Assistant Professor of Biology and Environmental Studies
I received my B.A. in Computer Science and Earth and Planetary Sciences from Johns Hopkins University in 2003 and a Ph.D. in Earth and Planetary Sciences from Harvard University in 2009. After two years as an O K Earl Postdoctoral Scholar in Geology and Postdoctoral Scholar in Geobiology at Caltech, I joined the Biology Department at Haverford in 2011 as an Environmental Biologist. I enjoy field-based work, whether geological or botanical in nature, and try to spend my summers near some combination of plants, rocks, and trout.
My lab investigates the coevolution of plants and the environment over the last 475 million years. We employ mathematical models, chemical analysis of fossils, comparative anatomy, and experimental plant physiology to reanimate extinct plants, focusing on examples with unusual anatomy, development, or growth forms. These insights shed light on past (and future) climate change, the evolutionary trajectory of plant adaptations, and ecosystem responses to mass extinctions. You can download my papers here.
Active research areas
Physiological properties of extinct plants
The major ongoing project in my lab is employing a series of mathematical models to calculate fluid dynamics properties of plant tissues based upon measurements of cells within their water transport system, the xylem. These mathematical models and statistical relationships take as their inputs various dimensions of xylem cells, including cell width, length, and porosity, and the outputs are hydraulic conductivity and resistance to damage from water stress. Taken together, conductivity and stress resistance allow extinct plants to be categorized and quantified physiologically: some plants have high hydraulic conductivity but low resistance to damage from water stress, whereas others are the reverse. The focus of this project has been to identify trends and patterns in plant physiological evolution in the time since the evolution of water transport tissue in plants, approximately 410 million years ago. My work has been able to identify extinct plant groups that employed hydraulic strategies that are absent from terrestrial ecosystems today, including physiologies that are comparatively risky, where high transport rates come at the cost of reduced safety from drought stress damage. The focus of this project has been to identify trends and patterns in plant physiological evolution in the time since the evolution of water transport tissue in plants, approximately 410 million years ago. My work has been able to identify extinct plant groups that employed hydraulic strategies that are absent from terrestrial ecosystems today, including physiologies that are comparatively risky, where high transport rates come at the cost of reduced safety from drought stress damage.
Evolution of plant biomineralization and the terrestrial silica cycle
The global carbon cycle is tied to a variety of other elemental cycles on the surface of the Earth, including the cycle of silicon. Today, approximately 50% of all photosynthetic activity is conducted by diatoms, small marine algae that build shells out of silicon, and these microscopic algae are responsible for converting carbon dioxide into organic carbon that sinks to the bottom of the ocean, reducing the concentration of CO2 in the atmosphere. Silicon is a necessary nutrient for these diatoms, and it is supplied to the ocean by plant tissues: certain plants accumulate silicon in their organs from soils and rocks they grow upon, and when these plants die, their leaves, stems, and roots decay and silicon flows to the ocean, fertilizing diatoms. Our preliminary work has shown that silicon accumulation is not distributed in a uniform way throughout all land plants—some accumulate substantial silicon (and thus supply much silicon to the ocean) whereas others accumulate very little. Specifically, our preliminary work has shown that a number of ancient land plant lineages, particularly ferns, accumulate more silicon than any other living plant group, and also that many of these lineages have been ecologically important for more than four hundred million years. Our ongoing work has been to clone, sequence, and identify the genes that code for these novel silicon transporters in these early-diverging land plant lineages. A major portion of these results was published in the Proceedings of the National Academy of Sciences in March, 2015.
Multiple origins of leaves
Using a similar approach as the silicon transporter project, we are working to clone and sequence PIN1, one of the genes responsible for leaf and vascular development, in Equisetum, Psilotum, and Botrychium. Each of these lineages diverged from ferns via a set of leafless ancestors between 375 and 300 million years ago and independently acquired—and perhaps one evolved and then lost—leaves. PIN1 plays a key role in leaf initiation and leaf vein formation, but very little is known about its function in spore-bearing plants that have leaves; our project is aimed at shedding light on this early stage in the evolution of leaves. If we are successful in these efforts, we hope to learn whether the multiple origins of leaves early in the vascular plant lineages that led to modern ferns always occurred in the same fashion.
Microbial diversity on leaves (the “phyllosphere”)
As part of an ongoing series of Superlab courses, we have been investigating the physiological and phylogenetic diversity of bacteria found on leaves. This microbial community has been called the “phyllosphere” and contains approximately as much bacterial diversity as the human gut. Students collected leaves from trees and plants across the Haverford campus and extracted DNA from the bacteria on leaves, using both culture-independent methods (extracting DNA directly from bacteria physically separated from the leaves using sonication) and culture-dependent methods (growing organisms from the leaves and isolating them in pure culture). Bacterial samples are identified using 16S rRNA sequencing, and students have identified a number of unusual organisms, including some that are new species. Ongoing work is focused on the abundance and diversity of rare bacteria on conifer leaves.
Extinct plant biomechanics
As a postdoc at Caltech, I developed a rapid, semi-destructive technique for determining the lignin content of fossil plant tissues using geochemical analysis, which, in turn, helps constrain the biomechanical properties of extinct plant tissues. The lignin biosynthetic pathway includes a step that contains an isotopic fractionation, where an enzyme preferentially incorporates the light stable isotope of carbon (12C) over the heavy stable isotope of carbon (13C), and measuring the relative abundance of these two isotopes against one another, called the stable carbon isotope composition (δ13C), can help distinguish between heavily lignified and lightly lignified tissue. By measuring unambiguously lignified tissue from fossil plants, such as plant xylem, and differentiating it from tissues of unknown biochemical composition, such as the external or cortical tissues in the same fossil, we can construct a map of the biomechanical properties of different tissues within the same plant fossil. If the cortical tissues are lignified, and are therefore contributing to the biomechanical stability of the plant, they will have a chemical signature more closely resembling xylem; if they are unlignified, and have values substantially different from those found within the xylem, they are unlikely to have played a role in the structural support of the plant. This project was published in International Journal of Plant Sciences in 2011.
Water/carbon tradeoffs in early vascular plants
One of the most interesting subfields of plant physiology (with serious implications for the study of past ecosystems) has been linking plant anatomical traits to physiological properties. A number of studies have shown that key parameters, including photosynthetic rate and leaf hydraulic capacity, are determined and controlled by the morphology and arrangement of plant vascular tissues and apertures on leaves, called stomata. The most interesting of these studies have shown that these relationships are quantitatively robust across all clades of vascular plants; that is, a fern, conifer, and angiosperm leaf that all happen to have the same vein density and distribution and size of stomata would all have similar photosynthetic rates, despite the great evolutionary distance between them. The implications for the study of extinct plants are vast: plant physiological traits in leaves can therefore be reconstructed quantitatively from leaf anatomy alone.
Life in a low-oxygen world: geobiology of the ~1.8 billion year old Duck Creek Dolomite, Western Australia
Several years ago, we investigated the geochemical, sedimentological, mineralogical, and stratigraphic context for a deposit of iron-coated bacteria from the Duck Creek Dolomite, a ~1.8 billion year old carbonate platform. By measuring the stable carbon and oxygen isotope composition of these rocks (at 2-meter intervals over >1km of section) and localizing the microbiota within their mineralogical and paleoenvironmental distribution, we were able to demonstrate that the Duck Creek Dolomite preserves a unique geobiological window into the low-oxygen world following the evolution of oxygenic photosynthesis. This project was published in Precambrian Research in 2010.
Paleoenvironmental distribution of early animal burrows: Treptichnus pedum in Namibia
With this project, we were able to demonstrate that Treptichnus pedum, the trace fossil that marks the global transition from the Ediacaran Period to the Cambrian Period, can be found in high abundance in a relatively narrow paleoenvironmental window within incised valley deposits in southern Namibia. This project was published in Palaios in 2012.