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 series of Earth observation data. Hotspot areas of high TP occurrence were identified, observed changes characterized and insights gained on potential drivers for these changes. Location: Arid and semi-arid regions. Time period: 1982-2015.Methods: We used a time series segmentation technique (breaks for additive season and trend) to detect breakpoints in rain-use efficiency as a means of analysing changes in ecosystem functioning. A new typology to characterize the detected changes was proposed and evaluated, at regional to local scales, for a set of case studies. Ancillary data on population and drought were used to provide insights on potential drivers of TP occurrence.Results: Turning points in ecosystem functioning were found in 13.6% (c. 2.1 × 10 6 km 2 ) of global drylands. Turning point hotspots were primarily observed in North America, the Sahel, Central Asia and Australia. In North America, the majority of TPs (62.6%) were characterized by a decreasing trend in ecosystem functioning, whereas for the other regions, a positive reversal in ecosystem functioning was prevalent. Further analysis showed that: (a) both climatic and anthropogenic pressure influenced the occurrence of TPs in North America; (b) Sahelian grasslands were primarily characterized by drought-induced TPs; and (c) high anthropogenic pressure coincided with the occurrence of TPs in Asia and Australia. Main conclusions: By developing a new typology targeting the categorization of abrupt and gradual changes in ecosystem functioning, we detected and characterized TPs in global drylands. This TP characterization is a first crucial step towards understanding the drivers of change and supporting better decision-making for ecosystem conservation and management in drylands. | 1231 BERNARDINO Et Al.
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 study, 30 m resolution historically consistent land cover and cover fraction maps are provided over the Sudano-Sahel for the period 1986–2015. These land cover/cover fraction maps are achieved based on the Landsat archive preprocessed on Google Earth Engine and a random forest classification/regression model, while historical consistency is achieved using the hidden Markov model. Using these historical maps, a multitude of variability in the dynamic Sudano-Sahel region over the past 30 years is revealed. On the one hand, Sahel-wide cropland expansion and the re-greening of the Sahel is observed in the discrete land cover classification. On the other hand, subtle changes such as forest degradation are detected based on the cover fraction maps. Additionally, exploiting the 30 m spatial resolution, fine-scale changes, such as smallholder or subsistence farming, can be detected. The historical land cover/cover fraction maps presented in this study are made available via an open-access platform.
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 drought events is essential. We used a phenology-based remote sensing approach coupled with field data to estimate the severity and recovery rates of a large scale die-off event that occurred in 2014–2015 in Senegal. Novel low (L-band) and high-frequency (Ku-band) passive microwave vegetation optical depth (VOD), and optical MODIS data, were used to estimate woody vegetation dynamics. The relative importance of soil, human-pressure, and before-drought vegetation dynamics influencing the woody vegetation response to the drought were assessed. The die-off in 2014–2015 represented the highest dry season VOD drop for the studied period (1989–2017), even though the 2014 drought was not as severe as the droughts in the 1980s and 1990s. The spatially explicit Die-off Severity Index derived in this study, at 500 m resolution, highlights woody plants mortality in the study area. Soil physical characteristics highly affected die-off severity and post-disturbance recovery, but pre-drought biomass accumulation (i.e., in areas that benefited from above-normal rainfall conditions before the 2014 drought) was the most important variable in explaining die-off severity. This study provides new evidence supporting a better understanding of the “greening Sahel”, suggesting that a sudden increase in woody vegetation biomass does not necessarily imply a stable ecosystem recovery from the droughts in the 1980s. Instead, prolonged above-normal rainfall conditions prior to a drought may result in the accumulation of woody biomass, creating the basis for potentially large-scale woody vegetation die-off events due to even moderate dry spells.
Abstract. 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 future climate scenarios. In this modelling study we assessed the impact of single years of extreme (high or low) rainfall on dryland vegetation in the Sahel. The magnitude and legacy of these impacts were quantified on both the plant functional type and the ecosystem levels. In order to understand the impact of differences in the rainfall distribution over the year, these rainfall anomalies were driven by changing either rainfall intensity, event frequency or rainy-season length. The Lund–Potsdam–Jena General Ecosystem Simulator (LPJ-GUESS) dynamic vegetation model was parameterized to represent dryland plant functional types (PFTs) and was validated against flux tower measurements across the Sahel. Different scenarios of extreme rainfall were derived from existing Sahel rainfall products and applied during a single year of the model simulation timeline. Herbaceous vegetation responded immediately to the different scenarios, while woody vegetation had a weaker and slower response, integrating precipitation changes over a longer timeframe. An increased season length had a larger impact than increased intensity or frequency, while impacts of decreased rainfall scenarios were strong and independent of the season characteristics. Soil control on surface water balance explains these contrasts between the scenarios. None of the applied disturbances caused a permanent vegetation shift in the simulations. Dryland ecosystems are known to play a dominant role in the trend and variability of the global terrestrial CO2 sink. We showed that single extremely dry and wet years can have a strong impact on the productivity of drylands ecosystems, which typically lasts an order of magnitude longer than the duration of the disturbance. Therefore, this study sheds new light on potential drivers and mechanisms behind this variability.
Tropical forests and savannas can co-occur in a range of macro-environmental conditions. In these conditions, disturbances and resource availability are thought to control savanna and forest transitions, although the mechanisms involved are disputed. We hypothesized that, in Neotropical regions where fire activity is high, fire is the main factor controlling functional differences between savanna and forest, as well as their relative resistance to biome shifts. We sampled plant functional traits and soil and determined fire history, for 198 plots distributed across three landscapes with distinct fire frequencies (high, mid, and low). In each landscape, plots covered a woody cover gradient (from wooded grasslands to forests). We tested whether the sharpness and the magnitude of the functional distinction between savanna and forest were affected by fire. We also computed the environmental hyperspace (niche space) to evaluate how biome relative stability changed in relation to fire. Functional thresholds were detected only in the high and mid landscapes, where savanna and forest plots formed a multidimensional bimodal distribution in functional trait space. The stability of savannas in relation to forest increased abruptly with fire, whereas functional differences between forest and savanna increased gradually. Our results suggest that savanna can occur as an alternative vegetation state to forest where a fire burns every 18 years (on average), but higher frequencies are required for savannas to occupy large unique portions of the environmental niche space.
<p>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., 2016). Such abrupt changes may have severe ecological and economic consequences, disturbing the livelihood of drylands inhabitants and causing increased poverty and food insecurity. Considering that drylands cover 40% of Earth&#8217;s land surface and are home to around one-third of the human population, detecting and characterizing hotspots of abrupt changes in ecosystem functioning (here called turning points) becomes even more crucial.</p><p>BFAST, a time series segmentation technique, was used to detect breakpoints in time series (1982-2015) of rain-use efficiency. An abrupt change in rain-use efficiency time series points towards a significant change in the way an ecosystem responds to precipitation, allowing the study of turning points in ecosystem functioning in both natural and anthropogenic landscapes. Moreover, we here proposed a new typology to characterize turning points in ecosystem functioning, which takes into account the trend in ecosystem functioning before and after the turning point, as well as differences in the rate of change. Case studies were used to evaluate the performance of the new typology. Finally, ancillary data on population density and drought were used to have some first insights about the potential determinants of hotspots of turning point occurrence.</p><p>Our results showed that 13.6% of global drylands presented a turning point in ecosystem functioning between 1982 and 2015. Hotspots of turning point occurrence were observed in North America (where 62.6% of the turning points were characterized by a decreasing trend in ecosystem functioning), the Sahel, Central Asia, and Australia. The last three hotspot regions were mainly characterized by a positive trend in ecosystem functioning after the turning point. The ancillary data pointed to an influence of both droughts and human action on turning point occurrence in North America, while in Asia and Australia turning point occurrence was higher in areas with higher anthropogenic pressure. In the grasslands of the Sahel, turning points were potentially related to drought.&#160;</p><p>By detecting where and when hotspots of turning points occurred in recent decades, and by characterizing the trends in ecosystem functioning before and after the turning points, we advanced towards better supporting decision making related to ecosystems conservation and management in drylands. Moreover, we provided first insights about the drivers of ecosystem functioning change in hotspots of turning point occurrence in global drylands (Bernardino et al., 2019).</p><p>&#160;</p><p><strong>References:</strong></p><p>Bernardino PN, De Keersmaecker W, Fensholt R, Verbesselt J, Somers B, Horion S (2019) Global-scale characterization of turning points in arid and semi-arid ecosystems functioning. Manuscript submitted for publication.</p><p>Horion S, Prishchepov A V., Verbesselt J, de Beurs K, Tagesson T, Fensholt R (2016) Revealing turning points in ecosystem functioning over the Northern Eurasian agricultural frontier. Global change biology, <strong>22</strong>, 2801&#8211;2817.</p>
Partindo do princípio de que vivemos numa civilização que se funda essencialmente na imagem, verifico que quanto mais me debruço sobre a noção de imagem e dos aparatos através do quais ela se faz aparecer mais me apercebo das inovações técnicas que auxiliam à sua construção – nomeadamente através de projeções e ecrãs –, mais relevância atribuo à contribuição das definições quer de Heraclito quer de Platão, em que as imagens são sombras, e por sombra se tem a figuração do outro através do mesmo. O termo imagem deixa de evocar apenas uma representação visual para se apoiar numa semelhança, tornando a imagem num objeto – seguindo um outro que ela representa de acordo com algumas leis particulares – onde estas facilmente se convertem em artefactos audiovisuais. Dessa forma influenciam nos processos de criação, orientando toda uma gramática visual fazendo depender a obra da surpresa e da intriga.
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