Competitive games plants play

Competition with other plants is intense in a forest, driving allocation to wood and fine roots at levels that decrease a plant's fitness in isolation.

With simplified models of plant physiology I investigate how interactions between competitive and environmental pressures drive individual strategies.

                      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 exists).



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. Why would the same plants respond to more than one resource? Why would plants respond to additions of a resource by increasing their investment in taking it up?
We added nitrogen and water in a factorial design to diverse prairie plots at Cedar Creek and 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 occured at low nitrogen levels, making it seem like the high nitrogen plots were water saturated, while the low nitrogen plots were not.

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.

Cedar Creek diverse prairie plots where water and nitrogen were added in a factorial design.

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.

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 and use less water.  But, sometimes this enhanced productivity does not translate into enhanced carbon storage of the whole forest. 

This increased productivity may not be allocated evenly among plant tissues and further plants may respond to the new environment by changing allocation strategy altogether. This can be incredibly important as these tissues have different residence times within the living biomass and the system. A unit of carbon allocated to wood stays in the tree on the order of 100 years, while a unit of carbon allocated to fine roots will be shed by the plant and begin to decompose after only 2 years. Thus changes in allocation can have significant impacts on carbon storage and the significance of plants for carbon sinks.

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.

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 different rainfall regimes.

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.

The mobile lab.