What is a tree?

“A tree is a big plant with a stick up the middle. Everybody knows that.” [Colin Tudge, The Secret Lives of Trees]

Everybody knows what trees are: Big, woody ent-like creatures that tend to form forests and house birds and bear fruit. A monkey’s playground, an elephant’s sparring partner and a rhinoceros’s back-scratching post. This all seems pretty obvious. But, what about the kokerboom (above) and the Aloe (below)? Are these trees?

The kokerboom seems like it should be called a tree; It just looks like one. It has a central stick-like feature and I could imagine a fairly small monkey hanging from those branches. The Aloe, on the other hand, seems too spongy and low growing to be a tree. But, then again, this raises the question must trees always be woody? Wood is often considered to be hard and rigid, rather than soft, fibrous and spongy, but kokerboom stems clearly fall into the latter category. They more resemble banana stalks:

“…they resemble palm trees, with a thick central stem and a whorl of huge leaves at the top. But the stem…is not of wood. [It] is formed largely from the stalks of the leaves, and its strength comes from fibres which are not bound together as in pines or oaks or eucalypts to form true timber; its hardness is reinforced, as in a cabbage stalk, by the pressure of water in the stem. So botanically the banana plant is a giant herb.” [Colin Tudge, The Secret Life of Trees]

Fortunately, the formal definition of wood helps us out. According to wikipedia, wood can be defined the following way:

Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic material – a natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin that resists compression. Wood is sometimes defined as only the secondary xylem in the stems of trees, or it is defined more broadly to include the same type of tissue elsewhere such as in the roots of trees or shrubs. In a living tree it performs a support function, enabling woody plants to grow large or to stand up by themselves. It also conveys water and nutrients between the leaves, other growing tissues, and the roots. Wood may also refer to other plant materials with comparable properties, and to material engineered from wood, or wood chips or fiber.

Ok, so the kokerboom is a tree and the Aloe is a succulent shrub. Clearly, the difference between them is that the Aloe growth form is low growing and sprawling… But what about Microcachrys, the Strawberry Pine that creeps along the ground (see below)? It has a woody central stick, but it is very low growing and shrub-like. Must all trees be tall? What about bonsai trees?

Microcachrys creeps along in Alpine environments of Tasmania.

On second thoughts, it is not as obvious as it initially seems to define a tree. Everyone has their own definition and internal category. What is clear is that trees are not a single, well-defined group of closely-related organisms, but rather a collection of unrelated organisms that resemble a vague category that we have developed in our minds. In The Secret Life of Trees Colin Tudge defines a tree thus:

A tree is just a way of being a plant.

Urban space-seeker seeks natural awe.

This passage was clearly written prior to 2020; at a time before corona inspired lockdown measures had been implemented.

Today I had a brief sortie into the countryside for work purposes and jislaak! it was invigorating to get out into the wilderness again! Myself and a colleague (Abri de Buys) traveled to the Jonkershoek Valley just outside Stellenbosch for some field work. Jonkershoek is, of course, famous for being the site of one of the longest ecological experiments in the world:

In 1938 the South African government started a network of hydrological experiments at Jonkershoek for the purpose of researching the impacts of afforestation on water supplies. The experimental design was based on the classic paired-catchment principle…that the streamflow from two untreated catchments is compared, so as to establish their natural relationship. One is then planted with trees. The change in the relationship between the two catchments after afforestation could then be ascribed to the treatment or influences of afforestation.

SAEON is mandated with maintaining the micrometeorological stations at the various sites, and we were there mostly to do some calibrations and to switch out some equipment. But the site is also beautiful and I managed to snap some shots of the mountain flora and scenery. Take a look at these photos that I took on our day out to satiate (or frustrate?) your sense of wanderlust:

The Berg River catchment
Scattered Proteas
Protea neriifolia
Abri at one of the micrometeorological stations at 1200 m a.s.l.

Abri informed me that this rain gauge located at 1200 m above sea level (below) holds the record for capturing the highest annual rainfall in South Africa. I forgot what the total was…something close to 2800 mm I think. If anyone knows better – or has a link to a site that records this – please let me know in the comments.

Presentation on how we can use plant hydraulics to understand plant response to droughts

I recently presented my research on plant physiological responses to water deficit to the Institute of Soil, Water and Environmental Sciences. I have included a link to the presentation below for those who might be interested.

This was my first presentation to a virtual audience and it got me thinking about the future of science presentations and how we currently consume science. In general, I think it is important to continue to share research and engage on these critical issues and virtual platforms, such as this type of online seminar, is one way to do so. It will certainly take some getting used to and we have a lot to learn from other disciplines which are currently better at attracting and maintaining people’s attention. One avenue that might be worth be exploring is how to make these types of seminars more interactive. One of the things I found most challenging in this presentation was having to “engage” with a screen, rather than an audience. This is somewhat different to in-person seminars where the presenter can feed off of the audience and engage with individuals, which I enjoy doing. It might be worth playing around with this idea, such as by making the question time longer and possibly having more of a discussion rather a full length presentation? Perhaps readers will have some ideas, which they might want to share in the comments below.

Another aspect that I started to consider is that in-person science conferences can be an incredibly effective way of connecting people. I was invited to present by Uri Hochberg, who I met a few years ago at a conference on plant hydraulics in Maine, USA. Uri and I were postdocs at the time and there is plenty of overlap in our research interests. These kinds of connections are great to maintain. But I wonder how moving to an online mode of interaction might influence the capacity to develop these kinds of connections. On the one hand it is making it easier to communicate remotely, across vast distances. But on the other, it might be more difficult to develop connections, because these are often made at in-person conference sessions. Perhaps I am overstating the importance of in-person events? Again, let me know what you think; I’m curious to hear other thoughts on this.

In praise of plants, and the three-billion-year-old trap

When the light shone on the greenness, the greenness welcomed it, and comprehended it, and put it to use.” [Oliver Morton, Eating the sun]

The sunlight’s energy bounced from one molecule to the next like a frog across lily pads before reaching the subtle trap at the pool’s centre, the three-bilion-year-old trap where the light becomes the stuff of the earth.[Oliver Morton, Eating the Sun]

We need to talk about Kevin plants. I discovered this yesterday after having a revealing (and humorous) conversation with my lovely niece (5 years old; K) and nephew (3 years old; L), which went like this:

L: What are you doing today?

Me: Working.

L: What’s your work?

Me: Looking at plants.

K: Why don’t you do real work?

Me: …but this is real work. Plants are great.

K: But plants can’t do anything!

So I want to put the record straight! Plants are, in fact, the most fascinating creatures to study, and for any number of reasons: Plants power the planet, they have shaped Earth’s history and climate, they eat the sun, they produce most of the food we and other animals eat, they form the backdrop to the most beautiful views…the list is endless.

I am fortunate enough to be able to study plants for a living; one of the most rewarding career/life choices that I have made! Richard Feynman once wrote about the pleasure of findings things out…and I am constantly inspired by finding out new things about plants and how they function. I have been helped in this pursuit by many wonderful books, which I highly recommend. My top five books are: Eating the Sun (Oliver Morton), The Emerald Planet (David Beerling), The Secret Life of Trees (Colin Tudge), In Praise of Plants (Francis Halle), and The Wollemi Pine (James Woodford).

As a quick aside, the story of the Wollemi pine (not an actual pine, of course) is one worth recounting. Wollemia nobilis (which is closely related to the Auracaria‘s, or Monkey Puzzle trees!) is called a “living fossil” because it was first described from fossils, similar to the Coelocanth and the Dawn redwood. Living specimens of the plant were only discovered in 1994 (!) in a refugial valley in Australia’s blue mountains. Notice how similar the branches of the living plant are to the fossil specimens in the photo below, taken from The Wollemi Pine. Although the exact location of the natural populations is a well-kept secret, many plants have been grown in botanic gardens around the world since it was discovered. While I was in Australia I was lucky enough to see Wollemi Pines in the Royal Botanic Garden in Sydney in 2016 (see below) and in the Royal Botanical Gardens in Hobart.

So why are plants often under-appreciated? My theory, which is my own, is that us humans are biased towards focusing on things that stimulate or trigger our senses. We are hard-wired to pay attention to and appreciate movement and new noises, which is why birds and animals appeal to us. There is possibly an adaptive (or survival) element to this: animals and birds that grab our attention can be hunted and eaten. At the same time, we have also evolved to ignore less mobile and more common organisms or items because it would require too much energy to constantly focus on everything. The colour green is a good example: it is ubiquitous and constant during daylight hours. The three-billion-year-old chlorophyll trap works relentlessly while the sun shines to generate carbon-based products, and we would be hard pressed to acknowledge this incredible (microscopic) dance all the time.

The good news is that this under-appreciation can be over-turned. There are many ways in which the exciting world of plants can be brought to life. We can use our innate senses: Red excites us (because this is the color of ripe fruit…which is consistent with the survival aspect of interest that I mentioned in the previous paragraph), as do new smells from flowers. We can also observe plants in motion: seedlings growing towards the light, or the venus fly trap clamping down on a hapless fly. But more than that, we can learn new and exciting aspects about how plants work. Did you know, for example, that plants can have heart attacks?

Ode to oaks

One of the pleasures of comparative biology is exploring new places to find the organisms that are the focus of your research. During my postdoctoral research at the University of California, Berkeley I was fortunate to find myself in a position where I could explore the western part of North America in an attempt to better characterize the drought tolerance of temperate woody angiosperm trees. My study group was the wonderfully diverse oaks of North America. Nineteen species of oaks occur in the western part of the region (see Figure 1) and my goal was to figure out how they varied in capacity to withstand embolism (which I have written about previously).

Figure 1: Nineteen species of oaks (Quercus spp.) from western North America. A. Q. kelloggii (black oak); B. Q. agrifolia (live oak); C. Q. parvula; D. Q. wislizenii (interior live oak) E. Q. palmeri F. Q. chrysolepis (golden cup oak) G. Q. vacciniifolia H. Q. tomentella I. Q. sadleriana J. Q. gambelii K. Q. lobata (valley oak) L. Q. garryana M. Q. durata N. Q. berberidifolia (scrub oak) O. Q. pacifica P. Q. john-tuckeri Q. Q. douglasii (blue oak) R. Q. cornelius-mulleri S. Q. dumosa T. Notholithocarpus densiflorus (tan oak; not an actual oak, but a closely-related species)

As you would expect just by looking at the leaves of the different oaks, many of these species occur in vastly different habitats, including moist temperate rainforest (e.g. Quercus sadleriana; note the moisture on the leaves in the photo) and semi-arid desert scrub or chaparral (e.g. Q. berberidifolia) (see Figure 2). This meant that I had to sample species ranging from the pacific northwest close to the border between Oregon and California to the deserts of southern California close to San Diego.

The measurements that I was taking on each species involved drying the plants down from a hydrated state and visually capturing the point at which they fail (i.e. emoblise) using repeat photographs taken of the xylem. A key part of this measurement process is ensuring that the plants are hydrated when the measurements start. To ensure this I was required to collect the plants in the early hours of the morning, and then place them in a bag to prevent them from drying out. I also needed to take the measurements as quickly as possible from the time when I first collected the plants. The best way to ensure this was to make the hydraulics lab mobile! So, I packed up all the gear into my trusty steed (including scanners, pole pruners, pressure chambers, stem psychrometers and a whole bunch of other equipment) and hit the road (see Figure 3).

“I went to the desert on a horse with no name; it felt good to be out of the rain” [Americas]

On my travels I was accompanied by several incredible assistants, companions and collaborators. Together, we sampled oaks from all sorts of exotic locations. We sampled species in the high elevation deserts in southern California (note the snow), the pristine Channel Islands (we got there by ferry) and slow moving Los Angeles. It was very special to see exciting and diverse habitats and to meet many wonderful people along the way.

You will have to wait to see what we found…as that is a post for another day! One exciting initial finding though is the discovery that science moves at about the same pace as the traffic in Los Angeles (see Figure below)…

It’s just an illusion

Illusions are a great way of getting us to think about how we are just biological creatures with biological systems at our disposal. No matter how weird and fantastic we think our brains are, the reality is that they are so much weirder and more fantastic. To paraphrase a famous astro-physicist (I think Neil deGrasse Tyson said the original quote): Our behaviour is not only weirder than we think, it is weirder than we can think! Take a look at two of my favourite (and very simple) optical illusions, which demonstrate in different ways how our brains process information about the world. The first is about how we process objects and the second is about how we process faces.

Lord of the Rings?

The first illusion is a very simple, but classic illusion involving concentric rings, which demonstrates quite nicely that the reality that we create in our minds about objects in space is simply an illusion created in our brains. Our brains have remarkable innate cognitive systems designed to re-create a 3-dimensional understanding of objects in space from a 2-dimensional image generated by the eyes.

Masked reality

The second optical illusion is the facial recognition test: can you tell which way the face is facing? There are two salient points to take from this illusion. The first point to note is that we have the ability to recognize faces from very simple masks, meaning that we must have some kind of inbuilt facial detection system in our brains. The second point is that our system works to detect faces even when we consciously know they are not there (as when we learn that the mask is the “wrong way round”, but we still see the face). Richard Feynman once said that Mother Nature cannot be fooled. But, here we see that our own natures can be!

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].


[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.


[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”: