Stéphanie Horion

• PhD in Science (Earth Observation)
• Professor (Associate) at University of Copenhagen

Excessive tree mortality is a global concern and remains poorly understood as it is a complex phenomenon. We lack global and temporally continuous coverage on tree mortality data. Ground-based observations on tree mortality, e.g., derived from national inventories, are very sparse, not standardized and not spatially explicit. Earth observation data...

The crossing of aridity thresholds triggers abrupt changes in multiple functional and structural ecosystem attributes across global drylands. While we understand the consequences associated with aridity thresholds, the key factors influencing dryland vegetation resistance when crossing them remain unclear. Here, we used field observations from 58 d...

Study region The Africa Sahel-Sudan region, defined by annual rainfall between 150 and 1200 mm. Study focus Understanding the mechanism of vegetation response to water availability could help mitigate the potential adverse effects of climate change on global dryland ecosystems. In the Sahel-Sudan region, spatio-temporal changes and drivers of the...

Interannual variability in climatic drivers can have a strong impact on dryland ecosystem functioning globally. While interannual variations in dryland ecosystem processes are mainly driven by rainfall, other global change drivers such as CO2 fertilization and rising temperatures can play an increasingly important role for these ecosystems. Yet, th...

In recent years, large-scale tree mortality events linked to global change have occurred around the world. Current forest monitoring methods are crucial for identifying mortality hotspots, but systematic assessments of isolated or scattered dead trees over large areas are needed to reduce uncertainty on the actual extent of tree mortality. Here, we...

The crossing of specific aridity thresholds has been shown to trigger abrupt changes in multiple functional and structural ecosystem attributes across global drylands. While we understand the consequences associated with aridity thresholds, a significant knowledge gap remains concerning the key biotic and abiotic factors that influence the resistan...

Aim Earth observation‐based estimates of land–atmosphere exchange of carbon are essential for understanding the response of the terrestrial biosphere to climatic change and other anthropogenic forcing. Temperature, soil water content and gross primary production are the main drivers of ecosystem respiration (R eco ), and the main aims of this study...

Increasing aridity is one major consequence of ongoing global climate change and is expected to cause widespread changes in key ecosystem attributes, functions, and dynamics. This is especially the case in naturally vulnerable ecosystems, such as drylands. While we have an overall understanding of past aridity trends, the linkage between temporal d...

Wetlands in drylands are vulnerable to degradation and disappearance due to the combined effects of increasing anthropogenic disturbances and climatic extremes. Such influences may drive non-linear shifts in surface responses that require long-term monitoring approaches for their study. Here, we used a piece-wise regression model to characterize lo...

Timely inputs for spatial planning are essential to support decisions about preventive or damage controlling measures, including flood. Climate change predictions suggest more frequent floods in the future, implying a need for flood mapping. The objectives of the study were to evaluate the suitability of Sentinel-1 SAR data to map the extent of flo...

Large-scale ecological restoration (ER) projects have been implemented in northwest China in recent decades as a means to prevent desertification and improve ecosystem services. However, previous studies have demonstrated adverse impacts in the form of widespread soil water deficit caused by intensive ER activities. Understanding the role of climat...

Changes in soil moisture strongly affect vegetation growth, which may in turn feed back on soil moisture by directly affecting evapotranspiration and indirectly regulating precipitation. Previous studies often focused on the unidirectional effects of soil moisture on temporal vegetation dynamics, yet bidirectional dependencies have rarely been stud...

Dryland ecosystems are a major source of land cover, account for about 40% of Earth's terrestrial surface and net primary productivity, and house more than 30 % of the human population. These ecosystems are subject to climate extremes (e.g. large-scale droughts and extreme floods) that are projected to increase in frequency and severity under most...

Global climate change is projected to lead to an increase in both the areal extent and degree of aridity in the world’s drylands. At the same time, the majority of drylands are located in developing countries where high population densities and rapid population growth place additional pressure on the ecosystem. Thus, drylands are particularly vulne...

Historical land cover maps are of high importance for scientists and policy makers studying the dynamic character of land cover change in the Sudano-Sahel, including anthropogenic and climatological drivers. Despite its relevance, an accurate high resolution record of historical land cover maps is currently lacking over the Sudano-Sahel. In this st...

Earth observation‐based estimates of global gross primary production (GPP) are essential for understanding the response of the terrestrial biosphere to climatic change and other anthropogenic forcing. In this study, we attempt an ecosystem‐level physiological approach of estimating GPP using an asymptotic light response function (LRF) between GPP a...

Dryland ecosystems are frequently struck by droughts. Yet, woody vegetation is often able to recover from mortality events once precipitation returns to pre-drought conditions. Climate change, however, may impact woody vegetation resilience due to more extreme and frequent droughts. Thus, better understanding how woody vegetation responds to drough...

Dryland ecosystems form a major land cover, accounting for about 40 % of Earth's terrestrial surface and net primary productivity, and housing more than 30 % of the human population. These ecosystems are subject to climate extremes (e.g. large-scale droughts) that are projected to increase in frequency and severity under most future climate scenari...

Although wave energy harnessing is still at the pre-commercial stage, accurate and up-to-date resource assessments are necessary to the development of a wave energy industry. Radar altimeters aboard spaceborne platforms provide extensive spatial coverage of wave height estimations but cannot compensate for the scarcity of in-situ measurements assoc...

Ecosystems in drylands are highly susceptible to changes in their way of functioning due to extreme and prolonged droughts or anthropogenic perturbation. Long-standing pressure, from climate or human action, may result in severe alterations in their dynamics. Moreover, changes in dryland ecosystems functioning can take place abruptly (Horion et al....

Aim Changes in dryland ecosystem functioning are threatening the well‐being of human populations worldwide, and land degradation, exacerbated by climate change, contributes to biodiversity loss and puts pressures on sustainable livelihoods. Here, abrupt changes in ecosystem functioning [so‐called turning points (TPs)] were detected using time serie...

Anthropogenic land use and land cover changes (LULCC) have a large impact on the global terrestrial carbon sink, but this effect is not well characterized according to biogeographical region. Here, using state-of-the-art Earth observation data and a dynamic global vegetation model, we estimate the impact of LULCC on the contribution of biomes to th...

Land use policies have turned southern China into one of the most intensively managed forest regions in the world, with actions maximizing forest cover on soils with marginal agricultural potential while concurrently increasing livelihoods and mitigating climate change. Based on satellite observations, here we show that diverse land use changes in...

Multiple ecological engineering projects have been implemented in semi‐arid and sub‐humid northern China since 1978 with the purpose to combat desertification, control dust storms, and improve vegetation cover. Although a plethora of local studies exist, the effectiveness of these projects has not been studied in a systematic and comprehensive way....

Extreme climate events and non‐sustainable land use are important drivers altering the functioning of European ecosystems, resulting in loss of the services provided. Yet, a consensus method for regular continental scale assessment of ecosystem condition in relation to land degradation (LD) is still lacking. Here we propose a new remote sensing bas...

The open-access paper with the results presented here can be found at https://doi.org/10.1111/geb.13099

The field of remote sensing is nowadays faced with huge amounts of data. While this offers a variety of exciting research opportunities, it also yields significant challenges regarding both computation time and space requirements. In practice, the sheer data volumes render existing approaches too slow for processing and analyzing all the available...

The field of remote sensing is nowadays faced with huge amounts of data. While this offers a variety of exciting research opportunities, it also yields significant challenges regarding both computation time and space requirements. In practice, the sheer data volumes render existing approaches too slow for processing and analyzing all the available...

Afforestation and reforestation projects in the karst regions of southwest China aim to combat desertification and improve the ecological environment. However, it remains unclear at what scale conservation efforts have impacted on carbon stocks and if vegetation regrowth occurs at a large spatial scale as intended. Here we use satellite time series...

It has been shown that vegetation growth in semi-arid regions is important to the global terrestrial CO2 sink, which indicates the strong need for improved understanding and spatially explicit estimates of CO2 uptake (gross primary production; GPP) in semi-arid ecosystems. This study has three aims: (1) to evaluate the MOD17A2H GPP (collection 6) p...

It has been shown that vegetation growth in semi-arid regions is an important sink for human induced fossil fuel emissions of CO2, which indicates the strong need for improved understanding, and spatially explicit estimates of CO2 uptake (gross primary productivity (GPP)) in semi-arid ecosystems. This study has three aims: 1) to evaluate the MOD17A...

Africa is a sink of carbon, but there are large gaps in our knowledge regarding the CO2 exchange fluxes for many African ecosystems. Here, we analyse multi-annual eddy covariance data of CO2 exchange fluxes for a grazed Sahelian semi-arid savanna ecosystem in Senegal, West Africa. The aim of the study is to investigate the high CO2 exchange fluxes...

Aim The present study models the response of vegetation to drought between 1982 and 2011 focusing on the growing season, considering: (1) vulnerable versus resistant ecosystems, (2) ecosystem resilience, (3) the delayed response of vegetation to accumulated precipitation deficits and (4) reduction in productivity due to drought. Location Continent...

The collapse of the Soviet Union in 1991 has been a turning point in the World history that left a unique footprint on the Northern Eurasian ecosystems. Conducting large scale mapping of environmental change and separating between naturogenic and anthropogenic drivers is a difficult endeavor in such highly complex systems. In this research a piece-...

This paper investigates how hyperspectral reflectance (between 350 and 1800 nm) can be used to infer ecosystem properties for a semi-arid savanna grassland in West Africa using a unique in situ-based multi-angular data set of hemispherical conical reflectance factor (HCRF) measurements. Relationships between seasonal dynamics in hyperspectral HCRF...

AimTo evaluate trend analysis of earth observation (EO) dense time series as a new way of describing and mapping changes in ecosystem functioning at regional to global scales. Spatio-temporal patterns of change covering 1982–2011 are discussed in the context of changes in land use and land cover (LULCC).LocationGlobal.Methods This study takes advan...

The Sahel has been the object of intensive international research since the drought of the early 1970s. A considerable part of the research has focused on environmental change in general and land degradation, land cover change and climate change in particular. Rich and diverse insights from many different scientific disciplines about these three do...

This paper investigates how seasonal hyperspectral reflectance data (between 350 and 1800 nm) can be used to infer ecosystem properties for a semi-arid savanna ecosystem in West Africa using a unique in situ based dataset. Relationships between seasonal dynamics in hyperspectral reflectance, and ecosystem properties (biomass, gross primary producti...

This chapter summarizes approaches to the detection of dryland vegetation change and methods for observing spatio-temporal trends from space. An overview of suitable long-term Earth Observation (EO) based datasets for assessment of global dryland vegetation trends is provided and a status map of contemporary greening and browning trends for global...

This chapter summarizes methods of inferring information about drivers of global dryland vegetation changes observed from remote sensing time series data covering from the 1980s until present time. Earth observation (EO) based time series of vegetation metrics, sea surface temperature (SST) (both from the AVHRR (Advanced Very High Resolution Radiom...

Changes in agricultural droughts were investigated using simulations of soil water deficit (SWD) and actual evapotranspiration (ETA) from a distributed semi-empirical soil water balance model – swbEWA. At European scale, both SWD and ETA did not change significantly between 1951 and 2011. However, significant increases in SWD were found in southern...

The Dahra field site in Senegal, West Africa, was established in 2002 to monitor ecosystem properties of semi-arid savanna grassland and their responses to climatic and environmental change. This paper describes the environment and the ecosystem properties of the site using a unique set of in situ data. The studied variables include hydroclimatic v...

Observing trends in global ecosystem dynamics is an important first step, but attributing these trends to climate variability represents a further step in understanding Earth system changes. In the present study, we classified global Ecosystem Response Types (ERTs) based on common spatio-temporal patterns in time-series of Standardized Precipitatio...

The co-existence of trees and grasses is a defining feature of savannah ecosystems and landscapes. During recent decades, the combined effect of climate change and increased demographic pressure has led to complex vegetation changes in these ecosystems. A number of recent Earth observation (EO)-based studies reported positive changes in biological...

Bridging the gap between theory and practise, between declarative and functioning knowledge, can be achieved through case-based learning activities (CBAs). However to be relevant cases need to address the intended learning outcomes ILOs in a sufficiently complex manner allowing the students to hypothesise, to reflect on their management of the case...

Drought affects more people than any other natural disaster but there is little understanding of how ecosystems react to droughts. This study jointly analyzed spatio-temporal changes of drought patterns with vegetation phenology and productivity changes between 1999 and 2010 in major European bioclimatic zones. The Standardized Precipitation and Ev...

The present study classified global Ecosystem Functional Types (EFTs) derived from seasonal vegetation dynamics of the GIMMS3g NDVI time-series. Rotated Principal Component Analysis (PCA) was run on the derived phenological and productivity variables, which selected the Standing Biomass (approximation of Net Primary Productivity), the Cyclic Fracti...

This study evaluates the relationship between the frequency and duration of meteorological droughts and the subsequent temporal changes on the quantity of actively photosynthesizing biomass (greenness) estimated from satellite imagery on rainfed croplands in Latin America. An innovative non-parametric and non-supervised approach, based on the Fishe...

The ‘rain use efficiency’ (RUE) may be defined as the ratio of above-ground net primary productivity (ANPP) to annual precipitation, and it is claimed to be a conservative property of the vegetation cover in drylands, if the vegetation cover is not subject to non-precipitation related land degradation. Consequently, RUE may be regarded as means of...

In this paper we investigated if and how a signature of climate control on vegetation growth can be individualized at regional scale using time series of SPOT-VEGETATION NDVI and ECMWF meteorological data. Twelve regions characterized by dominant and stable cropland or grassland covers were selected in Europe and Africa. Our results show that the r...

This study proposes a drought indicator that combines the Standardized Precipitation Index (SPI), the anomalies of soil moisture and the anomalies of the fraction of Absorbed Photosynthetically Active Radiation (fAPAR). Computed at the European level, the Combined Drought Indicator (CDI) gives a synthetic and synoptic overview of the drought situat...

The protection and the sustainable management of soil resources in Africa are of paramount importance, particularly in the context of the uncertain impact of climate change and the increasing pressures of human activities. From the perspective of a policy-maker interested in topics such as food security and land degradation in Africa, this situatio...

The protection and the sustainable management of soil resources in Africa are of paramount importance, particularly in the context of the uncertain impact of climate change and the increasing pressures of human activities. From the perspective of a policy maker interested in topics such as food security and land degradation in Africa, up-to-date an...

From the definition of drought to its monitoring and assessment, this report summarizes the main steps towards an integrated drought information system. Europe, Africa and Latin America are examples, based on the experience of the JRC, that illustrate the challenges for establishing continental drought observatory initiatives. The document is struc...

Many European countries have repeatedly been affected by droughts, resulting in considerable ecological and economic damage. Climate change studies indicate a trend towards increasing climate variability most likely resulting in more frequent drought occurrences also in Europe. Against this background, the European Commission's Joint Research Centr...

Cholera outbreaks have occurred in Burundi, Rwanda, Democratic Republic of Congo, Tanzania, Uganda, and Kenya almost every year since 1977-1978, when the disease emerged in these countries. We used a multiscale, geographic information system-based approach to assess the link between cholera outbreaks, climate, and environmental variables. We perfor...

Many studies already investigated the impact of climate change and climate variability on vegetation at global and continental scales. Using time series of remote sensing and climate data, Nemani et al. (2003) analyzed trends in Net Primary Production in relation with changes in climate and showed that, between 1982 and 1999, primary productivity i...

Drought is a complex and insidious natural hazard. It is hence difficult to detect in its early stages and to monitor its spatial evolution. Defining drought is already a challenge and can be done differently by meteorologists, hydrologists or socio-economists. In each one of these research areas, various indicators were already set up to depict th...

Determination of the spatial extent of peatlands and monitoring their status is important for the evaluation of soil carbon stocks and greenhouse gas fluxes. At European Level there is a need to provide accurate and updated estimate of the distribution of peatlands. Comparison of national data with EU wide land cover mapping shows that there is lim...

We observed a close correspondence between phytoplankton blooms and increased catches of S. tanganicae in the south. This could be the first application of remote sensing for fisheries at the African Great Lakes. The abundance of two main pelagic fish species in Lake Tanganyika (Stolothrissa tanganicae and Lates stappersii) has always been observe...

Our PhD research consists in analysing and modelling the vegetation response or sensitivity to climatic stresses with low satellite imagery. In that framework, the selection of optimal calibration sites is very important. These sites should be characterised by a stable and homogenous land cover over large area. Here we analyse the spatial heterogen...

The main fisheries of Lake Tanganyika are actually based on two sardines (clupeids) fishes (Stolotrissa tanganicae and Limnothrissa miodon) and one perch species (Lates stappersii). Several types of variability are observed in the abundance of those species (quasi monthly, annual, interannual (3-7 years) and long term changes). The fishing efforts...

The main fisheries of Lake Tanganyika are actually based on two sardines (clupeids) fishes (Stolotrissa tanganicae and Limnothrissa miodon) and one perch species (Lates stappersii). Several types of variability are observed in the abundance of those species (quasi monthly, annual, interannual (3-7 years) and long term changes). The fishing efforts...

Many studies already investigated the impact of climate change and climate variability on vegetation at global and continental scales. Low resolution satellite imagery is one of the main sources of information. In this paper, we describe a strategy to improve the quality of 10-daily time series of Normalized Difference Vegetation Index derived from...

Résumé Cette étude présente l'évaluation du stress hydrique des principales cultures de Belgique en 2003 grâce au modèle belge de suivi de croissance des cultures, nommé B-CGMS (Belgian Crop Growth Monitoring System). L'indicateur de référence est appelé Relative Soil Moisture Index (RSMI). Il est fondé sur un calcul intégrant des données de teneur...