Conséquences des invasions végétales sur le fonctionnement des écosystèmes riverains fluviaux

Dans le contexte des invasions, l'objectif de ce travail est d'évaluer dans quelle mesure l'origine des espèces végétales conditionne le fonctionnement d'un écosystème en situation de dominance. Cinq paires d'espèces autochtones dominantes (Agrostis stolonifera, Rubus caesius, Populus nigra, Urtica dioica et Salix alba) et introduites envahissantes (Paspalum distichum, Fallopia japonica, Buddleja davidii, Impatiens glandulifera et Acer negundo) ont été comparées en milieu riverain pour les processus de production primaire et de dégradation des litières. Ces études ont été complétées par une analyse détaillée des conséquences de l'invasion des ripisylves par A. negundo. Nos résultats montrent que si les espèces introduites envahissantes peuvent être plus efficaces dans la réalisation de certains processus écologiques, l'absence de coévolution entre les espèces introduites et les organismes des milieux récepteurs n'a pas d'implication systématique pour le fonctionnement des écosystèmes.The objective of this work is to evaluate plant species origin importance for biological invasion consequences, through the analysis of the influence of native and exotic dominant plant species for ecosystem functioning. Five pairs of native dominant species (Agrostis stolonifera, Rubus caesius, Populus nigra, Urtica dioica et Salix alba) and exotic invasive species (Paspalum distichum, Fallopia japonica, Buddleja davidii, Impatiens glandulifera et Acer negundo) were compared for litter breakdown process and primary production. A more detailed analysis evaluates the consequences of A. negundo invasion for riparian forests. Our results indicate that the lack of coevolution between exotic species and the organisms of recipient areas has little implications for ecosystem functioning, even if exotic invasive species can be more efficient in some ecological processes. No general pattern can be drawn for functional consequences of native dominant species replacement by exotic invasive ones

Enjeux de conservation d’une zone humide d’eau douce méditerranéenne exceptionnelle : la Garâa Sejenane (Tunisie septentrionale)

Cet article a pour but d’évaluer les enjeux de conservation concernant la végétation hydrophytique de la plus grande zone humide naturelle d’eau douce de Tunisie, la Garâa Sejenane. Les 55 relevés phytosociologiques réalisés révèlent une richesse floristique exceptionnelle, avec 74 hydrophytes dont 26 sont à forte valeur patrimoniale. Des analyses multivariées (AFC, CHA) effectuées sur ces données distinguent trois types d’habitats : des cultures inondables, des marais temporaires, et des pelouses humides/mares temporaires. Leur cartographie montre une structure en 3 zones concentriques : une ceinture externe constituée de pelouses humides, de mares temporaires et de prairies fourragères sèches, une zone intermédiaire occupée par des marais temporaires et des prairies fourragères humides, et une zone centrale formée de cultures inondables. Un Indice de Rareté des Espèces très élevé, et des espèces à fort enjeu conservatoire (1 endémique stricte à la Garâa Sejenane, 5 espèces dont elle constitue l’unique localité tunisienne, et 2 espèces dont elle abrite les plus grandes populations nord-africaines) révèlent l’intérêt écologique et conservatoire majeur du site, et ceci malgré de fortes pressions anthropiques subies depuis les années 60. Dans le contexte actuel d’augmentation de la pression de perturbation, la conservation à long terme de cette biodiversité exceptionnelle nécessite la mise en place de mesures de gestion appropriées intégrant les populations locales.This study addresses the conservation issues of the hydrophytic vegetation of Garâa Sejenane, the largest natural freshwater wetland in Tunisia. Fifty-five phytosociological relevés reveal the exceptional floristic wealth, including 74 hydrophytes of which 26 are patrimonial species. Multivariate analyses (CA, AHC) differentiate three types of habitats: flooded crops, temporary marshes, and wet lawns/temporary pools. Their mapping shows concentric zones, including an external belt of wet lawns, temporary pools and dry fodder meadows, an intermediate belt of temporary marshes and wet fodder meadows, and a central area of flood crops. A high Species Rarity Index and the occurrence of several species with high conservation values (1 strictly endemic species to the Garâa Sejenane, 5 others that exist in Tunisia only in this site, and 2 species whose local populations are the largest in North Africa) confirm the major ecological and conservatory interest of the site, despite anthropogenic pressures on the Garâa Sejenane since the sixties. In the present-day context of increasing human pressure, the long-term conservation of this exceptional biodiversity needs the implementation of targeted management measures integrating local populations

Light interception principally drives the understory response to boxelder invasion in riparian forests

Since several decades, American boxelder (Acer negundo) is replacing white willow (Salix alba) riparian forests along southern European rivers. This study aims to evaluate the consequences of boxelder invasion on understory community in riparian areas. We determined the understory species richness, composition and biomass in boxelder and white willow stands located in three riparian forests, representative of three rivers with distinct hydrological regimes. We investigated correlation of these variables to soil moisture and particle size, main soil nutrient stocks, potential nitrification and denitrification, tree canopy cover and photosynthetic active radiation (PAR) at the ground level. A greenhouse experiment was then conducted to identify the causal factors responsible for changes in the understory. The effect of soil type, PAR level and water level on the growth and the biomass production of Urtica dioica were examined. A lower plant species richness and biomass, and a modification of community composition were observed for boxelder understory in all sites, regardless of their environmental characteristics. The strongest modification that follows boxelder invasion was the decline in U. dioica, the dominant species of the white willow forest understory. These differences were mainly correlated with a lower incident PAR under boxelder canopy. The greenhouse experiment identified PAR level as the main factor responsible for the changes in U. dioica stem number and biomass. Our results indicate that adult boxelder acts as an ecosystem engineer that decreases light availability. The opportunistic invasion by boxelder leads to important understory changes, which could alter riparian ecosystem functioning

Late Glacial and Holocene vegetation history in the northern foothills of the Adirondack Mountains

International audiencehe postglacial history of vegetation at a regional scale is mainly governed by climate. However, the action of other environmental factors can lead to differentiation of the long-term vegetation dynamics in locations that are in proximity. In this context, the vegetation history of Covey Hill, at the northern tip of the Adirondack Mountains, was reconstructed using pollen analysis of a sediment core collected in a bog near the hilltop. This history was then compared with that of other sites in the Adirondacks, in the St. Lawrence Lowlands and in the Appalachian foothills to determine whether differentiations had occurred between sites across the landscape. Regionally, climate seems to have been the main driver of vegetation development. Differences between sites are most pronounced in regard to relative abundance of aspen and alder, and mostly occurred during the Late Glacial and early Holocene. Unlike other sites, fires occurred frequently on Covey Hill for several millennia, and most probably allowed the long-term maintenance of a rare pine barren. Finally, drier conditions are probably partly responsible for the hemlock decline evident around 5100 cal BP.L’histoire postglaciaire de la végétation à l’échelle régionale est principalement régie par le climat. Cependant, l’action d’autres facteurs écologiques peut mener à des différentiations dans la dynamique de la végétation pour des sites relativement proches spatialement. Dans ce contexte, l’histoire de la végétation de la colline de Covey, qui constitue l’extrémité septentrionale de la chaîne des Adirondacks, a été reconstituée grâce à l’analyse pollinique d’une carotte sédimentaire prélevée dans une tourbière près du sommet de la colline. Cette histoire a été comparée avec celle d’autres sites dans les Adirondacks, dans les basses-terres du St-Laurent et dans les contreforts des Appalaches afin de voir si des différenciations étaient survenues entre les sites à l’échelle du paysage. Régionalement, le climat semble avoir été le moteur principal de l’amorce des grandes étapes de végétation. Les principales différences entre les sites sont surtout survenues pendant le Tardiglaciaire et l’Holocène inférieur et concernent surtout l’abondance relative du peuplier et de l’aulne crispé. Des feux sont survenus fréquemment sur la colline. Ils ont probablement permis le maintien à long terme d’une rare lande de pins. Enfin, des conditions climatiques plus sèches seraient probablement en partie responsables du déclin de la pruche vers 5100 ans BP

Vegetation history of the western Rif mountains (NW Morocco): origin, late-Holocene dynamics and human impact

International audienc

History and conservation of Tunisia's largest freshwater wetland: Garâa Sejenane

International audienceThe Garâa Sejenane is an ancient lake in northern Tunisia that currently houses a mosaic of temporary wetlands. Comparison of modern pollen data and field vegetation surveys shows both a weak regional pollen signal and the homogenization of pollen assemblages at the scale of the garâa. Despite a hiatus between 19,000 and 2230 cal. BP, fossil pollen analyses provide the first Tunisian LGM record and attest to the ancient origin of the lake, which housed some taxa still present locally in the 1950s. The originality of the Pleniglacial surrounding landscape dominated by a juniper steppe contrasted with the mountain conifer forests in Kroumiria. Lastly, analyses of aerial photographs (1948, 1962, 1984), recent satellite photographs and previous botanical descriptions reveal that the lake has been drying since 1960 because of important anthropogenic activities (drainage, groundwater pumping, cultivation, grazing…). These disturbances have generated profound changes in hydrophytic plant communities (fragmentation, diversity loss, introduction of invasive species…) that are likely to threaten their long-term survival

Effects of hydrology on recruitment of Pilularia minuta Durieu (Marsileaceae), an endangered plant of Mediterranean temporary pools

International audienceHydrological requirements for germination and development of P. minuta were assessed through a 5-year field survey in Garâa Sejenane, Tunisia, and an experiment under controlled conditions. The spatial distribution of P. minuta was recorded in the field annually, while the experiment tested the effects of water levels and flooding dates on germination (the emergence of new individuals) and development. Water level was found to be the major factor affecting the germination and the development of P. minuta with flooding date as a secondary limiting factor. For germination to occur, the sediment must be completely waterlogged. Water depths of 5–10 cm are optimal for plant development. In addition, the plant needs a minimal flooding period of seven weeks to develop. Late-spring precipitation appears to be more critical than earlier rains

Conservation issues of an exceptional freshwater Mediterranean wetland in Northwest Tunisia: Garâa Sejenane.

International audienceThisstudy addressesthe conservation issues ofthe hydrophytic vegetation of Garâa Sejenane,the largest natural freshwater wetland in Tunisia.Fifty-five phytosociological relevés revealthe exceptional floristic wealth, including74 hydrophytes ofwhich26are patrimonialspecies.Multivariate analyses(CA, AHC) differentiatethree types of habitats: floodedcrops, temporary marshes, and wet lawns/temporary pools. Their mapping showsconcentric zones, includinganexternal beltof wet lawns, temporary pools and dry fodder meadows, an intermediate belt of temporary marshes and wet foddermeadows,and a central area offlood crops. A high Species Rarity Index and the occurrence of several species with high conservationvalues(1 strictly endemic species to the Garâa Sejenane, 5 others that exist in Tunisia only in this site,and 2 species whoselocal populations are thelargestin NorthAfrica) confirm the major ecological and conservatory interest of the site, despite anthropogenic pressures on the Garâa Sejenane since the sixties. In the present-day context of increasing human pressure, the long-term conservation of this exceptional biodiversity needsthe implementation of targetedmanagement measures integrating local populationsEnjeux de conservation d’une zone humide d’eau douce méditerranéenne exceptionnelle: la Garâa Sejenane (Tunisie septentrionale).—Cet article a pour but d’évaluer les enjeux de conservation concernant la végétation hydrophytique de la plus grande zone humide naturelle d’eau douce de Tunisie, la Garâa Sejenane. Les 55 relevés phytosociologiques réalisés révèlent une richesse floristique exceptionnelle, avec 74 hydrophytes dont 26 sont à forte valeur patrimoniale. Des analyses multivariées (AFC, CHA) effectuées sur ces données distinguent trois types d’habitats: des cultures inondables, des marais temporaires, et des pelouses humides/mares temporaires. Leur cartographie montre une structure en 3 zones concentriques: une ceinture externe constituée de pelouses humides, de mares temporaires et de prairies fourragères sèches, une zone intermédiaire occupée par des marais temporaires et des prairies fourragères humides, et une zone centrale formée de cultures inondables. Un Indice de Rareté des Espèces très élevé, et des espèces à fort enjeu conservatoire (1 endémique stricte à la Garâa Sejenane, 5 espèces dont elle constitue l’unique localité tunisienne, et 2 espèces dont elle abrite les plus grandes populations nord-africaines) révèlent l’intérêt écologique et conservatoire majeur du site, et ceci malgré de fortes pressions anthropiques subies depuis les années 60. Dans le contexte actuel d’augmentation de la pression de perturbation, la conservation à long terme de cette biodiversité exceptionnelle nécessite la mise en place de mesures de gestion appropriées intégrant les populations locale

Holocene history of peatland communities of central Rif (Northern Morocco)

International audienceThe Maghreb's Mediterranean coastal edge is home to many peatlands with boreal floristic affinities and high conservation value. The work aims at investigating the Holocene developmental history of these wetlands in Northern Morocco, based on pollen record, losson-ignition and radiocarbon dating. First, the comparison between modern pollen spectra and present-day hydrophytic vegetation makes it possible to identify the local pollen signal. Second, while peatlands have existed in the Rif for at least 13,000 years, their initiation mostly by paludification extends throughout the Holocene. Their plant composition remains relatively unchanged for millennia, but swamp undergrowth could locally experience changes in taxa dominance. Mid-to late-Holocene sedimentation changes appear to result from the human-induced regional decline of cedar. The major changes experienced by wetlands occurred recently, due to clearing, burning and overgrazing. Effective conservation policies should be urgently implemented in order to save the last relict peatlands of Northern Morocco