Evolutionarily stable strategies explain complex plant responses to simple resource addition experiments

Background/Question/Methods

Traditional theory of plant allocation strategies can be simple and intuitive, but plant responses in experiments often are not.  As belowground resources become more abundant it makes sense that competition should decrease, plants should have to invest less in roots to take up those resources, and  can invest more in stem and leaves to gather light aboveground.   In a simple nitrogen by water field experiment, performed on a mix of grassland species in Minnesota, we found this not to be the case.  Plants responded to nitrogen addition by increasing leaf biomass but had no root response.  With water addition these plants dramatically increased fine root biomass without a significant response in leaf biomass.  Additionally, the fertilization had a significant interaction with water, effectively removing the plant responses to water.

With a simple model of plant competition for light, water, and nitrogen, we have been able to derive analytical expressions for ESS strategies of plant allocation to fine roots, leaves, and structural biomass.   In this model, roots take up water and nitrogen from well mixed pools belowground.  Nitrogen is used to build the leaves, while water powers the photosynthesis that they can perform.  Leaves within a plant shade and thus reduce the photosynthetic potential of the leaves below them. 

Results/Conclusions

By solving this system for ESSs (i.e.:  strategies which cannot be invaded by other strategies) we find that the competitive strategy has more root biomass as water becomes more abundant and less root biomass as nitrogen increases.  The competitive interplay between the strategies for nitrogen and water also explains the result  that plant fine root biomass does not respond as strongly to water addition as nitrogen is increased. 

The potential for invasion by alternative strategies can drive plants communities to exhibit unintuitive strategies.  These competitive strategies result in dominant plants whose monoculture productivity is less than optimal (defining optimal as maximal).   Only with a mechanistic framework of the benefits of uptake in those resources and appropriate competition for them have we been able understand the complex dynamics of real plant communities.