Has nitrogen progressively limited shrub productivity over recent decades?
The overarching research aim of this thesis was to assess whether recently observed shrub productivity trends in Arctic ecosystems may have been influenced by long-term nitrogen dynamics. The thesis was completed in January 2020 and may be viewed on the Oxford Research Archive.
Ground-based observations indicate that an increase in woody deciduous shrub biomass – ‘shrubification’ – has occurred in the Arctic tundra over recent decades, through three processes: (1) increasing shrub height, (2) patch infilling, and (3) northward expansion. In addition, remotely-sensed ‘greenness’ – a proxy of vegetation productivity (NDVI) – suggests recent Pan-Arctic increases in productivity. However, since the 1980s ‘greenness’ has become progressively decoupled from rising air temperatures. Also, evidence syntheses suggest that shrub sensitivity to climate is spatially heterogeneous. These findings suggest that other environmental controls may be important in controlling shrubification processes.
Nitrogen is highly-limiting in Arctic environments. A key hypothesis to test is whether nitrogen (N) has progressively limited shrub productivity over recent decades. McLauchlan et al (2007) demonstrated the potential to use stable N isotopes from tree rings as an indicator of point-based N availability. This project adopts similar methods to create novel, high-resolution time series of N availability in tundra soils (at the individual-scale) from arctic shrubs, which have rings suitable for the use of dendrochronological techniques. Ring width and N data is used to assess the role of N in Arctic shrub productivity. The study area is the Yamal Peninsula, Russia, a region that contains each of the Arctic bioclimatic zones.
Publications
Andrew C Martin, Marc Macias‐Fauria, Michael B Bonsall, Bruce C Forbes, Pentti Zetterberg, Elizabeth S Jeffers, Common mechanisms explain nitrogen‐dependent growth of Arctic shrubs over three decades despite heterogeneous trends and declines in soil nitrogen availability, New Phytologist 233(2) 2022. https://doi.org/10.1111/nph.17529
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. Accepted Author Manuscript. doi: 10.1088/1748-9326/aa7989
H. Epstein, 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.
