Elevational shift in pollen quantity–quality trade-off and stress-tolerant morphology in Himalayan Thymus linearis

High-altitude environments impose multiple abiotic stressors that shape plant reproductive and morphological responses; however, within-species tests of pollen quantity–quality relationships along elevation remain rare. We investigated 16 vegetative and reproductive traits of Thymus linearis across five sites (1800–3499 m) in the Kashmir Himalaya. Fifteen individuals per site were sampled for vegetative and floral traits and three per site for pollen traits (n = 75 and n = 15, respectively). All measured traits differed significantly among sites ( p ≤ 0.003), with all except internodal length significant at p < 0.001. Leaf length and width showed pronounced site-level differences, and several leaf dimensions increased toward high elevation. The pollen–ovule ratio increased significantly with elevation (R 2 = 0.741, p = 0.0354), whereas pollen stainability decreased (R 2 = 0.780, p = 0.0227). Their direct relationship was strongly negative but non-significant (r = − 0.841, R 2 = 0.707, p = 0.0743), consistent with the proposed pollen quantity–quality relationship. High-elevation populations exhibited greater stem circumference (+ 71%), greater root circumference (+ 76%), wider flowers (+ 16%), and fewer inflorescences (− 64%). Principal component analysis explained 88.17% of the total variation, with PC1 (65.62%) representing vegetative structural differentiation and PC2 (22.55%) representing additional variation in plant architecture and reproductive morphology. Overall, the populations showed pronounced intraspecific differentiation in reproductive traits, leaf morphology and plant growth morphology, with high-elevation structural traits broadly consistent with stress-tolerant expectations. These findings provide a functional basis for understanding coordinated trait variation along Himalayan elevational gradients.