© 1996 Heron Publishing—Victoria, Canada
Optimality and nitrogen allocation in a tree canopy
D. Y. Hollinger (1, 2)
1. Manaaki Whenua–Landcare Research, P.O. Box 31-011, Christchurch, New Zealand / 2. USDA Forest Service, P.O. Box 640, Durham, NH 03824, USA / Received July 21, 1995
Summary
Physical and functional properties of foliage were measured at a variety of microsites in a broad-leaved Nothofagus fusca (Hook. f.) Ørst. canopy. The light climate of the foliage at these sites was monitored for 39 days in the late spring and
early summer with in situ sensors. Foliage nitrogen content (N), mean leaf angle, and gas exchange characteristics were all
correlated with the amount of light reaching the microsites during foliage development. Foliage N content on a leaf area basis
ranged between ~1 and 2.5 g N m–2 and was highest at the brightest sites. Light-saturated photosynthetic rates ranged between ~4 and 9 µmol m–2 s–1, increasing from the darkest to brightest sites.
A biochemical model of photosynthesis was fitted to foliage characteristics at the different microsites and used to integrate
foliage assimilation among the sites over 39 days. The actual arrangement of foliage physiological characteristics in the
observed microsites led to higher total canopy rates of net assimilation than > 99% of the combinations of observed foliage
characteristics randomly assigned to the observed microsites. Additional simulations first related the maximum rates of electron
transport (Jmax), ribulose bisphosphate turnover (Vc,max), and dark respiration (Rd) of Nothofagus fusca foliage to nitrogen content and then allowed foliage N (and consequently leaf gas exchange characteristics) to vary across
the canopy. The observed N allocation pattern results in greater total canopy assimilation than uniform or > 99% of the simulations
with random distributions of N among the microsites (constrained so that the total N allocated was equivalent to that observed
in the microsites). However, the observed pattern of N allocation places less N in the brightest microsites and results in
substantially less total assimilation than a simulated canopy in which N was allocated in an optimal manner where the N distribution
is such that the partial derivative of leaf assimilation (A) with respect to leaf nitrogen content, ∂A/∂N, is constant among microsites. These results suggest that other factors such as wind or herbivory reduce the integrated
assimilation of high-N foliage relatively more than lower-N foliage and that a ∂A/∂N optimality criteria based only on formulations of leaf gas exchange overestimate canopy assimilation.
Keywords:
light penetration, optimization, photosynthesis model, remote sensing, simulation model.
