Future atmospheric CO2 leads to delayed autumnal senescence

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 CO₂ Enrichment Experiments (AspenFACE and PopFACE), we present evidence from two continents and over 2 years that increasing atmospheric CO₂ acts directly to delay autumnal leaf coloration and leaf fall. In an atmosphere enriched in CO₂ (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 CO₂. This was supported by a significant delay in the decline of autumnal canopy leaf area index in elevated as compared with ambient CO₂, and a significantly smaller decline in end of season leaf chlorophyll content. Leaf level photosynthetic activity and carbon uptake in elevated CO₂ during the senescence period was also enhanced compared with ambient CO₂. The findings reveal a direct effect of rising atmospheric CO₂, 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 CO₂ world.