July..A blog by Andy Hirons. Twigs still spinning…
One of the challenges of researching drought tolerance with trees is as soon as I wave goodbye to students for the summer, I say hello to my research commitments. So, to dispel the myth that while the students are away I swan around barefoot on an exotic beach, I thought I would give you an insight into some of the research I have been doing over the last month.
As regular readers of this blog will be aware, I am particularly interested in tree selection in relation to drought tolerance. All of you actively involved in establishing trees in the urban environment, will recognise the value of planting drought tolerant species to aid tree survivorship. This is particularly true for street planting, where sealed surfaces interfere with infiltration and soil moisture recharging.
As with so many arboricultural challenges, answers to key questions come from collaborative working those in allied disciplines. Of course, understanding which trees are most threatened by drought is a vital question for those trying to understand how a changing climate will affect our forest ecosystems. It, therefore, makes sense then to adopt some of the techniques that forest scientists are using to determine which species are most threatened by drought and apply them to this key question for urban forestry.
One of the most instructive activities that can be pursued to determine the likely effect of water deficits on trees is to develop a species-specific vulnerability curve to embolism. This sigmoidal curve describes how the hydraulic conductivity of a stem (how easy it is for water to pass through the xylem) changes with declining stem water potential (a measure of the stem’s water status) (Figure 1). With this curve, it is possible to establish at what water potential the conductivity starts to decline, provide a quantitative data to compare species and indicate the water potential from which the tree is unlikely to be able to recover from.
Figure 1: A classic sigmoidal vulnerability curve. At high (less negative water potentials) the xylem is conducting very effectively and the percentage loss of hydraulic conductivity (PLC, %) is 0. As the xylem water potential declines (becomes more negative), conductivity is reduced as embolism in the stem increases. Eventually, no water is able to be moved through the xylem. Figure from Fichot et al., 2015.
There are a few ways to construct these curves. One of the most efficient is to use something known as the Cavitron (Figure 2). This is a rather rare piece of equipment, so in early July, I travelled to France to work with Hervé Cochard, a leading expert in xylem hydraulics.
To give you an idea of how the system works (and to help explain the title of this blog) it is probably worth explaining how the Cavitron works. Basically, it is an adapted centrifuge that is able to take sections of stem and spin them at a defined velocity. This generates a quantifiable force that mimics the negative tension that the stem experiences during a drying cycle. The faster the spin, the more negative the tension is. Hopefully, you are still with me… The really fancy bit is that whilst the stem is spinning in the centrifuge, it is simultaneously fed with water (plus a few nutrients to make it more sap-like), and the conductivity of the stem measured with the aid of image-analysis. This means that you can start a stem spinning rather gently to and establish the conductivity at ‘high water potentials’ (unstressed conditions) and then increase the rate of spinning in a stepwise fashion until the stem loses all conductivity – but before the centrifuge takes off! The data generated along the way indicates the water potential that leads to the loss of hydraulic conductivity as a result of embolism (gas blockage) within the stem.
Figure 2: The Cavitron is an adapted centrifuge which allows you to spin sections of stem in order to mimic the stress caused by drought.
Like all these things, the process of data collection does not inspire poetry, but the data just might (or at least a few academic papers). One of the key variables the data gives you is a standard measure of drought tolerance, the water potential a 50% loss of hydraulic conductivity (ΨP50). This ΨP50 is a standard measurement used widely in the ecological literature to help rank species according to their tolerance to drought.
I was able to process eleven species (Figure 3) in my week with Hervé and they varied widely. For example, the most sensitive species tested had a ΨP50 higher than -2 MPa and the most tolerant species had a ΨP50 lower than -4MPa. This is a big deal in terms of drought tolerance. It also shows that this provides a really useful trait to help inform selection decisions.
Figure 3: Processed samples of stem.
Having quantifiable data for traits that are known to influence tree survival in urban environments is critical to advance our approaches to tree selection and central to our ability to identify which species are most vulnerable to projected changes in climate.
I hope I will be able to give you more specific details on my summer’s research activities once the full analysis is done. This will include pairing my data with the drought tolerance of the leaf – currently being collected by Henrik Sjöman and his team in Sweden (Thanks Henrik ). In the meantime, if you have any questions, please feel free to contact me at email@example.com.