Journal
TREE PHYSIOLOGY
Volume 32, Issue 5, Pages 520-534Publisher
OXFORD UNIV PRESS
DOI: 10.1093/treephys/tps044
Keywords
acclimation; canopy carbon export; canopy nitrogen profile; leaf-area index; leaf lifespan; leaf nitrogen concentration; leaf-trait relationships; maximization; optimality; specific leaf area
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Funding
- Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS)
- Royal Society of Arts and Sciences in Gothenburg (KVVS)
- Knut and Alice Wallenberg Foundation
- Australian Research Council [DP0881765]
- Australian Research Council [DP0881765] Funding Source: Australian Research Council
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A long-established theoretical result states that, for a given total canopy nitrogen (N) content, canopy photosynthesis is maximized when the within-canopy gradient in leaf N per unit area (N-a) is equal to the light gradient. However, it is widely observed that N-a declines less rapidly than light in real plant canopies. Here we show that this general observation can be explained by optimal leaf acclimation to light subject to a lower-bound constraint on the leaf mass per area (m(a)). Using a simple model of the carbon-nitrogen (C-N) balance of trees with a steady-state canopy, we implement this constraint within the framework of the MAXX optimization hypothesis that maximizes net canopy C export. Virtually all canopy traits predicted by MAXX (leaf N gradient, leaf N concentration, leaf photosynthetic capacity, canopy N content, leaf-area index) are in close agreement with the values observed in a mature stand of Norway spruce trees (Picea abies L. Karst.). An alternative upper-bound constraint on leaf photosynthetic capacity (A(sat)) does not reproduce the canopy traits of this stand. MAXX subject to a lower bound on m(a) is also qualitatively consistent with co-variations in leaf N gradient, m(a) and A(sat) observed across a range of temperate and tropical tree species. Our study highlights the key role of constraints in optimization models of plant function.
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