Plant strategies for competition and evolutionary game theory

Background/Question/Methods: Many questions regarding biomass production among competing plants are necessarily game theoretic because optimal allocation strongly depends on the strategy of neighbouring plants. For example, competition belowground is said to be size symmetric. That is, the intensity of competition experienced by a focal plant depends on its root biomass relative to the root biomass produced by its neighbour. Similarly, competition for light is size asymmetric, and the intensity of competition experienced by a focal plant depends on its height relative to the height of its neighbour. Thus, whether plants experience competition aboveground, belowground or both, neighbouring individuals can influence each other’s fitness through  competition or facilitation. However, previous game theoretic models of plant competition have been met with criticism, and the validity of a game theoretic framework for plant competition is an area of active debate. Here I describe several new models that seek to move this debate forward and ask (i) how differences in uptake capacity or root costs among competing plants influences competition belowground, (ii) how competition for multiple substitutable resources (e.g. nitrate, ammonium) influences competition belowground, and (iii) how shoot and root competition for hemi-essential resources (e.g. carbon, nitrogen) interact to influence whole plant allocation strategies.

Results/Conclusions: In general, game theoretic models predict that competition among plants results in a tragedy of the commons. Plants must either over-proliferate roots or over-elongate stems in order to both pre-empt their neighbours and avoid being pre-empted by their neighbours. This is a tragedy of the commons because all individuals achieve lower fitness as a result of adopting the over-elongation/proliferation strategy, but a less proliferative strategy is not evolutionarily stable. However, the models described here demonstrate that there are a variety of scenarios that can ease the tragedy of the commons, and therefore lead to somewhat less proliferative strategies. (i) Plants that differ in traits such as uptake capacity may engage in less intense competition. For example, a plant with relatively low uptake capacity or high root costs will be at a competitive disadvantage, and can gain little from over-proliferating roots.  (ii) If plants partition substitutable resources then they avoid competition, and there is no reason to engage in a tragedy of the commons game. (iii) Plants compete both above and belowground for hemiessential resources. Strong competition in either or both areas can also limit the resources a plant has available to allocate to tissues, and should limit over-proliferation/elongation strategies.