Boomplantweek en die Internasionale Jaar van Woude - Dendro.co.za

DENDRON » No/Nr: 43 » November 2011
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UNDERGROUND TREES OF THE PONDOLAND CENTRE
Underground trees of the Pondoland Centre
The term ‘underground trees’ evocatively
presents a picture in one’s mind, perhaps of
a baobab buried deep in the ground, with just its
branch tips showing above the surface. Well now,
this is not quite how it is, but the concept is pretty
close. First the history, and there is not too much of
that. In 1922, Joseph Burtt Davy [1870–1940] wrote
a paper entitled ‘The suffrutescent habit as an
adaptation to environment’. Burtt Davy was quite a
remarkable fellow; he was born in England, worked
at Kew and later in California, having by then studied
both botany and agriculture. In 1903 he joined the
Transvaal Department of Agriculture and in 1925,
finally went as a lecturer at the Imperial Forestry
Institute, Oxford. This twofold academic training
together with his experiences in different parts of
the world gave him a solid feel for plant ecology.
In his paper Burtt Davy presents some possible
reasons for the evolution of what are now called
geoxylic suffrutices (singular: geoxylic suffrutex) or
“geosuffs”, a term coined by Marita Thornhill and
Ian Felton in their article in PlantLife No. 23. The
term ‘geofrutex’ is also occasionally encountered as
a synonym in the literature but I prefer geosuff.
According to the references I have, definitions of
suffrutex and geoxylic are: an undershrub (Jackson
1971): half shrub, sub-shrub, perennial plant with
only the lower part woody (Stearn 1983). The only
reference to “geoxyl” says: having a woody stem,
partly hypogeic (growing on or remaining below
ground), partly epigeic (spreading above the
surface (Jackson 1971).
Frank White [1927–1994], Curator of the Forest
tony abbott
Herbarium and Lecturer in Systematic Forest
Botany at Oxford University, held a lifelong interest
in the woody plants of southern Africa and was
renowned for his knowledge and work on the
vegetation of sub-Saharan Africa. White was the
next to look in more detail at this phenomenon and
in 1976 published his now classic paper on the
‘Underground Forests of Africa’. Here he suggests
that in many African grasslands, in particular those
of shallow wetlands and floodplains (‘dambos’) in
south-central Africa, the underground biomass of
‘underground trees’ exceeds that of the associated
grasses. He describes these peculiar ‘trees’ as
having massive woody underground parts….In the
majority, several axes radiate just beneath the surface
of the soil from the main vertical subterranean axis,
which, except in young plants, is relatively poorly
developed. Sometimes they extend for a distance of
several metres. In some species these axes can reach
a diameter of 10 cm. or more. They are usually very
hard and consist mostly of secondary xylem, the total
amount of which is probably no less than that of a
medium-sized woodland tree growing in the same
general region. These radiating axes are usually
referred to as rhizomes. Their true nature, however,
requires careful investigation since the arboreal
relatives of some suffrutices are said to sucker freely
from their extensive superficial roots.
Here is what Prof. Braam van Wyk had the following
to say in an address at a conference in Nelspruit on
the 13th November 2003 regarding the impact of
timber plantations on grasslands (transcript at www.
geasphere.co.za; accessed 24 April 2008):
Pyrogenic geoxylic suffrutices is a very peculiar growth form that is associated with our grasslands,
and it is very much a type of growth form in Africa. It is not found anywhere else in the world except
perhaps to a limited degree in South America. It is a growth form where you get plants, woody
plants that can be compared to underground trees, and all that you see are these green twigs
which can be compared with a canopy of the tree. And this is probably one plant sitting here, or
maybe even this whole area may be one plant, and it’s the canopy that just sticks out, the tips of the
branches above ground. They burn down every year, but the rest of the tree stays underground.
Why they have adopted this strategy… it is a very interesting challenge to come up with reasons.
Is it fire? We don’t think so. Is some of it frost? Shallow water table? Grazing? There are lots of interesting
things we can say about the reasons why it adopted this strategy and why it only evolved in Africa.
Now these clones, because we call them clones, they are essentially immortal, nothing can kill them.
Grazers can not kill them, fire can not kill them, they are drought resistant. They grow extremely
A geosuff is described by Yolande Steenkamp
and co-authors in PlantLife No. 25 as ‘basically
an underground tree, with a massive, woody,
underground, perennial central axis (often called a
rhizome) and many short-lived (annual) shoots above
ground’. In Burtt Davy’s paper he shows a drawing of
Parinarium capense. (now Parinari capensis. subsp.
capensis) with a huge stem driving down into the
depths. Today, however, it is considered that while
the initiating plant may have had a tap root, in most
instances it dies off and is superseded by an axillary
network of woody roots and stems (rhizomes) which
reach outwards from the point of origin supporting
aerial outgrowths or ramets (individual members of
a clone); see the picture of Eugenia albanensis (Vlei
myrtle). Considering the description by Steenkamp
and co-authors above, we should rather talk of an
extensive, rather than a massive, underground
network of interconnected woody axes. For the
present article, geosuff should be taken to refer to
a species which is woody in itself and mostly with
close relatives that are proper trees. These can be
divided into ‘facultative’ (when forced by outside
pressures) and ‘obligate’ (when compelled by
genetic requirement) geosuffs and here we enter
a rather grey area with the occasional doubt in
deciding what falls into which group. In Pondoland
we have, for example, two species which occur
as trees and also as possible facultative geosuffs,
namely Diospyros scabrida (Coastal bladdernut)
and Ochna natalitia (Natal plane). At present we
assume that the same species may display both life
forms, but future taxonomic investigation, especially
at gene level, might change this. Eugenia capensis
subsp. capensis (Dune myrtle) grows as a shrub or
small tree in dune forests but can occur as a geosuff
along forest fringes, especially when subjected to
frequent grassland fires.
We excavated part of an extensive stand of Eugenia
albanensis (Vlei myrtle), an obligate geosuff,
growing in red sands of the Berea Formation—a
system of ancient coastal dune cordons inland
from our present coastline. After exposing an area
about 3 m long, the growth pattern appeared to
be quite random with narrow (10–20 mm thick)
rhizomes running in all directions about 20 to 50
cm below ground level; we did not see any sign of
a root dropping vertically into the depths. It seems
UNDERGROUND TREES OF THE PONDOLAND CENTRE
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slowly, and if you look at the diameter of some of these clones, they must be the oldest inhabitants
of our grasslands. I would say easily more than a thousand years for many of these clones since the
first seed arrived for that particular species. But I would not be surprised if some of them are one
day shown to be perhaps more than 10 000 years old, amongst the oldest plants in the world, much
older than any tree that you are going to see. They are very peculiar plants and we have quite a
number of these species in our grasslands.
probable that the clone exists as a self-regenerating
organism where the rhizomes most likely have a
limited life (perhaps of some years) and are replaced
from time to time. I got the real impression that the
aerial shoots scattered over a large area (about a
hectare) could well prove to be interconnected—if
one could only have X-ray vision. Like the Eugenia
capensis subsp. gueinzii (Coast myrtle) mentioned
below, these clonal colonies must take at least
hundreds of years to establish over such large areas.
Herbaceous clones of non-grassy herbs (‘wild
flowers’) are quite common in fire-prone grassland
and are provided by several families, including
Asteraceae (Helichrysum), Euphorbiaceae
(Acalypha), Fabaceae (Eriosema, Indigofera),
Lamiaceae (Becium) and Rubiaceae (Pentanisia). The
Pondoland Centre endemic, Helichrysum pannosum
(Pondo everlasting), forms large clones several
metres across. A species such as Eriosemopsis
subanisophylla (Mock eriosema), however, seems to
qualify as a true geosuff, the heavy woody rootstock
of this plant and multiple shoots (note that in the
picture of this plant most of the aerial shoots had
been broken off during the excavation due to the
very brittle nature of the species) have a woody
appearance, but the species has no true arborescent
relatives as it is the only member of the genus. It
is nevertheless closely related to other groups of
woody Rubiaceae that include many proper trees.
The potential reasons put forward by the various
authors to explain the evolution of geosuffs from
arborescent ancestors are several and these
include: extreme cold, aridity, water logging,
edaphic constraints, herbivory, fire and soil fertility
(or lack thereof). Burtt Davy points out that a tree
could not invade grassland and then evolve a
suffrutescent habit. The ecotones of forest margins,
however, are dynamic habitats with adventurous
woody plants attempting to push out from the forest
into the grassland. Could these ecotones present
an opportunity for the evolution of geosuffs? A
walk in spring along our forest margins reveals
several otherwise arborescent species putting out
shoots in the ecotone, the endemic Rhynchocalyx
lawsonioides (Mock umdoni) being one of them. The
two species mentioned earlier, Diospyros scabrida
(Coastal bladdernut) and Ochna natalensis (Natal
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DENDRON » No/Nr: 43 » November 2011