Category Archives: Southern African trees

Interlude: What proportion of trees have we covered so far?

The aim of this post is to establish where we stand in terms of being able to identify the trees of Southern Africa. Thus far we have covered the top seven tree families: the Rubiaceae (coffee family), Fabaceae (the legumes), Celastraceae (the spike-thorns), Euphorbiaceae (Euphorbs or spurges), Anacardiaceae (mango family), Proteaceae (Proteoids) and Combretaceae (Bushwillows or Cluster-leafs). By my calculation this means we have covered families containing approximately 870 species, or just over 41% of all trees in Southern Africa! So we’re well on our way to meeting the challenge of identifying two thirds of the trees of the region.

Here is a quick (and pretty simple) breakdown of what we have covered and where we are going:

Bushveld Bushwillows

Number seven on our list of the largest families of trees in Southern Africa is the Combretaceae, commonly referred to as the Bushwillows. Globally, the Combretaceae is a large family of about seventeen genera containing more than 500 trees, shrubs and woody climbing plants, most of which occur in the tropics or warm subtropical areas. In Africa the richest variety of species occur in the tropics, but this diversity tends to decline as one moves southwards. By the time one reaches Southern Africa there are six genera containing just over 50 tree species. The vast majority of these species belong to only two genera: Combretum (thirty-four species) and Terminalia (twelve species). The four other lesser known genera occurring in the region contain five species between them: Pteleopsis has two species (P. myrtifolia and P. anisoptera), while the other three genera have a single species each (Meiostemon tetrandrus, Quisqualis parviflora and Lumnitzera racemosa).

Members of the Combretaceae, particularly Terminalia and Combretum, can be dominant species of certain vegetation types in Southern Africa. One of the more unusual species is Lumnitzera racemosa which forms a component of the mangrove forests on the east coast of Southern Africa and is one of the few species of mangroves extending as far south as South Africa. Overall the occurrence and diversity patterns within Southern Africa tend to mirror those of the family throughout Africa: both abundance and species diversity declines as one moves southwards. Only a handful of species occur in the sheltered coastal forests and riverine habitats of South Africa’s Eastern Cape Province – notably the Cape (C. caffrum) and River Bushwillows (C. erythrophyllum).

The leaves of all Combretaceae are simple and entire (in a few species they may sometimes be slightly toothed). In Combretum the leaves are usually arranged opposite or semi-opposite, and as a result these species can be mistakenly identified as members of the Rubiaceae. However, Combretum species lack the stipules characteristic of that family. Terminalia leaves are usually found in densely packed groups at the tips of the twigs and are sometimes referred to as Cluster-leaf trees (like the Lebombo Cluster-leaf, T. phanerophlebia, above [1]).

Combretum species have four to five winged fruits (one species, C. bracteosum, has a wingless nut and may very well be described as a new genus in the near future as a result of this oddity). The aptly named Large-Fruit Bushwillow (C. zeyheri; see image below) is the South African species with the largest fruit, and is easily recognized by this. Terminalia fruits (like the fruits of T. sericea below) tend to be flattened and have only two wings [2].

Notes

[1] The beautiful color plate was created by renowned botanical artist and botanist Elise Buitendag, of the Lowveld National Botanic Gardens in Mbombela. Much of the inspiration for this post came from her notes on Combretaceae made over four decades ago in the March 1974 Veld and Flora!

Interestingly, the plate shows a lesser known member of the Combretaceae, Terminalia phanerophlebia. In addition to the leaf clusters the image also shows another feature of the Combretaceae: species tend to have small flowers borne in bunches, of which the stamens are often the most noticeable part. Apart from a few Combretum species with red or light red or orange flowers, all South African species have white to yellow or greenish flowers. Occasionally, the flowers may spread a pleasant aroma, such as the flowers of Terminalia sericea.

[2] For this article I leaned on three excellent sources: Braam and Piet van Wyk’s Field Guide to Trees of Southern Africa (that I have mentioned in a previous post); Keith and Meg Coates Palgrave’s Trees of Southern Africa; and an article on the Combretaceae written by Elise Buitendag (see above).

If you are keen to learn more about Southern African trees (much more than I can provide) I highly recommend purchasing a copy of the two guides. Here is the cover of the Trees of Southern Africa:

Family of many forms

Although the Proteaceae is instantly recognisable to anyone familiar with the flora of southern Africa, few people would consider this family to be among the top ten tree families. Of the 330 or so species in the region, approximately 75 species (~22%) can be (somewhat generously) classified as trees, with the rest being small or large woody shrubs. But, for the Proteaceae, we will be accommodating with our definitions.

I have written about the Proteaceae previously, commenting on the derivation of the name from the greek god Proteus (the sea god of ever changing form), and how wonderfully diverse the different plant forms can be. In spite of this diversity of form, there are, of course, some underlying similarities among the Proteaceae species in southern Africa: All species have simple, alternate, entire, leathery leaves (naturally, there is one exception, which we will get to); and the flowers are collected in showy heads or spikes. Each flower has four stamens, which are often fused to the sepals, leaving only the anthers free. If you have ever looked closely at a single flower (by zooming in on one of the impressive inflorescences) you may have noticed the long style pushing through the closely formed sepals. This feature serves a very important function: by brushing up against the anthers and then extending through the sepals, the style presents pollen to pollinators. Large beetles and birds (e.g. the sugarbird, Promerops caffra) obligingly collect the pollen (although they are more interested in the nectar) by sitting on top of the inflorescences.

The two great subfamilies: Proteoideae and Grevilleoideae

There are two major subfamilies in southern Africa: the Proteoideae and the Grevilleoideae [1]. Most people are familiar with the former subfamily, due in no small part to the ecological, cultural and economic importance of Protea species from South Africa’s south-western region (these species constitute a major part of the Fynbos flora) [2]. Some Proteoideae species, like the silver tree (Leucadendron argenteum), even form genuine trees.

Pic 5: Leucadendron argenteum has remarkably silver leaves.

However, very few South Africans will be familiar with the other major subfamily of the Proteaceae: the Grevilleoideae. Most of the species in this subfamily are found in Australia and south east Asia and South America (e.g. Grevillea, Banksia and Macadamia). However, one of the Proteaceae species most deserving of “tree status” is in the Grevilleoideae subfamily: Brabejum stellatifolium. Brabejum stellatifolium is a small tree (<15m) found in moist habitats throughout the southern Western Cape region of South Africa. It has small spikes of white flowers, which when pollinated produce clusters of rusty-brown, velvet fruits. Brabejum is the only representative of the Grevilleoideae subfamily on the African continent, although Macadamia is commonly planted for it’s nuts (and apparently there are native species related to Macadamia on Madagascar!). Members of the Proteoideae and Grevilleoideae can be separated by their floral form: Proteoideae species have flowers borne singly in the axil of a bract, but in the Grevilleoideae each bract subtends two flowers. Thus, if a flowering spike of Brabejum is examined, it will be seen to have two flowers in the axil of each bract! Another feature of the Grevilleoideae is that the leaves are whorled and toothed, as you can see from the picture below:

Part of our unique heritage

Culturally and economically, the proteoids are a highly important group. The cut-flower industry uses many proteoids (e.g. proteas, pincushions, blushing brides, conebushes). Proteoids are a popular – and stunning – choice for bouquets for wedding couples. Stylised proteoids also adorn many cultural artifacts, including Protea cynaroides on South Africa’s 20c coin, and form the basis of institutional emblems, such as the Protea repens used for South Africa’s Botanical Society.

Notes

[1] There are five subfamilies in total: the Persoonioideae, Bellendenoideae, Symphionematoideae, Proteoideae and Grevilleoideae.

[2] I recommend John Rourke’s excellent book on Proteas to anyone who is interested in discovering more about these wonderful “trees”:

The most delicious family

Rounding off the top five (or is it six?) largest tree families in southern Africa is the Anacardiaceae or mango family. The Anacardiaceae contains about 80 native tree species, and most have either simple or compound, imparipinnate (i.e. pinnate with a single leaflet at the apex) leaves, and a watery or milky latex, which can cause irritation to the skin. The crushed leaves usually smell like turpentine or resin.

The largest and most familiar genus is Searsia (previously known as Rhus). Searsia species are trifoliolate (meaning that there are three leaflets) with small spherical or ovoid fleshy fruits (called drupes). The genus is named for Paul B. Sears (1891–1990), an American ecologist, who was head of the Yale School of Botany. Sears worked on the flora of North America, notably Ohio, where several Rhus species are found. In southern Africa there are approximately 47 described species, with many of these being very difficult to tell apart. Searsia burchellii (shown below) is named after William John Burchell (1782–1863), an English naturalist who traveled in southern Africa and collected thousands of plant specimens, including this species.

Another notable native genus in the Anacardiaceae occurring in southern Africa is Ozoroa, the resin trees. This genus of shrubs or small trees currently contains 14 species, some of which are very rare (e.g. O. namaquensis). Several other native genera are mono-specific, including Protorhus (the red beech), and Heeria (rockwood).

Culinary delights!

Many trees of the Anacardiaceae are often delicious! The most delectable native fruit is certainly that produced by the marula tree, Sclerocarya birrea. Although the marula is most commonly associated with an alcoholic drink (the fruits are often fermented and incorporated into a rich, creamy synonymous drink), the raw fruits are richly scented and taste delicious! I recall being initially skeptical when offered some of these fruits by my MSc supervisor (Prof. Jeremy Midgley from the University of Cape Town). But once I tasted the fruits, I could not get enough of them! A bonus is that they contain about four times as much vitamin C as an orange!

Many of the other culinary delights are produced by trees introduced into southern Africa from elsewhere. There are some really great nuts: pistachio nuts from the pistachio tree (Pistacea vera) and cashew nuts from the cashew tree (Anacardium occidentale). The latter nuts contain approximately 45% fat and 20% protein, which explains why they are so tasty. Pistachios were introduced from the middle east and the cashew was originally from tropical America. The most famous fleshy drupe is of course the mango from the mango tree (Mangifera indica). Mango trees were introduced from tropical east Asia and are now grown extensively in sub-tropical areas.

So the Anacardiaceae is the most delicious family. But beware! Not all species are palatable; some are highly toxic. The “pain bush” (Smodingium argutum) and “agony tree” (Trichoscypha ulugurensis) can both cause severe allergic rashes if touched (similar to the dreadful species that I encountered many times during field work in California: Toxicodendron diversilobum, otherwise known as poison oak). Smodingium has also been refered to as “the terrible tovana plant of Pondoland” (tovana is of Xhosa or Zulu derivation).

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.