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

Abstract

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 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.