Tropospheric 0₃ moderates responses of temperate hardwood forests to elevated CO₂: a synthesis of molecular to ecosystem results from the Aspen FACE project

Abstract

1. The impacts of elevated atmospheric CO₂ and/or O₃ have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, 0₃-sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera, and the determinate, late successional, 0₃-tolerant species Sugar Maple, Acer saccharum). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for 0₃-tolerant as well as 0₃-sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1?5 x ambient, with ambient averaging 34-36 nL L^-1 during the summer daylight hours), 0₃ offsets or moderates the responses induced by elevated CO₂. 3. After 3 years of exposure to 560 ?mol mol^-1 CO₂, the above-ground volume of Aspen stands was 40% above those grown at ambient CO₂, and there was no indication of a diminishing growth trend. In contrast, 0₃ at 1?5 x ambient completely offset the growth enhancement by CO₂, both for 0₃-sensitive and 0₃-tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO₂, and global models of forest productivity and climate change are presented.

Citation

Karnosky, D. F.; Zak, D. R.; Pregitzer, K. S.; Awmack, C. S.; Bockheim, J. G.; Dickson, R. E.; Hendrey, G. R.; Host, G. E.; King, J. S.; Kopper, B. J.; Kruger, E. L.; Kubiske, M. E.; Lindroth, R. L.; Mattson, W. J.; McDonald, E. P.; Noormets, A.; Oksanen, E.; Parsons, W. F. J.; Percy, K. E.; Podila, G. K.; Riemenschneider, D. E.; Sharma, P.; Thakur, R.; Sôber, A.; Sôber, J.; Jones, W. S.; Anttonen, S.; Vapaavuori, E.; Mankovska, B.; Heilman, W.; Isebrands, J. G. 2003. Tropospheric 0₃ moderates responses of temperate hardwood forests to elevated CO₂: a synthesis of molecular to ecosystem results from the Aspen FACE project. Functional Ecology 17:289-304