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.

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.

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).

 

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.

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