The rise of the Himalayas governed the Indian Summer Monsoon in Karewa basin during Plio-Pleistocene. A palynological study is presented to delineate the climate-vegetation relationship using an 8.5-m thick fluvio-lacustrine sequence of the Hirpur Formation (2.4–2.1 Ma). Our results suggest that the sediment sequence is mainly comprised of two units, namely, Unit 1 and Unit 2. Unit 1 shows the dominance of sub-tropical to broad-leaf temperate vegetation when mean annual temperature (MAT) was ${\sim}$ 17$^{\circ}$ C and mean annual precipitation (MAP) was 1025 mm. The subsequent increase in sand followed by a thin lignite layer with Trapa, megafossil (fruits) demarcates fluvial adjustments, suggesting a low altitude fluvio-lacustrine ecosystem. Conversely, Unit 2 shows a decline in rainforest pollen with a steady increase in conifers. The abrupt dominance of diatom species Tetracyclus lacustris and related species with MAT and MAP reducing to 10$^{\circ}$ C and 770 mm reveal a colder climate with the lacustrine ecosystem. This change of tropical to cool temperate vegetation could be attributed to the altitudinal rise of the Pir Panjal Mountains and consequent obstruction of the south-west monsoon, which resulted in lower precipitation and temperature during ${\sim}$ 2.4–2.1 Ma. Hence, the relic tropical flora of Palaeogene/Neogene transformed to Himalayan temperate flora sometime ${\sim}$ 2.1 Ma.

$\bf{Highlights}$

$\bullet$ Climate–vegetation relationship is established through a palynological record.

$\bullet$ Unit 1 comprises subtropical to broad-leaf temperate vegetation with warmer mean annual temperature and higher mean annual precipitation.

$\bullet$ Unit 2 comprises colder diatom species and is following cooler mean annual temperature and lower mean annual precipitation.

$\bullet$ A shift of tropical to cool temperate vegetation is related to the rise of the Pir Panjal Mountains, obstructing monsoon.

Carbon assimilation in plants depends on both atmospheric CO$_2$ uptake and internal (non-atmospheric) CO$_2$. To understand this mechanism in Strobilanthes kunthianus (an endemic pleitesial plant), we studied the leaf epidermal morphology, stomatal index (SI), cystoliths, carbon and nitrogen isotopes (${\delta}^{13}$C$_{leaf}$, ${\delta}^{15}$N$_{leaf}$) in herbarium samples collected during the bloom in 1958, 1970 and 2018 from Munnar region, India. The leaf epidermal morphology is xerophytic characterized by long, twisted and branched trichomes obscuring stomata to restrict evapotranspiration and thereby gaseous exchange. Our study shows that the values of ${\delta}^{13}$C$_{leaf}$ decreased by 4% (-26.0 to -30.0%) accompanied by a decline in ${\delta}^{15}$N$_{leaf}$ (+1.6 to -0.35%) from 1958 to 2018, respectively. The depleting trend in ${\delta}^{13}$C$_{leaf}$ values indicates increasing photosynthetic efficacy along with uptake of anthropogenically fixed nitrogen (${\delta}^{15}$N ${\cong}$ 0.0%). Albeit, we did not observe any alteration in SI (in response to present day increased atmospheric CO$_2$). Estimated intrinsic water use efficiency shows a reduction from 61 to 45 ${\mu}$mol·mol$^{-1}$ from 1958 to 2018, respectively. Leaf epidermal morphology remained static, low ${\delta}^{13}$C$_{leaf}$ suggests increased photosynthesis with the use of non-atmospheric CO$_2$ stored in enhanced number of cystoliths during 2018 channelized via ‘Alarm Photosynthesis’. Prolonged post-monsoon rainfall and soil amendment facilitated the mineral accumulationin S. kunthianus.

$\bf{Highlights}$

$\bullet$ Microscopic study in Strobilanthes kunthianus leaf shows no change in Stomatal Index (SI), but the density of cystoliths increased during 1958–2018.

$\bullet$ The ${\delta}^{13}$C leaf (carbon) and ${\delta}^{15}$N leaf (nitrogen) decreased by ${\sim}$ – 4‰ (– 26.0 to – 30.0‰) and 1.6 to – 0.35‰ in 2018, respectively.

$\bullet$ The intrinsic water use efficiency of S. kunthianus reduced from 61 to 45 μmol·mol−1 in 2018.

$\bullet$ An alternative process of ‘Alarm Photosynthesis’ seems to be active due to low stomatal conductance in the warmer climate.