Hydraulic tales from diverse fynbos functional types

A primary goal of ecophysiologists is to understand the major drivers of plant communities, which involves monitoring both plant response and environmental conditions Although fynbos communities, such as this one at Jonaskop, contain a high number of species, this diversity can often be split into three main functional types: restioid, ericoid and proteoid

"When you can measure what you are speaking about, and express it in numbers you know something about it." - Lord Kelvin

A part of me agrees with the sentiment expressed by Lord Kelvin; that you can only really know something when you can measure it. This part of me happens to be the same part interested in plant ecophysiology (the study of how plants function and how this influences dynamic community patterns).

Ideally, therefore, I should be able to measure plant functionality in response to changes in environmental conditions. This is, however, no simple task in any biome, and a particularly challenging one in the fynbos where the environmental conditions vary rapidly and communities contain incredibly high numbers of very different species.

Essentially, to get closer to an understanding of fynbos communities, it is necessary to capture the response of several different types of plants to environmental conditions at a remarkably high resolution.

Investigating drivers of plant response in fynbos

This, in brevity, was the broad aim of my PhD, which I initiated in February 2011. Of course, I was not the first to attempt an investigation into the drivers of plant response for diverse functional types in the fynbos. A more specific aim of the project was to build on those previous studies by providing a picture of plant response at a greater resolution.

Fortunately, recent advances in miniature sapflow¹ technology allowed me to embark upon capturing near-continuous sapflow data for coexisting fynbos species, which enabled me to record a proxy of transpiration at a very high resolution over fairly long periods. This provided an advance on the previous periodic campaign-based gas exchange measurements, where individuals had gone out every couple of weeks or months and recorded instantaneous measures of plant response.

To reduce the problem of high species diversity, I decided to make use of the "functional types" concept, where species that share similar (functional) traits are grouped into a single category; the idea being that similar looking species are likely to be analogous in ecophysiological functioning. I chose three species representative of the three major fynbos functional types: restioid, ericoid and proteoid.

The species I decided to include in the study were Cannomois congesta (Restionaceae), Protea repens (Proteaceae) and Erica monsoniana (Ericaceae), which all co-occur at Jonaskop in the Riviersonderend Mountains. Monitoring the sapflow of these species, coupled with monitoring of environmental variability using a micrometeorological station, is allowing me to gain an understanding of what the important environmental factors are and how response to them differs among fynbos species.

Different "strategies" of water use

From the outset of the project I suspected that with all this variability and diversity there may be different "strategies" of water use. I was hoping for different “hydraulic tales” for different species and I was not disappointed. I have been able to show that each of these species displays distinct reliance on summer and winter rainfall events. For example, the shallow-rooted Erica species is more reliant on infrequent, yet periodic summer rainfall events compared to Protea and Cannomois. The deep-rooted Protea species does not respond to those events, but appears to "recharge" only during the heavy winter rainfall season. Remarkably, the shallow-rooted Cannomois species appears to rely on episodic moisture sources, such as cloud or fog events.

The next step for me is to use these different responses to determine how vulnerable our communities are to potential long-term changes in environmental conditions. For instance, if we lose the summer rainfall events, does this mean that Erica species are vulnerable?

At least for now, however, I can say that I am one step closer to knowing something more about fynbos.

¹ Sapflow technology uses the movement of heat through a stem to monitor water movement and provides an idea of when a plant is turning on and off.