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Lewis, J.D., Robert B. McKane, David T. Tingey and Peter A. Beedlow. 2000. Vertical gradients in photosynthetic light response within an old-growth Douglas-fir and western hemlock canopy. Tree Physiology 20:447-456.  NHEERL-COR-2241J

We examined needle-level light response of photosynthesis across a vertical light gradient within 45-55 m-tall western hemlock (Tsuga heterophylla (RAF.) Sag.) And Douglas-fir (Pseudotsuga menziesii (mirb) Franco) trees growing in a 400-500-year-old mixed species stand. We determined: (1) whether light-saturated photosynthetic rates, light compensation points, and respiration rates varied from the upper to the lower canopy, and (2) if light-saturated photosynthetic rates, light compensation-points, and respiration rates varied between Douglas-fir and western hemlock. Over a 25-m gradient from the canopy top to the lower canopy, mean light-saturated photosynthetic rates, light compensation points, and respiration rates declined in overstory Douglas-fir and western hemlock needles, paralleling a 65% decline in the mean-daily photosynthetic photon flux density (PPFD). At the canopy top, increasing light-saturated photosynthetic rates relative to lower canopy needles increased carbon uptake at high PPFD. In the lower-canopy, reduced respiration rates relative to upper-canopy needles increased carbon uptake at low PPFD by reducing the light compensation point. At all canopy positions western hemlock had lower mean light-saturated photosynthetic rates, light compensation points and respiration rates than Douglas-fir. As a result, western hemlock had higher net photosynthetic rates at low PPFD, but lower-net photosynthetic rates at high PPED compared with Douglas-fir.

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