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!


*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.


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.


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


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.


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!

Low vulnerability to xylem embolism in leaves and stems of North American oaks

Have you ever wondered how plants become damaged and die during drought? As a post-doctoral researcher investigating plant physiological responses to water deficit, I spend most of my time thinking about this question. A very smart young lady once told me that it’s obvious…they run out of water. In one sense, she is absolutely correct: when the leaf mesophyll cells run out of water (desiccate), they become unable to maintain normal physiological function, which ultimately leads to death. But this explanation is not entirely satisfactory: It does not tell us about the precise processes that lead to desiccation, whether there is a particular threshold of desiccation that plants fail to recover from, and how different species might vary in their capacity to withstand desiccation. I have been investigating these questions in Californian oaks, and recently published my findings in the scientific journal Plant Physiology.

To find a satisfactory answer to the question, we need to revise some fundamentals about how plants function. Not having a heart, or a mechanism to pump water to the tops of their canopies, trees rely on gradients in water concentrations. Water naturally and passively moves from places of high concentration to areas of low concentration. As water is drawn out of the pores on leaves (stomata), it is replaced by water from the rest of the plant. Plants replace this water lost to the dry atmosphere by taking up moisture from the soil. Within the plant water is transported in xylem: from the roots, all the way to the leaf mesophyll cells. Think of this process as similar to an elastic band being stretched from the tops of the canopies. As the environment dries, plants find it harder and harder to extract moisture from the soil. The elastic band stretches…

Xylem embolism

We currently think that as the tension in the water column increases, air is eventually drawn into the xylem vessels and the elastic band snaps. This process creates an air pocket that blocks the transport of water, know as embolism. Xylem embolism represents a significant, drought-induced damaging process in land plants. As plants embolise they lose the capacity to hydrate the downstream cells in the leaves. So this partly answers our question: the point at which plants experience embolism is the point at which they begin to suffer (mostly irreversible) damage.

However, substantial debate surrounds the capacity of species with long xylem vessels to resist embolism. Some studies have shown that species are highly vulnerable to embolism, and regularly experience this during dry periods, while others suggest that plants are actually less vulnerable to embolism. Our paper investigated whether recent methodological developments could help resolve this controversy within oaks, a  charismatic, ecologically important, temperate angiosperm genus. We were hoping to shed further light on the importance of xylem vulnerability to embolism as an indicator of drought tolerance in long-vesselled trees. To do so we used a non-invasive optical technique to visualise embolism in leaves and stems of eight oak species from the Mediterranean-type climate region of California, USA. Basically we took a lot of photos of trees as they dried down and monitored when they embolised.

We show that the point at which air enters into leaves ranges from −1.70 MPa to −3.74 MPa, and from −1.17 MPa to −4.91 MPa in stems (pure water is 0 MPa and plants normally operate above -1.5 MPa). Consequently, our results show that long-vesselled North American oak species are more resistant to embolism than previously thought, and support the hypothesis that avoiding stem embolism is a critical component of drought tolerance in woody trees. So now we can say exactly when particular oak species start to fail during drought.

This information is useful, as it allows us to predict how oaks might respond to future drought events. It tells exactly when they will start to lose function, and become damaged. In fact, our study shows that accurately quantifying xylem vulnerability to embolism is essential for understanding species distributions along aridity gradients and predicting plant mortality during drought. All we need to do is monitor their water status to know whether they will recover or not. Ultimately, this is why physiological studies, such as ours, are important: They allow us to be precise in understanding how plants respond to climate change.

How plants respond to drought provides insights into climate change survival

Globally, droughts have had a negative impact on many plant species. This has led to much higher rates of mortality than usual. Understanding how different species are likely to respond to drought is crucial to accurately predicting the impact of future climate change on plant communities.

Plants in South Africa’s Western Cape area have tremendous variation in their sensitivity to drought. Photo: A. Betzelberger.
Plants in South Africa’s Western Cape area have tremendous variation in their sensitivity to drought. Photo: A. Betzelberger.

It can be extremely challenging to find meaningful ways to describe plants’ many different types of responses to drought, particularly in biodiverse areas. Scientists have been working to develop a new system in which plant functional traits can be used to assess the range of drought tolerance in diverse plant communities.

Why some plants shrivel and die and others don’t
Although it may be tempting to think that drought is bad for all plant species, there is tremendous variation in their sensitivity to drought. For several decades, plant scientists have been attempting to figure out what determines this variation. But there are no simple universal measurements of drought tolerance. Instead, scientists usually rely on long-term experimental manipulations.

The difficulty of assessing drought tolerance makes it challenging to work with large numbers of species. It is critically important that we do so, particularly within the world’s biodiversity hotspots – they are incredibly important systems for the planet. They include the tropical rainforests – the lungs of the planet – and many economically significant ecosystems such as South Africa’s fynbos.

Trade-off between water loss and carbon uptake
A recent paper offers a solution to the challenge. It proposes using simple plant characteristics or traits to identify the different strategies that species adopt to cope with drought. The beauty of using plant traits is that they are relatively easy to measure, and can be gathered for a large number of species. The system uses combinations of easily measured plant traits to get an index for drought tolerance in much the same way as body mass index works in humans. You can measure relatively simple traits like weight and height and combine these to determine how a person might react to stress or exercise.

To predict how different plant species respond to drought the researchers focused their trait selection on the classic water-loss versus carbon-uptake trade-off that plants face. When land plants open their stomata to allow carbon dioxide to enter the leaf for photosynthesis, they inevitably lose water. This creates a carbon-uptake-water loss trade-off, particularly under water limiting conditions. During periods of drought plants face a dilemma – to continue to photosynthesise and risk desiccation or close the stomata to conserve water but risk starvation.

Studies were conducted to determine how plants like the Protea adapted to drought. Photo: A. West.
Studies were conducted to determine how plants like the Protea adapted to drought. Photo: A. West.

Tracking two plant traits
The scientists recognised that two plant traits would be critical to determine which side of the trade-off different species tended to err towards. The first trait relates to how a plants’ internal plumbing responds to dehydration: the ability of the xylem to withstand desiccation. Water is transported from the soil through the stem to the leaves under tension, which gets greater under drought stress. When the plants become too dehydrated the water column breaks and air enters the stem, which blocks the vessel. This is similar in some sense to an embolism in human arteries and is potentially as fatal for a plant as it prevents them from moving sugars to where they are needed.

The second trait relates to the breathing apparatus, or stomatal response of a plant. Stomata operate like variable resistors on the leaf surface, opening and closing in response to desiccation. The sensitivity of the stomata varies between species. In some species they may be highly responsive to changes in water status and less so in others.

Scientists have known for a while that both xylem vulnerability and stomatal response varies between species. But the association between these traits and the ability of a species to survive droughts remained unclear. The key missing element was how the xylem and stomatal responses are co-ordinated within a species. Very few studies have addressed co-ordination between stomatal regulation and xylem vulnerability and then used this to predict how likely a species is to die during drought.

Lessons offered up by Fynbos
To test the system, the authors studied fynbos communities in South Africa’s Western Cape. These highly species-rich plant communities are recognised as one of the world’s biodiversity hotspots and contain three major plant functional types: the familiar proteas, restios and ericas.

Restios are highly diverse in fynbos, which makes these communities more resilient to change. Photo: R. Skelton
Restios are highly diverse in fynbos, which makes these communities more resilient to change. Photo: R. Skelton

Different species adopted a continuum of strategies. It ranged from a risky, low safety-margin response to a more conservative approach to water loss. A key finding was that species which were more conservative and closed their stomata earlier in response to desiccation were less likely to incur xylem embolism. On the other hand, species that closed their stomata later were more likely to incur xylem embolism.

It was also possible to make predictions about how different species might die during drought. Worryingly, the study suggests that many restios and ericas appear to be particularly sensitive to drought. This may lead to shifts in the future composition of fynbos communities.

On a more positive note, the study also showed that all three familiar fynbos plant types had representative species from the full range of responses. This suggests we are less likely to see any one plant type disappear. It highlights the importance of biodiversity for landscape resilience. Having species of each functional type respond differently to drought may allow a community to be more resilient in the face of change

This article was originally published on The Conversation. Read the original article here:

Also, the PNAS paper can be downloaded here: