The Arctic biome is at significant risk, with recent observations suggesting thatclimate change is warming the Arctic nearly four times faster than the globalaverage. Last decade, evidence from experimental warming studies andobservations of ambient warming over time shows how increasing airtemperature in the Arctic has led to changes to arctic vegetation, andencroachment of trees and shrubs into the tundra. Thus, this amplified Arcticwarming is threatening biodiversity, changing vegetation patterns, and thawingpermafrost with implications for carbon and nutrient dynamics. These are oneof the main concerns of observed plant biodiversity changes (except the lossof biodiversity itself) as they feedback on the global climate through theireffects on carbon cycling, albedo, and ecosystem energy balance. Studies ofArctic biodiversity have reported responses in either taxonomic, functional, orphylogenetic diversity, though phylogenetic has so far been understudied inthe Arctic. These different measures of quantifying biodiversity will vary intheir explanatory value and can have complementary value when looking atthe implications of vegetation changes. The overall aim of this thesis is todeepen the knowledge of the effect of ambient and experimental climatewarming on taxonomic, functional, and phylogenetic aspects of plant diversitywithin and between communities.In Latnjajaure (northern Sweden) I used a long-term passive warmingexperiment using open-top chambers, which include five distinct plantcommunities. The communities had distinct soil moisture conditions, leadingto community-specific responses of the plant growth forms (deciduous shrubs,evergreen shrubs, forbs, and graminoids) and phylogenetic dissimilarity. Moistcommunities tended to decrease in soil moisture, which drove similarity todryer, more nutrient-poor communities. Warming significantly affectedgrowth forms, but the direction of the response was not consistent across thecommunities. Evidence of shrub expansion was found in nearly allcommunities, with soil moisture determining whether it was driven bydeciduous or evergreen shrubs. These changes are expected to affect climatefeedback as the dry, evergreen-dominated heath community, has slower carboncycling. This slowdown in carbon cycling is at least partially due to theevergreen shrubs whose material is harder to decompose than most other arcticvegetation. As the studied communities are common in the region, it is likelythat future warming will drive community shifts in the tundra landscape.On a Pan-arctic dataset of warming studies, I explored the effect of scalingabundance weighting as well as the importance of deeper against shallownodes in the phylogeny on warming response and its interaction with soil moisture and site temperature in the tundra biome. For all metrics, we lookedat both plot level (α-diversity), and the difference between plots (βdissimilarity). We show that β-dissimilarity is more sensitive to warming thanα-diversity metrics. Furthermore, we show that sensitivity to abundance andphylogenetic weighting depends on local soil moisture conditions.In conclusion, the combined use of taxonomic, phylogenetic, and functionaldiversity measures enhances the quality of our assessment of the implicationsof arctic vegetation response to warming.