Shrub growth and expansion in the Arctic tundra: an assessment of controlling factors using an evidence-based approach.

Background Woody shrubs have increased in biomass and expanded into new areas throughout the Pan-Arctic tundra biome in recent decades, which has been linked to a biome-wide observed increase in productivity [1]. Experimental, observational, and socio-ecological research suggests that air temperature—and to a lesser degree precipitation—trends have been the predominant drivers of this change. However, a progressive decoupling of these drivers from Arctic vegetation productivity has been reported, and since 2010, vegetation productivity has also been declining. Method We explored the potential of using systematic evaluation methodologies [2] to determine the extent and characteristics of the published evidence for temperature-precipitation - and other - controls on Arctic shrub growth and expansion. We created a protocol to (a) identify the suite of controls that may be operating on shrub growth and expansion, and (b) map the evidence through space and time. Results We found evidence for a suite of 23 proximal controls that operate directly on shrub growth and expansion; the evidence base focused predominantly on just four controls (air temperature, soil moisture, herbivory, and snow dynamics). 65% of evidence was generated in the warmest tundra climes, while 24% was from only one of 28 floristic sectors. Temporal limitations beyond 10 years existed for most controls, while the use of space-for-time approaches was high, with 14% of the evidence derived via experimental approaches. Conclusions The findings suggest that the current evidence base is not sufficiently robust or comprehensive at present to answer key questions of Pan-Arctic shrub change [3]. We suggest future directions that could strengthen the evidence, and lead to an understanding of the key mechanisms driving changes in Arctic shrub environments. References 1. Epstein, H., U. Bhatt, M. Raynolds, D. Walker, B. C. Forbes, T. Horstkotte, M. Macias-Fauria, A. Martin, G. Phoenix, J. Bjerke, H. Tømmervik, P. Fauchald, H. Vickers, R. Myneni, C. Dickerson, 2017: Tundra Greenness [in Arctic Report Card 2017], http://www.arctic.noaa.gov/Report-Card. 2. Pullin A, Frampton G, Jongman R, Kohl C, Livoreil B, Lux A, Pataki G, Petrokofsky G, Podhora A, Saarikoski H et al. 2016. Selecting appropriate methods of knowledge synthesis to inform biodiversity policy. Biodiversity and Conservation 25(7): 1285-1300 DOI:10.1016/j.ecolecon.2016.03.023 3. Martin A.C., Jeffers E.S., Petrokofsky, G., Myers-Smith, I, and Macias-Fauria, M. (2017), Shrub growth and expansion in the Arctic tundra: an assessment of controlling factors using an evidence-based approach. Environmental Research Letters 12 (8). doi: 10.1088/1748-9326/aa7989