Abstract
Nitrogen (N) is essential for plant production, but N uptake imposes carbon (C) costs through maintenance respiration and fine-root construction, suggesting that an optimal C:N balance can be found. Previous studies have elaborated this optimum under exponential growth; work on closed canopies has focused on foliage only. Here, the optimal co-allocation of C and N to foliage, fine roots and live wood is examined in a closed forest stand. Optimal co-allocation maximizes net primary productivity (NPP) as constrained by stand-level C and N balances and the pipe model. Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N]), and stand-level photosynthesis and N uptake saturate at high foliage and fine-root density. Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N. Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]; in between, [N] increases with N availability. Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability. The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland. All predictions agree with our qualitative understanding of N effects on growth.
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Claims 9
Claim · model-written summary
Model photosynthesis and maintenance respiration rose with foliar nitrogen concentration.
Verbatim
e0“Photosynthesis and maintenance respiration increase with foliar nitrogen concentration ([N])”
e1“closed forest stand”
Claim · model-written summary
Stand-level photosynthesis and nitrogen uptake saturated at high foliage and fine-root density.
Verbatim
e0“stand-level photosynthesis and N uptake saturate at high foliage and fine-root density”
e1“closed forest stand”
Claim · model-written summary
Optimal NPP increased nearly linearly at low to moderate nitrogen availability and saturated at high availability.
Verbatim
e0“Optimal NPP increases almost linearly from low to moderate N availability, saturating at high N.”
e1“closed forest stand”
Claim · model-written summary
At very low or high nitrogen availability, predicted foliar nitrogen concentration was steady.
Verbatim
e0“Where N availability is very low or very high, the system resembles a functional balance with a steady foliage [N]”
e1“closed forest stand”
Claim · model-written summary
At intermediate availability, predicted foliar nitrogen concentration increased with nitrogen availability.
Verbatim
e0“in between, [N] increases with N availability”
e1“closed forest stand”
Claim · model-written summary
Predicted carbon allocation to fine roots decreased as nitrogen availability increased.
Verbatim
e0“Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability.”
e1“closed forest stand”
Claim · model-written summary
Predicted carbon allocation to wood increased as nitrogen availability increased.
Verbatim
e0“Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability.”
e1“closed forest stand”
Claim · model-written summary
Predicted carbon allocation to foliage remained stable as nitrogen availability increased.
Verbatim
e0“Carbon allocation to fine roots decreases, allocation to wood increases, and allocation to foliage remains stable with increasing N availability.”
e1“closed forest stand”
Claim · model-written summary
Predicted biomass density and foliage nitrogen relationships reasonably agreed with Finnish conifer stand data.
Verbatim
e0“The predicted relationships between biomass density and foliage [N] are in reasonable agreement with data from coniferous stands across Finland.”