Abstract:

Black spruce (Picea mariana [Mill.] B.S.P.) and white spruce (Picea glauca [Moench] Voss.) are congeneric species. Both are moderately shade tolerant and widely distributed across North American boreal forests.

To understand light effects on their ecophysiological responses to elevated CO₂, 1-year-old seedlings were exposed to 360 µmol mol−1 and 720 µmol mol−1 CO₂ at three light conditions (100%, 50% and 30% of full light in the greenhouse). Foliar gas exchanges were measured in the mid- and late-growing season.

Elevated CO₂ increased net photosynthesis (P_n) and photosynthetic water use efficiency, but it reduced stomatal conductance and transpiration. The stimulation of photosynthesis by elevated CO₂ was greatest at 50% light and smallest at 100%. Photosynthesis, maximum carboxylation rate (V_cmax) and light-saturated rate of electron transport (J_max) all decreased with decreasing light. Elevated CO₂ significantly reduced V_cmax across all light treatments and both species in mid-growing season. However, the effect of elevated CO₂ became insignificant at 30% light later in the growing season, with the response being greater in black spruce than in white spruce. Elevated CO₂ also reduced J_max in white spruce in both measurements while the effect became insignificant at 30% light later in the growing season. However, the effect on black spruce varied with time. Elevated CO₂ reduced J_max in black spruce in mid-growing season in all light treatments and the effect became insignificant at 30% light later in the growing season, while it increased J_max later in the season at 100% and 50% light. These results suggest that both species benefited from elevated CO₂, and that the responses varied with light supply, such that the response was primarily physiological at 100% and 50% light, while it was primarily morphological at 30% light.

Key words: black and white spruce, boreal forest, CO₂–light–species interaction