•Forest carbon storage is greatly influenced by understory vegetation, which serves as a crucial indicator of plant biodiversity.•The baseline diversity of understory vegetation (including shrubs and herbs) and carbon stock in the Western Himalayan Zabarwan Mountain Range.•The estimated average total understory carbon stock is 3.93 Mg C/ha.•The current research on understory vegetation presents compelling evidence in support of maintaining forest management practices. Carbon stock and diversity of species together address the two most pressing concerns for successful ecosystem functioning and management, namely climate change and biodiversity loss. Understory vegetation is a significant indicator of plant biodiversity, contributing to forest carbon storage, sustaining species, and providing many ecological services. However, little research has been done about the contribution of the understory carbon stock in the forests, most likely because these understory components only make up a small portion of the total carbon stock of the forest ecosystem. To fill this knowledge gap, the current study sought to study the baseline understory (shrub and herb) diversity and carbon stock from the Zabarwan Mountain Range. Data on the characteristics of the understory vegetation in five different forest types were gathered using a random sample method. Using the Pearson method, we calculated the correlation coefficient between the diversity parameters and the carbon stock. The calculated average total carbon stock in the understory was determined to be 3.93 Mg C/ha (1.55–9.2 Mg C/ha). The shrub layer contributed a maximum value of 82 % of carbon stock. The increasing trends in total understory carbon were Parrotiopsis forest < Broad-leaved forest < Oak forest < Acacia forest < and Pinus forest. Species-wise, the shrub Parrotiopsis jacquemontiana was found to contribute a maximum average carbon stock of 3.11 Mg C/ha (47.76 %) and can be regarded as a key indicator understory species for the restoration of Himalayan forests. The decreasing order of trends in Shannon–Wiener diversity index value in the study area was Oak forest > Broad-leaved forest > Acacia forest > Pinus forest > Parrotiopsis forest. Overall, the data show that coniferous-dominated woodlands had higher understory carbon stocks than other kinds of woodlands. The data acquired highlights the importance of shrub carbon stock storage in the understory layer in this Himalayan region. The current research on understory vegetation provides strong evidence for maintaining forest management practices that enable and encourage the planting of multiple layers of vegetation in landscape restoration as approaches to improve biodiversity while increasing resistance to climate change.