Fig. 1.

Possible evolutionary outcomes allowed by this model. The N fixation strategy (the units of which are equivalent to kilograms of N per hectare per y, given a 1,000 kilograms of C per hectare stand of this type) is plotted on the horizontal axis, and the fitness function (Eq. 6, where μ, ν, and ω are functions of F) is on the vertical axis (normalized so nonfixers are at 0). Eq. 6 is equivalent to 1/Ā, the reciprocal of the equilibrium soil available N pool. Evolution will maximize Eq. 6, locally if mutations are small or globally if they are large. (a) Nonfixation is the CSS (indicated by a closed circle) (7 is true, 8 is false). (b) Both nonfixation and F* are ESSs (open circles) but nonfixation wins (7 is true, 8 is false). (c) Both are ESSs, but F* wins (7 and 8 are true). (d) F* is the CSS (7 is false, 8 is true). These show a progression of decreasing N fixation costs from a–d. To plot Eq. 6, we used saturating functions with positive intercepts for μ(F), ν(F), and ω(F).