PLANT RESPONSES TO NUTRIENTS, WATER, AND UNCERTAINTY

Earth’s ecosystems emerge from interconnected biosphere, geosphere, and atmosphere processes. Changes to any one process ripple through the Earth system, affecting other processes. As global climate change continues, nitrogen deposition is anticipated to increase and precipitation is expected to have varied changes across the globe. These changes to the atmosphere and geosphere will have implications for the biosphere. Namely, vegetation will be impacted by changes to nutrient and precipitation regimes. Vegetation comprises the aggregate strategies of individual plants, which are also influenced by changes in nutrient and water availability. The responses of individual plants to nitrogen, water, and uncertainty are the main focus of this dissertation, as understanding those will be critical to scaling up to the aggregate.

 First, I describe a mathematical model that predicts grassland root and shoot biomass across carbon, nitrogen, and water gradients. The model simulates competition among plants by dynamically allocating carbon to either root or shoot growth depending on the growth strategy employed by the other plant. I show that the model accurately predicts root net primary productivity (NPP), but performs poorly for shoot and total NPP. At the biome scale, modeled NPP does not vary with water alone but rather water and nitrogen interact to influence NPP. Second, I conduct a greenhouse experiment using Eragrostis capillaris (L.) Nees to examine the predictions of the model mentioned above to answer the question: how do water and nitrogen affect fitness and biomass allocation in a drought-tolerant C4 grass? And ask: what is the nature of the relationship between water and nitrogen as resources? I show that water was important for increasing shoot and total biomass, but that root biomass and root:shoot ratio was influenced interactively by water and nitrogen as predicted by the model. I conclude that the nature of the relationship between water and nitrogen was that of either interacting or hemi-essential resources. That is, additional water was able to partially substitute for limited nitrogen to maintain biomass. Third, I explore how information theory can apply to plants that face uncertainty in resource availability and briefly review the types and sources of information and the mechanisms that plants use to perceive and respond to their environment. Overall, my framework posits that plants interpret information from their surroundings as an emergent property of distributed information processed by a network of cells. I end with a prospectus of directions for future research, including decoding signal from noise, storage of information, strategies to cope with information entropy, additional means of information transmission, and two-way information signaling with biotic partners. Finally, I use the information theory framework discussed above to answer the questions: can plants sense and respond to information entropy? I explore this question using data from an experiment which altered the temporal supply of nutrients and found no support that P. sativum can sense and respond to entropy. Understanding the relationships of water, nitrogen, and uncertainty is critical to predicting plant growth, especially as climate change continues to influence the global system.