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Le bocage constitue un des éléments essentiels du paysage dans la préservation de la ressource en eau, tant sur le plan qualitatif que quantitatif. Dans cet article, les auteures font le point des connaissances scientifiques sur le fonctionnement et le rôle des haies sur l’eau. Leur analyse concerne surtout l’Ouest de la France, une région à réseau hydrographique dense et nappes affleurant au niveaux des talwegs.

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Le bocage constitue un des éléments essentiels du paysage dans la préservation de la ressource en eau, tant sur le plan qualitatif que quantitatif. Dans cet article, les auteures font le point des connaissances scientifiques sur le fonctionnement et le rôle des haies sur l'eau. Leur analyse concerne surtout l'Ouest de la France, une région à réseau hydrographique dense et nappes affleurant au niveaux des talwegs.

To elucidate the functional relationship between intra-annual variations in vessel diameter and leaf emergence pattern of ring-porous and diffuse-porous deciduous trees in temperate forests, we determined the temporal relationships between leaf phenology and vessel formation. Cylindrical stem cores were periodically collected from each of five ring- and diffuse-porous species, and the leaf and vessel formation were observed simultaneously. In the ring-porous species, vessel formation began within 2 weeks of leaf appearance, and most wide vessels were formed within 2 weeks of full leaf expansion. More of the trees with intermediate-type formed medium-sized vessels or sparse wide vessels than trees with the flush-type leaf emergence, especially between full leaf expansion and the end of shoot elongation. Narrow vessel formation began 2-8 weeks after full leaf expansion in all specimens. The transition between the pore- and non-pore zones was abrupt in the flush-type species, and gradual in the intermediate-type species. In contrast, diffuse-porous species formed vessels 0-8 weeks after full leaf expansion. Our findings suggest that ring-porous species form leaves and vessels synchronously to accommodate water-transport requirements, whereas diffuse-porous species form these tissues asynchronously; thus, unlike the latter type, the former species change vessel diameters according to leaf formation. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to illustrate how some extreme events could affect forest ecosystems. Forest tree response can be analysed using dendroecological methods, as tree-ring widths are strongly controlled by climatic or biotic events. Years with such events induce similar tree responses and are called pointer years. They can result from extreme climatic events like frost, a heat wave, spring water logging, drought or insect damage… Forest tree species showed contrasting responses to climatic hazards, depending on their sensitivity to water shortage or temperature hardening, as illustrated from our dendrochronological database. For foresters, a drought or a pest disease is an extreme event if visible and durable symptoms are induced (leaf discolouration, leaf loss, perennial organs mortality, tree dieback and mortality). These symptoms here are shown, lagging one or several years behind a climatic or biotic event, from forest decline cases in progress since the 2003 drought or attributed to previous severe droughts or defoliations in France. Tree growth or vitality recovery is illustrated, and the functional interpretation of the long lasting memory of trees is discussed. A coupled approach linking dendrochronology and ecophysiology helps in discussing vulnerability of forest stands, and suggests management advices in order to mitigate extreme drought and cope with selective mortality. [Copyright &y& Elsevier]