Re: Savannah and Grassland

Harry Erwin (
Sat, 21 Oct 1995 14:37:23 -0400

In article <469t3i$9ha$>, Mark Fagan
<71640.2463@CompuServe.COM> wrote:

> RE: Fire and Maintenance of Open Savannah
> A recent special on PBS (I think) showed that the situation is much
> more complex than has been discussed in this thread.

It always is.

> During normal rainfall years, the grassland is maintained by the
> continuous grazing of large ungulate herds.

Not really. The ungulates prefer the annuals for the most part, so that
perennials do quite well if they are not burned over frequently. The
annuals are a successional system.

> During drought
> periods, the grazers move to other areas and trees become
> established and can convert the grassland to a broken forest type
> environment.

There's a yearly drought/wet-season cycle in the Serengeti. The ungulates
move out into the brushland during a good part of the year.

> Even when normal rainfall returns, ungulates cannot
> reopen the grassland. Elephants then enter the picture, as they
> are quite willing to push down trees to get to the higher leaves.
> This reopens the grassland so that the cycle can begin.

However, the elephants produce an open woodland, especially when they are
selective about which trees they browse from. The slave trade cleared the
human population from a good deal of East Africa for some centuries,
allowing the the elephants to dominate the ecosystem for that period. The
result was woodland, not grassland.

> All this presupposes that the grassland can support trees. Various
> environmental conditions could preculd this. The N. American
> plains are too cold, for example (I recently found out there are no
> hardwood decidious trees in Calgary, for example, for this reason).

There are a number of possible reasons for permanent grassland:
1. shallow soil,
2. regular flooding,
3. clines (marginal areas) between woodland and scattered brush.
4. temperature.
The Serengeti becomes woodland if protected, though, which is my point.

> So I think we need a wider outlook on this question.
> Mark Fagan, Toronto, Ontario


Harry Erwin
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PhD student in comp neurosci: "Glitches happen" & "Meaning is emotional"