Simulating the effects of temperature and precipitation change on vegetation composition in Arctic tundra ecosystems

Over the past decades, vegetation has changed significantly along with climatic changes in the Arctic. Deciduous shrub cover is often assumed to expand in tundra landscapes, but more frequent abrupt permafrost thaw resulting in formation of thaw ponds could lead to vegetation shifts towards graminoid dominated wetland. Which mechanisms drive vegetation changes in the tundra ecosystem is still not sufficiently clear. In this study, the dynamic tundra vegetation model NUCOM-tundra was used to evaluate the consequences of climate change scenarios of warming and increasing precipitation 15 for future tundra vegetation change, and to identify the mechanisms that drive these changes. The model includes three plant functional types (moss, graminoids and shrubs), carbon and nitrogen cycling, water and permafrost dynamics and a simple thaw pond module. Climate scenario simulations were performed for sixteen combinations of temperature and precipitation increases in five vegetation types representing a gradient from dry shrub dominated, to moist mixed and wet graminoid dominated sites. Vegetation composition dynamics in currently mixed vegetation sites was dependent on both temperature 20 and precipitation changes, with warming favouring shrub dominance and increased precipitation favouring graminoid abundance. Climate change simulations based on greenhouse gas emission scenarios in which temperature and precipitation increases were combined showed initial increases in graminoid abundance followed by shrub expansion with further climate change. The simulations suggest that the shrubs are better light competitors, but their growth can be limited by very wet soil conditions and low nutrient supply. Graminoids have the advantage of being able to grow in a wide range of soil moisture 25 conditions and having access to nutrients in deeper soil layers. Abrupt permafrost thaw initiating thaw pond formation led to complete domination of graminoids. However, due to increased drainage, shrubs could profit from such changes in adjacent areas. Both climate and thaw pond formation simulations suggest that a wetter tundra can be responsible for local shrub decline instead of shrub expansion.