The effects of elevated CO2 and canopy position on chlorophyll concentration in mature Quercus robur.

The timings of phenological events play an important role in determining the annual carbon uptake in key terrestrial carbon sinks, such as mature forests. With increases in atmospheric CO₂ expected to change physiological processes in plants, it is becoming increasingly important to monitor the changes in plant traits and subsequent phenological changes that may occur. Changes in photosynthetic pigments, such as chlorophyll, can be used as a proxy for physiological changes in leaves and can therefore be useful to monitor potential phenological change, such as autumnal leaf senescence. Non-destructive techniques allow for measurements of photosynthetic pigments without destructive sampling that would disturb the canopy. These methods are particularly useful in logistically difficult environments, such as high forest, or remote environments where traditional chlorophyll extractions are problematic and serve as ground-truthing for remote sensing of greenness. In the present study, we aimed to assess the effects of elevated CO₂ (150 mmol mol^-1 above ambient) and canopy position on chlorophyll concentrations of a common canopy-dominant species to identify potential implications on phenology. The study was conducted in a mature temperate forest situated at a Free Air Carbon Enrichment (FACE) experiment in the UK. Over 5,000 in-situ chlorophyll measurements were collected, across the 3^rd and 4^th season of CO₂ fumigation, in the canopy-dominant species Quercus robur (Q. robur). Additionally, 100 leaves were destructively sampled to verify chlorophyll concentrations using traditional chlorophyll extraction techniques. The established relationship between chlorophyll absorptance readings and leaf chlorophyll content allowed robust species-specific calibration equations to be calculated. Consistent with previous work, this study observed significantly higher chlorophyll concentrations at lower positions in the canopy in both sampling years (P < 0.001). Additionally, a reduction in foliar chlorophyll concentrations (-2 to -9%) when exposed to eCO₂ in both sampling years was observed, but this was only significant for the upper canopy (-7 to -9%, P < 0.05). This study found a marginally significant effect of CO₂ treatment on reducing the effective season length, with larger eCO₂-induced reductions in chlorophyll occurred through autumn. Overall, the research highlights a simple non-invasive method for monitoring changes in leaf traits of mature trees under eCO₂. The results suggest that leaves may be able to reallocate their resources away from light-harvesting apparatus in response to eCO₂, particularly in the upper canopy. Furthermore, the findings suggest direct consequences of rising atmospheric CO₂ to potential alterations of phenological events, such as leaf senescence, that may have implications for forest productivity and adaptation in a future high CO₂ world. Additionally, the research has shown the need to monitor potential changes in resource allocation to photosynthetic apparatus across the season as atmospheric CO₂ continues to rise. The information obtained in this study can be used to increase accuracy in the modelling of climate-carbon scenarios.