Borealization of tundra ecosystems with climate and land-use change

As the colder regions of the planet warm, species are moving northward and upward from the boreal forest to the tundra biome, a process that has been referred to as borealization. Here, we examine the diverse uses of the term borealization and propose the concept of ‘tundra borealization’ for terrestrial environments to specifically describe shifts and increases in abundance of boreal species into the tundra. We summarize the evidence to date for borealization of plant and animal communities in tundra ecosystems and the different approaches that can be used to quantify borealization. We discuss how land-use change is interacting with climate change, leading to species and community reorganization in colder biomes, and the consequences of borealization for food webs, ecosystem functions and northern livelihoods. Our review brings together the different definitions and lines of evidence for tundra borealization in terrestrial ecosystems to emphasize this important ecological process and rapidly evolving area of research.