A meta-analysis of leaf nitrogen distribution within plant canopies

Abstract

Background and aims: Leaf nitrogen distribution in the plant canopy is an important determinant for canopy photosynthesis. Although the gradient of leaf nitrogen is formed along light gradients in the canopy, its quantitative variations among species and environmental responses remain unknown. Here, we conducted a global meta-analysis of leaf nitrogen distribution in plant canopies.

Methods: We collected data on the nitrogen distribution and environmental variables from 393 plant canopies (100, 241 and 52 canopies for wheat, other herbaceous and woody species, respectively).

Key results: The trends were clearly different between wheat and other species; the photosynthetic nitrogen distribution coefficient (Kb) was mainly determined by leaf area index (LAI) in wheat, whereas it was correlated with the light extinction coefficient (KL) and LAI in other species. Some other variables were also found to influence Kb We present the best equations for Kb as a function of environmental variables and canopy characteristics. As a more simple function, Kb = 0·5KL can be used for canopies of species other than wheat. Sensitivity analyses using a terrestrial carbon flux model showed that gross primary production tended to be more sensitive to the Kb value especially when nitrogen content of the uppermost leaf was fixed.

Conclusion: Our results reveal that nitrogen distribution is mainly driven by the vertical light gradient but other factors such as LAI also have significant effects. Our equations contribute to an improvement in the projection of plant productivity and cycling of carbon and nitrogen in terrestrial ecosystems.

Keywords: Canopy photosynthesis; functional group; leaf area index; light distribution; light extinction coefficient; model; nitrogen allocation; nitrogen use; optimization.

Fig. 1.

Relationship between nitrogen distribution coefficients and light extinction coefficient (K_L). (A) The coefficient of leaf nitrogen distribution (K_N) and (B) the coefficient of photosynthetic nitrogen distribution (K_b). Circles, herbaceous species except wheat; triangles, wheat; and squares, woody species. Regression lines are K_N = 0·368K_L (r² = 0·22) and K_b = 0·499K_L (r² = 0·37), with data for wheat not included. Note that three data points that had K_b values higher than 1·5 were omitted from B (see Supplementary Data Fig. S1 for the whole data set).

Fig. 2.

Relationship between nitrogen distribution coefficients and leaf area index (LAI). (A) The coefficient of leaf nitrogen distribution (K_N) and (B) the coefficient of photosynthetic nitrogen distribution (K_b). Circles, herbaceous species except wheat; triangles, wheat; and squares, woody species.