The rugged Himalayan landscape results in large variations in site conditions that regulate plant response to warming. There is lack of deeper understanding on plant response to warming under differing site characteristics in the mountainous terrain. A 2-yr experiment was conducted in the high mountains of Bhutan. The objective was to investigate the effects of short-term artificial warming on high-altitude grassland vegetation at north- and south-facing sites. An artificially warmed environment was simulated using open-top chambers (OTCs) that were compared with control chambers experiencing ambient conditions. Variables measured were species diversity, species richness, proportions of plant functional groups, forage dry matter, and forage quality. Generally at the north-facing site, OTC treatment showed a lower species diversity (OTC treatment H' ≈2.35; Control treatment H' ≈2.75), species richness (OTC treatment MI ≈1.38; Control treatment MI ≈1.45), sedge abundance (OTC treatment sedge cover ≈14.5%; Control treatment sedge cover ≈25%), and crude protein content (OTC treatment CP ≈6.60%; Control treatment CP ≈7.70%). On both sites, OTC treatment had a higher grass abundance (OTC treatment grass cover ≈24.0%; Control treatment grass cover ≈17.0%) and higher dry matter content (OTC treatment DM ≈1.70 t ha−1; Control treatment DM ≈1.50 t ha−1). The study suggests that climate warming triggers shifts in vegetation characteristics of high-altitude grasslands in the rugged mountainous terrain, but the magnitude of shift varies according to site characteristics. Under warming, the north-facing site could experience greater vegetation change, characterized by reduced species diversity, species richness, proportion of sedge, and crude protein content.