Jag är universitetslektor i naturgeografi, med särskilda forskningsintressen i geomorfologi och marklära i tropiska miljöer. Jag har huvudsakligen arbetat i Östafrika med nedanstående forskningsprojekt:
- Jordskred och markanvändning i Kenyas och Tanzanias högländer
- Pre-koloniala bevattningssystem i Kenya, Tanzania och Sydafrika
- Naturresursutnyttjande och mangroveekologi i Rufijideltat, Tanzania
- Geomorfologisk utvecklingshistoria i Saveflodens delta, Mocambique
I forskningen samverkar jag med kulturgeografer, arkeologer, humanekologer och hydrologer.
Jag undervisar numera främst på Kandidatprogrammet i geografi, där jag ansvarar för undervisning i regionalgeografi, geovetenskaplig riskanalys och naturresurser/resursutnyttjande.
Jag undervisar också i marklära och geomorfologisk processlära, och jag har tidigare varit ansvarig för avancerade kurser om mark och vatten, tropisk miljö och tropisk naturresursshushållning.
I urval från Stockholms universitets publikationsdatabas
Environmental variability in northern Tanzania from c. AD 1000 to 1800, as inferred from diatoms and pollen in Lake Duluti
Helena Öberg (et al.).
Fossil pollen and diatoms have been analysed in a sediment sequence from a topographically closed crater lake in northern Tanzania(Lake Duluti), with the aim to reconstruct past changes in lake level and vegetation dynamics. The results contribute with a new paleoenvironmental record from equatorial Africacovering the period c. AD 1000 to AD 1800. Overall, the pollen and diatom records generate comparative stories of dry and wet periods. Dry conditions are inferred at c. AD 1040–1470, c. AD 1510–1640 and c. AD 1650–1670 with the lowest lake levels at c. AD 1260–1290 and AD 1600–1640. Wetter conditions occurred c. AD 1640–1650 and c. AD 1670–1790. The chronology is based on combined analyses of 210Pb activity and AMS 14C on bulk sediment, and a Bayesian model was applied to establish the age-depth relationship. The hydroclimatic record fromLakeDuluti shows good correlation with several East African lakes although precise comparison is hampered by dating uncertainties.
Multi-decadal L-band SAR observations of the Mangrove forests of the Rufiji Delta
Ian Brown, Simon L. A. Mwansasu, Lars-Ove Westerberg.
Mangroves are important habitats that face a range of threats, natural and anthropogenic. Synthetic aperture radar (SAR) images from 1994 to 2010 have been used to identify systematic changes in mangrove forest vegetation in the Rufiji Delta, Tanzania. The mangrove forest is a Forest Reserve and is protected from large scale exploitation, though there are settlements within the reserve. A dataset of five L-band SAR images spanning a 16-year time period was processed to identify spatio-temporal changes in mangrove forest extent and composition. SAR signatures are related to changes in water budget. The image data show minor expansion in cultivated land along the margins of established communities in the delta. Thinning of mangroves is detected on higher ground whilst along creeks and river channels SAR backscatter indicates an increase in biomass. Sea level height is found to exert a stronger influence on backscatter than minor differences in seasonality. Despite inaccuracies in older SAR images SAR time series are shown to provide valuable data on the spatio-temporal dynamics of East African mangrove forests.
Palaeogeography and dynamics of the deltaic wetland of Save River, Mozambique
Elidio Massuanganhe (et al.).
Many mangrove wetland systems in deltaic environments are negatively affected by massive sedimentation from river inflows. In this paper we use the example of the Save River delta to assess the palaeogeographic distribution of mangrove wetlands and to analyze their dynamics. To track past occurrences of mangrove wetlands in the study area we have integrated sedimentological data with siliceous microfossil analysis combined with AMS radiocarbon and OSL dating. The results show a fine-grained deposit with an approximate thickness of 2 m, present at different sampling sites. In the upper deltaic plain, the deposit is interbedded between sand layers, while in the lower deltaic plain the deposit occupies the uppermost stratigraphic position. In most of the sampling sites the deposit shows a succession with brackish-marine diatoms in the bottom while the upper part shows only scattered occurrences Based on sedimentological and microfossil characteristics we have interpreted the layer to represent a mangrove wetland deposit. The development of the deposit in the study area is suggested to have been initiated around 3100 cal. yrs BP, induced by sea-level rise. Thereafter, the development followed the combined effect of a sea-level fall and delta progradation processes. In some areas, particularly in the proximal part of the delta, the mangrove deposit has developed progressively on top of the delta-front. From around 1300 years ago (OSL) onwards, massive alluvial sedimentation impacted the mangrove ecosystem. However, the retreat of mangrove wetland coincided with a regional fall of sea-level. At the edges of the alluvial deposit, the current mangrove ecosystem has reclaimed the habitat in some sectors where gully erosion has exposed the once extinct mangrove habitat.
L-Band Polarimetric Target Decomposition of Mangroves of the Rufiji Delta, Tanzania
2016. Ian Brown, Simon Mwansasu, Lars-Ove Westerberg. Remote Sensing 8 (2)Artikel
The mangroves of the Rufiji Delta are an important habitat and resource. The mangrove forest reserve is home to an indigenous population and has been under pressure from an influx of migrants from the landward side of the delta. Timely and effective forest management is needed to preserve the delta and mangrove forest. Here, we investigate the potential of polarimetric target decomposition for mangrove forest monitoring and analysis. Using three ALOS PALSAR images, we show that L-band polarimetry is capable of mapping mangrove dynamics and is sensitive to stand structure and the hydro-geomorphology of stands. Entropy-alpha-anisotropy and incoherent target decompositions provided valuable measures of scattering behavior related to forest structure. Little difference was found between Yamaguchi and Arii decompositions, despite the conceptual differences between these models. Using these models, we were able to differentiate the scattering behavior of the four main species found in the delta, though classification was impractical due to the lack of pure stands. Scattering differences related to season were attributed primarily to differences in ground moisture or inundation. This is the first time mangrove species have been identified by their scattering behavior in L-band polarimetric data. These results suggest higher resolution L-band quad-polarized imagery, such as from PALSAR-2, may be a powerful tool for mangrove species mapping.
Precolonial agricultural terracing in Bokoni, South Africa
2016. Mats Widgren (et al.). Journal of African Archaeology 14 (1), 33-53Artikel
Earlier work on the terraced settlements of the Bokoni area (16th to 19th century, Mpumalanga province, South Africa) focussed on the homesteads, their contents, layout and chronology. This paper suggests a terminology and typology of agrarian structures in Bokoni in order to improve comparative approaches in Africa and beyond. The typology and an excavation of the terracing have made possible preliminary conclusions relevant for the further analysis of the terracing and stone-walling in Bokoni. The terracing developed incrementally, whereby cultivation, stone-clearing and terracing were intermixed processes. This is supported both by the organic content in a section of a terrace and by a phytolith analysis. The phytolith analysis furthermore indicated that maize was cultivated on the terraces, but this should be seen as a pilot study only, and presence of maize in Bokoni must be tested with other archaeobotanical methods.
Deltaic coasts under climate-related catastrophic events - Insights from the Save River delta, Mozambique
2015. Elídio A. Massuanganhe (et al.). Ocean and Coastal Management 116, 331-340Artikel
The deltaic coast of the Save River is characterized by mangrove wetland, one of the most important coastal ecosystems in Mozambique. This ecosystem provides direct services to the neighbouring communities and contributes to the productivity of the marine ecosystem. This region has, however, been hit by recurrent catastrophic events that have caused negative impacts on the ecosystem and in people's lives, posing challenges for its management. In this article we use this area as a case study to structure and propose an interactive and integrated approach for coastal zone management under recurrent climate-related catastrophic events. Our results show a need for systematic interaction between the decision makers (at the different levels) and the communities to set up adaptive measures for climate-related events. Also, we noticed that the presence of the neighbouring communities is a factor to capitalize on the adaptation activities by maximizing their participation as active actors in the process. Therefore, we conclude that a continuous process of adaptation and preparedness to climate-related catastrophic events (focused on both social and ecological systems) constitutes a leverage variable to be used for sustainable management of the coastal zones.
Biofuel potential and land availability
2014. Simon L. A. Mwansasu, Lars-Ove Westerberg. Journal of Ecology and the Natural Environment 6 (11), 389-397Artikel
Africa’s attractiveness to potential biofuel investors is based on the assumption that there is plenty of unused land available for investment in different countries of the continent. However, their postulations are not based on any concrete studies about land availability at country, regional or local level. This study investigates land availability for potential biofuel investment at the local level, using Rufiji district in Tanzania as a case study. We have analyzed different land cover/land use types and separated them into areas of potential biofuel investment and areas where biofuel investment is not possible by a process of elimination. The results suggest that land availability is inadequate to meet the needs of biofuel investors. The land assumed to be unused or underutilized by biofuel investors is either part of the fallow system or used to harvest natural resources and for other traditional uses. Expropriating the assumed idle land will adversely impact on the livelihoods of the local communities.
Labour, climate perceptions and soils in the irrigation systems in Sibou, Kenya & Engaruka, Tanzania
2014. Martina Angela Caretta (et al.).Rapport
This booklet presents the results of a 4 years project (2011-2015) by four geographers from the university of Stockholm. This research took place in two small villages: Sibou, Kenya and Engaruka, Tanzania. The overall project looks at three variables: soil, climate and labor. These aspects can give an indication of the type of changes that happened in these irrigation systems and what have been the triggers behind them. In this booklet results are presented according to location and focus on: agricultural practices, women´s and men´s labor tasks, soil and water characteristics, adaptation weather variability and how all of these aspects have changed over time.
Environmental variability in northern Tanzania from AD 1000 to 1800, as inferred from diatoms and pollen in Lake Duluti
2013. Helena Öberg (et al.). Palaeogeography, Palaeoclimatology, Palaeoecology 374, 230-241Artikel
Fossil pollen and diatoms have been analyzed in a sediment sequence from a topographically closed crater lake in northern Tanzania (Lake Duluti), with the aim to reconstruct past changes in lake level and vegetation dynamics. The results provide a new paleoenvironmental record from equatorial Africa covering the period c. AD 1000 to AD 1800. Overall, the pollen and diatom records generate comparable stories of dry and wet periods. Dry conditions are inferred at c. AD 1040-1470, c. AD 1510-1640 and C. AD 1650-1670 with the lowest lake levels at c. AD 1260-1290 and AD 1600-1640. Wetter conditions occurred c. AD 1640-1650 and c. AD 1670-1790. The chronology is based on combined analyses of Pb-210 activity and AMS C-14 on bulk sediment, and a Bayesian model was applied to establish the age-depth relationship. The hydroclimatic record from Lake Duluti shows good correlation with several East African lakes in a centennial time perspective, although comparison of high frequency variability in the region is hampered by dating uncertainties.
A diatom record of recent environmental change in Lake Duluti, northern Tanzania
2012. Helena Öberg (et al.). Journal of Paleolimnology 48 (2), 401-416Artikel
Lake Duluti is a small, topographically closed crater lake located on the flanks of Mt Meru, northern Tanzania. Analyses of diatoms in three short sediment cores and four modern samples from Lake Duluti were used to infer past environmental changes. 210Pb and 137Cs activity profiles combined with AMS 14C dates provide the chronological framework. Weak agreement between the 210Pb and 14C records, together with dating uncertainty, precludes construction of precise age models. The modern diatom flora, from plankton and three periphytic habitats, is dominated by Aulacoseira ambigua (Grunow) Simonsen, Gomphonema parvulum (Kützing) Grunow and Nitzschia amphibia Grunow. All three cores display similar stratigraphic succession, but the relative ratio of habitats represented by the diatoms varies substantially between cores. Diatoms indicate that the oldest part of the record is characterized by relatively low lake level and swampy vegetation. In the late nineteenth or early twentieth century there was a rapid lake level rise and the swamp turned into an open-water lake. High lake levels have prevailed since that time.
The development of the ancient irrigation system at Engaruka, northern Tanzania
2010. Lars-Ove Westerberg (et al.). Geographical Journal 176 (4), 304-318Artikel
Climate data from Empakaai Crater in northern Tanzania, covering the last 1200 years, are related to the establishment, development and decline of the ancient irrigation system at Engaruka. New dates for the system are linked to reconstructed climatic variations and historical data on long-distance and regional trade and migration patterns. A shift from a comparatively humid climate to drier conditions in the 1400s prompted the establishment of irrigated agriculture at Engaruka, and a flourishing long-distance trade increased its value as a water and food source for passing caravans. Once established, the land-use system at Engaruka was sufficiently resilient to survive and even intensify during much drier climate from c. 1500 to 1670 CE (Common Era) and during the decline of caravan trade between c. 1550 and 1750. The ancient land-use system probably reached its maximum extension during the humid conditions between 1670 and 1740, and was deserted in the early to mid 1800s, presumably as a result of the added effects of climate deterioration, the Maasai expansion, and change of livelihood strategies as agriculturalists became pastoralists. Towards the end of the 1800s irrigated agriculture was again established at Engaruka, in part driven by the transfer from pastoral to agricultural livelihoods caused by the Rinderpest.
Soil management and soil properties in a Kenyan smallholder irrigation system onnaturally low-fertile soils
2018. Martina Angela Caretta (et al.). Applied Geography 90, 248-256Artikel
In this study we examine the impact of soil management practices on soil properties in a landscape with naturally relatively poor soils on and below the dry slopes of a Rift Valley escarpment in Kenya that have been dominated by extensive smallholder investments in canal irrigation over the last 300 years. We show that farmers in the area have been able to keep up agricultural production in the face of growing population. The actual practices of soil management at one moment in time appear to be of minor importance to soil improvement, as indicated by the low correlation between Soil Management Index (SMI) and soil chemical data. However, cultivation triggers a process of slow soil improvement manifested by a positive correlation between nutrient levels and duration of irrigated cultivation and soil management, which likely explains farmers' confidence in soil productivity. However, we also identify sodicity as a risk connected to intensified irrigation in the area. Finally, we stress the need for further studies integrating investigations of local irrigation and soil management with soil and water quality analyses. These will be crucial to shape sustainable place-based and farmer-led solutions for African agricultural growth.
Late Holocene sea-level changes and paleoclimate recorded in Lake Lungué, southern Mozambique
2017. Sandra Raúl Sitoe (et al.). Palaeogeography, Palaeoclimatology, Palaeoecology 485, 305-315Artikel
We report on late Holocene paleoenvironmental and sea-level changes in southern Mozambique, based on analysis of diatom stratigraphy, mineral magnetic susceptibility, Saturation Isothermal Remnant Magnetization (SIRM) and organic carbon content in a sediment core from Lake Lungué, located ca. 30 km north of Xai Xai City in the Limpopo River floodplain. Eleven radiocarbon dates performed on terrestrial shells allowed establishment of an age-depth model. High content of brackish-marine taxa, especially Diploneis suborbicularis and Navicula yarrensis, suggests that the Lake Lungué basin was part of the Indian Ocean coastal zone between ca. 740 and 910 CE, suggesting higher relative sea-level during this phase. A similar diatom distribution in older parts of the sequence, of unknown age, indicates that the site was connected to the Indian Ocean also at some stage prior to 740 CE. Between ca. 910 and 1130 CE the basin was under a combined effect of sporadic marine water influx and Limpopo River actions. A freshwater lake was established sometime between 1130 and 1360 CE, when the basin became less affected by marine influences due to lower sea-levels. Instead, the lake system was more influenced by Limpopo River meandering dynamics and flooding events, and eventually isolated into a more or less independent lake ecosystem where diatom diversity most likely responded to lake levels fluctuations driven by shifts in relative humidity and rainfall amounts within the floodplain and also from upstream sources. During this phase, high abundance of freshwater planktonic species, i.e. Aulacoseira granulata and A. ambigua, indicates high lake levels and wetter conditions dated to ca. 1360–1560 CE. From 1560 CE until present, the lake has been subject to drier conditions and higher evaporation as indicated by lower lake levels, reported by a decline in freshwater planktonic taxa and increase in brackish taxa, e.g. Amphora robusta.
Vegetation dynamics within the savanna biome in southern Mozambique during the late Holocene
2017. Elin Norström (et al.). The HoloceneArtikel
This study explores temporal dynamics within grassland and Miombo woodland ecosystems in southern Mozambique and their potential coupling to hydro-climate change during the late-Holocene period. Palaeo-reconstructions are based on phytolith and diatom assemblages and mineral magnetic properties in fossil sediments from Lake Chilau, southern Mozambique. Phytolith interpretation was aided by previous ecological studies on modern plants and soils. The Lake Chilau record suggests high abundance of Panicoideae and other mesophytic grasses during the AD 1200s and 1300s, followed by an increase in Chloridoideae and grasses of more xerophytic affinity between ca. AD 1400 and 1550. This vegetation transition takes place during the early phase of the so-called ‘Little Ice Age’ (LIA), when regional palaeoclimate records report a shift from warmer and wetter towards drier and cooler conditions in southern Africa. Concurrent to these shifts within the grassland biome, the Chilau record reports an increase in phytoliths associated with arboreal vegetation (ca. AD 1400–1550), probably associated with the woody component of the Miombo savanna ecosystem. This supports previous studies hypothesizing that the forest component of the Miombo savanna was favoured by LIA dryness, although at Chilau, this expansion may have been amplified by a decline in fire disturbance. These tentative responses in the woody components of the savanna biome to shifts in moisture availability in the past have implications for future management and sustainability of the Miombo ecosystem in southern Mozambique under a changing climate.
Surface Area Variability of a North-Central Tanzanian Crater Lake
2016. Lindsey Higgins (et al.). Geosciences 6 (2)Artikel
A history of modern (1973–2015) surface area variability for Lake Basotu in north-central Tanzania has been reconstructed using archived Landsat images from the dry season between June and October. This record was compared to local weather data as well as larger scale weather patterns. The lake has been in a state of decline interrupted by major flood events since the beginning of the satellite record. From 1973 to 1997, the lake area was between 0.97 km2 and 4.28 km2. Lake extent abruptly increased to 13.86 km2 in 1998, when a co-occurrence of El Niño and a positive Indian Ocean Dipole led to extensive flooding. It is hypothesized that local agricultural practices leading to soil erosion and subsequent basin sedimentation have most likely increased the sensitivity of Lake Basotuto climatic fluctuations.