Cycads and more botanical revolutions I've missed...

Lake Natron Cycad, near Loliondo. Cycads look rather
palm-like, but are not true flowing plants at all.

Back in April I headed to Loliondo for a few days with a bunch of guides from Thompson Safaris. Along the long and bumpy route I was really pleased to spot some Cycads and jumped out to take a few photos. Spotted in action, I was forced to explain why I was taking pictures of some random tree. My answer at the time was based mainly on the evolutionary history of plants that I'd been taught at school and then probably on into university: Cycads form a remarkably early split from the branching evolutionary tree of seed-bearing plants, their ancestors somehow linking ferns to the much more modern flowering plants.  I was also keen to see this particular species (according to the IUCN commonly known as the Lake Natron Cycad) because it's one more of those remarkably restricted range species that fascinate me.

Now I have a confession to make - once again I was completely wrong!

Why do savanna trees have flat tops?

Umbrella Thorn, Serengeti: An icon of the savanna?

From sunsets behind a silhouetted acacia (properly Vachellia), to photos of rolling grasslands studded with isolated trees, a savanna landscape is immediately identifiable thanks to the flat-topped tree. But why is this? Why do so many Vachellia and other savanna trees have such a distinctive structure that they have become a virtual icon of the African savanna?

It's an interesting question that was given some answers in a nice paper by Sally Archibald and William Bond who studied one species called the Sweet Thorn (Vachellia karroo) that, rather like some of our Vachellia species in East Africa exhibits a range of different growth forms in different habitats. In the semi-desert of the Karroo, it grows as a medium-sized ball of thorns, whereas in the savanna it has a fairly typical medium-tall  flat-topped acacia look to it and in a forest it's a tall, thin tree. These differences are meditated mainly by genetic differences within the species, but equally could be caused in other species by a variable response to the environment - it's not really important to this discussion and, in fact, much of our discussion could focus on different species if we wanted. As always when we're thinking about what makes the savanna species, we'd be well advised to start with the savanna big four: nutrients, water availability, fire and herbivory.Now, the first two processes have impacts in all biomes, whereas it's the second two that are most distinctive about savanna and where we'll start our discussion.

Commelina, the Maasai Reconciliation Grass

Commelina sp, Mongo wa Mono, March 2011

It's surprisingly easy these days to find information on the medicinal use of plants (there's a great list for the Samuru people here, for example), such as the Commiphora uses we covered last week, but many plants have cultural significance beyond the simple medicinal uses and it's often much harder to find information about these uses. One of the 10 things I like to get people talking about when there are no lions is all to do with people, and talking about cultural uses of plants is often quite interesting. I was seriously impressed when one of the guides on our training course in November said that in 'circumcisim school' he'd had to learn to identify about 200 plant species and know their cultural and medicinal uses, so this knowledge is still very much alive out here - though he did confess to having forgotten many (before going on to hive an example of a rather harrowing use for one of the Euphorbia species that really isn't suitable for polite company...). So it's rather surprising how shy people can be about sharing the information, perhaps thinking it's not interesting, or somehow backward.

Myrrh trees (Commiphora) are useful things...

Most Commiphora have distinctive peeling bark, Eyasi Aug 2011.

Having last week given you the bad news about the biological warfare that plants with thorns are engaging in, I thought it only fair to share some tips that may help you stave off those tropical nasties threatening to kill you... So the good news is that some of those very same thorny trees that are out to get you also hold the cure in their sap. Traditional healers and many folk still living in the country have long known about the beneficial effects of the sap of Commiphora trees. Indeed, the earliest recorded use of Commiphora to treat infections goes back to 1100BC where Sumerians were recorded using myrrh (for that is what Commiphora is) to treat tooth infections and intestinal worms. It's use is also clear in the Bible, as one of the three famous gifts presented to the infant Jesus. Today it's still commonly used in village communities to treat an endless list of infections and maladies, and it's also pretty good as a mosquito repellant if you need emergency cover!

How old is a baobab?

Baobabs even make Giraffe look small! Tarangire, April 2010

I was lucky enough to see the New Year in, at Tarangire NP where, as always, I was impressed by the immensity of baobabs Adansonia digitata. When you see elephants dwarfed by a tree, you know it's impressive, and Tarangire is the place to spot both elephants and baobabs. But the two questions I'm often asked about baobabs are firstly, how old are they? And then, why don't we see baby baobabs? As they're perhaps both related issues, and because I found a nice paper that explains how we determine the age of baobabs (available, but not free I'm afraid, here), I thought it would make a nice post.

Room for one more? Take samples from the cavity...

The age of many trees is easy to estimate by simply counting annual rings: in temperate climates growth happens during the summer period, and slows during the winter, depositing a dark ring each year. In the tropics many trees have growth rings formed during the dry season. The problem with baobabs is that they have a succulent trunk that (a) gets stripped by elephants for water, (b) doesn't really have clear growth rings and (c) is often hollow. So a standard method of counting rings won't tell us how old the tree is. Instead we have to turn to radiocarbon dating. This is a method commonly used to age archaeological remains and replies on the fact that when a plant grows to 'fixes' CO2 from the atmosphere in its woody matter. Now, the carbon (C) in the CO2 of the atmosphere occurs in two forms which we call ¹⁴C and ¹²C. Now ¹⁴C is radioactive and changes ('decays') at a constant rate to Nitrogen, whilst ¹²C is stable. So if we compare the proportion of ¹⁴C and ¹²C in a sample with the proportion present in the atmosphere, we can calculate how much ¹⁴C has decayed, and therefore how old our sample is. So what Patrut and others have done, is to take samples of wood from with the hollow cavities within one particularly large baobab, and use radiocarbon dating methods to estimate the age of the tree. And they find some interesting results even for this one tree - the tree in question has two stems, one much larger than the other. We might suspect that the larger stem is, of course, the older one. But this isn't the case at all - the smaller trunk is estimated to be over 1060 years old, whilst the much fatter trunk is around 300 years newer. So once the tree is already pretty big, the size of the trunk is no useful guide to the age of the tree - and the authors also note several other similar studies they've done of other trees which confirm this pattern. They suggest that what matters is the initial conditions over the first 100 years of a stem's life - if it's particularly favourable the stem grows very quickly, and then keeps growing quickly for the rest of it's life (early life conditions are often very important for later growth rates in a range of organisms). So if a stem starts in a particular good time, its growth can easily overtake older trees who struggled to grow fast early on in life, so the biggest trees aren't necessarily the oldest. None the less, from this and other studies of baobabs it's still clear that very large baobabs are often over 1000 years old - the oldest known was over 1275 years old when it died.

You do see baby baobas - this nr Tarangire Nov 2011

Now, why is this longevity perhaps related to the lack of baobab recruitment (i.e. why don't we see baobab seedlings)? Well, firstly let's clarify - lots of people all over Africa have commented on the apparent lack of baobab recruitment (several cited in here), but it isn't actually true that we never see baobab seedlings, just that we don't see many - if you really get to know a park with baobabs well, you probably do know where there are one or two seedlings at least. And now think a little about population biology. All it takes for a population of a species to remain stable is that births equal deaths. If births exceed deaths it's obvious that we'll soon have a growing population, and if deaths exceed births we've got a population in decline. So how often does a baobab have to be successful in having a baby for the population of baobabs as a whole to remain stable? Only once in 1000 years, of course. But it may produce seeds (many!) every single year of that life - only for them all to die - let's say a mature tree produces 1000 pods per year, each with 100 seeds (guesses!) and it does that for 1000 years - that's 100 Million seeds, of which only 1 is expected to survive to maturity! So are we really likely to see lots of baby baobabs if the population is stable? No, I don't think so; especially as we've already got the suggestion that early conditions - for a baobab maybe periods as long as 10 or 100 years - are pretty important, and maybe those conditions only come around ever 4-500 years - nothing that we'd ever expect to experience in our lifetimes. So do I worry about baobabs? No, not really. But I do like to see them, and they're seriously important for the ecology of the areas where they occur, something I think we'll tackle in a later post...

Good for sunset photos too! Tarangire, Oct 2009

Main reference: Patrut, A., Reden, K., Pelt, R., Mayne, D., Lowy, D., & Margineanu, D. (2011). Age determination of large live trees with inner cavities: radiocarbon dating of Platland tree, a giant African baobab Annals of Forest Science, 68 (5), 993-1003 DOI: 10.1007/s13595-011-0107-x

Leopard Orchids

Leopard Orchid, Near Tarangire, Nov 2011

I'm just back from 2 weeks in the bush with Ethan and a bunch of other guide trainers and have a stack of photos to sort through (and loads of ideas for new posts, so there might be a flurry of activity on the blog again!). Whilst most of these are going to take a while to ponder and work though, I thought I'd start with a botanical story about the leopard orchid, since we found plants in all stages pre-flower, to ripe fruits and I got a nice set of photos.

Verreaux's Eagle Owl nesting in Leopard Orchid, Tarangire NP, June 2011

Leopard Orchids Ansellia africana are the only species in their genus, and occur widely throughout Africa, always growing as an epiphyte (i.e. growing on another plant, but not taking any nutrients from them - so not a parasite). Just like leopards, they're often seen lying on branches of big leafy trees and the flowers are yellow with brownish spots. They're very pretty, and often taken into cultivation (they're now considered threatened in the wild in South Africa, thanks to this trade), but they're also rather interesting ecologically. Three things particularly interest me: firstly, there's an association with ants - plants secrete nectar (not from the flowers though) that a wide variety of ant species drink. In return for the sugar, the plant gets effective protection from other insects. (Didn't seem to stop these Verreaux's Eagle Owls from using one in Tarangire as a nest site though!)

Leopard Orchid flowers provide no nectar to pollinators, Nov 2011

Next is that strange fact that mature flowers don't produce nectar. Actually, this isn't all that uncommon in orchids - they're well known for their 'deceptive' pollination strategies, a phenomenon noticed and described by Charles Darwin himself, in 1877. Most flowers produce nectar to encourage and reward pollinators, leopard orchids and many other orchid species don't - they rely on scent and appearance alone to persuade pollinators to visit. It turns out that leopard orchids are mainly pollinated by solitary bees, though we don't yet know if the scent the plant produces is similar to that of the pheromones produced by the bees to help them find mates - it certainly is what happens in other orchid species. In these species along comes a male bee expecting to find a female ready to mate, and instead finds a flower - in the case of the bee-orchids the deception even extends to the shape of the flower mimicing that of a female bee too - how cruel! Still, obviously works, as there were plenty of pods on one of the plants we found.

These pods could contain millions of seeds, Nov 2011

Which is where the thrid interesting fact comes in. This time it's general for all orchid species (I think?) - they are wind dispersed and have some of the smallest seeds of any plant - completely lacking any store of food for the new plant. They're so small that in some species each pod can hold over 4,000,000 seeds! I've not found the number for Ansellia, but we can be pretty sure there's lots in there. With so little energy provided, orchid seeds have no chance of germinating unless they can get it from somewhere else - and they all do it by hijacking a fungus. The seed, having settled in a crevase of a tree sits and waits - if a bit of the right fungus grows along the branch looking for something to digest (we call these little bits of fungus mycorrhiza) it infects the orchid, and instead of being digested the orchid grabs the fungus and steals it's nutrients in an act of parastism that enables the seedling to grow. In some species this later turns around and the orchid returns the favour by providing excesses sugar to the fungus (a sort of mutual parasitism if you like, or at least a symbiosis), but increasingly botanists are realising this isn't always the case and many mature plants also maintain this parasitism throughout their lives. So this stage of life is also rather interesting.

Leopard Orchid in Kigelia africana, Near Tarangire, Nov 2011

Happily these orchids still seem very abundant in much of Tanzania (though a quick google search confirms you can easily buy Tanzanian Leopard Orchids from a few suppliers at least, so how long this will last I don't know - I'm not giving you the links so you don't do it!), so you should be able to keep telling these stories for a while at least!

PS If you're here now because you were on the course, great to see you! Join the site, comment, ask questions and pass the address on to your friends too!

Phenology – the timing of biological events.

First rains arriving over Manyara Ranch, Nov 2011

This is one of my favourite times to be in the bush, as the rains arrive and the savannah turns green. I love the excitement of the birds as they greet the rain, and the miracle of new grass appearing in just a few days and I probably get as excited by the first thunderstorms as my children! But as we all know, the timing of these events can change year to year. In fact, never more so that recently – one of the first impacts of global climate change that we see here in East Africa. Despite the changing season being such a profound event in the savannah, there's a surprising amount that we don't know about the patterns of seasonal change that we see.

For example, it's obvious that grass growth responds directly to rainfall – or at least to soilmoisture. If the rains are late, the grass stays dry, if the rains are early, it turns green early. But how does it know? To all intents and purposes the grass (or the seed) seems completely dead until something tells it the soil is moist and it's time to start growing again. In this case, actually, it's fairly straightforward – the moisture in the soil is in direct contact with the grass roots (or seed) and as that moisture is absorbed the cell cycles are started up again.

Pre-rains green flush in Brachystigia woodland, Kafue NP, Oct 2011 (pic. H. Frederick)

Other patterns are harder to understand, and the one that fascinates me most is the green flush that we see in miombo woodlands (and on Commiphora and several Combretum species too) before the rains. Not just immediately before the rains either, but some weeks before. How, and why, do they do that?

Let's remember first that savannah woodlands are deciduous (the trees loose their leaves) because during the dry season their leaves would loose too much water to allow the tree to survive. Add water, and there's no problem, so you'll see evergreens in the savannah only in riverine and kopjie habitats. So why, just when water is in shortest supply, do some species 'choose' to use some of their remaining stores of water and put out leaves before the rains come – and not just before, but a long time before? One of the things that we do know that might help us understand this is that once the leaves are out, the plants once again 'switch off' until the rains arrive. They've got leaves out, but they're not photosynthesising and respiration (plants respiretoo, of course) is pretty much dormant too. But then, once the rains do come, they're active within 24hrs. And another clue might come from the fact that we know there's a flush of nutrients (particularlynitrogen) associated with the first rains, that rapidly declines after the first few days of rain. So there's obviously a strong advantage if you can be ready and waiting for the rain – other trees that aren't ready will spend those first few nutrient rich days busy growing leaves and not be able to take advantage of the nutrient flush. So as long as you can minimise the costs of having leaves before the rains come (by essentially shutting down as much as possible), it seems plausible that the benefits could outweigh the costs (and clearly, for some species they do, or they wouldn't survive!). One thing that suggests this idea might be right is the fact that legumes – like Vachellia and Senegalia (I will get you toforget about Acacias!) - don't do it, they respond to soil moisture and, as we know, being legumes hey have no shortage of nitrogen, unlike other savannah species.

More pre-rain greening, Kafue NP (H. Frederick)

But why, then, be so early – why not just wait until the week before the rain before growing leaves and further minimise your costs that way? And here is where we really run out of hard facts and enter the realms of interesting scientific speculation – my guess is that because the date when the rains start is variable, you can't predict it that accurately. If you want to take advantage of that first nutrient flush, you've got to be ready for the earliest possible date the rains might fall – which (like this year) might be several weeks before the rains begin in normal years. I'm far from certain this is right – among other things, it requires that the benefits of being ready for that first flush are extremely strong, such that plants that catch it every year have a meaningful evolutionary advantage over plants that only catch it most years, which is testable but not guaranteed. But it's a good theory to work on for now.

The next part of the story that I'm interested in, of course, is how they do it? How do these plants 'know' when it's October and the rain is coming in a few weeks time? Unlike the grasses that simply detect water, these plants must keep track of the changing date directly. In the north where these processes have been studied in extraordinary detail, plants (andanimals) use changes in day length to keep track of the seasons – in spring and autumn in Aberdeen where I used to live from one day to the next day length could change by as much as five or ten minutes. But I find it hard to conceive that the same process is possible here where day length changes only by two minutes across the entire year – the difference from one day to the next can only be measured in seconds or fractions of seconds, and I find it hard to believe this can actually be the cue. But, amazingly, no-one's studied it so we just don't know.

There are other biological events that depend on precise seasonal timing, of course – like the millions of birds that spend months here until April, then head north to breed, but even here we don't always know the signals that the birds are using and why, for example, so many species seem to have been rather late arriving this year. But this has already been a long enough blog for one day, so that will have to wait for another time...

Parasitic plants - a Commiphora mistletoe

Plicosepalus meridianus growing on Commiphora, Mwiba Ranch, Aug 2011

In the middle of the dry season out here there's precious little colour to break the greys and browns, but look closely at the Commiphora and you might be surprised. Not only do they flower themselves in the dry season - a small rather unexciting flower, but many of them (I think mainly C. schimperi, but it's tricky for me without leaves!) around Mwiba had been parasitised by Plicosepalus meridionalis, a shockly pink flower that on first glance appears to grow straight out of the tree's bare branches. (I don't know a common name, I'm afraid.)

So, how would we interpret this 'wildlife' sighting? Start by identifying it - I'm fairly sure this is what I've photographed (certainly this genus), but some of you are probably better botanists than me and might correct me. It belongs to the family Loranthaceae, most of which are parasites and which also includes all the African mistletoes (though not the European and North American ones). It's worth pointing out it's only growing on Commiphora too - like many, it's a pretty host specific parasite.

Note how it grows all along the host!

Then explain what it's doing. You probably don't need to say it's flowering - that should be obvious! But have a close look and you should spot a little green at the base, and have a fiddle with the Commiphora bark and you might find some more where the plant holds tight to it's host. It's green because, like most mistletoes, it can photosynthesize a bit - though it does get most of it's nutrients from it's host. So its also (probably) photosynthesizing and stealing nutrients and water from the Commiphora - though probably not much at the moment, as the host is in a dormant phase for the dry season. Find the plant in the wet season and the story is different - mistletoes use both active and passive mechanisms to tap into and extract nutrients and water from their hosts. This is clearly not to the hosts' advantage, but the impacts probably aren't so strong that it threatens the health of the host - a closely related species P. acacia (guess what it's host is!) has been found to only thrive when the host itself is thriving, a sick host means a sick parasite with few flowers and low growth.

That's what it's doing, but what about my third question, what's the role in the ecosystem? Well, in this case we can speculate a bit. The obvious thing it's doing right now, is flowing in the middle of a period where there are few other flowers available. (It can probably do this and remain active during the dry season because it has a ready source of water from it's host, the succulent Commiphora.) And the flowers are pink. So stick around a few minutes and see what happens - you'll almost certainly see a sunbird nip in to feed. Pink, red, orange and yellow flowers are often signals for birds (which have good colour vision), and during the dry season there are precious few flowers around. Some sunbirds move away for the dry season - Coppery Sunbirds are a well known migrant, for example - but many stay and make use of the few specialised flowers available during this lean season. And those few flowers that are available, of course, must get visited very regularly, with excellent pollination chances. So you could argue the plant is helping to maintain the pollinator community during the lean period - certainly if there weren't a few species during this, all the sunbirds would have to migrate to greener areas during the dry season.

Scarlet-chested Sunbirds (here Arusha, March 2011) require dry season flowers

Even more interestingly, we could talk about some of the general impacts of parasitic plants on biodiversity at large. As we've mentioned, parasitised plants aren't (usually) killed by other plants, but they are weakened. And if you were going to be a parasite you'd probably want to parasitise something fairly vigorous and strong that there's lots of, to ensure the next generation can also find some hosts to parasitise. Now, if you have one particularly strong and domiant plant in an ecosystem, it's likely to out compete all the others and you'll end up with a rather low diversity system. But if parasites are more common on these strong plants because it's in their interests to do so, then they'll weaken that dominant plant and allow other species that aren't usually able to compete to survive too, boosting the biodiversity of the ecosystem - a pattern that has been demonstrated experimentally in grasslands, using grass parasites. So although I don't know it happens with this species and Commiphora, you can certainly wheel the story out again in April when the Cycnium are flowering everywhere (you know them, the little white and pink flower that love short grass, I've not got a photo though, I'm afraid. You'll have to look here for one if you don't know it!), as these are also hemi-parasites (hemi-parasites being parasites that don't steal all the requirements they need from their hosts, but also make some nutrients themselves) on grass. Certainly plant parasites are interesting things with important impacts!

Vachellia tortilis, or why there are no Acacias in Africa

I'd promised to write about marine things this week, but here's an interruption that can't be ignored as I read today that there really aren't any Acacia in Africa. (Bad news for the Acacia Rat and Acacia Tit, etc!) What's that, you say? Well, you may (or may not) have heard that for the last few years there's been some discussion among plant taxonomists about the correct taxonomy and names for the genus Acacia. And what seems to be the final final word on the matter was decided on Monday at a meeting of the International Botanical Congress over in Australia – the result? Africa no longer has any Acacia.

Vachellia tortilis will always make good sunsets, whatever you call them!

Now, this is going to cause some confusion (especially to poor ornithologists like myself), and it probably helps to understand a little about where these scientific names come from in the first place. Let's say you find an odd little plant, look around a lot and can't find anything like it. After quite a lot of asking the experts you conclude you've got a new species, so you need to give it a name. Now, ever since Carl Linnaeus back in 1735, scientific names have had two parts – one genus, one species. You can add extras designating sub-species if you like, but when you see a scientific name the important parts are the genus (always capitalised) and the species (never capitalised, even if named after a person). The aim of the name you chose is to tell you something about the plant and the first thing you have to do is identify the genus. Does your plant have any obvious relatives? If so, I'm afraid you're not free to chose the full name – you'll have to name your species with the same genus, though you can chose the species name (btw, it's generally considered bad form to name it after yourself, sorry. You could name it for a friend though – or get them to name it for you, if you feel particularly narcissistic). So in a scientific name, the genus should tell us about evolutionary relationships. And if our understanding of the evolutionary relationship changes then, bad news, the name will have to change too. Which is what has happened with the genus Acacia. In fact, we touched on the issue yesterday, when I wrote about sponges not being monophyletic, but we didn't take that discussion very far.

Still, what's happened to Acacia? Well, the genus was first described back in 1754 (lots of these details from here, but it's behind a paywall I'm afraid) and until the recent changes referred to some 1300ish species, of which 960ish live in Australia, where they're known as wattles. It's long been noticed that within this large genus, there are some specific sub-groups (called subgenera) that are more closely related to one another - like those Australian wattles (two of which made it to Madagascar, btw) - but that's OK as long as the genus refers to all the relatives. The problem came when studies started showing that some of these sub-groups were more closely related to other members of the Mimosoidae subfamily, than to other members of the genus Acacia, which is what means the group is no longer monophyletic. Now, it's easiest to explain this with a little diagram, I think. So, here's what we used to think Acacia relationships were:

 You can see how all three groupings (subgenera) within the genus Acacia are grouped together, and all the Acacias are separate from the other members of the Subfamily Mimosoideae. That's good - it's a monophyletic group and everyone could have Acacias. (You know a few of the members of tribe Ingeae, btw - this is where Albizia are found.

Now, however, recent DNA evidence has shown a different pattern (it's not quite certain which pattern, so I've given an illustrative diagram from one paper):

Now this shows the problem - no matter how you twist and turn the branches, Acacia subgenus Phyllodineae comes out as more closely related to members of the Ingeae than other two groups of Acacia. As the genus therefore describes multiple branches of the tree, but not all the descendents, we know it's polyphyletic, and we're going to have to change some names. We could, of course, resolve the problem by naming all the plants within the tribe Ingeae as members of Acacia too - then the name would only refer to one group and would again be monophyletic. But imagine the chaos that would cause - suddely all the (obviously different) Albizia and their diverse relatives within the entire tribe, would have to become Acacia. No, it's much simpler to split the Acacia genus into several different genera, each of which are monophyletic. And because Australia has by far the most species in the old genus, they've got the Acacia name for their wattles, leaving the rest of the world to find some other names.

In fact, there are some names out there that look set to be standard. Many of our well known Acacia species look set to become Vachellia, whilst others will become Senegalia. Which become which, I've not sorted out yet. But, but I do know what was Acacia mellifera will be Senegalia, whilst A. tortilis will become Vachellia, and for many species it's fairly obvious which of these two are more similar, suggesting likely names - bushy, multi-stemmed versions look set to become Senegalia, big trees follow tortilis into Vachellia. The good news, of course, is that there's no reason to change the common names - we can still have Whistling-thorn Acacias if we want, even if their scientific name becomes Vachellia drepanolobium. Unless, of course, someone decides common names should change too, something I find much harder to understand (and I still see Crowned Plovers, even if they are Vanellus) - in which case there really will be trouble for the poor Acacia Rats...

Update: You can find a (nearly) complete list of the new species names here.

Plant signaling

Perhaps the subject that most surprised guides when we were chatting about things to talk about when there are no lions came up in my session on thorns. Thorns and other plant defences are quite fascinating and I'll certainly talk more about them in the future. But most people were more impressed to hear about how plants signal, than they were about thorns themselves, so I thought I'd give a bit more information about plan signaling in this post instead.


Now, imagine you're a plant that's getting browsed. Not much fun, huh? You'd want to do something about it if you could, wouldn't you? So, let's say you can detect browsing (how would you do that? Easy, as it happens, just look out for plant chemicals that should be contained within cells, in places they should be - if there's cell contents in places it shouldn't be, the chances are you'vebeen damaged - and we all know how damaged plants can smell), it would be nice to have a quick response and produce more nasty tasting chemicals straight away, and when you regrow, it would make sense to be extra thorny in this area. Even better if you could somehow warn other branches that you're been eaten and communicate with the other side of the tree, don't you think? And as it happens, plants can do this - rather than always producing lots of costly thorns and nasty chemical defenses, plants tend to just produce a minimal output, and then up the defenses if they actually come under attach. Very sensible really. And one of the key ways they have of signalling that they're under attack is through the use of a plant hormone. Now there are several plant hormones, but the simplest is called ethene (or ethylene, it's the same and I'll use the two interchangeably here) and is a colourless gas, consisting of two carbon atoms, and four hydrogen atoms:


H              H
    \          /
      C = C
    /           \
H              H


if you're into your chemical formulae. It's a very simple organic molecule, and plants use it for almost everything you can imagine: signalling that it's time to ripen fruit, so they all ripen at the same time (fruit importers in the west use this trick - they get people to pick unripe green fruit in the tropics, stick it on a boat to Europe (it last well if it's green), then gas it with ethene so everything ripens nicely before getting into the shop. Which explains why fruit tastes much better here than in UK... You can also use ripe bananas to speed up the ripening of other fruit, if you stick them together in a paper bag.); seed germination; fertilization, etc., etc. But for us, right now, we're interested in how they use it to signal stress, and in particular herbivory.

This avocado is ripening thanks to a ethylene signal from the ripe bananas

To be fair, it's not actually the main signal process for herbivory - it's probably too general in function for that task – but it does play a role and the concept is the same for the other signalling processes. Once chewed, a plant rapidly (within seconds) with start to produce ethylene. Being a gas, it can drift all around the plant, and receptors in other parts of the plant pick it up, decide what it means (“help!, I'm being eaten!”) and tell the plant to get on with appropriate defences.

All well and good. But the observant among you will already have picked up on one thing – there's nothing to stop the signal moving out of the plant being eaten and into the neighbouring plant. And if that plant can pick up the signal, then all of a sudden, plants in a neighbourhood can communicate with one another. And, in fact, this is what they do. Pretty impressive for a plant, I'd say! Since most plants use the more or less the same set of signals it's quite possible for the signal to come from an Acacia, but be picked up by a Balanites – inter-specifc plant communication. And you thought plants were just sitting there and taking it! I'll save the 'what can a plant do about it' question for another time, but for now just remember that plants can signal, even to other plants, that they're being eaten and perhaps there's something here to talk about next time you watch a bush being hammered by an elephant...

This poor Acacia xanthophloea is showing quite how many thorns it will grow when there are lots of nasty giraffe around. Arusha NP June 2011.