Scaling ozone responses of forest trees to the ecosystem level in a changing climate

Many uncertainties remain regarding how climate change will alter the structure and function of forest ecosystems. At the Aspen FACE experiment in northern Wisconsin, we are attempting to understand how an aspen/birch/maple forest ecosystem responds to long-term exposure to elevated carbon dioxide (CO 2 ) and ozone (O 3 ), alone and in combination, from establishment onward. We examine how O 3 affects the flow of carbon through the ecosystem from the leaf level through to the roots and into the soil microorganisms in present and future atmospheric CO 2 conditions. We provide evidence of adverse effects of O 3 , with or without co-occurring elevated CO 2 , that cascade through the entire ecosystem impacting complex trophic interactions and food webs on all three species in the study: trembling aspen ( Populus tremuloides Michx . ), paper birch ( Betula papyrifera Marsh), and sugar maple ( Acer saccharum Marsh). Interestingly, the negative effect of O 3 on the growth of sugar maple did not become evident until 3 years into the study. The negative effect of O 3 effect was most noticeable on paper birch trees growing under elevated CO 2 . Our results demonstrate the importance of long-term studies to detect subtle effects of atmospheric change and of the need for studies of interacting stresses whose responses could not be predicted by studies of single factors. In biologically complex forest ecosystems, effects at one scale can be very different from those at another scale. For scaling purposes, then, linking process with canopy level models is essential if O 3 impacts are to be accurately predicted. Finally, we describe how outputs from our longterm multispecies Aspen FACE experiment are being used to develop simple, coupled models to estimate productivity gain/loss from changing O 3 . Key-words : Populus tremuloides ; carbon accumulation and allocation; carbon dioxide; C and N cycling; climate change; ecosystem scaling; modelling; pest interactions; trembling aspen; tropospheric ozone. Abbreviations : AAQS, American air quality standards; AOT40, accumulated hourly O 3 exposure over 40 ppb; C 14 , radioactive isotope of carbon; CDIAC, Carbon Dioxide Information Analysis Center; CL, critical level; CWS, Canadian Wide Standard; ECOPHYS, an ecophysiological model to grow individual trees; FACE, Free-Air CO 2 Exposure; LAI, leaf area index; N, nitrogen; PAR, photosynthetically active radiation; ppb, parts per billion; ppbh, parts per billion hours; ppm, parts per million; SOM, soil organic matter; SUM60, accumulated hourly exposure over 60 ppb.