LTREB: Long-Term Consequences of Asymmetric Warming at the Tundra-Taiga Interface

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The location of the Arctic treeline (the northernmost extent of trees) has important implications for carbon cycling, land surface energy exchange, wildlife habitat, and the availability of subsistence resources to Arctic human communities. The prevailing theory of treeline formation states that growth of treeline trees is strictly limited by cold growing-season air temperature. While this perspective is superficially supported by global-scale correlations between summer air temperature and treeline elevation, it is not supported by the observation that only about 50% of global treelines have advanced in response to anthropogenic climate warming. There is increasing evidence that other factors can determine treeline position, particularly in the Arctic. This project tests the hypothesis that warming winters with deepening snowpacks increase the growth and reproduction of treeline trees, thereby fueling treeline advance into Arctic tundra. Results are improving predictions of where and under what circumstances future treeline advance can be expected. The project provides education and training opportunities for graduate students and native residents of northwest Alaska. The Arctic is warming much faster than the rest of the globe and the warming is seasonally asymmetrical, with modest warming during the summer months and extreme warming during the winter months. In many regions of the Arctic, extreme winter warming is coupled with increasing snowfall. The combination of extreme winter warming and deeper, insulating winter snowpacks will lead to rapid winter soil warming, with wide-ranging consequences for ecosystem function. Warmer winter soils are generally associated with greater overwinter microbial activity, enhanced breakdown of soil organic matter, and a flush of nutrients that may be available for uptake by trees following snowmelt. This project leverages long-term observations of winter snowpack and corresponding tree growth in the Alaskan Arctic. It also extends a snowfence experiment, which builds deeper snowpacks around individual trees at three contrasting treelines. Long-term measurements are essential to identify the effects of seasonally asymmetric warming on treeline position in the Arctic, because of high interannual variability in winter weather and time lags associated with altered soil nutrient cycling, storage of resources in trees, and the episodic nature of tree investment in reproduction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.