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Sixty-two Years of Shrub Expansion onto Colorado Alpine Tundra

Paper Reviewed
Formica, A., Farrer, E.C., Ashton, I.W. and Suding, K.N. 2014. Shrub expansion over the past 62 years in Rocky Mountain alpine tundra: possible causes and consequences. Arctic, Antarctic, and Alpine Research 46: 616-631.

Formica et al. (2014) introduce their work by noting that "woody plants are encroaching into many herbaceous-dominated communities across the globe, including arctic and alpine tundra," citing Sturm et al. (2001), Stow et al. (2004), Tape et al. (2006), Cannone et al. (2007), Hallinger et al. (2010), Meyers-Smith et al. (2011) and Rundqvist et al. (2011)." And being thus intrigued by this phenomenon, they decided to see if it had also been occurring in the Rocky Mountain alpine tundra of Colorado, USA. This they did by analyzing ortho-rectified aerial photographs of the entirety of Niwot Ridge taken in 1946, 1953, 1972, 1985, 1990, 1999, 2002, 2004, 2005, 2006 and 2008 by Manley et al. (2009). And what did they learn from this endeavor?

The four researchers determined that shrub cover consisting predominantly of two species of willow (Salix planifolia and Salix glauca) increased by an amazing 441% over the 62-year time-span of their database, which they say is "consistent with measurements of shrub increase in the alpine Swedish Scandes," citing increases there of 41% over 13 years (Hedenas et al., 2011), 56%-87% over 33 years (Rundqvist et al., 2011), and 1370% over 98 years (Van Bogaert et al., 2010). And they also write, in this regard, that willow coverage on Niwot Ridge is currently "increasing at an exponential rate, corresponding to increases in carbon storage of 137 kg/ha," which acceleration they additionally note is consistent with the findings of Van Bogaert et al. in Sweden.

And so it is that Formica et al. ultimately conclude - in stark contrast to the doom-and-gloom prognostications of the world's climate alarmists - that "the exponential expansion seen in the aerial photographs represents a gradual recovery of woody vegetation from a history of grazing that is intensified by global change in recent decades."

References
Cannone, N., Sgorbati, S. and Guglielmin, M. 2007. Unexpected impacts of climate change on alpine vegetation. Frontiers in Ecology and the Environment 5: 360-364.

Hallinger, M., Manthey, M. and Wilmking, M. 2010. Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia. New Phytologist 186: 890-899.

Hedenas, H., Olsson, H., Jonasson, C., Bergstedt, J.,Dahlberg, U. and Callaghan, T.V. 2011. Changes in tree growth, biomass and vegetation over a 13-year period in the Swedish sub-Arctic. Ambio 40: 672-682.

Manley, W.F., Parrish, E.G. and Lestak, L.R. 2009. High-resolution ortho-rectified imagery and digital elevation models for study of environmental change at Niwot Ridge and Green Lakes Valley, Colorado. University of Colorado at Boulder, INSTAAR, Niowot Ridge LTER, digital media.

Myers-Smith, I.H., Hik, D.S., Kennedy, C., Cooley, D., Johnstone, J.F., Kenney, A.J. and Krebs, C.J. 2011. Expansion of canopy-forming willows over the twentieth century on Herschel Island, Yukon Territory, Canada. Ambio 40: 610-623.

Rundqvist, S., Hedenas, H., Sandstrom, A., Emanuelsson, U., Eriksson, H., Jonasson, C. and Callaghan, T.V. 2011. Tree and shrub expansion over the past 34 years at the tree-line near Abisko, Sweden. Ambio 40: 683-692.

Stow, D.A., Hope, A., McGuire, D., Verbyla, D., Gamon, J., Huemmrich, F., Houston, S., Racine, C., Sturm, M., Tape, K., Hinzman, L., Yoshikawa, K., Tweedie, C., Noyle, B., Silapaswan, C., Douglas, D., Griffith, B., Jia, G., Epstein, H., Walker, D., Daeschner, S., Petersen, A., Zhou, L.M. and Myneni, R. 2004. Remote sensing of vegetation and land-cover change in Arctic tundra ecosystems. Remote Sensing of Environment 89: 281-308.

Sturm, M., McFadden, J.P., Liston, G.E., Chapin, F.S., Racine, C.H. and Holmgren, J. 2001. Snow-shrub interactions in Arctic tundra: a hypothesis with climatic implications. Journal of Climate 14: 336-344.

Tape, K., Sturm, M. and Racine, C. 2006. The evidence for shrub expansion in northern Alaska and the pan-Arctic. Global Change Biology 12: 686-702.

Van Bogaert, R., Jonasson, C., De Dapper, M. and Callaghan, T.V. 2010. Range expansion of thermophilic aspen (Populus tremula L.) in the Swedish subarctic. Arctic, Antarctic, and Alpine Research 42: 362-375.

Posted 7 January 2015