Fine roots and leaves are the organs responsible for harvesting resources belowground and aboveground, respectively, but our understanding of leaf function vastly exceeds our understanding of fine-root function. The lack of generalized theory on fine roots is likely causing structural uncertainty in the coupled carbon-nitrogen vegetation models used within larger Earth system models. Part of the reason for the discrepancy between our understanding of fine-root and leaf function is the much greater difficulty of working with fine roots versus leaves, and another part of the reason is the more complicated biotic and abiotic environment of soil versus air. An additional and underappreciated difference is the game-theoretic selective pressures placed on fine roots versus leaves, which tend to exaggerate traits past the point of optimality when considered from a non-game-theoretic viewpoint. Since most ecologists do not yet take a game-theoretic perspective, fine-root function may be especially difficult to understand. Here, I review theoretical and empirical evidence both in favor of and against a game-theoretic understanding of fine-root function.
Results/Conclusions
Uniquely game-theoretic mechanisms are manifest when the success of a given trait depends on the traits of the individuals with which it interacts. For example, a 5m-tall tree in competition with 4m trees would do quite well, whereas the same 5m-tall tree in competition with 10m trees would do poorly. Plants compete belowground for limiting resources such as water, nitrogen, and phosphorus, and because competition requires that plants influence the local resource environment of other plants, it must be, in part, game-theoretic. I present recently-published and as-yet unpublished work that relates plant nitrogen uptake rate with fine-root mass and nitrogen availability. Plants with relatively more fine-root mass take up a greater share of nitrogen, but the absolute fine-root mass of the community does not influence the absolute amount of nitrogen taken up, consistent with game-theoretic predictions of fine-root mass under uptake driven by diffusion and mass flow. Recent work indicates that this finding is unaffected by the presence or absence of mycorrhizal fungi. However, other fine-root functions, such as rhizosphere priming, may exert sufficiently localized effects that one plant’s priming may not directly affect any other plant’s success. If true, that would make rhizosphere priming non-game-theoretic.