Summary

Islands are particularly vulnerable to global change due to their limited size and remoteness. Sustainable land use practices based on nature-based solutions can help local communities to adapt their agricultural systems to climate change. This doctoral thesis research focused on Santa Cruz Island located in the Galapagos Archipelago-. About 72% of its continental surface area is nowadays protected. On the island’s windward side, the non-protected area was originally designated for the development of a self-sufficient rural community After characterising the land use dynamics over the last 60 years, this study analysed the potential impacts and benefits of agroforestry management practices on physical and hydraulic soil properties, and soil nutrient stocks . An intensive monitoring programme was implemented in 2019 to monitor hydrometeorology, soil moisture and temperature, and essential soil hydrophysical and biogeochemical properties, including soil nutrients.

Land use changed drastically in the central part of the island. While 94% of the non-protected area was still covered by native vegetation in 1961, the agricultural expansion converted the forests to an anthropogenic landscape having only 7% forest, 67% agricultural land and 26% invasive species. While the early settlements were large, isolated farmsteads, the average size of the farms decreased over time as the number of farms doubled. Over the last two decades, the rapid rise of tourism activities alleviated pressure on natural resources and led to the abandonment of agricultural land.

The agroforestry management practices have a significant effect on soil temperature, moisture availability, and nutrient contents. When forest vegetation protects the soil from direct solar radiation, the soil is about 12% cooler than in soils that were converted to agricultural land. Soil moisture is, on average, 20% higher under forest than under traditional agroforestry or abandoned farmland, and forest soils have a lower bulk density, lower saturated hydraulic conductivity, higher soil organic stocks and higher water retention capacity. The loss of soil organic carbon in agricultural sites is related to soil mixing due to tillage, which increases soil pore connectivity and facilitates the decomposition of soil organic carbon and leaching of base cations. At the same time, fertilization with organic manure and nitrogen-phosphorus-potassium changed the soil pH and enhanced soil nutrient leaching.

The present work illustrates how soil fertility, access to markets, and alternative incomes play an important role in land use decision making. Preserving forest remnants in an agricultural landscape has measurable effects with a reduction of soil warming by 12 %, a reduction of soil drying by 20%, and better preservation of soil organic carbon stocks compared to traditional agroforestry. Future land policy needs to account for the diversity in livelihoods in the rural communities, and the impacts and benefits of agroforestry management practices on soil and environmental health

Summary

Cosmogenic radionuclides accumulate at the Earth’ surface as a function of time and depth. They can be used to constrain the age and the deposition mode of fluvial deposits.

Occupied by the braided channels of the Meuse during the Middle Pleistocene, the (eastern) Campine plateau resisted the overall erosion that affected the Campine area, and nowadays stands out of its environment.

In this study, we applied cosmogenic radionuclides on the eastern Campine plateau to constrain its depositional and post-depositional history. Main results show that the deposition occurred in different phases, interrupted by thousands of years long hiatuses that correspond to climatic fluctuations.

These first numerical ages of the eastern Campine plateau could be used in the future to better constrain the geomorphological evolution of the Campine region and the European Lowlands in general.

A major concern for sustainable development in the Tropical Andes is the growing imbalance between ecosystem service supply and demand, particularly with regard to freshwater ecosystem services. While the capacity of the Tropical Andes to provide freshwater ecosystem goods and services is already under pressure, the demand is rapidly increasing as a result of demographic growth, urbanization and evolving socio-economic conditions. Coupled with future projections of climate change, urban growth and continued socio-economic development, these environmental questions surrounding sustainable development in the Ecuadorian Andes call for urgent action.

This thesis contributed with evidence-based research on conservation and restoration of young volcanic ash soils through an analysis of soil-water-plant interactions at the pedon, toposequence and landscape scale. The empirical data were collected over a 5-year period in the Jatunhuaycu experimental station, located in the northern Ecuadorian Andes at 4200 m a.s.l. The monitoring programme was specifically designed to test for potential differences in soil hydrophysical and biogeochemical properties and their changes over time between topographic positions and vegetation types.

At the soil pedon scale, the soil water balance was different between soils covered by tussock grasses and cushion-forming plants. The vegetation type had a measurable effect on the solute concentrations and fluxes in the uppermost horizon, while this effect strongly decreased with depth. The higher water infiltration rates of soils under tussock grasses resulted in higher chemical weathering rates than in soils under under cushion-forming plants.

At the toposequence scale, the soil pore structure and hydraulic properties of the A horizon varied by vegetation type across the studied profiles, while the studied topographic positions did not show a significant effect. The higher soil moisture content at higher matric potentials and total available water under cushion-forming plants can enhance soil water storage in the topsoil during prolonged rainfall events. However, its strong decrease in saturated hydraulic conductivity with depth can promote subsurface lateral flow during large rainfall events.

The connectivity of the geomorphic units at the landscape scale is time-dependent: storm runoff was observed at the outlet of the catchment when the moisture content in the upper horizons of the hillslope profiles reached field capacity. The geochemical signature of the solute export strongly varied over time, as a result of changes in the hydrological connectivity of geomorphic units.

The existence of strong interlinkages between plant functional types, soil and surface hydrology, and nutrient export has important implications for sustainable management of high Andean ecosystems. The results of this thesis can contribute to assess the potential impacts of vegetation conservation and restoration programmes on water resources in the Tropical Andes.

More information:

Páez-Bimos, S., Molina, A., Calispa, M., Delmelle, P., Lahuatte, B., Villacís, M., Muñoz, T., and Vanacker, V.: Soil-vegetation-water interactions controlling solute flow and transport in volcanic ash soils of the high Andes, Hydrol. Earth Syst. Sci. Discuss. [preprint], https://doi.org/10.5194/hess-2022-294, in review, 2022.

Páez-Bimos, S., Villacís, M., Morales, O., Calispa, M., Molina, A., Salgado, S., de Bievre, B., Delmelle, P., Muñoz, T., & Vanacker, V. (2022). Vegetation effects on soil pore structure and hydraulic properties in volcanic ash soils of the high Andes. Hydrological Processes, 36( 9), e14678. https://doi.org/10.1002/hyp.14678

The 9th of November, Beatriz will publicly defend her PhD thesis, entitled “Monitoring forest degradation patterns using multi-resolution spatial approaches”.

Summary

The subtropical dry forests of South-America have been impacted by deforestation and degradation over the last two centuries. The forests still provide important ecosystem services but overexploitation of the forested ecosystems may lead to irreversible loss of provision and regulation services. Remote sensing techniques enable us to monitor changes in forest cover at regional scale. Field-based forest inventories remain essential to measure more specific forest attributes and functions. However, the scale at which such field-based forest inventories can be organized is limited for the detection of slow and subtle changes in the forest at regional scale. Therefore, this thesis analyzed the potential of 3D vegetation models based on low-cost stereo-photos taken from unpiloted aerial vehicles to map forest degradation in dry forests. The ecosystem of the Dry Chaco in Argentina, which is considered as one of the major hotspots of deforestation worldwide, was taken as a case study. Therefore, new data on surface and vegetation height were collected during UAV-flights. Flights were realized at 44 locations in contrasting ecological settings of the Dry Chaco. The results of this research show that structure-from-motion algorithms are capable of reconstructing high resolution 3D surface models of the forest cover and this at a much lower cost than LIDAR derived products. Furthermore, this thesis proposes a set of relevant forest attributes that can be derived from these 3D surface models and compares these with indicators from traditional field-based forest inventories. In a next step, the correspondence between the forest structural indicators and degradation states was examined. Finally, the indicators were compared with high resolution laser data from a recently launched spaceborne sensor, which facilitates the assessment of forest degradation patterns at sub-continental scale.