The roles of competition and resource availability in the biomass allocation of plant communities

Abstract

Plants live in communities where their success is determined not only by the environment, but also by the traits of other individuals in the community. This sets up incentives for over-investment - investing more in taking up a resource than would be optimal in the absence of competition. To examine the effect of these incentives on biomass allocation in plant communities, I, with collaborators, built a tractable model of competition for water and light and compared its predictions with experimental and observational data. This model has two axes of water availability, which differ in their influence on the dominant plant allocation strategy. If plants spend more time in water saturation, plants invest less in fine-root biomass and more in leaves and woody biomass. Alternatively, if plants have more water while they are water-limited, they invest more in fine roots without changing their investment in leaves or woody biomass. These competitive responses to resource additions in the face of water and nitrogen limitation are capable of explaining complex plant responses to simple resource addition experiments. For example, experimentally, plants responded to increased water availability by increasing fine-root biomass, but this response decreased with nitrogen additions. To relate differences along these axes of water availability to real systems, I replaced the simple rainfall model with a stochastic one. This new model shows that the axes of water availability are intertwined with respect to common rainfall statistics, making predictions of plant responses across rainfall regimes complicated, a result consistent with experiments and observations. Finally, I tested this model's predictions against specific forest observations and found promising avenues for future model improvements, including variation in wood traits and leaf physiology that may correlate with allocation strategies.

Throughout the thesis, I make predictions about the potential for carbon sinks. The analytical model predicts that, because of competitive pressures on allocation patterns, water-limited plants will increase productivity but not carbon storage following increases in atmospheric carbon dioxide. The stochastic rainfall model predicts that this downregulation of the land carbon sink is strongest at dry and intermediate levels of total rainfall.