Competitive plant responses to elevated CO2 may significantly decrease a potential carbon sink

Background/Question/Methods

One of the most pressing applied questions in ecology today is determining the strength and persistence of the carbon sink in the terrestrial biosphere, the results of which could mean the difference between climate change as a solvable problem and climate change as a catastrophic acceleration of climate change (Cox et al. 2000, Friedlingstein et 2006).  Two well-documented leaf level mechanisms have been proposed to explain the sink: increased water-use efficiency and increased efficiency of photosynthesis. Extrapolating these leaf-level mechanisms to the stand level, researchers hypothesized that increases in carbon storage should be strongest in drier areas because of the water-use efficiency effect and weaker in areas where plants are limited by another nutrient, most importantly nitrogen (Strain and Bazzaz 1983, Luo et al. 2004). 

However, experimental and observational patterns often contradict these predictions. Across years of several different CO2 fertilization experiments, drier years did not have a stronger CO2 fertilization response (Nowak et al. 2004, McCarthy et al. 2010). Additionally, significant increases in WUE have been observed without corresponding increases in woody biomass growth in plots across the globe (Penuelas et al. 2011). Finally, strong fertilization responses have been shown to persist in several experiments despite nitrogen limitation (Norby et al. 2011). 

Results/Conclusions

Using a game theoretic model of tree competition for light, water (Farrior et al. In Prep), and nitrogen (Dybzinski et al. 2011), we predict changes in plant productivity and allocation patterns in response to the leaf-level CO2 fertilization mechanisms described above. If water-use-efficiency increases while plants are water-limited, plants are predicted, for competitive reasons, to invest the additional productivity into fine-roots, not structural biomass. This makes for a very weak carbon sink, explaining the lack of increased tree growth, despite increases in water-use-efficiency. Alternatively, when atmostpheric CO2 increases in a nitrogen-limited, and water-saturated ecosystem, the main leaf-level CO2 fertilization mechanism will be the increase in photosynthetic efficiency. In contrast to water, nitrogen uptake has diminishing marginal returns. Competition does not drive plants to invest the entire increase in productivity to fine roots; rather, a small portion of this enhanced productivity is invested in fine-roots, but a large portion is allocated to woody biomass, creating a strong carbon sink. We find that taking an individual-plant perspective in competition for water, light, and nitrogen is critical to predicting the strength and persistence of size and persistence of the carbon sink in the terrestrial biosphere.