Background
Remote sensing data suggest that tundra vegetation in North America may be responding to recent warming via enhanced photosynthetic activity (Goetz et al., 2005; Verbyla , 2008), and the authors write that "at a circumpolar scale, the highest photosynthetic activity and strongest growth trends are reported in locations characterized by erect shrub tundra (Reynolds et al., 2006)," noting that "live leaf phytomass from deciduous shrubs, shown to have increased in northern Alaska during the second half of the last century (Sturm et al., 2001; Tape et al., 2006), is believed to be a key driver of the observed trends (Jia et al., 2003; Goetz et al., 2005; Verbyla, 2008)."

What was done
Working with Salix lanata L. (sensu latu) -- an abundant deciduous dioecious willow with nearly circumpolar geographic distribution from the northern boreal forest to the northern limits of the Low Arctic -- Forbes et al. analyzed annual ring growth for 168 stem slices of 2- to 3-cm thickness that they collected from 40 discrete individuals spread across 15 sample sites within an area of approximately 3 x 2.3 km, which was located at about 68°40'N, 58°30'E.

What was learned
The three researchers say they found "a clear relationship with photosynthetic activity for upland vegetation at a regional scale for the period 1981-2005, confirming a parallel 'greening' trend reported for similarly warming North American portions of the tundra biome," and they state that "the standardized growth curve suggests a significant increase in shrub willow growth over the last six decades."

What it means
Noting that "the quality of the chronology as a climate proxy is exceptional," Forbes et al. state that their findings "are in line with field and remote sensing studies that have assigned a strong shrub component to the reported greening signal since the early 1980s," adding that the growth trend agrees with the qualitative observations of nomadic reindeer herders, which suggest there have been "recent increases in willow size in the region." In fact, they say that their analysis "provides the best proxy assessment to date that deciduous shrub phytomass has increased significantly in response to an ongoing summer warming trend," as the warming- and CO2-induced Greening of the Earth continues almost everywhere.

References
Goetz, S.J., Bunn, A.G., Fiske, G.J. and Houghton, R.A. 2005. Satellite-observed photosynthetic trends across boreal North America associated with climate and fire disturbance. Proceedings of the National Academy of Sciences USA 102: 13,521-13,525.

Jia, G.J., Epstein, H.E. and Walker, D.A. 2003. Greening of arctic Alaska, 1981-2001. Geophysical Research Letters 30: 31-33.

Reynolds, M.K., Walker, D.A. and Maier, H.A. 2006. NDVI patterns and phytomass distribution in the circumpolar Arctic. Remote Sensing of Environment 102: 271-281.

Sturm, M., Racine, C. and Tape, K. 2001. Increasing shrub abundance in the Arctic. Nature 411: 546-547.

Tape, K., Sturm, M. and Racine, C.H. 2006. The evidence for shrub expansion in northern Alaska and the Pan-Arctic. Global Change Biology 32: 686-702.

Verbyla, D. 2008. The greening and browning of Alaska based on 1982-2003 satellite data. Global Ecology and Biogeography 17: 547-555.