Caroline
                    Farrior

Competitive games plants play


A tree that is alone in a field can invest much more in reproduction if it forgoes some investment in wood. However, if that tree is in a forest in competition with other individuals, such a strategy would lead to suffering in the shade of neighbors. Competition for light motivates plant overinvestments in wood, investment that serves the individual in competition but hinders it in isolation. 
This can occur with other resources as well including water and nutrients and depending on the details of plant usage and competition can have different and sometimes unintuitive effects on plant communities.

With simplified models of plant physiology I investigate how interactions between competitive and environmental pressures drive individual strategies and scale up to landscape level properties.

 
model
                      treeCartoon of a model tree depicting the simplified physiology.

Trees are made out of leaves (green),  woody biomass (grey), and fine roots (brown).  Roots take up water (blue) and nitrogen (red).  Leaf photosynthesis is a function of light availability (yellow), which drops off exponentially through the crown of a tree due to self shading.  If
                                                                    water is limiting carbon assimilation is proportional to water uptake. If nitrogen is limiting the number of leaf layers is limited.


Evolutionarily stable strategy analysis 
Dominant strategy is predicted to be the strategy that, when in monoculture, resists invasion by all other strategies
(if such a strategy is possible).

ESS

References                                                                                                                                                                                                               
Farrior, CE. 2014. Competitive optimization models, attempting to understand the diversity of life.* New Phytologist 203(4):1025-1027.
* title in reference to Maynard Smith (1978) Optimization theory in evolution - an excellent read.

Farrior, CE, R Dybzinski, SA Levin, and SW Pacala. 2013. 
Competition for water and light in closed-canopy forests: a tractable model of carbon allocation with implications for carbon sinks. The American Naturalist 181(3): 314-330.
 

Dybzinski, R, C Farrior, A Wolf, PB Reich, and SW Pacala. 2011.  Evolutionarily stable strategy carbon allocation to foliage, wood, and fine roots in trees competing for light and nitrogen: An analytically tractable, individual-based model and quantitative comparisons to data. The American Naturalist 177(2):153-166.


Sequential resource limitation in plant communities


Simple resource addition experiments often have surprising results. Plants respond to more than one essential resource. Plants respond to additions of a resource by increasing their investment in taking it up. etc..
We added nitrogen and water in a factorial design to diverse prairie plots at Cedar Creek and again found such confusing responses. 

Plants responded to both water and nitrogen additions. With water additions, plants increased their fine-root biomass without concurrent increases in aboveground biomass. Further, this response only occurred at low nitrogen levels, making it seem like the high nitrogen plots were not water limited, while the low nitrogen plots were water limited.

We found explanations for these responses through a comparison with theoretical predictions for competitive dominant allocation strategies of plants in competition for water, nitrogen, and space.
Because water availability varies throughout the growing season, it is often the case that plants switch between water limitation and water saturation throughout the summer.
Cedar Creek diverse prairie plots where water and nitrogen were added in a factorial design.








(below)
Separating leaf biomass from structural and reproductive biomass.



Plant biomass responses to resource additions


We found plants are likely sequentially limited by nitrogen and water. They are saturated with water during periods immediately following rain and limited by water during hot summer days in between storms. During the periods of water saturation, plant photosynthesis is limited by the photosynthetic machinery it has, powered by nitrogen uptake. Plants increase fine-root biomass when water is added because competition for this nitrogen becomes more important. As nitrogen becomes more available, however, the effect is less significant.
References                                                                                                                                                                                                              

Farrior, CE, D Tilman, R Dybzinski, PB Reich, SA Levin, and SW Pacala, 2013.  Resource limitation in a competitive context determines complex plant responses to experimental resource additions. Ecology 94(11): 2505-2517.


Resource limitation and forest carbon sinks


As the concentration of CO2 in the atmosphere rises, plant leaves become more productive while using less water.  But, enhanced efficiency may not necessary translate into carbon storage of the whole forest. 

To further our understanding of carbon sinks and better predict the roles of forests in the global carbon cycle, I am working to understand the effect of competitive allocation under CO2 fertilization for plants in competition for essential resources.
 




Figure 4 of Farrior et al. 2013 AmNat. 

(A)  Effects of enhanced leaf-level water use efficiency during water-limitation on carbon storage for constant allocation (straight line) and competitive allocation (dashed line).
(B) Effects of enhanced leaf-level maximum photosynthetic rate on carbon storage.

References                                                                                                                                                                                                              
Papers in review, and


Farrior, CE, R Dybzinski, SA Levin, and SW Pacala. 2013.  Competition for water and light in closed-canopy forests: a tractable model of carbon allocation with implications for carbon sinks The American Naturalist 181(3): 314-330.
 


Allocation strategies in forests across a variety of rainfall regimes in the US


In an effort to understand the role of competition and resource availability in forest allocation patterns, I set up 22 plots across differing rainfall regimes in the Midwestern US.

Within these forests, I have taken data on growth, leaf area index, and fine root biomass as well as net-nitrogen mineralization rate and rainfall.  

Visiting these forests over the course of four summers has motivated and grounded my theoretical work in many ways.












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