text: Clearly, elevated O 3 has a number of significant negative impacts on the well-being of North America's most widely distributed tree species, while elevated CO 2 has a number of significant positive impacts. In addition, elevated CO 2 often completely eliminates the negative impacts of elevated O 3 . Hence, if the tropospheric O 3 concentration continues to rise in the future - and it is indeed expected to do so (the authors note that "damaging O 3 concentrations currently occur over 29% of the world's temperate and subpolar forests but are predicted to affect fully 60% by 2100") - we had better hope that the air's CO 2 content continues to rise as well. Without it, we will be in a world of hurt; for the authors report that "since the mid-1990s, aspen use has increased almost exponentially in the US Great Lakes Region and in the boreal mixed wood region of Canada, where the resource is now almost fully utilized."