© 1999 Heron Publishing—Victoria, Canada
Interrelationships among light, photosynthesis and nitrogen in the crown of mature Pinus contorta ssp. latifolia
A. W. Schoettle (1) and W. K. Smith (2, 3)
1. Rocky Mountain Research Station, 240 W. Prospect Road, Fort Collins, CO 80526, USA (Schoettl@lamar.colostate.edu) / 2. Department of Botany, University of Wyoming, Laramie, WY 82070, USA / 3. Department of Biology, Wake Forest University, Winston-Salem, NC 27109, USA / Received October 24, 1997
Summary
Scaling leaf-level measurements to estimate carbon gain of entire leaf crowns or canopies requires an understanding of the
distribution of photosynthetic capacity and corresponding light microenvironments within a crown. We have compared changes
in the photosynthetic light response and nitrogen (N) content (per unit leaf area) of Pinus contorta Dougl. ssp. latifolia Engelm. (lodgepole pine) leaves in relation to their age and light microenvironment. The vertical gradient in integrated
daily photosynthetic photon flux density (PPFD) from the upper to the lower crown of lodgepole pine was similar in magnitude
to the horizontal gradient in daily PPFD along shoots from young to old leaves. The relationship between light-saturated net
photosynthesis (Amax) and daily PPFD was significant for both young and old leaves. However, old leaves had a lower Amax than young leaves in a similar daily irradiance regime. For leaves of all ages from throughout the crown, Amax was linearly related to the estimated daily net carbon gain that leaves could achieve in their natural PPFD environment (estimated
Aday) (r2 = 0.84, P < 0.001, n = 39), indicating that estimated Aday may be dominated by carbon fixed when leaves are light-saturated and operating at Amax. Comparison of the PPFD required to achieve Amax and the PPFD available to the leaves showed that all of the measured leaves (n = 39), regardless of their position in the crown or age, were in light environments that could light-saturate photosynthesis
for a similar proportion of the day. For all data pooled, foliar N was weakly correlated with daily PPFD. Analyzing each leaf
age class separately showed that foliar N was significantly related to daily PPFD, Amax, and estimated Aday for the youngest leaves but not for middle-aged or old leaves. Therefore, the general theory that foliar N is allocated within
a crown according to total daily light availability was supported only for young (1–4 years old) leaves in this study.
Keywords:
conifers, crown architecture, evergreens, leaf age, light acclimation, light gradients, light microenvironments, lodgepole
pine, nitrogen partitioning.
