• Energy Research

Cette étude étudie le comportement d'éloignement des éléphants par rapport à la dynamique de la végétation induite par les précipitations. Les données sur les mouvements ont été acquises pour cinq troupeaux de célibataires et cinq troupeaux familiaux de femelles pendant trois ans dans l'aire protégée de Marsabit au Kenya et les changements dans la végétation ont été cartographiés à l'aide de la série temporelle de l'indice de végétation à différence normalisée MODIS (NDVI). Dans la zone d'étude, des altitudes de 650 à 1100 m.a.s.l connaissent deux périodes de croissance par an, tandis que des périodes de croissance supérieures à 1100 m.a.s.l. durent un an ou plus. Nous constatons que les éléphants réagissent rapidement aux changements dans la disponibilité du fourrage et de l'eau, effectuant des migrations en réponse à des événements pluviométriques importants et mineurs. La migration en altitude des éléphants individuels correspondait étroitement aux modèles de verdissement et de sénescence de la végétation dans leur domaine vital. Les éléphants occupaient des altitudes plus basses lorsque l'activité de la végétation était élevée, alors qu'ils se retiraient dans la forêt à feuilles persistantes à des altitudes plus élevées tandis que la végétation sénescait. Les domaines vitaux des éléphants ont diminué en taille et se chevauchaient moins avec l'augmentation de l'altitude. Une hypothèse récente selon laquelle les migrations des ongulés dans les savanes résultent de précipitations saisonnières compensatrices et de gradients de fertilité est démontrée et étendue aux migrations à plus courte distance. En d'autres termes, le compromis entre la mauvaise qualité du fourrage et l'accessibilité dans la forêt avec ses sources d'eau toute l'année, d'une part, et le fourrage de meilleure qualité dans la garrigue de basse altitude avec sa disponibilité saisonnière en eau, d'autre part, entraîne les migrations relativement courtes (les deux principaux corridors sont de 20 et 90 km) des éléphants. En outre, une compétition intra-spécifique accrue semble influencer l'utilisation de l'habitat des animaux pendant la saison sèche, ce qui indique que l'empiètement humain sur la forêt affecte la population d'éléphants. Este estudio investiga el comportamiento de los elefantes en relación con la dinámica de la vegetación impulsada por las precipitaciones. Los datos de movimiento se adquirieron para cinco solteros y cinco rebaños familiares femeninos durante tres años en el área protegida Marsabit en Kenia y los cambios en la vegetación se mapearon utilizando la serie de tiempo del índice de vegetación de diferencia normalizada MODIS (NDVI). En el área de estudio, las elevaciones de 650 a 1100 msnm experimentan dos períodos de crecimiento por año, mientras que los períodos de crecimiento superiores a 1100 msnm duran un año o más. Encontramos que los elefantes responden rápidamente a los cambios en la disponibilidad de forraje y agua, haciendo migraciones en respuesta a eventos de lluvia grandes y pequeños. La migración en altura de los elefantes individuales coincidió estrechamente con los patrones de reverdecimiento y senescencia de la vegetación en su área de distribución. Los elefantes ocupaban elevaciones más bajas cuando la actividad de la vegetación era alta, mientras que se retiraban al bosque de hoja perenne en elevaciones más altas mientras la vegetación se senescía. Los rangos de hogar de los elefantes disminuyeron de tamaño y se superpusieron menos con el aumento de la elevación. Se demuestra una hipótesis reciente de que las migraciones de ungulados en las sabanas son el resultado de la compensación de las precipitaciones estacionales y los gradientes de fertilidad, y se extiende a las migraciones de distancias más cortas. En otras palabras, el equilibrio entre la mala calidad del forraje y la accesibilidad en el bosque con sus fuentes de agua durante todo el año, por un lado, y el forraje de mayor calidad en los matorrales de baja altitud con su disponibilidad estacional de agua, por otro lado, impulsa las migraciones relativamente cortas (los dos corredores principales son de 20 y 90 km) de los elefantes. Además, el aumento de la competencia intraespecífica parece influir en el uso del hábitat de los animales durante la estación seca, lo que indica que la invasión humana del bosque está afectando a la población de elefantes. This study investigates the ranging behavior of elephants in relation to precipitation-driven dynamics of vegetation. Movement data were acquired for five bachelors and five female family herds during three years in the Marsabit protected area in Kenya and changes in vegetation were mapped using MODIS normalized difference vegetation index time series (NDVI). In the study area, elevations of 650 to 1100 m.a.s.l experience two growth periods per year, while above 1100 m.a.s.l. growth periods last a year or longer.We find that elephants respond quickly to changes in forage and water availability, making migrations in response to both large and small rainfall events. The elevational migration of individual elephants closely matched the patterns of greening and senescing of vegetation in their home range. Elephants occupied lower elevations when vegetation activity was high, whereas they retreated to the evergreen forest at higher elevations while vegetation senesced. Elephant home ranges decreased in size, and overlapped less with increasing elevation.A recent hypothesis that ungulate migrations in savannas result from countervailing seasonally driven rainfall and fertility gradients is demonstrated, and extended to shorter-distance migrations. In other words, the trade-off between the poor forage quality and accessibility in the forest with its year-round water sources on the one hand and the higher quality forage in the low-elevation scrubland with its seasonal availability of water on the other hand, drives the relatively short migrations (the two main corridors are 20 and 90 km) of the elephants. In addition, increased intra-specific competition appears to influence the animals' habitat use during the dry season indicating that the human encroachment on the forest is affecting the elephant population. تبحث هذه الدراسة في سلوك تحديد المدى للفيلة فيما يتعلق بديناميكيات الغطاء النباتي التي يحركها هطول الأمطار. تم الحصول على بيانات الحركة لخمسة عزاب وخمس قطعان عائلية من الإناث خلال ثلاث سنوات في منطقة مارسابيت المحمية في كينيا وتم تعيين التغييرات في الغطاء النباتي باستخدام السلسلة الزمنية لمؤشر الاختلاف الطبيعي للغطاء النباتي (NDVI). في منطقة الدراسة، تشهد الارتفاعات من 650 إلى 1100 ميللي ثانية في السنة فترتين للنمو، بينما تستمر فترات النمو التي تزيد عن 1100 ميللي ثانية في السنة أو أكثر. نجد أن الفيلة تستجيب بسرعة للتغيرات في العلف وتوافر المياه، مما يؤدي إلى هجرات استجابة لكل من أحداث هطول الأمطار الكبيرة والصغيرة. تتطابق الهجرة الارتفاعية للفيلة الفردية بشكل وثيق مع أنماط تخضير وشيخوخة النباتات في نطاقها الأصلي. احتلت الفيلة ارتفاعات منخفضة عندما كان النشاط النباتي مرتفعًا، في حين تراجعت إلى الغابة دائمة الخضرة على ارتفاعات أعلى بينما كان الغطاء النباتي يشيخ. انخفضت نطاقات منازل الأفيال من حيث الحجم، وتداخلت بشكل أقل مع زيادة الارتفاع. تظهر الفرضية الحديثة القائلة بأن هجرات ذوات الحوافر في السافانا ناتجة عن موازنة هطول الأمطار وتدرجات الخصوبة المدفوعة موسمياً، وتمتد إلى هجرات لمسافات أقصر. وبعبارة أخرى، فإن المفاضلة بين جودة الأعلاف الرديئة وإمكانية الوصول إليها في الغابة مع مصادر المياه على مدار السنة من ناحية والأعلاف عالية الجودة في الأراضي المنخفضة الارتفاع مع توافرها الموسمي للمياه من ناحية أخرى، تدفع الهجرات القصيرة نسبيًا (الممران الرئيسيان هما 20 و 90 كم) للفيلة. بالإضافة إلى ذلك، يبدو أن المنافسة المتزايدة بين الأنواع تؤثر على استخدام موائل الحيوانات خلال موسم الجفاف مما يشير إلى أن التعدي البشري على الغابة يؤثر على عدد الأفيال.

Global vegetation models predict that boreal forests are particularly sensitive to a biome shift during the 21st century. This shift would manifest itself first at the biome's margins, with evergreen forest expanding into current tundra while being replaced by grasslands or temperate forest at the biome's southern edge. We evaluated changes in forest productivity since 1982 across boreal Alaska by linking satellite estimates of primary productivity and a large tree-ring data set. Trends in both records show consistent growth increases at the boreal-tundra ecotones that contrast with drought-induced productivity declines throughout interior Alaska. These patterns support the hypothesized effects of an initiating biome shift. Ultimately, tree dispersal rates, habitat availability and the rate of future climate change, and how it changes disturbance regimes, are expected to determine where the boreal biome will undergo a gradual geographic range shift, and where a more rapid decline.

AbstractRussia's boreal (taiga) biome will likely contract sharply and shift northward in response to 21st century climatic change, yet few studies have examined plant response to climatic variability along the northern margin. We quantified climate dynamics, trends in plant growth, and growth–climate relationships across the tundra shrublands and Cajander larch (Larix cajanderi Mayr.) woodlands of the Kolyma river basin (657 000 km2) in northeastern Siberia using satellite‐derived normalized difference vegetation indices (NDVI), tree ring‐width measurements, and climate data. Mean summer temperatures (Ts) increased 1.0 °C from 1938 to 2009, though there was no trend (P > 0.05) in growing year precipitation or climate moisture index (CMIgy). Mean summer NDVI (NDVIs) increased significantly from 1982 to 2010 across 20% of the watershed, primarily in cold, shrub‐dominated areas. NDVIs positively correlated (P < 0.05) with Ts across 56% of the watershed (r = 0.52 ± 0.09, mean ± SD), principally in cold areas, and with CMIgy across 9% of the watershed (r = 0.45 ± 0.06), largely in warm areas. Larch ring‐width measurements from nine sites revealed that year‐to‐year (i.e., high‐frequency) variation in growth positively correlated (P < 0.05) with June temperature (r = 0.40) and prior summer CMI (r = 0.40) from 1938 to 2007. An unexplained multi‐decadal (i.e., low‐frequency) decline in annual basal area increment (BAI) occurred following the mid‐20th century, but over the NDVI record there was no trend in mean BAI (P > 0.05), which significantly correlated with NDVIs (r = 0.44, P < 0.05, 1982–2007). Both satellite and tree‐ring analyses indicated that plant growth was constrained by both low temperatures and limited moisture availability and, furthermore, that warming enhanced growth. Impacts of future climatic change on forests near treeline in Arctic Russia will likely be influenced by shifts in both temperature and moisture, which implies that projections of future forest distribution and productivity in this area should take into account the interactions of energy and moisture limitations.

AbstractInsect and disease outbreaks in forests are biotic disturbances that can profoundly alter ecosystem dynamics. In many parts of the world, these disturbance regimes are intensifying as the climate changes and shifts the distribution of species and biomes. As a result, key forest ecosystem services, such as carbon sequestration, regulation of water flows, wood production, protection of soils, and the conservation of biodiversity, could be increasingly compromised. Despite the relevance of these detrimental effects, there are currently no spatially detailed databases that record insect and disease disturbances on forests at the pan‐European scale. Here, we present the new Database of European Forest Insect and Disease Disturbances (DEFID2). It comprises over 650,000 harmonized georeferenced records, mapped as polygons or points, of insects and disease disturbances that occurred between 1963 and 2021 in European forests. The records currently span eight different countries and were acquired through diverse methods (e.g., ground surveys, remote sensing techniques). The records in DEFID2 are described by a set of qualitative attributes, including severity and patterns of damage symptoms, agents, host tree species, climate‐driven trigger factors, silvicultural practices, and eventual sanitary interventions. They are further complemented with a satellite‐based quantitative characterization of the affected forest areas based on Landsat Normalized Burn Ratio time series, and damage metrics derived from them using the LandTrendr spectral–temporal segmentation algorithm (including onset, duration, magnitude, and rate of the disturbance), and possible interactions with windthrow and wildfire events. The DEFID2 database is a novel resource for many large‐scale applications dealing with biotic disturbances. It offers a unique contribution to design networks of experiments, improve our understanding of ecological processes underlying biotic forest disturbances, monitor their dynamics, and enhance their representation in land‐climate models. Further data sharing is encouraged to extend and improve the DEFID2 database continuously. The database is freely available at https://jeodpp.jrc.ec.europa.eu/ftp/jrc‐opendata/FOREST/DISTURBANCES/DEFID2/.

Terrestrial disturbances are accelerating globally, but their full impact is not quantified because we lack an adequate monitoring system. Remote sensing offers a means to quantify the frequency and extent of disturbances globally. Here, we review the current application of remote sensing to this problem and offer a framework for more systematic analysis in the future. We recommend that any proposed monitoring system should not only detect disturbances, but also be able to: identify the proximate cause(s); integrate a range of spatial scales; and, ideally, incorporate process models to explain the observed patterns and predicted trends in the future. Significant remaining challenges are tied to the ecology of disturbances. To meet these challenges, more effort is required to incorporate ecological principles and understanding into the assessments of disturbance worldwide.

AbstractSatellite‐derived indices of photosynthetic activity are the primary data source used to study changes in global vegetation productivity over recent decades. Creating coherent, long‐term records of vegetation activity from legacy satellite data sets requires addressing many factors that introduce uncertainties into vegetation index time series. We compared long‐term changes in vegetation productivity at high northern latitudes (>50°N), estimated as trends in growing season NDVI derived from the most widely used global NDVI data sets. The comparison included the AVHRR‐based GIMMS‐NDVI version G (GIMMSg) series, and its recent successor version 3g (GIMMS3g), as well as the shorter NDVI records generated from the more modern sensors, SeaWiFS, SPOT‐VGT, and MODIS. The data sets from the latter two sensors were provided in a form that reduces the effects of surface reflectance associated with solar and view angles. Our analysis revealed large geographic areas, totaling 40% of the study area, where all data sets indicated similar changes in vegetation productivity over their common temporal record, as well as areas where data sets showed conflicting patterns. The newer, GIMMS3g data set showed statistically significant (α = 0.05) increases in vegetation productivity (greening) in over 15% of the study area, not seen in its predecessor (GIMMSg), whereas the reverse was rare (<3%). The latter has implications for earlier reports on changes in vegetation activity based on GIMMSg, particularly in Eurasia where greening is especially pronounced in the GIMMS3g data. Our findings highlight both critical uncertainties and areas of confidence in the assessment of ecosystem‐response to climate change using satellite‐derived indices of photosynthetic activity. Broader efforts are required to evaluate NDVI time series against field measurements of vegetation growth, primary productivity, recruitment, mortality, and other biological processes in order to better understand ecosystem responses to environmental change over large areas.