Graham von Maltitz "Too few, too many or the wrong type of trees - economic implications of changes in tree cover in Southern Africa"
Too Many, Too few or the wrong treesA 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
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 mappingMapping and understanding the dynamics of tree-baseddegradation – 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
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
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)• .
Estimated biomass of Invasive alien trees in South Africa andSwaziland (le Maitre and Forsyth 2011)
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
A BUSH ENCROACHED EAST1955 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 20111993 (near King William’s Town)
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 land2010 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
Attempting to map bush encroachment Sankaran et al 2005) Based on Bester 1999
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
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