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

Abstract1. 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... [ABSTRACT FROM AUTHOR]