Interrelationships among light, photosynthesis and nitrogen in the crown of mature Pinus contorta ssp. latifolia

Scaling leaf-level measurements to estimate carbon gain of entire leaf crow ns or canopies requires an understanding of the distribution of photosynthe tic capacity and corresponding light microenvironments within a crown. We h ave compared changes in the photosynthetic light response and nitrogen (N) content (Der unit leaf area) of Pinus contorta Dougl, ssp. latifolia Engelm . (lodgepole pine) leaves in relation to their age and light microenvironme nt. The vertical gradient in integrated daily photosynthetic photon flux de nsity (PPFD) from the upper to the lower crown of lodgepole pine was simila r in magnitude to the horizontal gradient in daily PPFD along shoots from y oung to old leaves. The relationship between light-saturated net photosynth esis (A(max)) and daily PPFD was significant for both young and old leaves. However, old leaves had a lower A(max) than young leaves in a similar dail y irradiance regime. For leaves of all ages from throughout the crown, A(ma x) was linearly related to the estimated daily net carbon gain that leaves could achieve in their natural PPFD environment (estimated A(day)) (r(2) = 0.84, P < 0.001, n = 39), indicating that estimated A(day) may be dominated by carbon fixed when leaves are light-saturated and operating at A(max). C omparison of the PPFD required to achieve A(max) and the PPFD available to the leaves showed that all of the measured leaves (n = 39), regardless of t heir position in the crown or age, were in light environments that could li ght-saturate photosynthesis for a similar proportion of the day. For all da ta pooled, foliar N was weakly correlated with daily PPFD. Analyzing each l eaf age class separately showed that foliar N was significantly related to daily PPFD, A(max), and estimated A(day) for the youngest leaves but not fo r middle-aged or old leaves. Therefore, the general theory that foliar N is allocated within a crown according to total daily light availability was s upported only for young (1-4-year-old) leaves in this study.