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Quantification of elevated CO2/N impacts on canopy foliage photosynthetic acclimation

A recent study of old-growth forest, suggests that there is no increase in carbon retention by the trees under elevated [CO2]. If mature natural forests are unable to accumulate more C, then this leaves plantation forests as the only option for partial mitigation of rising [CO2] by woody systems. Fast growing coppice poplar systems producing repeated crops of timber could be one system that would remove CO2 from the atmosphere at least over a period of decades, potentially slowing the rate of rise in atmospheric [CO2] and buying time for longer-term solutions. We demonstrated that in the three poplar clones grown under Free Air CO2 Enrichment (EuroFACE) revealed, over 3 seasons, a significant and large increase in light saturated photosynthesis (Asat) was sustained.

However, some transient acclimatory down-regulation of photosynthetic potential at elevated [CO2] occurred in the season following coppice, when net photosynthesis was initially no higher with growth at elevated [CO2] compared to growth at current ambient [CO2]. The lack of sustained and significant acclimation of photosynthetic potential, increased gross primary production and increased growth rates, at both leaf and canopy level, suggest that poplar trees may be able to escape long term acclimatory changes that lead to more severe down regulation of photosynthesis.

Furthemore, we examined the hypothesis whether poplars, grown throughout a complete growth cycle in the field under elevated [CO2], show evidence of biochemical acclimation in terms of accumulation of leaf carbohydrates and decreased expression of key enzymes of photosynthetic carbon metabolism.

We demonstrated that:
- Poplar trees sustain close to the predicted increase in leaf photosynthesis when grown under long term elevated CO2 concentration ([CO2]).
- Young expanding leaves of P.x euramericana become photosynthetically competent more rapidly in elevated CO2. This aspect of has not been considered in previous studies and may contribute significantly to CO2 uptake.
- No increase in the levels of soluble carbohydrates was observed, in young expanding or mature sun leaves.
- Substantial increases in starch levels were observed in the mature leaves of all three poplar genotypes
- No changes in the expression of photosynthetic Calvin cycle proteins, or in the starch biosynthetic enzyme AGPase, were observed.
- No long-term photosynthetic acclimation to CO2 occurred in these plants.
- Poplar trees are escaper from long term, acclimatory down regulation of photosynthesis through a high capacity for starch synthesis and carbon export.

These results are very significant with respect to managing carbon sequestration. They show that acclimatory loss of the initial increase in photosynthetic rate under elevated [CO2] is not inevitable at least when trees have a high genetic productivity potential and are grown with adequate nutrients and water. Plantations may therefore be developed that could show a continued stimulation of CO2 uptake in proportion to the rise in atmospheric [CO2]. Given that canopy photosynthetic carbon gain in these species is proportional to wood increment, this implies that the increased photosynthesis will result in more carbon in wood. The residence of the additional carbon in the biosphere will depend on subsequent use of the harvested wood. Importantly, and in sharp contrast to old-growth forest, these results suggest that there are management options for creating forest plantations which can achieve increased sequestration of carbon into wood as atmospheric [CO2] rises. Specifically, these results indicate that poplar species, selected for rapid growth, may be well suited to a future elevated [CO2] environment and particularly suited to afforestation projects aimed to increase carbon uptake into wood in the near-term.

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Reported by

Wivenhoe Park John Tabor Laboratories
United Kingdom
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