs BP) the NPP assemblage indicates a warm, humid climate with stable wetland conditions and well-developed forest cover. The dominance of water-loving and plant-pathogenic fungi, along with zoological remains and pteridophyte spores, supports the presence of mesotrophic freshwater systems and limited human or grazing activity, in contrast to other regional records of aridity during this period (Prasad et al., 2014; Staubwasser et al., 2003). Between 2,260–1,460 cal yrs BP, the NPP assemblage indicates a warm, humid climate with high precipitation, aligning with the RWP. Increased coprophilous fungi and open vegetation indicate intensified grazing and human activity, indicating significant anthropogenic influence. Despite some regional contrasts, the overall evidence suggests climatic fluctuation, forest degradation and expanding land use driven by early civilisation development. A transition phase has been observed in Zone III from 1,460 to 1,100 cal yrs BP, which indicates a marked shift to colder, drier conditions, reflected in stress-tolerant fungal taxa, reduced forest cover, minimal aquatic activity and low evidence of grazing. This phase signifies ecological regression, with simplified, drought-adapted vegetation and declining biodiversity, consistent with global and regional records of climatic deterioration and reduced human activity. A retrieval of favourable climatic conditions and vegetation was observed during Zone IV, that is, from 1,100 to 500 cal yrs BP, corresponding to global climatic changes during the MCA. The NPP assemblage during this phase reflects warm, humid conditions, moderate vegetation cover and intensified human activities, such as grazing and agriculture. The presence of diverse fungal taxa, coprophilous spores and aquatic indicators suggests a dynamic landscape shaped by mixed land use, localised wetland management and ongoing ecological stabilisation amid environmental changes. The NPP assemblage during the last NPP zone (Zone V), which coincides with the LIA, reflects a cooler, slightly moist climate with reduced human and grazing activity. Ecological indicators suggest slower organic matter decomposition, relatively stable but less productive aquatic habitats and a landscape shaped predominantly by natural vegetation dynamics under subdued climatic conditions, consistent with historical records of reduced agricultural activity. These NPP-based findings reconstruct the past ecological trajectory of Majuli Island while providing a much-needed data set for northeastern India. NPPs offer insights into local moisture regimes, vegetation dynamics and anthropogenic influences, often revealing nuances not captured by pollen alone. Their consistency with independent climatic proxies underscores their reliability and integrating multiple NPP indicators enhances accuracy despite preservation challenges. Overall, the study advances understanding of ecological and human-environment interactions, informing conservation and land management in the region.