McCulloh Lab Research

Our research focuses on the physiology of the xylem with particular emphasis on how it impacts the interactions between species. Below are descriptions of a few projects we’re involved in at the moment. While these projects are ongoing, we are generally interested in a wide range of topics and are always excited about new collaborations. Please feel free to contact Kate if you have ideas for projects!

Physiology of seedlings

The seedling lifestage is an extremely vulnerable one. A very large fraction of plants that germinate die before reaching maturity. Yet, we still know surprisingly little about what physiological mechanisms lead to death during this time. That’s unfortunate, because survival through this stage determines local species ranges—plants can only exist as adults in places they can survive in as seedlings. Our project is examining how different populations of the same species differ in their tolerance of water stress.

Drought tolerance and growth in Eucalyptus

The genus Eucalyptus is extremely diverse and is very ecologically and economically important. Along an aridity gradient in Victoria, Australia, we have set up four common gardens in each of which we are growing 10 eucalypt species whose ranges start and stop across the state. Using these species we are asking if the range limits are caused by differences in species’ tolerance to water stress and which physiological parameters are crucial in permitting survival under stress. Duncan Smith is the postdoc in charge of keeping this project running smoothly! This is a collaboration with Tom Givnish (UW-Madison, Botany), Mark Adams (U Sydney), and Tom Buckley (U Sydney). Photo credit: Michael Ryan (VicForests).

Differing hydraulic strategies in co-occurring species

Three continua exist with respect to strategies that woody species employ to tolerate normal diurnal water stress. First, plants differ in the amount that they regulate their leaf water potential. Some plants (referred to as “isohydric”) maintain roughly the same midday leaf water potential throughout the growing season, while others (“anisohydric”) allow their midday water potentials to decline with drying soils or greater vapor pressure deficit. Second, plants differ in the hydraulic safety margins, which are the difference between the midday stem xylem pressure and the pressure inducing a 50% loss of hydraulic conductivity in stems. A strong trend exists with species that have mild stem xylem pressures at midday (i.e., close to 0) having small safety margins, while those with more negative stem xylem pressures have large safety margins. Third, plants differ in their reliance on structural features within the xylem to avoid the propagation of emboli due to drought stress vs tolerating emboli during the day and refilling them over night. Our research is focused on how these three continua combine to form an overarching hydraulic strategy and how co-occurring species differ in these strategies.