What can sapflow tell us about plant communities in a changing world?

The author carefully considers the merits of a potential sample site. A meteorological station is set up to quantify climatic patterns.

Although the cold and wet winter experienced in the Western Cape this year might not suggest it, climate models predict that the region will become warmer and drier over the next century or so.

Inasmuch as scientists agree, most agree that climate change is a threat to biodiversity. Since the Western Cape’s fynbos biome contains an exceptionally high level of diversity and endemism, it seems prudent to investigate how vulnerable our communities are to the predicted changes.

Currently, our knowledge of how vulnerable different species are to drought and increased temperatures is limited. This information is critical - one can easily see that if certain species are more susceptible to desiccation than others, that changing conditions could drastically alter community composition.

In a nutshell, this was the rationale behind my PhD. A little over a year ago I set out to assess whether different species respond differently to drought and whether I can use these data to infer which species are more likely to be affected by future change.

Sapflow technology

As a plant physiologist I tend to think of community level responses as functions of mechanistic processes occurring at an individual level. This bottom-up approach to community ecology has a strong influence on the type of methods (and instruments) that I choose to collect data with. In particular, I chose to blend more traditional methods of plant water relations, such as leaf level gas exchange and water potential, with some new methods, such as sapflow technology.

Sapflow technology uses heat as a tracer for the movement of sap through a plant stem and has the benefit of being able to remotely capture almost continuous measures of sap velocity. However, working in the fynbos presents some unique problems for sapflow studies. A large proportion of our diversity has small stems (e.g. Erica) or photosynthetic culms in lieu of stems (e.g. Restionaceae).

Developing miniature sensors to study sapflow

Since traditional sapflow gauges consist of large needles, they are unsuitable for the majority of fynbos species and functional types. Consequently, we had to develop miniature sensors using a design which had only been published as recently as 2009 and had previously only been shown to work on fruit pedicels (think apple stalks).

Fortunately, we were able to obtain some sample gauges from collaborators at the University of California at Berkeley who were doing some work on small flower pedicels. Using these, we were able to manufacture gauges in our own laboratory and set about testing them on several fynbos species in controlled settings. Our species were selected to ensure that they adequately represented the three main functional types in the fynbos and covered a wide range of stem morphologies.

Selecting a suitable study site

After much time spent in the lab we finally convinced ourselves that the gauges were working effectively and that they were ready to be tested in the field. Now all we needed was to select a study site; no simple task when you have sensitive (and relatively expensive) electrical equipment. The merits of a variety of different sites had to be considered with Goldilocks-type scrutiny.

Since fire is a common occurrence in fynbos communities, we had to find a site that was of the appropriate age. Some were not quite old enough to have plants of the right stature; others were too old and very likely to burn. We also had to ensure that the site was sufficiently remote to be secure against mischievous human activity. Eventually, after much searching and deliberation, we found what we were looking for - Jonaskop in the Riviersonderend mountains of the Western Cape.

Testing our sapflow gauges in the field

But for the lack of swimming sites towards the end of summer this site is the middle bowl - just right. It also has the added bonus of being the site of much previous climate change type research. With this in mind, I set out to contribute in my own way to the legacy of research in South Africa.

We tested the effectiveness of our sapflow gauges in the field by investigating the relationship between leaf level gas exchange and sapflow velocity. This would provide a strong measure of how effective the gauges are, since gas exchange instruments have been shown to reliably measure plant response. For all species there was a strong linear relationship between sapflow and gas exchange, indicating that it captured dynamics of water movement through the plant as we had hoped.

Identifying how different species respond to environmental conditions

The next step of the study was to identify how different species are responding to environmental conditions. To effectively quantify climatic patterns, we set up a meteorological station, which currently towers above the vegetation. We also extended the number of individuals that were instrumented with sapflow gauges, to gain a more representative idea of what each species is doing.

We are currently analysing these data to determine exactly how different species are responding throughout the stressful summer period. Preliminary data analysis has shown that there is variation in the response of different species and suggests some interesting trends ... but you will have to wait until the end of this summer to find out exactly what those trends are, how vulnerable our communities are and how they are likely to change in the future.

About the author

Robert Skelton is a SAEON graduate student, receiving a bursary from the SAEON Fynbos Node. He is studying towards his PhD in the Botany Department at the University of Cape Town under the supervision of Dr Adam West.