Light availability and photosynthesis of Pseudotsuga menziesii seedlings grown in the open and in the forest understory.

The light environment, photosynthetic dynamics and steady-state net photosynthetic rates of lateral branch shoots of Pseudotsuga menziesii var. glauca (Beissn.) Franco seedlings growing in the open and in the forest understory were investigated in situ. Mean incident photosynthetic photon flux density (PPFD) was 702.5 micro mol m(-2) s(-1) on open-grown branches and 52.0 micro mol m(-2) s(-1) on understory-grown branches. Mean daily durations of PPFD greater than 500, 200, and 50 micro mol m(-2) s(-1) were 8.5, 31.5, and 270.3 min, respectively, on understory-grown branches, and 559.1, 700.7, and 803.3 min, respectively, on open-grown branches. Sunflecks accounted for 32.4% of total daily photosynthetically active radiation incident on understory branches. Following 10 min at a PPFD of 50 micro mol m(-2) s(-1), the induction time required for net photosysnthesis to reach 50 and 90% of steady-state rates was shorter at a PPFD of 200 than at a PPFD of 500 micro mol m(-2) s(-1) and shorter in understory-grown branches than in open-grown branches. On a leaf area basis, dark respiration rates of understory-grown branches were lower and net photosynthetic rates were higher than those of open-grown branches exposed to low PPFD. However, at high PPFDs, understory-grown branches had lower photosynthetic rates than open-grown branches. When measurements were expressed on a leaf dry mass basis, there was no difference in dark respiration rates between understory branches and open-grown branches, but net photosynthetic rates of understory branches were equal to or higher than those of open-grown branches at all PPFDs.