Open-top chambers were used to maintain the four treatment combinations of ambient or elevated (ambient + 300 ppm) atmospheric CO2 crossed with ambient or warmed (ambient + 3°C) air temperatures.
In case of conditioning, lower thermal thresholds at cold ambient temperatures compared to warm ambient temperatures would have been expected, but results showed the opposite with higher TT in November.
Within each tunnel, three plant sections (3 × 1.5 × 1.5 m each) were established where the air temperature was either kept at ambient, or a constant warming of +3.0 °C above ambient (constant warming), or +2.2 °C warming above ambient during the day and +4.0 °C warming during the night (high night-time warming).
In this study, soluble carbohydrate concentrations were enhanced by elevated atmospheric CO2 but decreased by warming under ambient CO2 conditions.
The increase in leaf N concentration under warming at ambient CO2 was reflected in increased litter N concentration.
Tissue C concentrations (g C g−1) through time as affected by atmospheric CO2 concentration, warming treatment, ambient, higher night-time warming and continuous warming, and clipping frequency.
Tissue C : N ratios through time as affected by atmospheric CO2 concentration, warming treatment, ambient, higher night-time warming and continuous warming, and clipping frequency.
Tissue N concentrations (mg N g−1) through time as affected by atmospheric CO2 concentration, warming treatment, ambient, higher night-time warming and continuous warming, and clipping frequency.
Our finding that green leaf tissue N concentration increased in response to warming under ambient atmospheric CO2 concentrations differs from An et al. (2005), who found that warming decreased tissue N concentrations in five grassland species.
The increase in tissue N in response to warming at ambient atmospheric CO2 levels is consistent with the idea that shoot nitrate assimilation depends on photorespiration (Rachmilevitch et al. 2004; Bloom et al. 2010, 2012).
