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Long-term land use change analysis in south-central Angola. Assessing the trade-off between major ecosystem services with remote sensing data
Abstract
Dry tropical forests are facing large-scale conversion and degradation processes and are the most endangered forest type worldwide. We analyse these processes in the dry tropical forest type of miombo woodlands in a rural area of south-central Angola. We show that large-scale conversion to agricultural areas takes place in this area, as does modification of woodland areas, i.e. by degradation due to the extraction of natural resources. By using remote sensing data, spatial drivers of this conversion and its effects may be assessed for the time period 1989-2013. We identify settlement dynamics and the location and quality of streets as major underlying determining factors for conversion processes. Since the 1980s, the rate of agricultural expansion has strongly depended on socioeconomic background factors and is currently on a level of ca. 9 000 ha/year in the study area. Fallows were found to only slowly regenerate, and there is a change in cultivation pattern to more permanent forms of cultivation. Large portions of the study area are undergoing degradation processes, leading to an additional loss of biomass. The results indicate that there is high pressure on the natural ecosystems of the study area, which will probably aggravate in the future with a high likelihood of emerging land use conflicts.
... However, the use of satellite imagery has made some general measures possible. Schneibel et al. (2018) and Mendelsohn (2019) provide syntheses of available knowledge, and demonstrate the widespread deterioration of forest and woodland cover, and of trends in soil erosion, in Angola. While much of the Huambo, Bié and Huíla planalto was cleared in the colonial era for crop farming, the rate of deforestation has accelerated since independence, especially since the peace accords of 2002, after which many displaced rural people returned to their original homesteads. ...
This Chapter provides an introduction to basic elements of soil science, from an understanding of the soil profile, its develop and its importance to plant growth. The processes of weathering and the development of laterites, calcretes, salinised and other major soil types and their distribution in Angola are described. Soil water relations and soil chemistry and thus the availability of water and nutrients are fundamental determinants of plant growth, species composition and productivity. The differences between dystrophic (low base status) and eutrophic (high base status) soils and the distribution of the mesic/dystrophic savanna biome and the arid/eutrophic savanna biome, which dominate Angolan landscapes (totaling over 90% of the vegetation mantle of the country) are emphasised. The Key Soil Groups of Angola are mapped and their characteristics summarised. Sandy arenosols cover 53% of Angola, mainly comprising the Kalahari sands of the eastern half of Angola. Ferralsols cover 23% of Angola, occupying the spine of crystalline rocks along the western highlands. Both are of low nutrient status but carry dense miombo woodlands where they have not been transformed by human activities. Richer soils occur along the escarpment and hot coastal lowlands. The processes of land degradation, due to inappropriate soil management threaten the livelihoods of communities living on these fragile soils, are described.
... Second is the clearing of woodlands around urban areas. Many had already been cleared of tree cover by 2000, after which clearings expanded as trees were removed progressively further from the town centres, a trend illustrated by Schneibel et al. (2018). Examples of recent clearings between 2000 and 2015 are conspicuous as 'red bands' around Dundo, Menongue, Luena, Malange, Cafunfo, Cubal and Caimbambo in Fig. 8.1. ...
Landscape changes in Angola are dominated by woodland and forest losses due to clearing for crops, bush fires (which convert woodland into shrubland) and the harvesting of fuel (as wood and charcoal) and timber. Rates of clearing for small-scale dryland crops are high over much of Angola as a result of poor soil fertility. Erosion is also a severe problem, which has caused widespread losses of topsoil, soils nutrients and ground water. Rates of erosion are greatest in areas with steep slopes, sparse plant cover and high numbers of people, as well as around diamond mines in Lunda-Norte. Patterns of river flow and water quality have been changed, largely as a result of soil erosion and plant cover loss, as well as large irrigation schemes and dams. High rates of urban growth and the production of untreated urban waste have led to large concentrations of contamination around towns. Further research is needed, for example to assess the environmental impacts of the fishing and petroleum industries offshore, the effects of large volumes of urban waste being washed into and down major rivers to the sea, and landscape changes in an around areas of highland forests and grasslands that support populations of rare and endemic species.
... Pröpper et al. 2015). The main drivers of deforestation and degradation of woodlands and the general loss of pristine vegetation cover in Angola are the clearing of new fields for shifting cultivation, industrialised agricultural schemes and the production of charcoal (Cabral et al. 2010;Hansen et al. 2013;Schneibel et al. 2013Schneibel et al. , 2016Schneibel et al. , 2018Röder et al. 2015;Wallenfang et al. 2015;Mendelsohn 2019). Without adequate knowledge of the spatial distribution and extent of vegetation types, their species composition and the environmental drivers of vegetation patterns (climate, geology, soils, landuse) sound landuse management is not feasible. ...
Spatial information about plant species composition and the distribution of vegetation types is an essential baseline for natural resource management planning. In Angola, the first countrywide vegetation map was elaborated by Gossweiler in 1939. Subsequently, Barbosa published a revised map with much higher detail in 1970 and his work has remained the main reference for the vegetation of Angola until today. However, these early maps were expert drawn and were not based on systematic surveys. Instead, the delimitation of vegetation units was based on many years of field observations and also incorporated results of local studies carried out by other authors. In spite the rich history of the scientific exploration of Angola’s vegetation in colonial times, quantitative and plot based studies were rare. After the end of the armed conflict, new vegetation surveys making use of new methodological developments in numerical approaches to vegetation classification in combination with modern remote sensing imagery have provided spatial information of unprecedented detail. However, vast areas of the country still remain seriously understudied. At the same time, sustainable land management strategies are urgently needed due to the increasing pressure on natural resources driven by socio-economic development and global change, thus calling for a new era of vegetation surveys that will enable data-based landuse and conservation planning in Angola.
... Of particular interest is the impact of precipitation patterns preceding the 2013 period, which caused the signifi cant extension of seasonal fl ooding areas and of Lake Liambezi, with an associated disappearance of all agricultural fi elds that had still been there in the earlier period. Again, this is a temporal phenomenon with process length corresponding to mid-term precipitation variation, and which might be better resolved using continuous time series (see for instance Schneibel et al., 2018). ...
In southern African drylands, an important driver of deforestation is the ongoing conversion of woodland to smallholder agriculture. Our study in NE Namibia and SW Zambia evaluated the potential of operational earth observation satellites to characterize land-use change processes and quantifi ed their impact on soil organic carbon (SOC) and nutrient concentrations. We found that the area under agricultural use increased by 24% from 2002 to 2013, mainly at the expense of natural vegetation (i.e., woodland). This conversion caused a decline in SOC and total N and tended to increase plant-available P in the soils of old agricultural fields. The eff ects were most pronounced in NE Namibia, where the total SOC stocks were 19.6% (±18.4 SD) lower in agricultural land compared to woodland. Moreover, the losses in SOC and total N tended to result in a decline of predicted maize yields calculated with the QUEFTS model by ~15% when comparing soils of old agricultural fi elds and woodland. Overall, our results indicate that long-term continuation of low-input arable farming can reduce soil fertility.
The distribution and density of people in the highlands and escarpments of Angola and Namibia (HEAN) is largely a product of proximity to urban areas, climate and soil fertility. The highest rural densities in HEAN are in central Angola, and the lowest in southern Namibia. In northern and central Angola most rural households grow crops for domestic consumption and sale, whereas pastoralism prevails in the HEAN areas of southern Angola and northern Namibia. Remittances, social grants and revenues from tourism provide most household income in northern Namibia. Angola is divided and administered via provinces, munícipios and comunas, whereas Namibia is administered through regions and local authorities for urban areas. Shifting cultivation has led to the clearing of large areas of forest, woodland and grassland. Other major human impacts are the harvesting of trees for charcoal and timber, the hunting of wildlife for the sale of bushmeat, soil erosion, and the loss of forests and woodlands and soil nutrients as a result of frequent fires.
The spatial extension of the countries covered by SASSCAL, the diversity of their landscapes, and the range of social and ecological processes, constitute a challenge to environmental research. The latter have sometimes needed to focus on small test sites for very specific questions, or else required data and methods that allowed large area assessments. In either situation it is important that the studies are founded on consistent and comparable data. Responding to this requirement, a range of products based on operational earth observation satellite systems has been developed in the frame of SASSCAL. Here, we introduce the most relevant primary and derived products at coarse (250-500 m MODIS) and medium (30 m Landsat) spatial resolution, describe their basic properties, and provide examples of application as an impetus for further research. At the same time, alternative sources of data and advances in sensor systems offer high potential in complementing information from operational products, or provide further insights into specific local questions. We thus briefly touch upon the potential of such systems, including active sensing and/or airborne technologies such as Synthetic Aperture Radar, Light Detection and Ranging, use of Unmanned Aerial Vehicles (UAV), and hyperspectral imaging, and introduce studies carried under SASSCAL using these systems.
The spatial extension of the countries covered by SASSCAL, the diversity of their landscapes, and the range of social and ecological processes, constitute a challenge to environmental research. The latter have sometimes needed to focus on small test sites for very specific questions, or else required data and methods that allowed large area assessments. In either situation it is important that the studies are founded on consistent and comparable data. Responding to this requirement, a range of products based on operational earth observation satellite systems has been developed in the frame of SASSCAL. Here, we introduce the most relevant primary and derived products at coarse (250-500 m MODIS) and medium (30 m Landsat) spatial resolution, describe their basic properties, and provide examples of application as an impetus for further research. At the same time, alternative sources of data and advances in sensor systems offer high potential in complementing information from operational products, or provide further insights into specific local questions. We thus briefly touch upon the potential of such systems, including active sensing and/or airborne technologies such as Synthetic Aperture Radar, Light Detection and Ranging, use of Unmanned Aerial Vehicles (UAV), and hyperspectral imaging, and introduce studies carried under SASSCAL using these systems.
Dry tropical forests undergo massive conversion and degradation processes. This also holds true for the extensive Miombo forests that cover large parts of Southern Africa. While the largest proportional area can be found in Angola, the country still struggles with food shortages, insufficient medical and educational supplies, as well as the ongoing reconstruction of infrastructure after 27 years of civil war. Especially in rural areas, the local population is therefore still heavily dependent on the consumption of natural resources, as well as subsistence agriculture. This leads, on one hand, to large areas of Miombo forests being converted for cultivation purposes, but on the other hand, to degradation processes due to the selective use of forest resources. While forest conversion in south-central rural Angola has already been quantitatively described, information about forest degradation is not yet available. This is due to the history of conflicts and the therewith connected research difficulties, as well as the remote location of this area. We apply an annual time series approach using Landsat data in south-central Angola not only to assess the current degradation status of the Miombo forests, but also to derive past developments reaching back to times of armed conflicts. We use the Disturbance Index based on tasseled cap transformation to exclude external influences like inter-annual variation of rainfall. Based on this time series, linear regression is calculated for forest areas unaffected by conversion, but also for the pre-conversion period of those areas that were used for cultivation purposes during the observation time. Metrics derived from linear regression are used to classify the study area according to their dominant modification processes. We compare our results to MODIS latent integral trends and to further products to derive information on underlying drivers. Around 13% of the Miombo forests are affected by degradation processes, especially along streets, in villages, and close to existing agriculture. However, areas in presumably remote and dense forest areas are also affected to a significant extent. A comparison with MODIS derived fire ignition data shows that they are most likely affected by recurring fires and less by selective timber extraction. We confirm that areas that are used for agriculture are more heavily disturbed by selective use beforehand than those that remain unaffected by conversion. The results can be substantiated by the MODIS latent integral trends and we also show that due to extent and location, the assessment of forest conversion is most likely not sufficient to provide good estimates for the loss of natural resources.
Fire spread information on a large scale is still a missing key layer for a complete description of fire regimes. We developed a novel multilevel object-based methodology that extracts valuable information about fire dynamics from Moderate Resolution Imaging Spectroradiometer (MODIS) burned area data. Besides the large area capabilities, this approach also derives very detailed information for every single fire regarding timing and location of its ignition, as well as detailed directional multitemporal spread information. The approach is a top–down approach and a multilevel segmentation strategy is used to gradually refine the individual object membership. The multitemporal segmentation alternates between recursive seed point identification and queue-based fire tracking. The algorithm relies on only a few input parameters that control the segmentation with spatial and temporal distance thresholds. We present exemplary results that indicate the potential for further use of the derived parameters.
Large parts of sub-Saharan Africa are experiencing rapid changes in land use and land cover, driven largely by the expansion of small-scale shifting cultivation. This practice creates complex mosaic landscapes with active agricultural fields and patches of mature woodland, interspersed with remnant patches in various stages of regrowth. Our objective here was to examine the rate and extent to which carbon stocks in trees and soils recover after cultivation, and detail how this disturbance and regrowth affect patterns in tree species composition and diversity over 40 years of succession in a miombo woodland landscape in southeast Tanzania. We sampled 67 areas, including plots previously cleared for cultivation, active fields, and mature woodlands for reference purposes. Sites were further stratified by soil texture to test for associated effects. Tree carbon stocks accumulated at an average rate of 0.83 +/- 0.10 Mg C.ha(-1).yr(-1), with soil texture having no clear impact on accumulation rates. Bulk soil carbon stocks on both soil types appeared unaffected by both the initial land clearance and the subsequent regrowth, which resulted in no significant changes over time. Tree species diversity in regrowing plots developed rapidly and within similar to 10 years was equivalent to that of mature woodland. Many of the species found in mature woodlands reappeared relatively quickly after abandonment, although species composition is expected to take considerably longer to recover, with at least 60-80 years required for the compositional similarity between regrowing and mature woodlands to reach levels similar to that among nearby mature woodlands. Through impacts on beta-diversity, disturbance was also found to increase the total number of tree species present in the landscape, with many of the recorded species only found in regrowing woodlands. Our results are of relevance to carbon sequestration projects by helping to inform the potential future carbon and biodiversity benefits of restoring disturbed habitats (REDD+). At a time where the use of shifting cultivation is threatened by shifts to larger-scale, commercial agriculture, we show that secondary woodland habitats can retain considerable biodiversity value, and act as carbon sinks.
This study aimed at developing allometric models from destructive sample field data for estimating both aboveground and belowground tree biomass and assessing changes in root biomass after old-growth Brachystegia-Julbernardia (miombo) woodland clearing in central Zambia. Logarithmic linear models were selected for estimating tree biomass because they gave the most accurate (low mean error) predictions. On average aboveground and belowground biomass in regrowth woodland represented 29% and 41%, respectively, of the biomass in old-growth woodland. The root:shoot ratios were 0.54 and 0.77 in old-growth and regrowth woodland, respectively. Ten years after clear-cutting old-growth woodland, root biomass loss was about 60% of the original biomass. The main cause of post clearing root biomass loss was fire which at the study sites occurred annually or biannually. Control of fire in cleared sites should be encouraged in forest management for carbon storage and sequestration in miombo woodland of southern Africa.
Tropical dry forests provide globally important ecosystem services and host exceptionally high biodiversity. These biomes are currently under immense pressure, particularly for conversion to agriculture, and already experience high global deforestation rates. Miombo forests in Southern Angola are affected by deforestation, fragmentation and degradation, caused mainly by an increasing rural population who follows a traditional farming system of shifting cultivation with slash-and-burn agriculture. After the termination of the civil war in 2002, population growth and resettlements have accelerated the use of woody resources, selective logging and clearing for cultivation purposes and led to an exceedance of sustainability thresholds. Large scale projects are expected to put further pressure on the forests and increase the potential of conflicts regarding land resources and competition with local subsistence farming.
The repopulation of abandoned areas in Angola after 27years of civil war led to a fast and extensive expansion of agricultural fields to meet the rising food demand. Yet, the increase in crop production at the expense of natural resources carries an inherent potential for conflicts since the demand for timber and wood extraction are also supposed to rise. We use the concept of ecosystem services to evaluate the trade-off between food and woody biomass. Our study area is located in central Angola, in the highlands of the upper Okavango catchment. We used Landsat data (spatial resolution: 30×30m) with a bi-temporal and multi-seasonal change detection approach for five time steps between 1989 and 2013 to estimate the conversion area from woodland to agriculture. Overall accuracy is 95%, user's accuracy varies from 89-95% and producer's accuracy ranges between 92-99%. To quantify the trade-off between woody biomass and the amount of food, this information was combined with indicator values and we furthermore assessed biomass regrowth on fallows. Our results reveal a constant rise in agricultural expansion from 1989-2013 with the mean annual deforestation rate increasing from roughly 5300ha up to about 12,000ha. Overall, 5.6% of the forested areas were converted to agriculture, whereas the FAO states a national deforestation rate for Angola of 5% from 1990-2010 (FAO, 2010). In the last time step 961,000t per year of woodland were cleared to potentially produce 1240t per year of maize. Current global agro-economical projections forecast increasing pressure on tropical dry forests from large-scale agriculture schemes (Gasparri et al., 2015; Searchinger and Heimlich, 2015). Our study underlines the importance of considering subsistence-related change processes, which may contribute significantly to negative effects associated with deforestation and degradation of these forest ecosystems.