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.

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.