The cactus-like trees of the Euphorbiaceae

The Euphorbiaceae is one of my favourite plant families. Not only are many Euphorbia trees instantly recognisable by their candelabra-like growth form, but they also form unique and characteristic components of the Subtropical Thicket Biome in the region where I spent my childhood. These trees – especially Euphorbia triangularis – remind me of my original home!

The Euphorbiaceae contains approximately 90 native southern African tree species. Most of the species have succulent stems with (often absent) simple, alternate or spirally-arranged leaves. The flowers are usually small, yellow and bird pollinated. Although Euphorbia species (the largest genus in the family) are often cactus-like in growth form, Euphorbs and cacti are quite unrelated, being an example of remarkable convergent evolution to arid environments. True Euphorbias can be distinguished by the paired spines and poisonous, corrosive milky latex-like sap. I recall one incident from my childhood where I was dared by a friend to taste the sap from one of these plants growing on our school’s premises. I was tempted, made a gash in the side of one of the plants and licked some latex from my finger. A human would have to drink large quantities of the stuff to notice serious effects, but I can still remember the horrible taste and the way it left my mouth dry for a few hours.

Su Abraham’s beautiful illustration (above) shows quite clearly the characteristic traits of a true Euphorbia, including the succulent stem, paired spines, and reduced yellow flowers (with three-lobed fruit capsules).

Non-succulent Euphorbs

Not all Euphorbiaceae species are succulent. In fact, most Euphorbs are non-succulent: only two out of the thirty nine genera have succulent trees (in addition to Euphorbia, the other genus containing succulent trees is Synadenium). Some of the remaining thirty-seven genera contain some familiar trees, such as Tambotie (Spirostachys africana) and several Clutia species. I have a stink-ebony tree (Heywoodia lucens) growing in my garden.

The wonky spike-thorns

The Celastraceae is the fourth largest tree family in southern Africa, containing just less than 100 species (~94 species). Yet, despite a few notable and abundant species, the Celastraceae is “a rather indistinct family” according to the Field Guide to Trees of Southern Africa [1]. Fortunately, the guide goes on to add that as one becomes more familiar with the family you can start to recognise…

…a distinct, though difficult to describe, celastraceous ‘look’.

van Wyk and van Wyk [1]

So what might that look be? Well…the guide mentions that the leaves are simple and arranged spirally, or opposite, or clustered in fascicles… And when the leaves are opposite in the adult tree, they are often arranged alternately in the juvenile saplings of the same species. Indistinct indeed. The guide also mentions that the

young twigs tend to be greenish and somewhat angular…

…and that many species appear to have spines or spike-thorns (hence the popular name). Overall, I get the distinct impression that species of the Celastraceae are like tall, skinny people: somewhat edgy, a bit wonky and knobbly, and, most distinctly, all elbows and knees.

Gymnosporia and Maytenus

Unsurprisingly, the generic relationships within the Celastraceae family are still somewhat uncertain, including among two of the largest genera, Gymnosporia and Maytenus. The largest genus in southern Africa (Gymnosporia) did not exist until recently and is still in a state of “taxonomic flux”. Prior to the early 2000’s most of the species now contained within Gymnosporia were considered to be a part of Maytenus. Taxonomic investigations conducted by researchers at the University of Pretoria (most notably Marie Jordaan and colleagues) recognised Gymnosporia based on the presence of several “distinguishing” features, or, as I think of it, based on features pertaining to the classic celastraceous “look”. Gymnosporia can be recognised by the…

…truncated branchlets and spines, alternate leaves or fascicles of leaves, an inflorescence that forms a dichasium*, mostly unisexual flowers, and fruit forming a dehiscent capsule, with an aril on the seed.

So, the next time you find yourself in the field wondering what that common, knobbly, wonky spiny looking tree is, you can feel confident that it is a Gymnosporia (like this Gymnosporia heterophylla above). But if someone asks, best to call it a spike-thorn.

Tomorrow, we move on to cover one of my favourite families, the much more easily identifiable Euphorbiaceae!

Notes:

*A dichasium is a cyme where each flowering branch gives rise to two or more branches symmetrically.

[1] According to the Braam van Wyk and Piet van Wyk in the Field Guide to Tree of Southern Africa published by Struik Nature. This is an excellent guide and I would urge anyone interested in southern African trees to go out and purchase a copy.

Fabulous butterflies, small yellow pom-poms and flamboyant pride.

Today’s post will be all about the wonderful Fabaceae. Broadly defined, the Fabaceae is the third largest family in the world in terms of number of species, but tied first (with the Poaceae, or grasses) in terms of ecological and economic significance. The Legume or Pea family (as it is commonly referred to) contains over 18 800 species in 630 genera, behind only the Asteraceae (asterids) and the Orchidaceae (orchids). Many of these species provide staple foods, either directly (e.g. pulses, beans and peas) or indirectly (e.g. alfalfa or lucerne, which provides grazing or fodder for cattle). The term “faba-” itself comes from Latin for “bean”. The reason for the high nutritional value of legumes is that they contain nitrogen-fixing bacteria (known as Rhizobia) in nodules in their roots, which allows them to be self-nourishing.

In southern Africa, the Fabaceae (again, broadly defined) contains more than 280 species, many of which are important tree species. Why do I keep mentioning “broadly defined”? The Fabaceae is such a large family that some taxonomists split it into three “narrowly defined” families: the Papilionaceae (or Fabaceae, narrowly defined), the Caesalpiniaceae and the Mimosaceae.

Papilinionaceae

The Fabaceae (narrowly defined) is still a very large family, containing approximately 133 tree species in southern Africa. Many people will be familiar with this group, as most species are instantly recognizable by their butterfly-shaped flowers (“papillon” is French for “butterfly”). The flowers have a keel (shaped like a boat), an uppermost “banner” and two side lobes (or wings). Erythrina and Virgilia are commonly encountered trees in southern Africa.

Caesalpiniaceae

The Caesalpiniaceae (or flamboyant family) is the nineth largest group of trees in southern Africa, containing approximately 50 species. The region that I grew up in along South Africa’s south coast has one particularly spectacular ornamental tree species from this group: Bauhinia galpinii, also known as “the pride of the Cape”.

According to Coetzer (University of Pretoria) the popular name for this tree species was first used in November 1889 by E.E. Galpin:

The popular name of Pride of the Cape may have been used for the first time in November 1889 by Galpin, a dedicated plant collector, while he was introduced to this plant in the Cape during a botanical excursion. During the flowering periods of the plant that stretch from October to May (June), the flowers with their brick to orange-red color are very noticeable. Therefore, one can understand why it made a big impression on Dr. Galpin made when he first observed and collected the plants.

Coetzer (1974) Veld and Flora

The species was given it’s scientific name in England by Dr. N.E. Brown who studied all available specimens of the “Flame of the Cape” and placed it under the genus Bauhinia L. “mainly on the basis of the shape and hand-shaped bearing of the leaves”. Dr. Brown was also the person who decided to honour Dr. Galpin, publishing his description of Bauhinia galpinii in the London Gardener’s Chronicle in June 1891. As a side note, the two common names should again make one think twice about the value of popular names for a species. I have mentioned this before in a previous post, but Coetzer, writing about Bauhinia galpinii, shared similar sentiments:

In the vernacular, where the more popular names originate, no scientific facts such as morphological features are taken into account when giving a name. The names usually differ from region to region and in many cases the same plant has more than one popular name. These many names for the same plant create confusion and make communication very difficult.

Coetzer (1974) Veld and Flora

Mimosaceae

Of the three narrowly defined families of the Fabaceae, the Mimosa family contains the most number of tree species in southern Africa (about 133 species). Many of these species will be recognisable to most people who have ever gone on safari as the thorn trees with small yellow pom-pom-like flowers that obscure their views of charismatic herbivores. Whether you refer to them as Acacia, Vachellia or Senegalia will depend on your knowledge of taxonomy (a story for another time)…

With these three narrowly-defined plant “families” we have covered the second (Fabaceae), third (Mimosaceae), and nineth (Caesalpiniaceae) largest tree families in southern Africa. Added to the Rubiaceae, we’re off to a good start in covering the tree flora of the region.

Start your day off with the Rubiaceae

Like any morning should, we start off with the Rubiaceae, also known as the coffee family. Globally, the family is the fourth largest family of flowering plants (after the Asteraceae, Orchidaceae, and Fabaceae), containing approximately 13 686 species. The species are identifiable by their opposite leaves that have entire margins, and interpetiolar stipules. The flowers are generally tubular with fused petals and have an inferior ovary (meaning that it sits below the point of connection with the petals). Although we are fortunate to have many wonderful, native species, one very important species has been imported from South America: coffee.

Our indigenous species are no less impressive. In southern Africa the Rubiaceae contains about 200 native species. Of these, the best known are contained in the genus Gardenia, since many species are grown as ornamental garden plants. One of the most familiar is the “wild gardenia” (also known as the “wildekatjiepiering” or “buffelsbol” in Afrikaans, and the “mutarara” in Shona). In 1974, Grobler (then of Kirstenbosch National Botanic Gardens) wrote the following (in Afrikaans) about the “Wildekatjiepiering” (Gardenia thunbergia):

This particular plant with its large white fragrant flowers and large hard fruits is one of our most beautiful tree shrubs. The natural home of the wild kitten saucer [I am not 100% convinced that this is the correct translation…but I will stick with it for now] is the forests and thickets found in the Eastern Cape and Kwa-Zulu Natal. The flowers are borne singly in late spring or early summer on the ends of the sturdy pale white twigs. The sweet scent that spreads from the flowers is especially noticeable at night. The large egg-shaped fruits that range from 50 mm to 80 mm but can grow up to 120 mm are gray, smooth and very hard. The fruit remains on the tree for years and it is not certain how the seed is distributed. It may be that large antelope or baboons eat the fruit and that the seed then passes intact through the digestive tract of the animals. The tree is fairly frost resistant and can be grown from seed or pole cuttings.

The drawing that accompanied the text quite nicely shows the showy flowers, the large, gray egg-shaped fruits, and general growth form of Gardenia thunbergia. Although it is not entirely clear who drew the piece, it could have been Emily Thwaits, the daughter of the art master at the Rev. James Beck’s school in Roeland Street in Cape Town in the late 19th century. Emily Thwaits was a fine artist; she won a medal for the best water colour painting at the South African Fine Arts Association Exhibition in 1880. Clearly artistic talent ran in the family: her sister Florence Thwaits was an art mistress at Wellington and painted some of the illustrations in Marloth’s Flora of South Africa (most of those not done by Ethel May Dixie).

Another interesting note is that the scientific name of Gardenia thunbergia honors two people. The genus is named after Alexander Garden (1730-1791), a medical practitioner in America who sent plants to the great Swedish botanist, Linnaeus. The species name honours Carl Thunberg, one of Linnaeus’s students who made several collecting trips in southern Africa in the seventeenth century.

How to wrap your head around southern Africa’s remarkable diversity of trees.

Southern Africa is home to many trees: by some estimates there are over 2100 tree species native to the region. This number is more than double the number of bird species (~900 species) and more than five-fold the number of mammals (~400 species). With such a large number it is easy to become overwhelmed by the diversity; to see a sea of green rather than a forest of individual species. Fortunately, there is a surprisingly simple way to wrap your head around southern Africa’s trees. Approximately 1345 species of the region’s tree flora – roughly 65%…about two-thirds…or (ahem) most of the trees – are contained in just 20 plant families. If you learn these twenty families, you can just about call yourself a budding arborist.

Over the course of my next few blogposts I want to introduce you to southern Africa’s remarkable trees. I will try to describe the twenty largest tree families in an engaging way: by briefly introducing some of their identifying features, describing commonly occurring or familiar species and some of their uses, and bringing to life some of the unique aspects of their ecology.

Southern Africa has some truly remarkable trees, like this Aloe dichotoma growing in the Richtersveld.

We’ll start with the Rubiaceae.

Then move to the Fabaceae (defined broadly…and then narrowly as well, just for good measure…).

Third on our list (if you’re counting broadly…) is the wonky Celastraceae.

Fourth is the Euphorbiaceae.

Rounding off the top five is the Anacardiaceae, the most delicious family.

Just missing out on the top five is the Proteaceae…but only because most proteoids are shrubs.

And the Andes!

My love affair with Chile continued today when I went on a field excursion to the Chilean Andes (Figure 1). I joined a group of delegates attending a conference on Hydrology on an outing to the San Pablo de Truega experimental watershed. The tour took us past some experimental stream gauges, designed to test the impact of forest thinning on stream flow. The site has two streams with monitoring gauges: a control stream in a relatively undisturbed natural forest stand and a treatment stream in a thinned forest stand. So far their results indicate that stream flow is vastly increased in the thinned site, primarily because fewer trees intercept rainwater and return less water to the atmosphere via transpiration, so more water ends up as run-off. Consequently results from this watershed are assisting foresters to manage the land, and also assist scientists in predicting future changes in water supply, an important ecosystem service.

DSC_8106

The highlight of the trip for me was definitely our hike to El Presidente – the largest Nothofagus tree in the forest (Figure 2). On the hike I saw magnificent Saxagothea, Nothofagus and Laureliopsis trees, and a Darwin Frog (Rhinoderma darwinii)! This unusual little creature relies on camouflage to avoid being detected by predators, resembling a fallen leaf (Figure 2). Another interesting aspect about the frog is that the male Darwin’s frog protects the eggs for about three to four weeks. After the female lays the eggs in the leaf litter the male guards them until the developing tadpoles begin to move, and then he ingests the eggs and retains them in his vocal sac! The baby frogs hatch about three days later after which the male frog continues to carry them in his vocal sac until they hop out and disperse. Remarkable.

After the hike we had a traditional Chilean barbecue. It was excellent, and I was absolutely stuffed with delicious steak, potato salad and blood sausage. Needless to say that most of the tour group slept on the bus on the way home.

I was pleased that the Andes lived up to (and exceeded) my expectations, both in terms of the diversity of plants, but also the beauty of the forests. These ecosystems really are magnificent, and I very much hope to return to them one day.

Journey to another distant world: Chile

Visiting South American temperate rainforests had always been on my wishlist, and recently I had the opportunity to go see some of these beautiful ecosystems in Chile. The opportunity arose when Rocio Urrutia, a researcher at the Universidad Austral de Chile in Valdivia, invited me to collaborate with her on a research project investigating the potential impacts of future climate change on temperate Chilean rainforest species (Figure 1). The focal species of the project is Alerce (Fitzroya cuppresoides), a conifer species often referred to as the “redwood of the south”. This magnificent tree is threatened by drought events, habitat destruction, and illegal logging, and Rocio and her colleagues are doing some fantastic work with the aim of promoting conservation of the trees and the forests that they occur in.

Upon my arrival in Validivia, I was immediately struck by the trees; Many of them seemed oddly familiar. On the way to the city from the airport my taxi drove past Chilean firetrees in full bloom (Embothrium coccineum, members of the Proteaceae), and magnificent southern beech trees (several Nothofagus species). I am particularly fond of species of the Proteaceae, mostly as a result of having grown up in South Africa where Protea, Leucospermum and Leucadendron are common features of the Fynbos flora (and also as a consequence of having studied the Fynbos flora for my dissertation).

One of the reasons for my trip to Chile was to participate in a discussion around the conservation of Alerce (Fitzroya cuppresoides). The mini conference was excellent (Figure 2). Among other things, I learned that even though the mean annual rainfall of Alerce Costera is over 4000 mm, Alerce trees can experience water stress during dry years as a consequence of the shallow, sandy soils. I was also highly impressed to learn that Fitzroya is the second longest living tree in the world (and I met the individual who dated the tree – the wonderful Antonio Lara!). I found this remarkable because these trees are quite different in appearance from the longest living trees in the world, the Bristlecone Pines. Alerce is tall, and flourishes in its native habit (moist temperate rainforest); in stark contrast the Bristlecone Pines appear to persevere rather than grow in arid mountains of North America. It really is unfortunate that these Alerce forests are threatened by climate change and deforestation. Overall the conference was incredible. It was fantastic to learn about the ongoing research at the University.

Screenshot 2018-11-03 10.34.09

After the formalities of the conference were completed, my wonderful hosts, Rocio and Aldo took me and a few other colleagues (Jarmilla Pittermann and Jonathan Barichivich) on a field excursion to see the Alerce Costero forest (Figure 3). The forest is a two-hour drive from Valdivia and the site of much long-term monitoring and experimental research conducted by Rocio, Jonathan and their colleagues.

When we arrived I was awestruck to be standing amongst Nothofagus, Fitzroya, Pilgerodendron and Saxagothea. Incredible. My mind wandered to the last time I was standing in a mixed Nothofagus-conifer temperate forest. That time, I was travelling in New Zealand, but the similarities between the two forests are striking. Of course, the similarity between the southern hemisphere flora is no coincidence. The splitting of the ancient continent of Gondwana resulted in many shared families between the countries, including the Proteaceae, Myrtaceae, and Podocarpaceae. Antarctica was also a part of this supercontinent, as demonstrated by the abundance of fossils of tree species that occur on the cold continent. Remarkably, some of the fossil species still exist in the Alerce Costero temperate rainforest; I was amazed to see Nothofagus antarctica trees for the first time, surviving in a small drainage basin frequently exposed to very cold conditions (Figure 4). Its occurrence on Hoste Island (one of the southernmost islands in Chile) earns Nothofagus antarctica the wonderful distinction of being the southernmost tree on earth! Tangible, living evidence of these ancient geographic links between the southern hemisphere land masses.

After our walk through the forest and the research sites we retired to a small cafe located on the outskirts of the Alerce Costero National Park. Jonathan’s mother runs a tiny cafe where she provides warm refreshment in the form of coffee and baked goods (Figure 5). It was delightful. We crowded into the cozy, warm room and had an incredible lunch.

Finally, we decided to see the big Grandpa of the forest: the largest Alerce of the region (Figure 6). This tree is one of the few remaining large trees of the forest, with most of the large trees having been cut down in previous centuries for timber. It is incredible! It has a known diameter of at least 4m and is at least 2000 years old according to tree rings obtained by Antonio Lara (It is probably much older, since the inner core has rotted away with time).  Jonathan mentioned that the sex of the tree is actually unknown (Fitzroya is dioecious, meaning that, like humans, individuals can have seperate sexes). However, they aim to find this information out later this year….and Jonathan suspects that the tree is actually the Grandma of the forest.

So far my experience of Chile has been wonderful, and I still have a week to go here. It is great to be among southern hemisphere trees again and to learn about more remarkable southern hemisphere forests!