Trade-offs in low-light CO₂ exchange: A component of variation in shade tolerance among cold temperate tree seedlings

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^-2s^-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 x 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^-2s^-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.