What was done
The authors evaluated the process-based LPJ-GUESS model of vegetation dynamics and biogeochemistry (Smith et al., 2001; Hickler et al., 2004) via a site-by-site comparison with the results of four temperate forest FACE experiments (Norby et al., 2005). Then, after demonstrating that the model simulations adequately reproduced the magnitude of the FACE site measurements -- a mean model-derived net primary productivity (NPP) increase of 25.9% for CO2 raised to a value of 550 ppm vs. a mean measured NPP increase of 27.8% for the same CO2 increase -- they conducted what they called a "global forest FACE experiment" to see what the successfully-reality-tested model suggested about CO2 enrichment effects on the NPP of earth's boreal and tropical forests, as well as its temperate forests.

What was learned
For the world as a whole, the model suggested that raising the air's CO2 concentration to 550 ppm would increase the NPP of its temperate forests by an average of 25.7%, but that its boreal forests would only have their NPP raised by 15.1%, while its tropical forests would experience an NPP increase of 35.1%.

What it means
The results of this exercise clearly demonstrate that the growth-promoting effects of atmospheric CO2 enrichment are strongly temperature dependent, more than doubling when moving from boreal forests to tropical forests, in harmony with the findings of numerous other studies described in Journal Reviews we have archived under the heading of Growth Response to CO2 with Other Variables (Temperature - Woody Plants) in our Subject Index. However, Hickler et al. note that warming "is likely to increase NPP more in cold northern regions than close to the equator because of a greater proportional growing season extension in temperature-limited environments." Consequently, in the case of concomitant CO2 enrichment and warming -- the latter of which is typically greater at higher latitudes -- all of earth's forests might well be equally hugely benefited.

References
Hickler, T., Smith, B., Sykes, M.T., Davis, M., Sugita, S. and Walker, K. 2004. Using a generalized vegetation model to simulate vegetation dynamics in northeastern USA. Ecology 85: 519-530.

Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King, S.J., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., De Angelis, P., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarasci-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences 102: 18,052-18,056.

Smith, B., Prentice, I.C. and Sykes, M.T. 2001. Representation of vegetation dynamics in the modeling of terrestrial ecosystems: comparing two contrasting approaches within European climate space. Global Ecology & Biogeography 10: 621-637.