UNCCD 2nd Scientific Conference

1. Too Many, Too few or the wrong
trees
A southern African case study on one aspect of
desertification for the World Atlas on
Desertification (WAD)
10 April 2013
Graham von Maltitz (CSIR)
With contributions from Timm Hoffman

2. Why focus on trees?
• Trees are only one aspect of land degradation in southern Africa
• The loss of trees is typically a sign of degradation and planting of trees is
often a strategy to counter degradation,
BUT increased trees can also be a sign of degradation and carry an
economic cost
• Unique considerations for mapping
Mapping and understanding the dynamics of tree-based
degradation – a case study from southern Africa for the WAD
• Alien invading tree: the wrong trees
• Encroaching tree: too many trees
• Overharvesting: The loss of trees

3. Invasive alien trees (IAP)
• Biggest threat to biodiversity after direct land use
change
• Reduce runoff of South Africa by an estimated 7%.
Use 13% of available surface water.
• Cover 10% of surface area
• Change fire leading to soil destruction
• ~ US$ 50 million / year on public works based
eradication programme BUT too little to solve
problem
• Some benefits such as fuelwood, dune stabilisation

5. Mapping IA trees
• National Invasive Alien Plant Survey (NIAPS, Kotzé et
al. 2010)
• Homogenous environmental units
• Systematic aerial photography subsamples of 100 X
100m
• Mean percentage cover and frequency of the IAP
• Estimate biomass based on case study data (Le
Maitre and Forsyth 2011)
• .

6. Estimated biomass of Invasive alien trees in South Africa and
Swaziland (le Maitre and Forsyth 2011)

7. Bush encroachment – too many trees
• Thickening of trees within the savanna biome and
invasion of trees into grassland
• Occurring globally, a major problem in Namibia,
Botswana, Zimbabwe and arid areas of South Africa
• Reduces grazing capacity – millions of ha of grazing
lost – up to 10 fold reduction
• Cost to clear cannot be recovered from clearing
• Reduced hydrology
• Biodiversity loss
• Soil erosion

8. A BUSH ENCROACHED EAST
1955
Rate of increase in woody plant
thickening has been
substantial especially in last 20
years as small scale farmers
have abandoned cultivated
fields
Slide supplied by Timm Hoffman
2011
1993
(near King William’s Town)

9. 1957 WHY BUSH THICKENING?
Extent of thickening is strongly
influenced by land use history at a
site (e.g. abandoned fields such as
in this photograph taken near Kei
River Bridge).
However, several studies have also
shown that woody thickening has
occurred irrespective of land
2010 tenure (communal, private,
conservation) which suggests a
global driver such as CO2 might be
important.
Rate of thickening in reality might
be far greater than predicted from
global change models, certainly for
lowland savanna sites.
Slide supplied by Timm Hoffman

10. Attempting to map bush
encroachment
Sankaran et al 2005)
Based on Bester 1999

11. Deforestation
• Rates of deforestation country dependent and
site dependent
• A diversity of drivers, most linked to poverty:
– Clearing for agriculture (including slash and burn)
– Overharvesting for fuelwood
– Charcoal production
– Changed fire regimes
• Now mostly on communal land
• Some from commercial agriculture

12. Closing thoughts
• Whilst deforestation remains a key concern to the
region.
• Increased trees through IAPs and bush
encroachment is also a major form of degradation
• Land tenure and land use partly, but not fully
explains the difference
• Mapping is complex and difficult.
• Results from mapped trends may be counter intuitive

13. Thank you