1. Does enhanced whole-plant CO2 exchange in moderately low to high light occur at the cost of greater CO2 loss rates at very-low light levels? We examined this question for first-year seedlings of intolerant Populus tremuloides and Betula papyrifera, intermediate Betula alleghaniensis, and tolerant Ostrya virginiana and Acer saccharum grown in moderately low (7·3% of open-sky) and low (2·8%) light. We predicted that, compared with shade-tolerant species, intolerant species would have characteristics leading to greater whole-plant CO2 exchange rates in moderately low to high light levels, and to higher CO2 loss rates at very-low light levels.
2. Compared with shade-tolerant A. saccharum, less-tolerant species grown in both light treatments had greater mass-based photosynthetic rates, leaf, stem and root respiration rates, leaf mass:plant mass ratios and leaf area:leaf mass ratios, and similar whole-plant light compensation points and leaf-based quantum yields.
3. Whole-plant CO2 exchange responses to light (0·3–600 µmol quanta m−2 s−1) indicated that intolerant species had more positive CO2 exchange rates at all but very-low light (< 15 µmol quanta m−2 s−1). In contrast, although tolerant A. saccharum had a net CO2 exchange disadvantage at light > 15 µmol quanta m−2 s−1, its lower respiration resulted in lower CO2 losses than other species at light < 15 µmol quanta m−2 s−1.
4. Growth scaled closely with whole-plant CO2 exchange characteristics and especially with integrated whole-plant photosynthesis (i.e. leaf mass ratio × in situ leaf photosynthesis). In contrast, growth scaled poorly with leaf-level quantum yield, light compensation point, and light-saturated photosynthetic rate.
5. Collectively these patterns indicated that: (a) no species was able to both minimize CO2 loss at very-low light (i.e. < 15 µmol quanta m−2 s−1) and maximize CO2 gain at higher light (i.e. > 15 µmol quanta m−2 s−1), because whole-plant respiration rates were positively associated with whole-plant photosynthesis at higher light; (b) shade-intolerant species possess traits that maximize whole-plant CO2 exchange (and thus growth) in moderately low to high light levels, but these traits may lead to long-term growth and survival disadvantages in very-low light (< 2·8%) owing, in part, to high respiration. In contrast, shade-tolerant species may minimize CO2 losses in very-low light at the expense of maximizing CO2 gain potential at higher light levels, but to the possible benefit of long-term survival in low light.