Fig. 1.

Regression trends of plant N:P ratios (molar basis) as a function of absolute latitude (degrees): Reich and Oleksyn (ref. 8; blue), Kerkhoff's group (green), and McGroddy et al. (ref. 9; red). Line A, Reich and Oleksyn's (8) study of leaf nutrient content in 1,280 plant species from 452 locations worldwide (log N:P = 1.30985 – 0.00377 · latitude – 0.00006 · latitude²; r² = 0.78; P < 0.005). Line B, Kerkhoff's group's study of leaf nutrient content in 1,054 plant species worldwide, binned across 16 latitudes (N:P = 49 – 0.33 · latitude; r² = 0.55; P < 0.001). Line C, McGroddy et al.'s (9) study of foliage nutrient content across 59 undisturbed forests worldwide (N:P = 43.1 – 0.338 · latitude; r² = 0.28; P < 0.0001). Line D, McGroddy et al.'s (9) study of litterfall nutrient content across 106 undisturbed forests worldwide (N:P = 111.1 · latitude^–0.389, r² = 0.43; P < 0.0001).