Tundra plant communities under woody vegetation expansion: Current knowledge and research gaps

Rapid warming in high-latitude regions is having drastic impacts on vegetation in terrestrial arctic ecosystems. As temperatures rise, warm-adapted boreal species are expanding into tundra, a process termed “borealisation”. Woody plants have been especially successful in exploiting these warming conditions and the northward and upward expansion of the treeline into treeless tundra ecosystems as well as increasing shrub extent, cover, and height (termed “shrubification”) have been widely documented in recent decades. These changes are projected to continue spreading in the future and increases in woody vegetation may, in turn, be associated with subsequent changes in composition of the underlying vegetation, but these changes remain poorly understood. Here, we conducted a systematic literature review to synthesise existing knowledge on vegetation change in the tundra and the bordering treeline ecotone in high latitudes, with a particular emphasis on how increasing woody vegetation affects the underlying vegetation. Our review shows that while there is extensive research on the patterns of shrubification and advances of the treeline, as well as on changes in tundra vascular plant communities in response to climate change and herbivory, these two topics are seldom linked. Among the 200 papers reviewed, only 13 cover this intersection, most of which originate from northern Fennoscandia. Across these studies, the impact of woody vegetation is strongest on cryptogams and tundra specialists, with forbs exhibiting differing responses. While most studies report a decline in diversity, mostly measured as species richness, with increasing woody vegetation, others report a hump shaped curve. This decline is often assumed to be caused by increased competition, especially for light. As these findings are based on a small number of studies from few locations, this review therefore highlights a knowledge gap in this area and underscores the need for ecosystem-level studies of borealisation.