Detection and mapping of burnt areas from time series of MODIS-derived NDVI data in a Mediterranean region

Moderate resolution remote sensing data, as provided by MODIS, can be used to detect and map active or past wildfires from daily records of suitable combinations of reflectance bands. The objective of the present work was to develop and test simple algorithms and variations for automatic or semiautomatic detection of burnt areas from time series data of MODIS biweekly vegetation indices for a Mediterranean region. MODIS-derived NDVI 250m time series data for the Valencia region, East Spain, were subjected to a two-step process for the detection of candidate burnt areas, and the results compared with available fire event records from the Valencia Regional Government. For each pixel and date in the data series, a model was fitted to both the previous and posterior time series data. Combining drops between two consecutive points and 1-year average drops, we used discrepancies or jumps between the pre and post models to identify seed pixels, and then delimitated fire scars for each potential wildfire using an extension algorithm from the seed pixels. The resulting maps of the detected burnt areas showed a very good agreement with the perimeters registered in the database of fire records used as reference. Overall accuracies and indices of agreement were very high, and omission and commission errors were similar or lower than in previous studies that used automatic or semiautomatic fire scar detection based on remote sensing. This supports the effectiveness of the method for detecting and mapping burnt areas in the Mediterranean region.This work was supported by the research projects FEEDBACK (CGL2011-30515- C02-01), funded by the Spanish Ministry of Innovation and Science, CASCADE (GA283068), funded by European Commission under the Seventh Framework Program, and GVPRE/2008/310, funded by the Valencia Regional Government (Generalitat Valenciana)

Connectivity-Mediated Ecohydrological Feedbacks and Regime Shifts in Drylands

Identified as essential mechanisms promoting alternative stable states, positive feedbacks have been the focus of most former studies on the potential for catastrophic shifts in drylands. Conversely, little is known about how negative feedbacks could counterbalance the effects of positive feedbacks. A decrease in vegetation cover increases the connectivity of bare-soil areas and entails a global loss of runoff-driven resources from the ecosystem but also a local increase in runoff transferred from bare-soil areas to vegetation patches. In turn, these global resource losses and local resource gains decrease and increase vegetation cover, respectively, resulting in a global positive and a local negative feedback loop. We propose that the interplay of these two interconnected ecohydrological feedbacks of opposite sign determines the vulnerability of dryland ecosystems to catastrophic shifts. To test this hypothesis, we developed a spatially explicit model and assessed the effects of varying combinations of feedback strengths on the dynamics, resilience, recovery potential, and spatial structure of the system. Increasing strengths of the local negative feedback relative to the global positive feedback decreased the risk of catastrophic shifts, facilitated recovery from a degraded state, and promoted the formation of banded vegetation patterns. Both feedbacks were most relevant at low vegetation cover due to the nonlinear increase in hydrological connectivity with decreasing vegetation. Our modelling results suggest that catastrophic shifts to degraded states are less likely in drylands with strong source–sink dynamics and/or strong response of vegetation growth to resource redistribution and that feedback manipulation can be useful to enhance dryland restoration.This work was supported by the research Projects DRYEX (CGL2014-59074-R) and DRYEX2 (CGL2017-89804-R), funded by the Spanish Ministry of Economy and Competitiveness, and the EC-funded Project CASCADE (GA283068). FR and SB acknowledge the financial support from the “Programa Estatal de Promoción del Talento y su Empleabilidad en I + D+i, Subprograma Estatal de Movilidad, del Plan Estatal de Investigación Científica y Técnica y de Innovación 2013-2016” (PRX14/00691 and PRX16/00583), funded by the Spanish Ministry of Education, Culture and Sports (MECD). FR also acknowledges the financial support from the Valencia Regional Government, Generalitat Valenciana (BEST/2014/285). AGM was supported by the EC-funded Marie Skłodowska-Curie Action ECOHYDRY (GA660859)

Comparative assessment of goods and services provided by grazing regulation and reforestation in degraded Mediterranean rangelands

Several management actions are applied to restore ecosystem services in degraded Mediterranean rangelands, which range from adjusting the grazing pressure to the removal of grazers and pine plantations. Four such actions were assessed in Quercus coccifera L. shrublands in northern Greece: (i) moderate grazing by goats and sheep; (ii) no grazing; (iii) no grazing plus pine (Pinus pinaster Aiton) plantation in forest gaps (gap reforestation); and (iv) no grazing plus full reforestation of shrubland areas, also with P. pinaster. In addition, heavy grazing was also assessed to serve as a control action. We comparatively assessed the impact of these actions on key provisioning, regulating and supporting ecosystem services by using ground‐based indicators. Depending on the ecosystem service considered, the management actions were ranked differently. However, the overall provision of services was particularly favoured under moderate and no grazing management options, with moderate grazing outranking any other action in provisioning services and the no grazing action presenting the most balanced provision of services. Pine reforestations largely contributed to water and soil conservation and C sequestration but had a negative impact on plant diversity when implemented at the expense of removing natural vegetation in the area. Heavy grazing had the lowest provision of ecosystem services. It is concluded that degraded rangelands can be restored by moderating the grazing pressure rather than completely banning livestock grazing or converting them into pine plantations

The role of ecohydrological (dis)connectivity in dryland functioning and management

La conectividad se define como el grado en que la estructura espacial del paisaje facilita el flujo de organismos y substancias. Diversos modelos conceptuales y evidencias empíricas atribuyen a la conectividad hidrológica un papel clave en el funcionamiento de los ecosistemas y paisajes áridos, pero aún falta una perspectiva integrada sobre los distintos aspectos y procesos hidrológicos y ecológicos que vinculan la conectividad con la degradación y la recuperación de zonas áridas. Aquí, describimos la conectividad ecohidrológica como una propiedad emergente de los ecosistemas y paisajes que captura la interacción entre conectividad hidrológica y procesos ecológicos, y discutimos cómo la conectividad ecohidrológica controla el funcionamiento de los ecosistemas áridos, subyace a las dinámicas de desertificación y puede manejarse para facilitar la recuperación de las zonas áridas degradadas. Una conectividad ecohidrológica disfuncional sería aquella en la que la mayor parte del flujo de recursos asociado a la conectividad del suelo desnudo a escala de ladera o paisaje no puede ser retenido y explotado por el efecto sumidero de la vegetación. Las actuaciones para la recuperación del sistema degradado irían encaminadas a interrumpir la conectividad general de la matriz suelo desnudo, fomentar la dinámica fuente-sumidero y potenciar la capacidad de sumidero de las manchas o teselas de vegetación. A escala de paisaje, la interrupción de la conectividad hidrológica puede, a su vez, crear teselas de hábitat y corredores para una gran variedad de especies, facilitando sus desplazamientos y, de este modo, su conservación y adaptación al cambio climático.Connectivity is defined as the degree to which the spatial structure of the landscape facilitates the flow of organisms and materials. Various conceptual models and empirical evidence attribute a key role to hydrological connectivity in the functioning of dryland ecosystems and landscapes, yet an integrated perspective on the different facets and processes that link connectivity with the degradation and recovery of drylands is still lacking. Here, we describe ecohydrological connectivity as an emergent property of ecosystems and landscapes that captures the interaction between hydrological connectivity and ecological proceses, and discuss how ecohydrological connectivity controls dryland ecosystem functioning, underlies the dynamics of desertification, and can be managed to facilitate the recovery of degraded drylands. A dysfunctional ecohydrological connectivity would be one in which most of the resource flow associated with bare soil connectivity at the slope or landscape scale cannot be retained and exploited by the sink effect of vegetation. Actions for the recovery of the degraded system would be aimed at interrupting the general connectivity of the bare soil matrix, promoting source-sink dynamics and enhancing the sink capacity of the vegetation patches or tiles. At the landscape scale, the disruption of hydrological connectivity can, in turn, create habitat patches and corridors for a wide variety of species, facilitating their movements and thus their conservation and adaptation to climate change.Este trabajo se ha realizado en el marco del proyecto DRYEX2 (CGL2017-89804-R), financiado por el Ministerio de Ciencia e Innovación

A null model for assessing the cover-independent role of bare soil connectivity as indicator of dryland functioning and dynamics

Recent research has identified the connectivity of the bare-soil interpatch areas as a key pattern attribute that controls resource conservation and structure-function feedbacks in dryland ecosystems, and several indices have been developed for this attribute. We aimed to characterize the dependence of bare-soil connectivity on vegetation cover and provide a null model that helps differentiate the independent roles of vegetation pattern and cover in hydrological connectivity and dryland functioning. Using a simple hydrological connectivity index, Flowlength, we developed explicit theoretical expressions for its expected value and variance under a null model of random vegetation cover distribution and constant slope. We also obtained the expected value of Flowlength for a model including an aggregation parameter. We found a non-linear inverse relationship between bare-soil connectivity and vegetation cover, which accounts for sharp increases in runoff and sediment yield for low cover values. The expressions for the mean values and standard errors for the random model allow the construction of confidence intervals, and thus testing for deviations from the null random model in experimental data. We found that positive deviations of Flowlength from the expected values, either under random or aggregated-pattern null models, sharply increase before transitions to a degraded state in a spatially-explicit dryland vegetation model, suggesting that an extraordinary increase in bare-soil connectivity may lead to unavoidable degradation. Our results show that increased deviation from the expected cover-dependent bare-soil connectivity may serve as indicator of ecosystem functional status and imminent transitions.This work was supported by the research projects FEEDBACK (CGL2011-30515-C02-01) and DRYEX (CGL2014-59074-R), funded by the Spanish Ministry of Economy and Competitiveness, and the EC-funded project CASCADE (GA283068). FR and SB acknowledge financial support from the “Programa Estatal de Promoción del Talento y su Empleabilidad en I+D+i, Subprograma Estatal de Movilidad, del Plan Estatal de Investigación Científica y Técnica y de Innovación 2013-2016”, (PRX14/00691 and PRX16/00583), funded by the Spanish Ministry of Education, Culture and Sports (MECD). FR also acknowledges financial support from the Valencia Regional Government, Generalitat Valenciana (BEST/2014/285). AGM was supported by the EC-funded Marie Skłodowska-Curie Action ECOHYDRY (GA660859)

Connectivity-Mediated Ecohydrological Feedbacks and Regime Shifts in Drylands

Identified as essential mechanisms promoting alternative stable states, positive feedbacks have been the focus of most former studies on the potential for catastrophic shifts in drylands. Conversely, little is known about how negative feedbacks could counterbalance the effects of positive feedbacks. A decrease in vegetation cover increases the connectivity of bare-soil areas and entails a global loss of runoff-driven resources from the ecosystem but also a local increase in runoff transferred from bare-soil areas to vegetation patches. In turn, these global resource losses and local resource gains decrease and increase vegetation cover, respectively, resulting in a global positive and a local negative feedback loop. We propose that the interplay of these two interconnected ecohydrological feedbacks of opposite sign determines the vulnerability of dryland ecosystems to catastrophic shifts. To test this hypothesis, we developed a spatially explicit model and assessed the effects of varying combinations of feedback strengths on the dynamics, resilience, recovery potential, and spatial structure of the system. Increasing strengths of the local negative feedback relative to the global positive feedback decreased the risk of catastrophic shifts, facilitated recovery from a degraded state, and promoted the formation of banded vegetation patterns. Both feedbacks were most relevant at low vegetation cover due to the nonlinear increase in hydrological connectivity with decreasing vegetation. Our modelling results suggest that catastrophic shifts to degraded states are less likely in drylands with strong source–sink dynamics and/or strong response of vegetation growth to resource redistribution and that feedback manipulation can be useful to enhance dryland restoration

Variation in soil enzyme activity as a function of vegetation amount, type, and spatial structure in fire-prone Mediterranean shrublands

Fire-prone Mediterranean shrublands may be seriously threatened by land degradation due to progressive opening of the vegetation cover driven by increasing drought and fire recurrence. However, information about the consequences of this opening process for critical ecosystem functions is scant. In this work, we studied the influence of vegetation amount, type, and spatial pattern in the variation of extracellular soil enzyme activity (acid phosphatase, β-glucosidase, and urease) in fire-prone shrublands in eastern Spain. Soil was sampled in vegetation-patch and open-interpatch microsites in 15 shrubland sites affected by large wildfires in 1991. On average, the activities of the three enzymes were 1.5 (β-glucosidase and urease) to 1.7 (acid phosphatase) times higher in soils under vegetation patches than in adjacent interpatches. In addition, phosphatase activity for both microsites significantly decreased with the fragmentation of the vegetation. This result was attributed to a lower influence of roots -the main source of acid phosphatase- in the bigger interpatches of the sites with lower patch cover, and to feedbacks between vegetation pattern, redistribution of resources, and soil quality during post-fire vegetation dynamics. Phosphatase activity was also 1.2 times higher in patches of resprouter plants than in patches of non-resprouters, probably due to the faster post-fire recovery and older age of resprouter patches in these fire-prone ecosystems. The influence on the studied enzymes of topographic and climatic factors acting at the landscape scale was insignificant. According to our results, variations in the cover, pattern, and composition of vegetation patches may have profound impacts on soil enzyme activity and associated nutrient cycling processes in fire-prone Mediterranean shrublands, particularly in those related to phosphorus.This study was funded by the research projects CGL2004-03627, INDEX2 (CGL2005-07946-C02-01/BOS), and FEEDBACK (CGL2011-30515-C02-01), from the Spanish Ministry for Economy and Competitiveness. The authors also acknowledge support from the European Union's Seventh Framework Program (FP7/2007-2013) under grant agreement 283068 (CASCADE)

Disentangling the independent effects of vegetation cover and pattern on runoff and sediment yield in dryland systems – Uncovering processes through mimicked plant patches

There is strong empirical evidence on the importance of the spatial pattern of vegetation in dryland hydrologic and geomorphologic dynamics. However, changes in vegetation cover and spatial pattern are often linked, making it difficult to disentangle and assess their independent hydro-geomorphologic roles. We used synthetic sponges placed on the soil surface to mimic the aboveground structure of vegetation patches, and manipulated patch cover and pattern as well as the sink capacity of the patches on a set of 24 (2 × 1 m) runoff plots. Combining natural-rainfall and simulated-rainfall experiments, we aimed to test that (1) both vegetation cover and pattern independently control runoff and sediment yield; (2) for any given cover, coarsening the vegetation pattern entails increasing runoff and sediment yield; and (3) pattern effect is mostly exerted by modulating the source-sink dynamics of the system. We found that increasing either patch cover or patch density decreased runoff and sediment yields from natural rainfalls, yet the effect of patch density largely disappeared when the effect of the co-varying patch cover was removed. Simulated-rainfall experiments on plots with equal medium-low patch cover showed however that coarser patterns (lower patch density; higher patch size) increased runoff coefficients and reduced time to runoff as compared with finer patterns. The effect of patch density was particularly clear when the sink function of vegetation patches was also mimicked. Rainfall interception and direct soil protection proved to be critical mechanisms underlying the effects of patch cover, yet they barely contributed to the effects of patch pattern. The control of overland flow by patch pattern was exerted through changes in the level of runoff disruption. However, physical obstructions to runoff hardly reduced runoff unless coupled to mimicked soil sinks. Overall this work demonstrates the independent effects of patch cover and pattern on the hydro-geomorphologic functioning of patchy landscapes, with patch cover being the primary hydrologic control factor and patch pattern exhibiting its full potential for low and medium low patch cover values. Our findings provide useful information for modelling and understanding dryland vegetation dynamics, and for designing management and restoration measures that take into account the critical role played by source-sink dynamics and hydrological connectivity in dryland landscapes.This work was supported by the Spanish Ministry of Science and Innovation (DRYEX2 project; grant number CGL 2017-89804-R), and the European Union's Seventh Framework Programme (CASCADE project; grant number GA283068)

Proyecto PRACTICE: evaluación de alternativas de gestión frente a la desertificación incorporando la participación y experiencias locales

El proyecto PRACTICE (Prevention and Restoration Actions to Combat Desertification. An Integrated Assessment) del 7º programa marco de la Comisión Europea, ha desarrollado metodologías para la evaluación de alternativas de gestión destinadas a la lucha contra la desertificación, incorporando el conocimiento y opinión de los agentes locales, es decir, personas y colectivos que conocen, hacen uso, o de alguna u otra forma tienen que ver con la gestión del territorio. La metodología desarrollada en PRACTICE fomenta el intercambio de conocimientos con los agentes implicados

Feedbacks between vegetation pattern and resource loss dramatically decrease ecosystem resilience and restoration potential in a simple dryland model

Conceptual frameworks of dryland degradation commonly include ecohydrological feedbacks between landscape spatial organization and resource loss, so that decreasing cover and size of vegetation patches result in higher water and soil losses, which lead to further vegetation loss. However, the impacts of these feedbacks on dryland dynamics in response to external stress have barely been tested. Using a spatially-explicit model, we represented feedbacks between vegetation pattern and landscape resource loss by establishing a negative dependence of plant establishment on the connectivity of runoff-source areas (e.g., bare soils). We assessed the impact of various feedback strengths on the response of dryland ecosystems to changing external conditions. In general, for a given external pressure, these connectivity-mediated feedbacks decrease vegetation cover at equilibrium, which indicates a decrease in ecosystem resistance. Along a gradient of gradual increase of environmental pressure (e.g., aridity), the connectivity-mediated feedbacks decrease the amount of pressure required to cause a critical shift to a degraded state (ecosystem resilience). If environmental conditions improve, these feedbacks increase the pressure release needed to achieve the ecosystem recovery (restoration potential). The impact of these feedbacks on dryland response to external stress is markedly non-linear, which relies on the non-linear negative relationship between bare-soil connectivity and vegetation cover. Modelling studies on dryland vegetation dynamics not accounting for the connectivity-mediated feedbacks studied here may overestimate the resistance, resilience and restoration potential of drylands in response to environmental and human pressures. Our results also suggest that changes in vegetation pattern and associated hydrological connectivity may be more informative early-warning indicators of dryland degradation than changes in vegetation cover.This work was supported by the research projects PATTERN (AGCL2008/-05532-C02-01/FOR) and FEEDBACK (CGL2011-30515-C02-01) funded by the Spanish Ministry of Innovation and Science, the EC-funded projects PRACTICE (GA226818) and CASCADE (GA283068), and the project RESINEE funded by the ERA-Net on Complexity. AGM was supported by a postdoctoral contract (APOSTD/2011/005) from the Valencia Regional Government (Generalitat Valenciana), Spain