Impact of climate warming on Arctic plant diversity: phylogenetic diversity unravels opposing shrub responses in a warming tundra

Abstract: The Arctic biome is at significant risk, with recent observations suggesting that climate change is warming the Arctic nearly four times faster than the global average. Last decade, evidence from experimental warming studies and observations of ambient warming over time shows how increasing air temperature in the Arctic has led to changes to arctic vegetation, and encroachment of trees and shrubs into the tundra. Thus, this amplified Arctic warming is threatening biodiversity, changing vegetation patterns, and thawing permafrost with implications for carbon and nutrient dynamics. These are one of the main concerns of observed plant biodiversity changes (except the loss of biodiversity itself) as they feedback on the global climate through their effects on carbon cycling, albedo, and ecosystem energy balance. Studies of Arctic biodiversity have reported responses in either taxonomic, functional, or phylogenetic diversity, though phylogenetic has so far been understudied in the Arctic. These different measures of quantifying biodiversity will vary in their explanatory value and can have complementary value when looking at the implications of vegetation changes. The overall aim of this thesis is to deepen the knowledge of the effect of ambient and experimental climate warming on taxonomic, functional, and phylogenetic aspects of plant diversity within and between communities. In Latnjajaure (northern Sweden) I used a long-term passive warming experiment using open-top chambers, which include five distinct plant communities. The communities had distinct soil moisture conditions, leading to community-specific responses of the plant growth forms (deciduous shrubs, evergreen shrubs, forbs, and graminoids) and phylogenetic dissimilarity. Moist communities tended to decrease in soil moisture, which drove similarity to dryer, more nutrient-poor communities. Warming significantly affected growth forms, but the direction of the response was not consistent across the communities. Evidence of shrub expansion was found in nearly all communities, with soil moisture determining whether it was driven by deciduous or evergreen shrubs. These changes are expected to affect climate feedback as the dry, evergreen-dominated heath community, has slower carbon cycling. This slowdown in carbon cycling is at least partially due to the evergreen shrubs whose material is harder to decompose than most other arctic vegetation. As the studied communities are common in the region, it is likely that future warming will drive community shifts in the tundra landscape. On a Pan-arctic dataset of warming studies, I explored the effect of scaling abundance weighting as well as the importance of deeper against shallow nodes in the phylogeny on warming response and its interaction with soil moisture and site temperature in the tundra biome. For all metrics, we looked at both plot level (α-diversity), and the difference between plots (β- dissimilarity). We show that β-dissimilarity is more sensitive to warming than α-diversity metrics. Furthermore, we show that sensitivity to abundance and phylogenetic weighting depends on local soil moisture conditions. In conclusion, the combined use of taxonomic, phylogenetic, and functional diversity measures enhances the quality of our assessment of the implications of arctic vegetation response to warming.