Leaf nitrogen from the perspective of optimal plant function

Abstract

Leaf dry mass per unit area (LMA), carboxylation capacity (V _cmax) and leaf nitrogen per unit area (N_area) and mass (N_mass) are key traits for plant functional ecology and ecosystem modelling. There is however no consensus about how these traits are regulated, or how they should be modelled. Here we confirm that observed leaf nitrogen across species and sites can be estimated well from observed LMA and V _cmax at 25°C (V _cmax25). We then test the hypothesis that global variations of both quantities depend on climate variables in specific ways that are predicted by leaf-level optimality theory, thus allowing both N_area to be predicted as functions of the growth environment.A new global compilation of field measurements was used to quantify the empirical relationships of leaf N to V _cmax25 and LMA. Relationships of observed V _cmax25 and LMA to climate variables were estimated, and compared to independent theoretical predictions of these relationships. Soil effects were assessed by analysing biases in the theoretical predictions.LMA was the most important predictor of N_area (increasing) and N_mass (decreasing). About 60% of global variation across species and sites in observed N_area, and 31% in N_mass, could be explained by observed LMA and V _cmax25. These traits, in turn, were quantitatively related to climate variables, with significant partial relationships similar or indistinguishable from those predicted by optimality theory. Predicted trait values explained 21% of global variation in observed site-mean V _cmax25, 43% in LMA and 31% in N_area. Predicted V _cmax25 was biased low on clay-rich soils but predicted LMA was biased high, with compensating effects on N_area. N_area was overpredicted on organic soils. Synthesis. Global patterns of variation in observed site-mean N_area can be explained by climate-induced variations in optimal V _cmax25 and LMA. Leaf nitrogen should accordingly be modelled as a consequence (not a cause) of V _cmax25 and LMA, both being optimized to the environment. Nitrogen limitation of plant growth would then be modelled principally via whole-plant carbon allocation, rather than via leaf-level traits. Further research is required to better understand and model the terrestrial nitrogen and carbon cycles and their coupling.

Keywords: coordination hypothesis; ecosystem model; leaf mass per area; leaf nitrogen; least‐cost hypothesis; nitrogen cycle; photosynthetic capacity.

FIGURE 1

Partial residual plots of leaf nitrogen per unit area (N_area, g m⁻²) as an additive function of (a) leaf mass per area (LMA, g m⁻²) and (b) carboxylation capacity at 25°C (V _cmax25, μmol m⁻² s⁻¹) across all species, based on the main regression equation of Table 1 (N_area ~ LMA + V _cmax25 + LMA × N_fixer). Each plot shows values of N_area corrected by holding the other variable constant at its median value. Nitrogen‐fixers (N_fixer) are in blue, non‐nitrogen‐fixers in red. Coloured lines are partial regression lines; shading indicates 95% confidence bands.

FIGURE 2

Relative importance of each variable in explaining the variation of leaf nitrogen per unit area (N_area, g m⁻²) across all species_. Leaf mass per unit area (LMA, g m⁻²), carboxylation capacity at 25°C (V _cmax25, μmol m⁻² s⁻¹).

FIGURE 3

Partial residual plots of log‐transformed carboxylation capacity at growth temperature (V _cmax,gt, μmol m⁻² s⁻¹) (a–c) and at 25°C (d–f) versus (a, d) growing‐season daytime mean temperature (T, °C), (b, e) log‐transformed growing‐season daytime mean photosynthetic photon flux density (I _L, μmol m⁻² s⁻¹), and (c, f) growing‐season daytime vapour pressure deficit (D, kPa) across all species. Black lines are partial regression lines; grey shading indicates 95% confidence bands.

FIGURE 4

Partial residual plots of log‐transformed leaf mass per unit area (LMA, g m⁻²) versus growing‐season daytime mean temperature (T, °C), log‐transformed growing‐season mean daily total photosynthetic photon flux density (I _L*, mol m⁻² day⁻¹), growing‐season length as a fraction of the year (f, day day⁻¹), and actual/equilibrium evapotranspiration (α, dimensionless) in evergreen (a–d) and deciduous (e–h) woody plants. Black lines are partial regression lines; grey shading indicates 95% confidence bands.

FIGURE 5

Predicted versus observed all‐species (a–c) and site‐mean (d–f) carboxylation capacity at 25°C (V _cmax25, μmol m⁻² s⁻¹), leaf mass per unit area (LMA, g m⁻²) and leaf nitrogen per unit area (N_area, g m⁻²). The dashed lines are the 1:1 lines. Coloured lines are regression lines; shading indicates 95% confidence bands. Blue: evergreen species, red: deciduous species.