Dan JohnsonNicholas School

Research Interests

I'm interested in most anything having to do with trees, shrubs, woody vines, etc. (even some herbaceous things are interesting).  When I was a kid, my parents would say "Get out of the house! Go play in the woods!", so I guess it makes sense that I still spend a lot of time in the woods. It's amazing how much we learn and experience as a kid impacts our adult lives. In fact, I have another non-plant related research project (named Winston) that is directly related to that idea.

Mostly what I do is physiology and ecology of woody plants and plant functional morphology (i.e. adaptive traits). Currently, my primary focus is on water transport (hydraulic architecture and function) in tree, shrub and liana species. My other interests include physiological ecology, anatomy, developmental biology and population biology of young seedling establishment (critical for forest regeneration and migration with climate change) and growth/distribution of tree species in the Appalachians, and at treeline in the Rocky Mountains.


Plant Hydraulics – From the cell to the canopy

Plant water transport is critical in determining species distributions and responses to climate change.  I am currently working on comparative studies of xylem cavitation resistance, capicatance and anatomy. We are developing (and/or modifying) techniques to measure hydraulic conductance and capacitance of very small samples, including single conifer needles and newly-germinated seedlings in the field. We can also use these methods to analyze small wood cores from large trees, negating the need for cutting down entire trees.  These techniques include rehydration kinetics, cryo-temperature SEM to determine of water distribution inside tissues, acoustic detection of cavitation events in leaves/needles/wood cores, and osmometric and psychrometric determination of tissue water storage capacity (i.e. buffering capacity).  These methods will allow us to more mechanistically address water stress limitations (including limitations to seedling establishment) to photosyntheis, growth and survival.

Additionally, I am interested in assessing the impacts of leaf hydraulic architecture and function on gas exchange and growth.  The pathway of water from the leaf petiole to the sites of evaporation can account for as much 95% of the total resistance to water transport in individual plants. Therefore leaf hydraulic design can have a large impact on photosynthesis, transpiration and growth. Studies that are underway or are planned for the next 6-12 months range from the impact of aquaporins on post-drought leaf hydraulic conductance recovery, how leaf xylem anatomy is related to embolism and gas exchange, and the function of leaf transfusion tissues (primarily in the Pinaceae) in maintenance of hydraulic conductance (likely as a source of capacitance). 

I have recently begun working with Rob Jackson on questions addressing deep (~20 m) versus shallow (< 0.5 m) root systems in arid environments. This work has only recently started (as of July 2011), but we are starting to explore a variety of new questions related to the topic, including 1) What impact do aquaporins have on overall deep and shallow root conductivity?, 2) How much overall tree water use comes from deep versus shallow roots in these systems?, and 3) How will the record drought of 2011-2012 impact the productivity and survival of these trees?


Seedling establishment and climate change, or "how do seedlings survive?"

As our realization of climate change and its impacts on vegetation increases, a more thorough understanding of young seedling biology and mechanisms of establishment becomes critical.  With human population growth and greater resource demands, our need for efficient regeneration of forests is becoming ever more important, and our need for alternative energy source production (e.g., biofuels) becomes more acute. Additionally, many natural plant pop ulation distributions are predicted to change over the next century, leading to altered regional ecosystem services. The woody plant life stage with the greatest mortality, by far, is the germinant (1st year) seedling stage and young seedling performance may be more important than seed germination for determination of species distributions, and potentially forest regeneration and production of woody fuel crops.  However, very little work has focused on the ecophysiology of naturally-occurring current year seedlings (newly emerged). 

            The primary determinant of vegetation patterns and species distributions on a global scale is aridity gradients, and moisture regimes (either via precipitation changes or temperature changes and related evaporative demands) have shifted over last 40 years and are predicted to continue to change. Species distributions can shift over time through adult mortality and less subsequent regeneration (shrinking) or through seedling establishment beyond current distribution boundaries (expansion). Additionally, population studies have shown that the woody plant life stage with the greatest impact on population growth or decline is the survival of seedlings, not adult mortality or reproductive output. Therefore, a critical component of our ability to predict future species distributions is an understanding of the mechanisms of seedling establishment, which requires a fundamental knowledge of seedling physiology.

            Current research in this area includes field trials, rainout shelters, isotopic determination of reliance of young seedlings on seed reserves versus photosynthetic carbon gain, vulnerability of young seedlings to hydraulic dysfunction.

Now that I am a father, I have a new found interest in broader scale questions that are important for humanity in general (like drought-induced adult tree mortality and it's impact on regional carbon budgets) and natural history (so I can tell him all about different things we see during our walks in the woods). ]