Thicket restoration and frost: the forgotten enemies

By Robbert Duker


Thicket restoration by planting of spekboom cuttings in the Eastern Cape has, in some cases, been quite unsuccessful.

In some areas a dense canopy cover of spekboom now thrives, with thicket biodiversity returning of its own accord. However, just a short distance away one might only find a handful of stunted survivors, barely clinging to life in an area with seemingly identical environmental conditions.

Why this happens is a question that has left many feeling that "spekboom planting for thicket restoration is an expensive waste of time". This really is not the case, it's just that we have been somewhat forgetful and a bit hasty.

So why do some spekboom die, while others thrive a short distance away?

Well, a fact that is often overlooked/forgotten, is that spekboom really doesn’t enjoy the cold. 


For spekboom, those calm starry winter nights in the mountains that might make such great photographs of the Milky Way are an absolute nightmare. If temperatures fall near 0°C, spekboom is in real danger, and even a light frost can wipe out hundreds of cuttings.
So frost kills spekboom cuttings.

Is frost really killing your spekboom cuttings?

It's easy to find out really, because there are some classic symptoms:

• Moisture leaking from discoloured leaves, and stems flopping over (Figure 1a). This happens when cell membranes have been broken by ice crystals inside the leaves.

• Dry blackened leaves and stems with cracked bark peeling away in flakes by the end of winter are classic signs of frost damage (Figure 1b).
• Dead leaves and stems will often remain attached to the plant for a long time, giving the plant the appearance of being half dead and half alive. It is unlikely that lack of water will cause these symptoms.

Frost damage spekboom showing floppy discolored leaves and stems (a), and complete die-off (b)

How to plant spekboom in the right spot to avoid frost damage and restoration failure?
Simple. Don’t bother buying frost netting and trying to come up with fancy techniques, just take heed of the following three principles.

1) Avoid valley floors and flat areas.
Why? Because cold air pools (Figure 2). Cold air is denser than warm air, and on calm starry nights in winter, will flow downslope and accumulate on valley floors and in flat areas (often called frost hollows). This is where frost will wipe out your restoration efforts (Figure 3a & b).

Figure 2: Cold air pools on valley floors and in flat areas, often causing frost 

2) Hide your spekboom from the stars.

Why? Because exposure to the cloudless night sky means loss of heat. Have you ever stepped under the canopy of a tree on a calm starry night? It’s often noticeably warmer underneath the canopy than outside in the open. This can be the difference between success and failure for spekboom planting. When planted underneath the sheltering canopy of not only trees, but even small shrubs, spekboom can survive in areas where frost kills those planted in the open. The insulating microclimate generated by the nurse plants canopy is the cause of this frost exclusion. 

3) Nurse plants are great.

Why? Because nurse plants protect spekboom from the cold (and also from herbivores). Even on frost-free valley slopes, temperatures may fall to just a few degrees above zero. Spekboom doesn’t even like temperatures below 5°C, and the insulation of nurse plant canopies can protect cuttings from unnecessary stress. Survival and growth rates will be much higher inside bush-clumps, which will in turn benefit from the presence of spekboom, as it improves the soil quality under the canopy.

Figure 3: The do's and don'ts of where to plant spekboom

In summary, plan before you plant!

Don't plant on valley floors and flat areas (and sometimes even on gradual slopes), and use nurse plants as much as possible!

The wondrous world of Independent Contrasts

By Megan Smith

In my recent efforts to understand Phylogenetic Independent Contrasts (PIC), I came across a lecture that was done by the man who proposed the method himself, Joe Felsenstein. So who better to explain the method than the creator himself? PIC is a comparative method that statistically compares traits between species or clades by using phylogenies. However,  this poses a problem: how can one compare traits in an independent manner when phylogenies are hierarchical? The answer: you cannot. As Felsenstein explains, it creates biases in the statistical analyses and although it seems like one is comparing traits individually, you are not. As a response to these faulty comparative methods, Felsenstein created a method that takes into account weighted differences of branch lengths to calculate a variance of each trait. Because the branch lengths are taken into account, non-independence of species is circumvented. The phylogenies are assumed to work under Brownian Motion i.e. the probability of  a trait to move upward or downward in a tree is the same and traits evolve in a linear manner.

Figure 1:  PIC explained graphically. The  differences in distances  between branches of nodes are calculated using the Brownian Motion model. These values are then used to plot the differences between traits and a regression can be calculated. (http://www.life.umd.edu/classroom/zool360/L5_evolution/img023.gif)

The algebraic side to PIC is a little more complicate than what has come across in my post, but the idea is "fairly" simple. PIC's are highly useful in evolutionary biology and one can find correlations between traits and the environment through evolutionary time. Its kind of ... magical. 

To understand PNC in a more in depth manner, I have attached the link to the lecture conducted by Felsentein. The lecture is 59 minutes long, but it is worth the watch. 

Prepare to be fascinated. 

References

Felsenstein, J., 1985. Phylogenies and the comparative method. American Naturalist, 1-15.

The Gap in Behavioural Science - Post 1

By Maxine Whitfield

I remember as a child, fascinated by all things non-human, collecting garden snails in my little glass terrarium. I would throw in a bunch of lettuce leaves and spend hours watching the snails glide around the slime-streaked glass, with their strange eye-stalks contracting and extending like a cheerleader’s arms in slow motion. Occasionally, I would stick my finger out and tentatively touch an eye – and jump ever so slightly when the eye disappeared into the snail’s gelatinous head. What had prompted the snail to retract its eye? Was it afraid of me, or had I possibly hurt it? Might I have threatened our burgeoning friendship? What was it thinking?

To this day, as an adult, I am still fascinated by animal behaviour, cognition and consciousness. What I had never considered, however, and what struck me this morning while I scanned Science Daily’s “Behavioural” section, was that behaviour isn't limited to the animal kingdom. I read, with my mouth slightly ajar, about a unicellular protist that displayed evidence of habituation – an unmistakable, if rudimentary, form of learning. 

Plasmodium of Physarum polycephalum on water agar in a Petri dish after about four hours at room temperature. https://www.uoguelph.ca/~gbarron/MISCE2002/plasmppa.htm

The original paper was published just a couple of days ago in Proceedings of the Royal Society of London B. Boisseau et al. (2016) barred the slime mould cells’ passage to a food source with a bitter, but unharmful substance, and showed that, while initially hesitant to move through the threatening substance, they moved with increasing speed after “realising” that the substance was harmless. Within six days, they were behaving in the same way as the control group. Two days after removing the chemical obstacle, the cells once again became “distrustful” of the substance. I had to know more. A cursory search of the recent literature was edifying – this particular species has been shown to optimise its nutrition when given the choice (Dussutour et al. 2010) and solve mazes (Nakagaki et al. 2010) using Hansel and Gretel’s bread crumb approach (substitute bread crumbs with a trail of slime). They also form complex, interconnected networks of slime between food sources which allow them to move between these sources at maximum efficiency – to the point that researchers are suggesting they be utilised in the future planning of transport systems (Adamatzky et al. 2016). This particular species has also been shown to foresee predictable changes in its environment, and prepare for them physiologically in order to prevent unnecessary energy expenditure (Saigusa et al. 2008). Incredible. And here I sit in the office in winter, shivering and expending way too much energy because I once again forgot my jersey at home. Perhaps it’s time we stop considering intelligence to be synonymous with a nervous system.

If a single-celled Protist can accomplish so much, I imagine that certain highly complex, multicellular, but seemingly behaviour-less organisms might hold a wealth of secrets hidden away from the observer’s keen eye. After everything I've read today, I think it’s high time I explored the hitherto unknown (by me, anyway) field of plant behaviour.. Stay posted.

Inspired by biological design and self-organizing systems, artist Heather Barnett co-creates with Physarum polycephalum, a eukaryotic microorganism that lives in cool, moist areas. TED Talks https://www.youtube.com/watch?v=2UxGrde1NDA

References

Boisseau RP,  Vogel D, Dussutour A. 2016. Habituation in non-neural organisms: evidence from slime moulds. Proceedings of the Royal Society of London B 283: DOI: 10.1098/rspb.2016.0446.

CNRS. 2016. A single-celled organism capable of learning. ScienceDaily. Retrieved April 29, 2016 from www.sciencedaily.com/releases/2016/04/160427081533.htm

Dussutour A, Latty T, Beekman M, and Simpson SJ. 2010. Amoeboid organism solves complex nutritional challenges. Proceedings of the National Academy of Sciences 107:4607- 4611.

Nakagaki T, Yamada H, Tóth A. 2000. Intelligence: Maze-solving by an amoeboid organism. Nature 407:470.

Saigusa T, Tero A, Nakagaki T, Kuramoto Y. 2008. Amoebae anticipate periodic events. Physical Review Letters 100(1):018101.

The number one invasive species in arid lands globally: Spekboom?

By Alastair Potts

In a word: No.

Spekboom (Portulacaria afra) does not appear to be invading in any arid land across the globe, or at least I can find no report of it being considered invasive.

I find this simply astounding.

Spekboom is an immensely popular garden plant and has been transported around the world by gardeners. This includes an influx into arid areas. In a recent visit to Arizona State University, USA, I cycled past rows of Spekboom hedges (example of spekboom in Phoenix here). A common theme on international gardening websites is how easy it is to propagate Spekboom from cuttings (note that most overseas websites refer to this species as "Jade" plants, but also lump Crassula ovata under this common name). Thus, we can rule out a lack of invasion simply because it is has not reached exotic destinations.

One plausible reason that it may not have invaded arid lands, is that many arid areas are not only dry but also cold (i.e. experience frost events), and we now know that Spekboom is highly intolerant to frost events (Duker et al. 2015a, 2015b). Nonetheless, there are still plenty of frost-free arid areas. For example, the Sonoran Desert is predominantly frost-free, except in mountainous areas. The Saguaro Cactus (Carnegiea gigantea), an emblematic species of this desert, is sensitive to frosts (although likely to be hardier than spekboom). Parts of the Sonoran Desert experience winter and summer rainfall, similar to the native distribution of Spekboom in the Eastern Cape. The Sonoran Desert therefore seems like an ideal habitat for this ecosystem engineering species to invade and change the ecology of this landscape. Yet it hasn't. Why?

Saguaro Cacti in the Sonoran Desert (Photo credit: Alastair Potts)

I suggest that the reason is that it has no means of dispersal. Although gardeners routinely highlight the mass flowering of this species in overseas gardens as part of its aesthetic appeal, it appears that this is not sufficient for invasive propagation. As mentioned in a previous post, I have yet to find a small individual in the wild that I consider to be a true seedling (i.e. arose from seed and not clonally); this observation is also recorded by Stuart-Hill (1992). Other succulents that grow clonally have proved to be invasive (e.g. members of Opuntia). If we look at Cylindropuntia bigelovii, a native of the Sonoran Desert, which sheds stems with barbed spines that readily attach to, and thus are dispersed by, animals. These "cholla balls" readily take root and most cholla stands are clones of a single individual. Unsurprisingly, this species has become invasive in arid lands beyond its native range (Novoa et al. 2015).

Cylindropuntia bigelovii, sardonically named the "teddy-bear" cholla - pronounced "CHOY-A", and the "cholla balls" that are shed around the parent plant. These either get stuck onto unwary animals, and thus are transported away from the parent plant, or simply take root in situ.

So, why does Spekboom fail to disperse readily? I suspect that this species requires megaherbivores – specifically elephants (Loxodonta africana)  – for effective dispersal. In the recently-opened southern sections of Addo Elephant National Park, elephants have readily removed invasive Opuntia infestations, yet Spekboom continues to flourish throughout the park. Elephants are fairly messy eaters, and will break off Spekboom branches and continue walking; and while they eat the branch there is a rain of dropping broken spekboom branchlets.

Thus, not only does Spekboom survive elephant browsing, but I suggest that it is also dependent on it for effective dispersal of clonal propagules. The removal of elephants from the majority of their historical distribution may have had profound effects on Spekboom recruitment. This could be a factor explaining why recruitment is so incredibly slow in areas where domestic livestock pressures have been removed, but remain elephant free.

So, in summary, my hypothesis is that the primary dispersal agents for Spekboom are elephants. If correct, there are a number of predictions generated from this hypothesis.

• Spekboom recruitment will be low wherever elephants are absent as elephants generate the bulk of the propagule dispersal events.
• Long-distance dispersal of Spekboom will be rare wherever elephants are absent. In this case, long-distance dispersal is in terms of meters to hundreds of meters. The exception to this may be on steep slopes where propagules are transported downslope by gravity. 

This hypothesis would be most easily tested in Addo Elephant National Park by...

• Observing elephant feeding behaviour and tracking the distance that elephants move with Spekboom branches. Observations of other animals, such as Greater Kudu, will also be necessary (but, admittedly, probably more difficult). 
• Comparing the density of 'small' – i.e. recently recruited – individuals between the long-term browser exclusion areas and the general park. 
• Simulating elephant browsing in a degraded spekboom-removed (and browser-free?) landscape, i.e. by seeding the landscape with lots of small bits of broken Spekboom, and recording recruitment. 

Note that Stuart-Hill (1992) suggested that Spekboom is adapted to elephant browsing, but proposed a model where spreading occurs vegetatively from side branches that take root and eventually form new individuals when connecting branches become detached.

References

Duker, R., Cowling, R.M., Preez, D.R.d. & Potts, A.J. (2015a) Frost, Portulacaria afra Jacq., and the boundary between the Albany Subtropical Thicket and Nama-Karoo biomes. South African Journal of Botany, 101, 112-119.

Duker, R., Cowling, R.M., du Preez, D.R., van der Vyver, M.L., Weatherall-Thomas, C.R. & Potts, A.J. (2015b) Community-level assessment of freezing tolerance: frost dictates the biome boundary between Albany Subtropical Thicket and Nama-Karoo in South Africa. Journal of Biogeography, 42, 167–178.

Novoa, A., Le Roux, J.J., Robertson, M.P., Wilson, J.R.U. & Richardson, D.M. (2015) Introduced and invasive cactus species: a global review. AoB Plants, 7

Stuart-Hill, G.C. (1992) Effects of elephants and goats on the Kaffrarian Succulent Thicket of the Eastern Cape, South Africa. Journal of Applied Ecology, 29, 699-710.