19 July 2025

 

When Old Patterns Stop Making Sense: Shifts in Western Himalayan Plant Assemblages, 2020-2024

by Sudipto Majumdar, Pragya

The first signs of change were not spotted by scientists. They were noticed by people who walk these slopes every day. Women collecting Ajwain (Trachyspermum ammi), herders gathering fodder & men pruning terrace edges. They noticed small but telling absences in their environment. The small, useful herbs that used to grow in the fields along bunds, near terraces & between crops had started to disappear. These included Trifolium repens and Medicago lupulina for fodder, and even Ajwain, the spice that flavours several local foods, which had once thrived in bund crevices and terrace risers. In their place came coarse, wiry colonisers and bitter, fast-spreading invasives. Lantana camara, Ageratina adenophora, and Parthenium hysterophorus began appearing in clusters where farmers once gathered medicine or fodder. These are not just unfamiliar plants but culturally out of place for being non-palatable & non-medicinal.

Many of these shifts echo broader range transitions observed in mid-altitude ecotones across the Western Himalaya, where thermophilic (heat-loving) and ruderal (disturbance-adapted) species now dominate microhabitats that were once shaded & moist but are increasingly dry & open. The shift in plant cover they are witnessing has a deeper unravelling of known patterns very much revered amongst ecologists. Seasonal cues had grown unreliable since the hills are warming early. Pre-monsoon showers have become erratic & long dry spells stretch into what used to be moist months. Forest fires are now common in belts where they were once rare, especially above 1800 metres. As these stressors converge, plant assemblages that held together for decades are breaking up. Where known herbs and legumes once grew, hardy colonisers & invasive species are now taking over. In fire-affected zones, soil pH had shifted upward, due to ash deposition after the surface litter burns out, a spike that fades away but not until invasive species have moved in. These areas also showed a near-complete disappearance of litter-binding herbs like Leucas & Plantago, whose loss is likely to accelerate runoff & reduce topsoil retention in early monsoon phases. These shifts also correlated with fire exposure, cattle tread, and observable soil changes, specifically reduced organic matter and increased compaction in bund toes and terrace risers.


From 2020 to 2024, we carried out biodiversity & habitat monitoring studies in districts of Chamoli, Uttarkashi, and Chamba in the western Indian Himalayas. These covered ecotones between 1320 and 1950 metres and targeted microhabitats like nala-fed gullies, bund drop-offs, springfed terraces & degraded ridges. Taking lead from the community reports, elevational shift analysis was conducted by quadrat-wise altitudinal centroids for 8 focal herbs: Trachyspermum ammi (Ajwain, native aromatic and medicinal herb), Ajuga bracteosa (Neelkanthi, native midslope medicinal herb used in skin and respiratory remedies), Swertia chirayita (Chirayita, high-altitude bitter herb of medicinal and cultural value), Urtica dioica (stinging nettle, native leafy perennial herb used for food and fibre) and Parthenium hysterophorus (Congress grass, aggressive invasive herb with known allelopathic effects), Lantana camara (woody, thorny invasive shrub with allelopathic properties and fire-prone growth habit), and Ageratina adenophora (Crofton weed, aggressive invasive Forb, dominant in disturbed mid- to lower-elevation slopes with dense cover & allelopathic effects) each with 100 replicates.
To identify medicinal species and observe the changes in their populations and their habitats we established 300 1 × 1 m quadrats along 23 belt transects, tracking shifts in species composition, phenology, and vegetation structure across microhabitats. These were supplemented by detailed conversations with community elders, whose insights on changes in leaf flush timing, plant smell, and root texture often served as early indicators of species loss or upslope movement. Their observations helped interpret not just presence or absence, but changes in phenological windows (timing of life cycle events), altered associations among species, and signs of functional turnover in the plant community. To quantify these patterns, we used relative abundance across all quadrats. Species compositional shifts were tested using Mann-Whitney U-tests & Kruskal-Wallis for slope and aspect classes. Range shifts were assessed via altitudinal centroid tracking between 2020 & 2024. Differences were considered significant at p < 0.05. All quadrats were georeferenced (±5 m) and linked to aspect & slope gradient metadata.

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A typical mid-altitude Himalayan terrain with steep southern slopes dominated by Chir pine (Pinus roxburghii). Most visible habitats, including settlements and forest patches, are encircled by expanding pine cover. This widespread pine dominance especially on sun-facing slopes has important implications for fire risk, soil moisture retention, native biodiversity, and regeneration of broadleaf species. Spreading pine cover means greater understory accumulation of flammable litter which drives vegetation change.
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Once dominated by herbs like Trifolium and Medicago, this mid-altitude slope in Chamba (typical to Western Himalayas) now shows soil degradation driving replacement of native legumes by coarse grasses and woody colonisers.
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This hillside shows surface erosion, with exposed soil and rocky outcrops where vegetation cover has degraded. Shallow-rooted plants dominate the slope but fail to bind soil. Mature oak trees persist above.

The results showed consistent upslope range shifts of medicinal herbs: Ajwain has moved higher by over 300 m, no longer seen below 1500 m in Kalauta (Chamba district, 1400-1500m). The mean upslope shift between 2020 and 2024 was 301.6 m. Similarly, Valeriana jatamansi (Tagar), a much-valued medicinal herb, found in shaded forest pockets is now absent from lower homestead zones (below 1650 m) where it was once being very common, and is only found in moist upper corners of oak-pine patches above 1800 m, often in low- canopy pockets protected from surface heat. Ajuga bracteosa has also moved upslope. It was once common along old field walls & orchard fringes near Kalauta, but is now only seen above 1866 m. Ajuga prefers shaded, less-trampled trailbanks & micro-sites now more frequent upslope, as mid-slopes have dried & degraded. These shifts in elevation are driven by habitat squeeze because viable zones have shrunk, pushing these species into narrower upslope refuges. These are consistent with temperature-linked phenological displacement observed in other temperate herb species, possibly accelerated by declining under-canopy moisture and soil crusting. Thus, Ajuga and Valeriana exhibit the highest upslope ratesamong all observed taxa (mean 6.5-7.8 m/year), with consistent vertical displacement across both southern and northern aspect belts.


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Ageratina adenophora is now advancing into mid-elevation oak forests once considered resistant to such invasion. In field plots like this one, Ageratina covers over 60-80% of the ground layer in some 1×1 m quadrats, with visible suppression of native herbs like Trifolium, Vigna & Geranium. Community members note that this patch remained free of Ageratina a few years ago but now shows rapid spread following canopy opening and past fire events. The understorey here is becoming increasingly homogenous & litter decomposition has slowed, with drier, matted layers observed across the slope. In Ageratina-invaded oak patches we observed bulk density rise to 1.42-1.56 g/cm³ (vs. 1.18–1.26 g/cm³ in intact areas) & soil organic carbon drop to 0.58% (from 0.92-1.15%). Litter decomposition is generally not very slow but the shear volume of it overwhelms the native herbs while Ageratina itself does very well under these induced conditions.

Across the ridge & mid-slope ecotones in Sidoli and Kwarit (Chamoli district, 1500-1800 m), moisture-loving Quercus leucotrichophora belts now record drought/fire-tolerant Pinus roxburghii (Chir pine) saplings in 30- 35% of plots. These belts had near-zero Chir in forest maps before 2015. It signals a possible regime shift toward fire-adapted canopy types, reinforced by pine needle accumulation, drier soil surfaces & heat- facilitated seedling survival. Plot metadata from Pilang (Chamoli district, 1380-1680 m) and Sidoli further confirms Chir seedlings establishment co-occuring with Eragrostis nigra & Parthenium hysterophorus which dominate post-fire trailbeds & compacted edges. We now consider this a landscape-scale structural transition, with Chir oak inversion patterns confirmed via forest-type boundary reclassification between 2015 and 2024. Some species are riding this change and are expanding their altitudinal range by creeping upslope. For example, ruderal disturbance-loving Parthenium hysterophorus & Urtica dioica have expanded their elevational range and now also seen in higher reaches (92.9 m and 51.0 m on average between 2020 and 2024).


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Healthy forest floor ground cover is essential for nutrient cycling, soil stability, and native regeneration. In this patch, it has been largely replaced by Lantana camara, an invasive shrub forming a monodominant understorey. The litter layer is thick but poorly decomposed, signalling low microbial activity. Dry, compacted soil with low organic content further reflects ecological decline and limits the return of native herbaceous species.

Species abundance also displays a statistically significant decline (in more than 60% of quadrats) in native herbs and legumes, paired with a rise in graminoid (grass family) colonisers (Cynodon, Eragrostis, Setaria) and invasive species, particularly along bund crests and scraped margins, suggesting a shift towards shallow- rooted, disturbance-favoured assemblages. Abundance ratios showed a 2.7-fold rise in graminoid presence post-2021, primarily driven by Eragrostis spp. and Cynodon dactylon, which replaced former legume- dominated ground layers such as Trifolium repens & Medicago lupulina. In Silagrath (Chamoli district, 1420- 1600 m), populations of Berberis aristata, a medicinal herb, fell dramatically (from 7.4% to 1.8% mean relative abundance) across trails below 1450 m and were absent in around 80% of its previously recorded quadrats. In the same patches, the invasive species Lantana camara and Ageratina adenophora rose to over 22% combined coverage, and the non-native grasses Eragrostis nigra & Cyperus rotundus dominate with >35% ground coverage. In adjacent degraded fields, soil-enriching native herbs such as Trifolium repens, once abundant, are now rare (present in 85% of quadrats in 2020, now in only 23%) implying a loss of nitrogen-fixing soil organisms. Across fire-affected ridge zones between 1550-1800 m in Chamoli (Pilang-Dungra belt) and Chamba (Kalauta-Sidoli belt), Lantana camara (more than 50% coverage in some plots) and Ageratina (up 20%) have expanded between 2021 and 2024. In Pilang, native legumes like Medicago lupulina declined by 78% (in 147 quadrats). Once common in moisture-retaining bund zones and shaded stone margins, they appear only in fragmented clumps today, mostly below terrace risers. Field teams recorded a 63% replacement of low-growing legumes by wiry colonisers like Cynodon dactylon and Cyperus rotundus, with mean abundance of legumes dropping by nearly 40%. These trends indicate replacement of native herbs by invasive competitors that disproportionately benefit from fast colonisation dynamic driven by soil exposure and moisture stress, with opportunistic graminoids taking hold in once-legume-dominated areas. A lot of this is due to changes in soil- increased compaction and reduced nutritional content (soil bulk density in Eragrostis-dominated plots was 1.4–1.6 g/cm³, beyond compaction threshold for root penetration, and soil Organic Matter levels were below 1.2%). Soil organic matter was significantly lower (by 27.8%) and pH higher (by 0.36 units) in fire-affected zones compared to intact forest-edge control sites. Agricultural patches and fire- affected zones showed soil compaction near bund toes and nitrate leaching in post-harvest zones, suggesting that both water retention and nutrient cycling are being compromised, and that nutrient loss and compaction are jointly driving these assemblage shifts [significant differences in both soil OM and nitrate-N of fire-affected & scraped zones, when compared with intact zones (χ² = 14.5, p < 0.01)] in favour of disturbance-adapted invaders, which outcompete & limit regeneration niches for native species.


In Gaila (Chamba district, 1350-1500 m), a bamboo-like grass, identified in the field as Arundinella bengalensis or a Pogonatherum species- not recorded during surveys in 2018-2019, now covers 34% of plots along degraded terrace edges & abandoned bund margins. It is unpalatable, spreads by basal tillers, and dominates post-fire and cattle-tread zones. Communities call it “nangal ghaas” and associate its spread with soil scraping and fire. Its basal spread indicates rhizome-driven expansion which better utilises the soil nitrogen in disturbed sites, especially in areas with footfall compaction or exposure post burning. Sampling from 2024 shows basal cover ratios >60% for Arundinella in >40% of fire-exposed quadrats, confirming its role in post-disturbance dominance. Species richness in these belts has more than halved (avg. 9 species/ m² pre-2020, now below 4). Across Chamoli and Uttarkashi, Urtica dioica has increased by more than 40% (in 68 plots), replacing herbs like Viola and Leucas in trail-edge and scrub zones. Parthenium hysterophorus now forms monocultures with over 60% coverage in Ranbirpur (Uttarkashi district, 1440 m).
These shifts may stem from nutrient pulses (temporary nitrogen surges in runoff collection zones due to sediment inflow from burnt sites with loose soil) are contributing to niche openings for aggressive ruderal taxa. Soil profiles in Parthenium-dominated plots also showed elevated nitrate levels, because of runoff pouring in. Spatial modelling confirmed the expansion zones of Parthenium closely matched low-lying moisture catchments & bund breaches with visible sediment accumulation.
“Earlier we had to search for Bicchu Buti (Urtica dioica). Now it grows where the goats walk.”- goat herders

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Parthenium hysterophorus
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Lantana camara
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Ageratina adenophora
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Urtica dioica
These quadrats reflect the ongoing shift in understorey composition across disturbed mid-Himalayan forest sites. Dominant species include Ageratina Adenophora, Lantana camara, Urtica dioica & Parthenium hysterophorus. All of these are generalist, disturbance-adapted (ruderal) species expanding their range uphill. Their aggressive spread is associated with canopy thinning, past fire events & soil degradation. These plants form dense, monodominant stands that displace of native herbs like Trifolium, Vigna & Geranium which require cooler & undisturbed microhabitats. Soils in these quadrats are typically dry, compacted & low in organic carbon with poor litter decomposition. These new dominants thrive in these exposed, degraded patches, limiting floristic diversity and reducing habitat suitability for pollinators and soil organisms. Species richness in these plots is visibly low, with 1-3 species per 1×1 m quadrat, often with a declining trend in native herb abundance with increasing density of these species.

Several native herbs now show compressed growth periods and altered reproductive timing. Trachyspermum ammi (Ajwain) flowers earlier- in Pilang, it initiated flowering approximately 18.5 ± 2.7 days earlier than the 2010-2015 baseline, with first blooms recorded in early April instead of the typical late April– early May window; the flowering phase also concluded prematurely by late May, nearly 35 ± 4.2 days earlier than the historical July endpoint. This phenological acceleration coincided with its complete disappearance below 1500 m in Kalauta, suggesting both thermal sensitivity to spring warming & altitudinal contraction due to failure to complete its reproductive cycle at lower elevations. Ajuga bracteosa displays a significantly shortened flowering duration- in upper Kwarit (above 1866 m), its inflorescences persist for 20.8 ± 3.4 days which earlier was 42.1 ± 5.1 days; concurrently, its post-May vegetative growth declined noticeably along with reduced leaf health. Here we see both a shorter blooming period and weaker plant growth after flowering, suggesting seasonal stress or nutrient drawdown following early flowering. In Dungra, Swertia chirayita started sprouting nearly 3 weeks later than usual, with first shoots showing up only around mid-May instead of late April. On exposed ridges, flowering was patchy & sparse, and in Silagrath, it was off by over two weeks and didn’t match the oak leafing like it always used to, which local harvesters used as a timing cue. Plot records between 2020 and 2024 confirm that first blooming has shifted nearly 18 days earlier than it used to between 2010 and 2015. Trifolium repens too has a shorter cyclein Dungra,


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Trifolium repens
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Vigna unguiculata
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Oxalis corniculata
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Geranium nepalense
Native herbs such as Trifolium repens, Vigna unguiculata, Oxalis corniculata, and Geranium nepalense. continue to thrive in a few intact, moist plots with stable canopies and low disturbance. These species once dominated mid-elevation oak and oak-pine understorey zones, contributing to high ground cover, fast litter turnover, and healthy soil conditions. However, they are now disappearing from most surveyed sites replaced by exotic invasives. This shift from diverse functional herb layers to disturbance-tolerant dominance reflects a broader ecological transition driven by canopy thinning, soil compaction & moisture loss across the landscape.

populations had far fewer flower heads than before, dropping by more than half compared to 2015, and also senesced early, ending its season by mid-June, its flowering window shortened by over two weeks, likely due to the intense March to May heat. Medicago lupulina in Pilang set seed almost three weeks earlier than expected, peaking in early May instead of early June, but dried up fast with the early rains. It has now nearly vanished from most Pilang plots. Near Chamoli, Valeriana jatamansi barely showed up below 2200 m and local monitors reported flowering either missing or delayed by nearly a month, many saying the plant may have disappeared from lower belts altogether. Leaf flushing is earlier in Ajuga and Phoenix. In upper Dungra, Ajuga integrifolia flushed leaves about four weeks ahead of schedule, but flower spikes were mostly absent in over 80 % of plots. So it grew leaves but skipped flowering entirely. In Sidoli, Phoenix dactylifera began leafing out in late February, over a month early compared to before 2020, but no fruits set in over 70% of plots, perhaps due to moisture stress and cold night temperatures.

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This is a typical balanced oak–pine mixed forest, with high species diversity and healthy understorey vegetation. These habitats retain soil moisture well and support a rich layer of native herbs and shrubs. Litter decomposes rapidly due to active microbial processes with a higher soil organic carbon. Bulk density stays moderate (around 1.1– 1.2 g/cm³) indicating that the soil is safe from compaction.

The ruderal Urtica and Opuntia display stress adapted behaviour. In Sidoli in the last few years Urtica dioica had leaf size and height much reduced, even in richer soils, and faded by the end of April, a month earlier than expected. But it still shows up all over trail edges and disturbed sites, suggesting it’s able to spread in exposed spots even when stressed. Opuntia in Pilang began budding in early April, nearly a month ahead of normal and in some patches, it flowered twice, in April and again in September. This two-time blooming seems to be its way of coping with climate stress, and it now covers many abandoned maize fields and dry fallows. The data only affirms what villagers had sensed all along. Plant groupings are falling apart; familiar pairings no longer hold. It's not the same herbs or grasses, not the same scent in the soil. Neither is the same leafing rhythm. The oak no longer sets the seasonal clock & the vegetation no longer responds in the same way when the rain sets in.













  When Old Patterns Stop Making Sense: Shifts in Western Himalayan Plant Assemblages, 2020-2024 by Sudipto Majumdar, Pragya T...