| 1. |
- Lembrechts, Jonas J., et al.
(författare)
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Global maps of soil temperature
- 2022
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Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 28:9, s. 3110-3144
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Tidskriftsartikel (refereegranskat)abstract
- Research in global change ecology relies heavily on global climatic grids derived from estimates of air temperature in open areas at around 2m above the ground. These climatic grids do not reflect conditions below vegetation canopies and near the ground surface, where critical ecosystem functions occur and most terrestrial species reside. Here, we provide global maps of soil temperature and bioclimatic variables at a 1-km2 resolution for 0–5 and 5–15cm soil depth. These maps were created by calculating the difference (i.e. offset) between in situ soil temperature measurements, based on time series from over 1200 1-km2 pixels (summarized from 8519 unique temperature sensors) across all the world's major terrestrial biomes, and coarse-grained air temperature estimates from ERA5-Land (an atmospheric reanalysis by the European Centre for Medium-Range Weather Forecasts). We show that mean annual soil temperature differs markedly from the corresponding gridded air temperature, by up to 10°C (mean=3.0±2.1°C), with substantial variation across biomes and seasons. Over the year, soils in cold and/or dry biomes are substantially warmer (+3.6±2.3°C) than gridded air temperature, whereas soils in warm and humid environments are on average slightly cooler (−0.7±2.3°C). The observed substantial and biome-specific offsets emphasize that the projected impacts of climate and climate change on near-surface biodiversity and ecosystem functioning are inaccurately assessed when air rather than soil temperature is used, especially in cold environments. The global soil-related bioclimatic variables provided here are an important step forward for any application in ecology and related disciplines. Nevertheless, we highlight the need to fill remaining geographic gaps by collecting more in situ measurements of microclimate conditions to further enhance the spatiotemporal resolution of global soil temperature products for ecological applications.
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| 2. |
- Ahmed, Mohamed, et al.
(författare)
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Dynamic response of NDVI to soil moisture variations during different hydrological regimes in the Sahel region
- 2017
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Ingår i: International Journal of Remote Sensing. - : Informa UK Limited. - 1366-5901 .- 0143-1161. ; 38:19, s. 5408-5429
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Tidskriftsartikel (refereegranskat)abstract
- Over the last few decades, the African Sahel has become thefocus of many studies regarding vegetation dynamics and theirrelationships with climate and people. This is because rainfalllimits the production of biomass in the region, a resource onwhich people are directly dependent for their livelihoods. In thisstudy, we utilized a remote-sensing approach to answering thefollowing two questions: (1) how does the dynamic relationshipbetween soil moisture and plant growth vary across hydrologi-cal regimes, and (2) are vegetation-type-dependent responsesto soil moisture availability detectable from satellite imagery? Inorder to answer these questions, we studied the relationshipbetween monthly modelled soil moisture as an indicator forwater availability and the remotely sensed normalized differ-ence vegetation index (NDVI) as a proxy for vegetation growthbetween a“recovery rainfall period”(1982 to 1997) and a“stable rainfall period”(1998 to 2013), at different time lagsacross the Sahel region. Using windowed cross-correlation, wefind a strong significant positive relationship between NDVI andsoil moisture at a concurrent time and at NDVI lagging behindsoil moisture by 1 month for grassland, cropland, and decid-uous shrubland vegetation–the dominant vegetation classes inthe Sahel. South of the Sahel (the Sudanian and Guinean areas),wefind longer optimal lags (soil moisture lagged by 1–3 months) in association with mixed forest and deciduousshrubland. Wefind no major significant change in optimal lagbetween the recovery and stable periods in the Sahelian region;however, in the Sudanian and Guinean areas, we observe atrend towards shorter time lags. This change in optimal lagsuggests a vegetation change, which may be a response to aclimatic shift or land-use change. This approach of identifyingspatiotemporal trends in optimal lag correlations between mod-elled soil moisture and NDVI could prove to be a useful tool formapping vegetation change and ecosystem behaviour, in turnhelping inform climate change mitigation approaches and agri-cultural planning
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| 3. |
- Ghilain, Nicolas, et al.
(författare)
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A new retrieval algorithm for soil moisture index from thermal infrared sensor on-board geostationary satellites over Europe and Africa and its validation
- 2019
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Ingår i: Remote Sensing. - : MDPI AG. - 2072-4292. ; 11:17
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Tidskriftsartikel (refereegranskat)abstract
- Monitoring soil moisture at the Earth'surface is of great importance for drought early warnings. Spaceborne remote sensing is a keystone in monitoring at continental scale, as satellites can make observations of locations which are scarcely monitored by ground-based techniques. In recent years, several soil moisture products for continental scale monitoring became available from the main space agencies around the world. Making use of sensors aboard polar satellites sampling in the microwave spectrum, soil moisture can be measured and mapped globally every few days at a spatial resolution as fine as 25 km. However, complementarity of satellite observations is a crucial issue to improve the quality of the estimations provided. In this context, measurements within the visible and infrared from geostationary satellites provide information on the surface from a totally different perspective. In this study, we design a new retrieval algorithm for daily soil moisture monitoring based only on the land surface temperature observations derived from the METEOSAT second generation geostationary satellites. Soil moisture has been retrieved from the retrieval algorithm for an eight years period over Europe and Africa at the SEVIRI sensor spatial resolution (3 km at the sub-satellite point). The results, only available for clear sky and partly cloudy conditions, are for the first time extensively evaluated against in-situ observations provided by the International Soil Moisture Network and FLUXNET at sites across Europe and Africa. The soil moisture retrievals have approximately the same accuracy as the soil moisture products derived from microwave sensors, with the most accurate estimations for semi-arid regions of Europe and Africa, and a progressive degradation of the accuracy towards northern latitudes of Europe. Although some possible improvements can be expected by a better use of other products derived from SEVIRI, the new approach developped and assessed here is a valuable alternative to microwave sensors to monitor daily soil moisture at the resolution of few kilometers over entire continents and could reveal a good complementarity to an improved monitoring system, as the algorithm can produce surface soil moisture with less than 1 day delay over clear sky and non-steady cloudy conditions (over 10% of the time).
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| 4. |
- Müller, Mitro, et al.
(författare)
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Features predisposing forest to bark beetle outbreaks and their dynamics during drought
- 2022
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Ingår i: Forest Ecology and Management. - : Elsevier BV. - 1872-7042 .- 0378-1127. ; 523
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Tidskriftsartikel (refereegranskat)abstract
- Climate change is estimated to increase the risk of the bark beetle (Ips typographus L.) mass outbreaks in Norway Spruce (Picea abies (L.) Karst) forests. Habitats that are thermally suitable for bark beetles may expand, and an increase in the frequency and intensity of droughts can promote drought stress on host trees. Drought affects tree vigor and in unison with environmental features it influences the local predisposition risk of forest stands to bark beetle attacks. We aimed to study how various environmental features influence the risk of bark beetle attacks during a drought year and the following years with more normal weather conditions but with higher bark beetle populations. We included features representing local forest stand attributes, topography, soil type and wetness, the proximity of clear-cuts and previous bark beetle attacks, and a machine learning algorithm (random forest) was applied to study the variation of predisposition risk across a 48,600 km2 study area in SE Sweden.Forest stands with increased risk of bark beetle attack were distinguished with high accuracy both during drought and in normal weather conditions. The results show that during both study periods, spruce and mixed coniferous forests had elevated risk of attack, while forests with a mix of deciduous and coniferous trees had a lower risk. Forests with high average canopy height were strongly predisposed to bark beetle attacks. However, during the drought year risk was more similar between stands with lower and higher canopy height, suggesting that during drought periods younger trees can be predisposed to bark beetle attacks. The importance of soil moisture and position within the local landscape were highlighted as important features during the drought year.Identifying areas with increased risk, supported by information on how environmental features control the predisposition risk during drought, could aid adaptation strategies and forest management intervention efforts. We conclude that geospatial data and machine learning have the potential to further support the digitalization of the forest industry, facilitating development of methods capable to quantify importance and dynamics of environmental features controlling the risk in local context. Corresponding methods could help to direct management actions more effectively and offer information for decision-making in changing climate.
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| 5. |
- Tagesson, Torbern, et al.
(författare)
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Dynamics in carbon exchange fluxes for a grazed semi-arid savanna ecosystem in West Africa
- 2015
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Ingår i: Agriculture, Ecosystems & Environment. - : Elsevier BV. - 1873-2305 .- 0167-8809. ; 205, s. 15-24
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Tidskriftsartikel (refereegranskat)abstract
- The main aim of this paper is to study land-atmosphere exchange of carbon dioxide (CO2) for semi-arid savanna ecosystems of the Sahel region and its response to climatic and environmental change. A subsidiary aim is to study and quantify the seasonal dynamics in light use efficiency (epsilon) being a key variable in scaling carbon fluxes from ground observations using earth observation data. The net ecosystem exchange of carbon dioxide (NEE) 2010-2013 was measured using the eddy covariance technique at a grazed semi-arid savanna site in Senegal, West Africa. Night-time NEE was not related to temperature, confirming that care should be taken before applying temperature response curves for hot dry semi-arid regions when partitioning NEE into gross primary productivity (GPP) and ecosystem respiration (R-eco). Partitioning was instead done using light response curves. The values of epsilon ranged between 0.02 g carbon (C) MJ(-1) for the dry season and 2.27 g C MJ(-1) for the peak of the rainy season, and its seasonal dynamics was governed by vegetation phenology, photosynthetically active radiation, soil moisture and vapor pressure deficit (VPD). The CO2 exchange fluxes were very high in comparison to other semi-arid savanna sites; half-hourly GPP and R-eco peaked at -43 mu mol CO2 m(-2) s(-1) and 20 mu mol CO2 m(-2) s(-1), and daily GPP and R-eco peaked at -15 g C m(-2) and 12 g C m(-2), respectively. Possible explanations for the high CO2 fluxes are a high fraction of C4 species, alleviated water stress conditions, and a strong grazing pressure that results in compensatory growth and fertilization effects. We also conclude that vegetation phenology, soil moisture, radiation, VPD and temperature were major components in determining the seasonal dynamics of CO2 fluxes. Despite the height of the peak of the growing season CO2 fluxes, the annual C budget (average NEE: -271 g C m(-2)) were similar to that in other semi-arid ecosystems because the short rainy season resulted in a short growing season. Global circulation models project a decrease in rainfall, an increase in temperature and a shorter growing season for the western Sahel region, and the productivity and the sink function of this semi-arid ecosystem may thus be lower in the future. (C) 2015 Elsevier B.V. All rights reserved.
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| 6. |
- Tagesson, Torbern, et al.
(författare)
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Spatiotemporal variability in carbon exchange fluxes across the Sahel
- 2016
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Ingår i: Agricultural and Forest Meteorology. - : Elsevier BV. - 0168-1923. ; 226-227, s. 108-118
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Tidskriftsartikel (refereegranskat)abstract
- Semi-arid regions play an increasingly important role as a sink within the global carbon (C) cycle and is the main biome driving inter-annual variability in carbon dioxide (CO2) uptake by terrestrial ecosystems. This indicates the need for detailed studies of spatiotemporal variability in C cycling for semi-arid ecosystems. We have synthesized data on the land-atmosphere exchange of CO2 measured with the eddy covariance technique from the six existing sites across the Sahel, one of the largest semi-arid regions in the world. The overall aim of the study is to analyse and quantify the spatiotemporal variability in these fluxes and to analyse to which degree spatiotemporal variation can be explained by hydrological, climatic, edaphic and vegetation variables. All ecosystems were C sinks (average ± total error -162 ± 48 g C m-2 y-1), but were smaller when strongly impacted by anthropogenic influences. Spatial and inter-annual variability in the C flux processes indicated a strong resilience to dry conditions, and were correlated with phenological metrics. Gross primary productivity (GPP) was the most important flux process affecting the sink strength, and diurnal variability in GPP was regulated by incoming radiation, whereas seasonal dynamics was closely coupled with phenology, and soil water content. Diurnal variability in ecosystem respiration was regulated by GPP, whereas seasonal variability was strongly coupled to phenology and GPP. A budget for the entire Sahel indicated a strong C sink mitigating the global anthropogenic C emissions. Global circulation models project an increase in temperature, whereas rainfall is projected to decrease for western Sahel and increase for the eastern part, indicating that the C sink will possibly decrease and increase for the western and eastern Sahel, respectively.
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