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