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Dépérissement forestier en vallée du Rhin. Etude du bilan hydrique des chênaies de la forêt domaniale de La Harth (Haut-Rhin) et impact des épisodes de sécheresse sur la croissance radiale des chênes

Abstract : The purpose of this study was to identify and quantify soil water constraints in oaks stands from the Harth Forest and to evaluate their possible involvement in the recent decline of oaks (tree mortality and reduction of growth). Water shortage in oak-forest ecosystems in the Harth Forest has been quantify by soil water balance modelling (Granier et aL, 1999). taking into account both local pedological and climatic condition. Rainfall is highly variable form north (640 mm) to south (800 mm) of the forest. Extractable water from the soil of each of the 70 plots used for tree-ring measurements (Bréda. 1998) has been calculated from soil description. Soils were sandy with high stones amount leading to low extractable water (80-100 mm). Leaf area index (LAI) of stand canopy and duration of leafly period have been measured. Transpiration of covering herb layer was measured using close chambers at different dates in the year. Transpiration was strongly related to the radiation reaching the herb layer : maximum transpiration of herb was observed before oak budburst but sharply decreased as LAI of oak canopy developed. The daily water balance proposed to ca1culate seasonal pattern of soil water content during the year was validated for the local condition by direct measurements of soil water content (TDR) and throughfalI. Extractable water was estimated in situ from soil water content measurements and water uptake was observed till 2 m depth. Soil water dynamic computed by the model compared weIl with observed measurements. Water balance was then reconstructed from 1964 by modelling. using daily climatic data. Radial growth as previously measured from tree-rings was reconstructed from monthly temperature and rainfall. Cumulated rainfaIl during May, June and July explained the largest part of year-to-year growth variability (46% for sessile oak. 41% for pedonculate oak). Nevertheless. during the recent time 0964-1994) exhibiting a decrease in radial growth and a dieback in oaks from both species. monthly c1imatic data were not suitable to reproduce year-to-year variability in tree-rings and differences between oak species. Drought events were then computed from retrospective modeIling of soil water balance for each of the 70 plots. using its own extractable water and LAI. An index of soil water deficit was computed from duration and intensity of water shortage during the leafly period. occurring below a threshold of soil water content inducing stomatal closure « 40% of extractable water). Soil water deficit was significantly correlated to radial growth. The reduction in radial growth of pedonculate oaks started during the 1972 drought event and then progressively decreased as compared to the one of sessile oaks after 1976, 1983 and 1985 period of water shortage. The most severe drought event from the last 30 years was quantify in 1989. which was probably responsible for oak ecosystems dieback leading to the establishment of the "Observatoire Ecologique de la Harth". Moreover, this study pointed out specifie response to climate of each oak species. pedonculate oak being more sensitive to the growth from the previous year. Finally. daily water balance ca1culation was significantly more efficient than monthly c1imatic data to detect water shortage reducing radial growth of both oak species.
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Submitted on : Sunday, June 7, 2020 - 1:42:40 PM
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  • HAL Id : hal-02839880, version 1
  • PRODINRA : 118145



Nathalie Bréda, Marianne Peiffer. Dépérissement forestier en vallée du Rhin. Etude du bilan hydrique des chênaies de la forêt domaniale de La Harth (Haut-Rhin) et impact des épisodes de sécheresse sur la croissance radiale des chênes. [Contrat] 1999. ⟨hal-02839880⟩



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