© 2000 Heron Publishing—Victoria, Canada
Vertical gradients in photosynthetic light response within an old-growth Douglas-fir and western hemlock canopy
J. D. Lewis (1, 3), R. B. McKane (2), D. T. Tingey (2) and P. A. Beedlow (2)
1. National Research Council, Corvallis, OR 97333, USA
/ 2. U.S. Environmental Protection Agency, Western Ecology Division, Corvallis, OR 97333, USA / 3. Louis Calder Center—Biological Field Station, Fordham University, 53 Whippoorwill Road, P.O. Box K, Armonk, NY 10504, USA (jdlewis@fordham.edu) / Received June 13, 1999
Summary
We examined needle-level light response of photosynthesis across a vertical light gradient within 45–55-m-tall western hemlock
(Tsuga heterophylla (Raf.) Sarg.) 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 PPFD
compared with Douglas-fir.
Keywords:
carbon assimilation, leaf irradiance, light compensation point, photosynthesis, Pseudotsuga menziesii, respiration, Tsuga heterophylla.
