Spring leaf flush in aspen (Populus tremuloides) clones is altered by long-term growth at elevated carbon dioxide and elevated ozone concentration

Early spring leaf out is important to the success of deciduous trees competing for light and space in dense forest plantation canopies. In this study, we investigated spring leaf flush and how long-term growth at elevated carbon dioxide concentration (CO₂]) and elevated ozone concentration (O₃]) altered leaf area index development in a closed Populus tremuloides (aspen) canopy. This work was done at the Aspen FACE experiment where aspen clones have been grown since 1997 in conditions simulating the CO₂] and O₃] predicted for ∼2050. The responses of two clones were compared during the first month of spring leaf out when CO₂ fumigation had begun, but O₃ fumigation had not. Trees in elevated CO₂] plots showed a stimulation of leaf area index (36%), while trees in elevated O₃] plots had lower leaf area index (−20%). While individual leaf area was not significantly affected by elevated CO₂], the photosynthetic operating efficiency of aspen leaves was significantly improved (51%). There were no significant differences in the way that the two aspen clones responded to elevated CO₂]; however, the two clones responded differently to long-term growth at elevated O₃]. The O₃-sensitive clone, 42E, had reduced individual leaf area when grown at elevated O₃] (−32%), while the tolerant clone, 216, had larger mature leaf area at elevated O₃] (46%). These results indicate a clear difference between the two clones in their long-term response to elevated O₃], which could affect competition between the clones, and result in altered genotypic composition in future atmospheric conditions.