Summary• Expression of 4600 poplar expressed sequence tags (ESTs) was studied over the [2001][2002] growing seasons using trees of the moderately ozone (O 3 )-tolerant trembling aspen ( Populus tremuloides ) clone 216 exposed to elevated CO 2 and /or O 3 for their entire 5-yr life history.• Based on replication of the experiment in years 2001 and 2002, 238 genes showed qualitatively similar expression in at least one treatment and were retained for analysis. Of these 238 genes, 185 were significantly regulated (1.5-fold) from one year to the other in at least one treatment studied. Less than 1% of the genes were regulated 2-fold or more.• In the elevated CO 2 treatment, relatively small numbers of genes were upregulated, whereas in the O 3 treatment, higher expression of many signaling and defense-related genes and lower expression of several photosynthesis and energyrelated genes were observed. Senescence-associated genes (SAGs) and genes involved in the flavanoid pathway were also up-regulated under O 3 , with or without CO 2 treatment. Interestingly, the combined treatment of CO 2 plus O 3 resulted in the differential expression of genes that were not up-regulated with individual gas treatments.• This study represents the first investigation into gene expression following longterm exposure of trees to the interacting effects of elevated CO 2 and O 3 under field conditions. Patterns of gene-specific regulation described in this study correlated with previously published physiological responses of aspen clone 216.

Growing seasons are getting longer, a phenomenon partially explained by increasing global temperatures. Recent reports suggest that a strong correlation exists between warming and advances in spring phenology but that a weaker correlation is evident between warming and autumnal events implying that other factors may be influencing the timing of autumnal phenology. Using freely rooted, field-grown Populus in two Free Air CO 2 Enrichment Experiments (AspenFACE and PopFACE), we present evidence from two continents and over 2 years that increasing atmospheric CO 2 acts directly to delay autumnal leaf coloration and leaf fall. In an atmosphere enriched in CO 2 (by $ 45% of the current atmospheric concentration to 550 ppm) the end of season decline in canopy normalized difference vegetation index (NDVI) -a commonly used global index for vegetation greenness -was significantly delayed, indicating a greener autumnal canopy, relative to that in ambient CO 2 . This was supported by a significant delay in the decline of autumnal canopy leaf area index in elevated as compared with ambient CO 2 , and a significantly smaller decline in end of season leaf chlorophyll content. Leaf level photosynthetic activity and carbon uptake in elevated CO 2 during the senescence period was also enhanced compared with ambient CO 2 . The findings reveal a direct effect of rising atmospheric CO 2 , independent of temperature in delaying autumnal senescence for Populus, an important deciduous forest tree with implications for forest productivity and adaptation to a future high CO 2 world.