Background
Since we recently reviewed a study of tropical productivity trends over the last two decades of the 20th century (Ichii et al., 2005), we here do the same for a study of boreal productivity trends.

What was done
The authors transformed satellite-derived Normalized Difference Vegetation Index (NDVI) data obtained across boreal North America (Canada and Alaska) for the period 1982-2003 into photosynthetically-active radiation absorbed by green vegetation and treated the result as a proxy for relative June-August gross photosynthesis (Pg), stratifying the results by vegetation type and comparing them with spatially-matched concomitant trends in surface air temperature data.

What was learned
Over the full course of the study, area-wide tundra experienced a significant increase in Pg in response to a similar increase in air temperature; and Goetz et al. note that "this observation is supported by a wide and increasing range of local field measurements characterizing elevated net CO2 uptake (Oechel et al., 2000), greater depths of seasonal thaw (Goulden et al., 1998), changes in the composition and density of herbaceous vegetation (Chapin et al., 2000; Epstein et al., 2004), and increased woody encroachment in the tundra areas of North America (Sturm et al., 2001)."  In the case of interior forest, on the other hand, there was no significant increase in air temperature and essentially no change in Pg, with the last data point of the series being essentially indistinguishable from the first.

What it means
Plant productivity typically rises with increases in air temperature and atmospheric CO2 concentration; and when both rise together, plants are benefited most of all (see Temperature x CO2 Interaction in our Subject Index).  This is the situation in which North American tundra ecosystems found themselves over the past couple of decades.  If temperatures are fairly constant and on the cool side, however, there is obviously no temperature-induced increase in photosynthesis, and the growth-promoting effects of increasing atmospheric CO2 levels are often very small or even non-existent, which is what appears to have been the case with North American boreal forests over the same time period.  As a result, Canada's and Alaska's tundra ecosystems exhibited increasing productivity over the past couple of decades, while their boreal forests did not.

References
Chapin III, F.S., McGuire, A.D., Randerson, J., Pielke, R., Baldocchi, D., Hobbie, S.E., Roulet, N., Eugster, W., Kasischke, E., Rastetter, E.B., Zimov, S.A., and Running, S.W.  2000.  Arctic and boreal ecosystems of western North America as components of the climate system.  Global Change Biology 6: 211-223.

Epstein, H.E., Calef, M.P., Walker, M.D., Chapin III, F.S. and Starfield, A.M.  2004.  Detecting changes in arctic tundra plant communities in response to warming over decadal time scales.  Global Change Biology 10: 1325-1334.

Goulden, M.L., Wofsy, S.C., Harden, J.W., Trumbore, S.E., Crill, P.M., Gower, S.T., Fries, T., Daube, B.C., Fan, S.M., Sutton, D.J., Bazzaz, A. and Munger, J.W.  1998.  Sensitivity of boreal forest carbon balance to soil thaw.  Science 279: 214-217.

Oechel, W.C., Vourlitis, G.L., Verfaillie, J., Crawford, T., Brooks, S., Dumas, E., Hope, A., Stow, D., Boynton, B., Nosov, V. and Zulueta, R.  2000.  A scaling approach for quantifying the net CO2 flux of the Kuparuk River Basin, Alaska.  Global Change Biology 6: 160-173.

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