Evidence of a general 2/3-power law of scaling leaf nitrogen to phosphorus among major plant groups and biomes

Abstract

Scaling relations among plant traits are both cause and consequence of processes at organ-to-ecosystem scales. The relationship between leaf nitrogen and phosphorus is of particular interest, as both elements are essential for plant metabolism; their limited availabilities often constrain plant growth, and general relations between the two have been documented. Herein, we use a comprehensive dataset of more than 9300 observations of approximately 2500 species from 70 countries to examine the scaling of leaf nitrogen to phosphorus within and across taxonomical groups and biomes. Power law exponents derived from log-log scaling relations were near 2/3 for all observations pooled, for angiosperms and gymnosperms globally, and for angiosperms grouped by biomes, major functional groups, orders or families. The uniform 2/3 scaling of leaf nitrogen to leaf phosphorus exists along a parallel continuum of rising nitrogen, phosphorus, specific leaf area, photosynthesis and growth, as predicted by stoichiometric theory which posits that plants with high growth rates require both high allocation of phosphorus-rich RNA and a high metabolic rate to support the energy demands of macromolecular synthesis. The generality of this finding supports the view that this stoichiometric scaling relationship and the mechanisms that underpin it are foundational components of the living world. Additionally, although abundant variance exists within broad constraints, these results also support the idea that surprisingly simple rules regulate leaf form and function in terrestrial ecosystems.

Figure 1.

Relationships of leaf N (N_L) to leaf P (P_L) for all data pooled, and for plants grouped by both phylogeny (angiosperm, gymnosperm) and life form within the angiosperm group. The details of these relations using reduced major axis (RMA) regressions are presented in table 1.

Figure 2.

Relationship between the scaling exponent (the RMA slope) of the N_L–P_L relationship and the sample size, for 50 angiosperm orders (closed circles). The mean (0.667) and 95% CI for the scaling exponent are shown in the solid and dashed lines. The mean for the gymnosperm order ‘Pinales’ is shown in the open circle for comparison.

Figure 3.

Relationship of leaf N (N_L) and leaf P (P_L) to specific leaf area and net photosynthetic capacity (A_mass) for all angiosperm data pooled. (a) For leaf N (N_L) and (b) leaf P (P_L), the relationships to specific leaf area (SLA) were log N_L = −0.326 + 0.759 (log SLA); n = 1819, r² = 0.54, p < 0.0001; and log P_L = −2.28 + 1.141 (log SLA); n = 1819, r² = 0.45; p < 0.0001. Statistics for specific leaf area relations for subgroups and biomes are listed in table 2. (c) For leaf N (N_L) and (d) leaf P (P_L), the relationships to net photosynthetic capacity were log N_L = −0.090 + 0.681 (log A_mass); n = 391, r² = 0.37, p < 0.0001; and log P_L = −2.21 + 1.131 (log A_mass); n = 391, r² = 0.30; p < 0.0001.