Background
The authors write that "the boreal forest historically has been considered a carbon sink," but that "autotrophic respiration is [supposedly] more sensitive than photosynthesis to increases in temperature (Ryan, 1991; Amthor, 1994)," and, therefore, that in response to global warming, "most models predict autotrophic respiration will increase at a greater rate than photosynthesis, which infers decreased carbon use efficiency and net primary production (Ryan, 1995; VEMAP Members, 1995; Ryan et al., 1996)," which implies that "a substantial increase in temperature could turn the boreal forest into a carbon source (Goulden et al., 1998)," which positive feedback phenomenon could lead to an intensification of the warming of the globe.

What was done
Working about 20 km south of Thompson, Manitoba, Canada (55°53'N, 98°20'W), within large enclosed greenhouse chambers containing black spruce trees (Picea mariana (Mill.) B.S.P.) and the majority of their fine roots, along with soil-heating cables that were used to warm air and soil temperatures about 5°C over ambient control temperatures, Bronson and Gower measured light-saturated net photosynthesis, foliage respiration and stem respiration in heated and control forest plots during the 2005, 2006 and 2007 growing seasons.

What was learned
The two researchers report that throughout their entire study, "both the older foliage, which developed before the experiment, and the new foliage, developed during the experiment, had similar rates of light-saturated net photosynthesis, foliage respiration and stem respiration across all treatments," which, in their words, "underscores the ability of black spruce to maintain homeostasis in a 5°C warmer environment."

What it means
Noting that many global change models predict a doubling of respiration for every 10°C increase in temperature, Bronson and Gower state, in the concluding sentence of their paper, and in no uncertain terms, that "the results from this and other whole-ecosystem warming experiments do not support this model assumption."

References
Amthor, J.S. 1994. Scaling CO2 photosynthesis relationships from the leaf to the canopy. Photosynthesis Research 39: 321-350.

Goulden, M.L., Wofsy, S.C. and Harden, J.W. 1998. Sensitivity of boreal forest carbon balance to soil thaw. Science 279: 214-217.

Ryan, M.G. 1991. Effects of climate change on plant respiration. Ecological Applications 1: 157-167.

Ryan, M.G., Hunt, E.R., McMurtrie, R.E., Agren, G.I., Aber, J.D., Friend, A.D., Rastetter, E.B., Pulliam, W.M., Raison, R.J. and Linder, S. 1996. Comparing models of ecosystem function for temperate conifer forests. In: Greymeyer, A.I., Hall, D.O., Agren, G.I. and Melillo, J.M. (Eds.), Global Change: Effects on Coniferous Forests and Grasslands. John Wiley, New York, New York, USA, p. 313-361.

VEMAP Members. 1995. Vegetation Ecosystem Modeling and Analysis Project: Comparing biogeography and biogeochemistry models in a continental-scale of terrestrial ecosystem response to climate change and CO2 doubling. Global Biogeochemical Cycles 9: 407-437.