The Spekboom Sponge hypothesis

By Alastair Potts

We know that Portulacaria afra, commonly known as "Spekboom" (and the focus of much misguided hype around carbon sequestration), is an ecosystem engineer in its native range in the Eastern Cape province (and the Little Karoo of the Western Cape). 

An ecosystem engineer is any organism that significantly contributes to the creation, modification or destruction of a habitat. In this case, P. afra, increases the water availability in the landscape (van Luijk et al. 2013) for other subtropical thicket tree species that would not normally be able to survive in that landscape (Wilman et al. 2014). It does this by increasing the rainwater infiltration and creating a thick mulch leaf litter (Lechmere-Oertel et al., 2008) which traps water and decreases evaporation.

Our research group was recently in the field exploring how this dynamic helps subtropical trees ​— i.e. what effect does having P. afra around a tree help its water dynamics. Our focus species was Pappea capensis (Jacket-Plumb) and we were measuring the tension of the water column in trees that were either emerging from Spekboom clumps or were in herbivore-degraded parts of the landscape, and therefore standing alone.

Examples of Pappea capensis trees that we assessed for water pressure potential (a measure of plant water stress). Trees were on the same slope and within ~200 meters of one another. Exposed trees occurred lower on the slope where overbrowsing by goats and sheep has removed P. afra. The exposed trees have likely been in this state for >50 years. 

What we found was unsurprising...

The mean water pressure potential (uMPA= mean MPa of 3 replicates per plant) of trees of Pappea capensis occurring alone or within a clump of P. afra (Exposed: n=9, In-clump=11). {Don't ask about the difference in sample size! All I can say, this measurement mission was done between 12h00 and 15h00 with our highest temperature of 47°C ​— our brains were a tad fried}. More negative values means greater water stress!

Thus, the subtropical trees within a P. afra clump, in general, had significantly lower tension (more +ve values on the figure) on their water column than those on exposed slopes without P. afra. {Note: these are preliminary results that require replication ​— that's up to Daniel B.}

But, while we were waiting around for the poor sod who was fetching the latest samples (we tag-teamed as to who had to venture forth from our basecamp — the centre of large P. afra clump — to collect the next sample) ​— we decided to also measure the water tension in P. afra. We were expecting it to be quite a bit lower, but it was consistently around -0.83 to -0.96 MPa.

This is quite a difference in water pressure potential between the succulent P. afra and canopy tree P. capensis. Which got us thinking...! What if P. afra increased water availability in the landscape more than simply by increasing infiltration and decreasing evaporation? If the roots of P. capensis and P. afra grew close together (i.e. touching), then the difference in water pressure potential might be enough for the tree to suck water out of the succulent!

We call this the "Spekboom Sponge" hypothesis: specifically, as trees become more stressed and the tension on their water columns increased, that they might ​— at some point ​— be able to suck water out of P. afra. 

P. afra is able to suck up a lot of water during a rainfall event ​— data from Dr Kathleen Smart's PhD found that P. afra increased its stem diameter by 5-7% post rainfall (×1.05-1.07 of original size). Based on the size of the stems she was analysing (ø: 84.5 & 145.5 mm),  that's ~0.4-0.8 mm³ per cm of stem. The subtropical trees do not have this same ability, and cannot store much water in their stems, and are thus largely reliant on soil moisture for their physiological requirements {ref needed}. Thus, the P. afra, may provide the surrounding plant community with additional water via touching roots systems as the environment becomes drier and drier.

This is quite an exciting hypothesis, as it suggests that P. afra may be more than just an ecosystem engineer but also a "nurse plant" for subtropical trees during the common droughts and generally low rainfall conditions experienced by arid and valley subtropical thicket in the Eastern Cape landscapes.

And of course, Daniel B. (the Duracell Bunny in our research group) found this paper within hours of our return from the field:

"Nurse shrubs can receive water stored in the parenchyma of their facilitated columnar cacti" by Alicia Montesinos-Navarro and colleagues published in the Journal of Arid Environments in 2019.

In this paper, these researchers demonstrate that the columnar cacti, Neobuxbaumia tetetzo, provides water to neighbouring shrubs, Mimosa luisana. 

We'll be testing this hypothesis in the coming year using the same methods as Montesinos-Navarro et al. We'll provide an update as to what we find.

Alastair out...

Update: 20-Feb-2020. After chatting to Ed February, we're also going to be looking at leaf SLA of trees in and out of P. afra clumps as well as leaf δ13C to determine if there have been long-term water contraints (beyond short hot and dry periods, like our pressure chamber measurement mission).

Update: 22-May-2020. Thanks Daniel for picking that I don't know how to convert PSI to MPA! Excuse the crazy readings in the previous figure version...!

References

Lechmere-Oertel, R.G., Kerley, G.I.H., Mills, A.J., Cowling, R.M., 2008. Litter dynamics across browsing-induced fenceline contrasts in succulent thicket, South Africa. South African Journal of Botany 74, 651-659.

Montesinos-Navarro, A., Verdú, M., Querejeta, J.I., Valiente-Banuet, A., 2019. Nurse shrubs can receive water stored in the parenchyma of their facilitated columnar cacti. Journal of Arid Environments 165, 10-15.

van Luijk, G., Cowling, R.M., Riksen, M.J.P.M., Glenday, J., 2013. Hydrological implications of desertification: Degradation of South African semi-arid subtropical thicket. Journal of Arid Environments 91, 14-21.

Wilman, V., Campbell, E.E., Potts, A.J., Cowling, R.M., 2014. A mismatch between germination requirements and environmental conditions: Niche conservatism in xeric subtropical thicket canopy species? South African Journal of Botany 92, 1-6.