Rangelands: Monitoring land degradation with long-term satellite data¶
Konrad J. Wessels (2011). Rangelands: Monitoring land degradation with long-term satellite data. Observations on Environmental Change in South Africa (States and Trends in the Terrestrial Environment chapter).
| Type | book chapter |
| Scope | Wider apartheid literature |
| Method | descriptive quantitative |
| Period studied | 1913–2005 |
| Regions | South Africa, Limpopo province, former homelands / communal areas |
| Themes | bantustans removals influx, agriculture land, methods data sources, inequality poverty unemployment |
| Citation key | wessels2011 |
Argument¶
Land degradation in South Africa's former homelands (now communal areas) is principally a result of high population densities and apartheid-era forced resettlement, not of traditional communal pastoralism. Earlier degradation maps rested on expert opinion or single-date imagery and could not track change over time. The author uses 18 years of AVHRR NDVI data (1985-2003), plus MODIS, with rainfall surfaces to compare degraded and adjacent non-degraded rangelands with the same soils and climate. Degraded areas show consistently lower vegetation production per unit rainfall, yet they support many livestock and appear functionally stable, so catastrophic collapse seems unlikely. The method is presented as a regional indicator that needs field validation.
Evidence and method¶
National Review on Land Degradation survey (453 extension workers, 367 magisterial districts); the 1995-96 National Land Cover map from Landsat TM; AVHRR NDVI at 1 km2 resolution (1985-2003); MODIS data (2000-2005); rainfall surfaces; helicopter and field validation by the Department of Agriculture.
Place in the debates¶
- Causes of land degradation in the homelands: overgrazing by communal pastoralism vs. apartheid-induced overcrowding (see Agricultural policy, productivity, farm size and land reform). Attributes degradation mainly to high population densities aggravated by the oppressive apartheid system, not to traditional communal pastoralism.
- Whether degraded rangelands face catastrophic collapse (see Agricultural policy, productivity, farm size and land reform). Argues degraded areas look functionally stable and resilient, so there is no apparent danger of the historically predicted collapse; economic loss is undetermined.
- The 'expanding Karoo' theory (see Agricultural policy, productivity, farm size and land reform). Notes that an extensive 1990s review found no evidence for it.
Findings¶
-
The homelands were created under the 1913 and 1936 Land Acts, and apartheid-era forced resettlement confined Africans to them, leading to unsustainable land use. (p. 1)
Stable communities were uprooted and compelled to settle in areas where the unsustainable land use degraded the local resource base upon which their rural livelihoods depended.
-
Degradation is attributed to population density and apartheid rather than to communal pastoralism. (p. 1)
The degradation observed in the communal lands is thus principally a consequence of high population densities aggravated by the oppressive apartheid system rather than the outcome of traditional communal pastoralism.
-
The underlying drivers are socio-economic and institutional. (p. 1)
The underlying cause of degradation is a combination of unemployment, poverty and an absence or failure of land-use regulation.
-
The National Land Cover map found a small share of the country degraded, mostly within communal areas. (p. 2)
4,8% (5,8 million ha) of the country was mapped as degraded.
-
Most large continuous degraded areas lie in communal areas. (p. 2)
Most of the large continuous areas of degradation mapped by the NLC occur within the communal areas.
-
Degraded areas have consistently lower vegetation production than adjacent non-degraded areas regardless of rainfall variation. (p. 2)
non-degraded areas had consistently higher vegetation production as indicated by the seasonal total of NDVI obtained from satellite observations despite a six-fold variation of annual rainfall
-
Degraded areas appear stable and resilient, with no sign of catastrophic collapse. (p. 3)
they consistently support large numbers of livestock and results suggest that they are functionally stable and resilient. Therefore, there does not appear to be any danger of a catastrophic collapse
-
Satellite monitoring cannot detect degradation that predates the 1985 record, which explains why some known degraded areas show no negative trend. (p. 3)
since the homelands were created as long ago as 1913 or 1936, much of the degradation could have occurred before the start of the satellite record in 1985 and may not have worsened since.
-
Negative trends in Limpopo were largely associated with communal lands. (p. 3)
In the Limpopo province, negative trends were largely associated with the degraded communal lands, although some well-known degraded areas did not show continued negative trends
Evidence ledger¶
Sources used¶
| Source | Kind | Period | How used |
|---|---|---|---|
| National Review on Land Degradation (NRLD) survey of 453 extension workers and conservation technicians on 367 magisterial districts | author survey or fieldwork | c. 1990s-2000s (not specified) | Cited as an earlier perception-based survey that identified severe degradation in former homelands/communal areas, and as a baseline that is not repeatable. |
| National Land Cover map (NLC) 1995-96, from single-date Landsat TM imagery | maps spatial data | 1995-96 | Supplies the mapped 'degraded' classes (4.8% of the country, mostly in communal areas), which serve as the degraded study areas compared against non-degraded neighbours. |
| AVHRR NDVI data, 1 km2 resolution, NOAA satellite series, 1985-2003 | maps spatial data | 1985-2003 | Seasonal total NDVI gives the long-term vegetation production of degraded and non-degraded rangelands and the trends in production per unit rainfall. |
| MODIS satellite data 2000-2005 | maps spatial data | 2000-2005 | Supplementary five-year NDVI record used alongside AVHRR to compare degraded and non-degraded areas. |
| Rainfall surfaces (gridded rainfall data) | official statistical series | 1985-2003 | Used with NDVI to compute vegetation production per unit rainfall and to separate human-induced degradation from drought effects. |
| Department of Agriculture helicopter and field verification visits (2008) | author survey or fieldwork | 2008 | Informal validation of the satellite degradation product, with positive results in some areas and conflicting results in others. |
| Natives Land Acts of 1913 and 1936 | legislation gazette | 1913, 1936 | Cited as the legal origin of the homelands, which provides the historical context for the degradation and dates the start of possible degradation. |
New data¶
Author-processed analysis of 18 years of AVHRR NDVI (1985-2003) and 5 years of MODIS NDVI, producing paired degraded vs non-degraded NDVI time series for Limpopo and a national map of trends in NDVI per unit rainfall for the summer rainfall region.
Inference¶
Quantification: Descriptive tables.
Remote-sensing comparison: matched degraded and adjacent non-degraded rangelands with the same soils and climate are compared over time, and rainfall-adjusted NDVI trends are mapped. Historical attribution to apartheid-era settlement is asserted rather than tested, and validation is partial.
Key claims¶
| Claim | Basis | Strength |
|---|---|---|
| Degraded communal rangelands in Limpopo have consistently lower vegetation production (seasonal NDVI) than adjacent non-degraded areas with the same soils and climate, despite a six-fold variation in annual rainfall. (p. 2) | AVHRR NDVI 1985-2003 and MODIS 2000-2005, with degraded areas taken from the NLC map, compared against adjacent areas. | Partial data |
| Degraded areas appear functionally stable and resilient, with no danger of catastrophic collapse, because they still support large numbers of livestock. (p. 3) | The stable NDVI gap over time, plus the assertion that livestock numbers are large. No livestock data are presented. | Illustrative or anecdotal |
| In Limpopo, negative trends in vegetation production per unit rainfall were largely associated with degraded communal lands, though some well-known degraded areas show no continued decline. (p. 3) | AVHRR NDVI and rainfall surfaces 1985-2003, trend map (Fig. 3.39). | Partial data |
| Degradation in communal lands is principally a consequence of high population densities aggravated by the apartheid system, not of traditional communal pastoralism. (p. 1) | Asserted by reference to the NRLD and earlier literature, with no direct data analysis in the chapter. | Asserted |
| Much of the degradation may predate the 1985 start of the satellite record and may not have worsened since, which explains why not all degraded areas show negative trends. (p. 3) | Reasoning from the dates of homeland creation (1913/1936) and the satellite record start. No pre-1985 data are used. | Asserted |
| The NLC mapped 4.8% (5.8 million ha) of the country as degraded, with most large continuous areas of degradation falling within communal areas. (p. 2) | NLC map from 1995-96 Landsat TM imagery, photo-interpretation and fieldwork. | Partial data |
| The earlier 'expanding Karoo' theory of degradation from commercial sheep overgrazing found no support in an extensive 1990s review. (p. 1) | Cited secondary review (ref 3), with no data presented here. | Asserted |
The author's caveats¶
The author notes that remote sensing can only detect change within the time series and cannot capture pre-1985 degradation. Other land-cover and land-use change or natural processes can also reduce production per unit rainfall. A single-sensor method cannot unequivocally map degradation, and the trend maps need extensive field verification. Validation results are conflicting, and long-term field observations are lacking. The author also states that any economic loss from lower production remains to be determined.
Gaps in the evidence¶
The argument links degradation to apartheid-era resettlement but uses no historical data on population density, stocking rates, resettlement dates or land-use regulation, so the causal attribution is not tested. Long-term field data on vegetation, soils and livestock numbers, and pre-1985 aerial photography or archival records, would be needed to validate the satellite signal and capture earlier degradation. Some of these records exist (historical aerial photographs, agricultural census livestock counts), while SAEON-style long-term field plots only accumulate from now.
Replicability: Data described but not public.