Aidan Matthews

Aidan is currently pursuing a PhD in Environmental Science and Engineering from Harvard University under Professor Kaighin McColl. Aidan's research interests are in ecohydrology and land-plant-atmosphere interactions. He graduated Summa Cum Laude from Princeton University with a Bachelors in Civil and Environmental Engineering. In his spare time he enjoys writing about himself in the third person and designing amateurish websites. Challenge me in chess, link is the horse icon.

Resume

Ecohydrology

Do you know how water flows up a tree?

Ecohydrology is the study of plant (eco) water (hydrology) interactions from the global to the leaf scale. The reason water is able to flow against gravity up a tree is the same reason a puddle will evaporate from a liquid to water vapor. The atmosphere is relatively dry (even on humid days) and acts like a sponge pulling the water out from the leaves. When the water is sucked out of the leaves it acts like a rope pulling up on the water in the rest of the tree down to the roots, which ultimately suck water from the soil. The drier the soil is the more tension is required to pull it up. When water is under tension like this its pressure is negative. A negative pressure means that if exposed to the atmosphere it would instantly boil! The reason the trees are able to stop water from spontaneously boiling at negative pressures is still not well understood and an active area of research.

So far my work has been on models of stomata -- the tiny mouths on the leaves of plants that open to allow CO2 to diffuse in. When stomata open water can escape however, which means a plant will not want to keep stomata open for longer than it needs to. Collectively, stomata act as a critical valve for both the carbon and water cycles. Thus understanding them and when and why they open is critical. In this paper I develop an optimization theory that explains mathematically why 'optimal' stomatal behavior is coupled to a plant's resilience to water stress. I also demonstrate an intuitive but previously unquantified idea, that it is optimal for plant resistance to water stress to increase with aridity.

Geochemical Cycles

Simple, non-dimensional models can be hugely important for understanding complex and multivariate environmental phenomena like nutrient cycles in the soil. Plants need nutrients (the big ones are nitrogen, phosphorus, and potassium) to perform vital functions. A lack of nutrients can limit agricultural productivity. The soil, however, is highly complex and each nutrient can exist in a variety of compounds and redox states. Nitrogen for instance, can range from having plus four electrons in nitrate to having minus three electrons in ammonia! The transitions between states are spontaneous due to favorable conditions or are mediated by organisms, usually bacteria. A good example is nitrogen fixing bacteria which provide natural nitrogen fertilization for plants by converting nitrogen gas (N2), which is abundant in the atmosphere, to ammonia, a form of nitrogen that is available for plant uptake.

A much less studied soil chemical cycle is the iron cycle, a micro nutrient for plants. The iron cycle is significant not only as a nutrient but also for its varied interactions with soil organic carbon (SOC). SOC is a major store of carbon that can serve as a sink of CO2, or a source. The lifetime of SOC in the soil can vary from days to millenia and has major ramifications for the rate of climate change. Iron is hugely important because in its mineral form (Fe3) it 'protects' SOC that latches on from being decomposed back into CO2. However, when the soil is flooded Fe3 can be reduced to Fe2, which (1) releases the protected carbon and (2) serves as an oxidizing agent for further production of CO2. Thus, the effect that iron has on the carbon cycle is complex and depends on soil moisture fluctations. I wrote my thesis on modeling the soil iron cycle and its interactions with the carbon cycle. The main conclusions were: (1) an analytical solution to a stochastic differential equation which solves for what fraction of the time iron is in its Fe2 vs Fe3 state given a few parameters (rate of oxidation, average rainfall...); and (2) that rewetting previously drained wetlands (seen as generally a good thing) can lead to increased rates of carbon emissions by reducing the iron from Fe3 to Fe2 and releasing the attached SOC.

Land-Atmosphere Interactions

Land, known far and wide as the bottom boundary condition, interacts with the atmosphere to determine the temperature and humidity that plants and animals feel. The atmospheric boundary layer (ABL) is the bottom layer of the atmosphere going from surface level to several kilometers. During the day is defined by turbulent mixing and grows in height as the day progressess and more heat is transfered from the ground to the air. Heat can be transferred in from the ground to the air as radiation, sensible heat, and latent heat. Latent heat is just the energy that is released when water evaporates. In other words, the evaporation of water does not just affect the water cycle but is also critical for the earth's energy balance. About 2/3 of land evaporation comes from plants (called transpiration), but plants are able to control when they evaporate using their stomata (we are back to ecoyhdrology!). Plants, as well as ice, different colored rocks, and human land use change can all affect the land surface in several ways. First, it can change the albedo (reflectiveness) of surfaces which can change how much radiation is absorbed by the land. Asphalt is always hot in the summer because its black and has a low albedo, meaning it absorbs much more light than it reflects. Another way land properties affect the atmosphere is through surface roughness. Grasses, corn fields, towns, sky scrapers, and mountains all introduce roughness into the path of wind and change the dynamics in the lower atmosphere.

Feedback cycles can occur in the land-plant-atmosphere system. For instance rainforests have dense vegetation that is incredibly efficient at evaporating water compared to bare soil. This means that more water is put back into the atmosphere when it rains over the Amazon. More water in the atmosphere increases the rate of precipitation, which promotes the growth of dense vegetation. You can read more about it here. This feedback also means that disrupting the cycle, i.e. deforestation, will disrupt the feedback loop and lead to amplified droughts and deforestation. As an undergraduate I developed a coupled model of the plant-atmosphere system using a simplified boundary layer model for the atmosphere and the Photo3 model for plants. I found that CAM plants (e.g. succulents/cacti) can significantly alter the energy and water conditions of the ABL because they transpire at night, rather than during the day. The code for that project can be found here, and the original Photo3 model (credit Sam Hartz) can be found here.

Publications

Aidan has several years of tutoring and mentoring experience for a variety of STEM topics. He was a tutor for two years at the McGraw Center for teaching and learning at Princeton University where he taught multivariable calculus, physics, and data analysis in R. Aidan enjoys teaching math and science at all levels (up to his own limit of knowledge?). He is also available for SAT/ACT prep and general college/graduate school advice. If you are interested please email at amatthews@g.harvard.edu.