Background
The authors of this ground-breaking investigation note that "although there have been many studies on the effects of elevated CO2 on the interaction between plants and their insect herbivores (see Bezemer and Jones, 1998; Hunter, 2001), comparable studies on mammalian herbivores are lacking altogether, even though mammals play important roles in dynamics of many ecosystems (McNaughton and Sabuni, 1988; Pastor and Naiman, 1992)."

What was done
In one experiment, Mattson et al. grew one-year-old seedlings of silver birch (Betula pendula) in closed-top chambers for one summer and autumn in pots containing an unfertilized commercial peat maintained at three different soil nitrogen (N) levels (low = 0 kg N ha^-1, medium = 150 kg N ha^-1, high = 500 kg N ha^-1) and two temperature (T) levels (ambient and ambient + 3°C) in air of either 362 or 700 ppm CO2 concentration, after which feeding trials with caged Eurasian hares (Lepus timidus) were carried out and a number of chemical analyses were made of the tops of the seedlings and the basal parts of their stems.  In a second experiment, they grew paper birch (Betula papyrifera) from seed for two 140-day growing seasons in well-watered and fertilized pots placed within FACE rings maintained at atmospheric CO2 concentrations of either 362 or 562 ppm, after which (in an unplanned aspect of the study) North American eastern cottontail rabbits (Sylvilagus floridanus) fed ad libitum, consuming bark tissue down to and scoring the wood, on the basal third of the seedlings, which tissues were also tested for the presence of various herbivore-deterring chemical constituents.

What was learned
"As expected," in the words of the authors, "elevated CO2 substantially increased the above-ground woody biomass growth of both paper birch (63%) and silver birch (21%)."  In addition, noting that "numerous studies have shown that elevated atmospheric CO2 often, but not always, elicits increases in carbon partitioning to carbon-based secondary plant compounds," which often act as deterrents to herbivory, they say that their findings "confirm this general pattern in silver and paper birch."  Last of all, they report that high CO2 reduced hare feeding on silver birch shoots by as much as 48%, and that it reduced rabbit feeding on paper birch stems by about 51%, while neither temperature nor severe early-season defoliation (another experimental treatment) affected tree resistance against these mammalian herbivores.

What it means
Calling the anti-herbivory effect of elevated CO2 "remarkably strong," and noting that rabbits "overwhelmingly preferred ambient CO2 plants," Mattson et al. say their data "clearly suggest that the defensive biochemistry of paper birch twigs as well as the main stem were [positively] altered as the result of elevated CO2."  Hence, we can expect that as the air's CO2 content continues to rise, at least these two species of birch trees will have a significantly easier time of getting established and growing to maturity, in that they will likely not be hassled nearly as much by rabbits and hares munching away at their trunks and branches while in their early growth years.  It will be interesting to see just how general this response proves to be as other similar experiments are conducted on still other woody species.

References
Bezemer, T.M. and Jones, T.H.  1998.  Plant-insect herbivore interactions in elevated atmospheric CO2: quantitative analyses and guild effects.  Oikos 82: 212-222.

Hunter, M.D.  2001.  Effects of elevated atmospheric carbon dioxide on insect-plant interactions.  Agricultural and Forest Entomology 3: 153-159.

McNaughton, S.J. and Sabuni G.A.  1988.  Large African mammals as regulators of vegetation structure.  In: Werger, M.J.A., Van der Aart, P.J.M., During, H.J. and Verhoeven, J.T.A.  (Eds.), Plant Form and Vegetation Structure.  SPB Academic Publishing, The Hague, The Netherlands, pp. 339-354.

Pastor, J. and Naiman, R.J.  1992.  Selective foraging and ecosystem processes in boreal forest.  American Naturalist 139: 690-705.