1. The structure and function of xylem: safety vs. efficiency
All else aside, the key role of xylem tissue is water transport to the leaves and the maintenance of hydraulic continuity in the face of drought and/or freezing stress. We call this this the efficiency vs. safety tradeoff, with 'safety' referring to a species' resistance to the formation of air-vapour emboli in the xylem conduits. As you can imagine, riparian species that have reliable access to water during the growing season, such as a water birch are much less resistant to drought-induced embolism than xeric-adapted chaparral shubs or some high-desert conifers. Characterizing the morphological and physiological traits that explain the safety-efficiency trade-off is a key focus of our research programme. For example, how is vulnerability to embolism related to xylem conduit size, conduit traits, species' life history strategy, evolutionary trajectory and biogeography? How adaptive are these traits under situations of variable atmospheric CO2, browsing by large herbivores and exposure to prolonged water deficit? Conversely, how have these traits shaped the evolutionary history of plant groups over deep time?
What 'else' does xylem do for a plant? In a conifer or woody angiosperm, the secondary xylem (wood) supports the canopy and functions in storage. How does storage and support trade-off with embolism resistance, hydraulic efficiency and life-history strategy? Could the storage function alleviate drought stress by facilitating the recovery of embolized conduits?
We tackle these questions using a combination of field and lab work, microscopy and modeling.
2. Old plants, new tricks: the evolutionary ecophysiology of Pteridophytes
Ferns are one of the oldest free-sporing vascular plant lineages with origins in the Devonian. They have undergone at least three major radiations since this time and were once a significant component of the Carboniferous floras that make up the extensive coal deposits mined in the Northern Hemisphere. Today, ferns continue to be a steadfast presence in a variety of habitats where they display a diverse array of semi-arborescent, climbing, and erect morphologies. Despite fern abundance, diversity and evolutionary signiﬁcance, fern physiology is conspicuously absent from modern plant physiology textbooks. Yet fern vasculature is fascinating because the xylem is based on single-celled conduits (like in conifers) and yet extant ferns do not develop secondary xylem, that is wood. How is vascular structure and function related to the evolutionary trajectory, life history strategy and biogeography of Pteridophytes and Lycophytes? My colleagues Eddie Watkins (Colgate University), Mike Windham and Kathleen Pryer (Duke University), James Beck (Kansas State), Lawren Sack and Van Savage (UCLA), Brian Enquist (Arizona State) and Steve Davis (Pepperdine) are working to answer these multifaceted questions by examining the hydraulic architecture, leaf function and anatomy of these ancient plants.
3. Climate change and the redwood forest understory: a mirror for the canopy?
Ferns can be very sensitive to drought with frond traits changing seasonally depending on water availability during development. As such, ferns can be harbingers of drought and ecosystem change. In California's coastal redwood forest, the sword fern is one such indicator in that it's size, frequency and perhaps even fertility can change over the water availability gradient from the northern to the southern range of the redwoods. Long-term monitoring coupled with an understanding of fern physiology can inform our understanding of the effects of drought on the redwood forest understory. Our campus is part of a large climate change initiative project spearheaded by the Save the Redwoods League, a non-profit organization in San Francisco. In particular, my colleague Dr. Emily Burns (Science Director at the League) and I are working towards a synthetic understanding of the role that plant physiology, life history strategy and ecology can play in forest management and conservation.