Can large animals direct the fate of the vast arctic soil carbon reserves – a review

Abstract Arctic areas store vast soil carbon reserves that are highly sensitive to be released into the atmosphere due to a warming climate. Large arctic herbivores may shape this sensitivity, but owing to high spatial and temporal variation in their ecosystem effects, the conditions under which herbivores might negate soil carbon losses have remained elusive. Here, we summarize the main pathways by which ungulates may counteract unwanted climatic feedbacks of the ongoing warming. Firstly, they may counteract the climate-induced shrubification; secondly, induce ecosystem state transitions from shrub and moss dominance to grass and forb dominance; and thirdly, contribute to colder winter soil temperatures. In non-permafrost soils, these pathways feed back on climate mostly via herbivory-induced increases in albedo and, potentially, decreased sensitivity to wildfire-induced soil carbon losses. In permafrost soils, herbivores may additionally enhance soil carbon storage as the colder winter soil temperatures, induced by vegetation change and trampling-associated compaction of snow, may prevent permafrost melting under warming. The role of current large animal populations in the first pathway (i.e., counteracted shrubification) is already documented in many parts of the Arctic. Yet, the second and third pathway (i.e., ecosystem state transitions and snow-mediated permafrost feedbacks have only limited occurrence today and would require drastic increases in the number and diversity of herbivores to change arctic climate feedbacks at a larger scale, imposing a high degree of uncertainty on the feasibility of such efforts. Given the alarming trends the arctic carbon stocks are facing, a better understanding of the contribution of large herbivores to the carbon cycle is more urgent than ever before, relevant if introducing animal populations into areas with large carbon reservoirs, and crucial when evaluating the net effect of current animal populations that already counteract shrubification and the warming-induced increase in albedo in many areas.