|Umbrella Thorn, Serengeti: An icon of the 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.
|It's not only Vachellia species that are flat-topped. Here are a |
range of species including Balanites aegyptiaca in Serengeti.
|Senegalia mellifera is often in drier areas and is typically a 'ball of thorns'.|
|Forest on Kilimanjaro - trees here are tall and have |
branches to catch the light passing through the top layer
|I love Vachellia tortilis! Another in Serengeti.|
Archibald, S., & Bond, W. (2003). Growing tall vs growing wide: tree architecture and allometry of Acacia karroo in forest, savanna, and arid environments Oikos, 102 (1), 3-14 DOI: 10.1034/j.1600-0706.2003.12181.x