Titre
Variations in riparian forests: Exploring interactions among plant communities, organic matter, soil, and water in the Grande Cariçaie nature reserves
Type
thèse de doctorat
Institution
UNIL/CHUV/Unisanté + institutions partenaires
Auteur(s)
Siegfried, Lila
Auteure/Auteur
Directrices/directeurs
Vittoz, Pascal
Directeur⸱rice
Verrecchia, Eric
Codirecteur⸱rice
Liens vers les personnes
Liens vers les unités
Faculté
Université de Lausanne, Faculté des géosciences et de l'environnement
Statut éditorial
Accepté
Date de publication
2025-04-14
Langue
anglais
Résumé
Floodplains, which include riparian forests, are important places for biodiversity, flood regulation,
climate impact mitigation, and recreation. They also represent strong cultural and historical
heritage. In Central Europe, these environments have been extensively destroyed and degraded
during the last centuries. For example, the water levels of most large lakes in Switzerland have
been regulated. Lake Neuchâtel is one of these lakes and constitutes a perfect case study. Its
water level was lowered by approximately 3 m 150 years ago, and then adjustments were made to
reduce water level variations, promoting vegetation colonization and soil formation on previously
submerged surfaces. These new lands began to be set aside in 1967, and in 2002, they became
the nature reserves of the Grande Cariçaie, the largest lake marsh complex in Switzerland.
Despite the importance of riparian ecosystem services, the effects of the reduction of the water
variations on forest dynamics have not been extensively studied. Therefore, it is necessary to
better understand these regulated riparian forests to efficiently preserve them. The objectives of
this research are: (i) to understand the recent evolution of the plant communities; (ii) to estimate
the importance of the different compartments of organic matter and to study its dynamics through
its production, decomposition, and accumulation; and finally (iii) to describe the different forms
of organic matter and its integration in soils in different types of riparian forest habitats.
For this purpose, historical floristic surveys (1970-1980) were compared to recent surveys (2020).
Inventories were grouped into plant community types and changes in environmental conditions
were assessed using ecological indicators. The species compositions of the tree and shrub strata
were compared to predict future forest evolution. In addition, precise measurements were
performed of the production, decomposition, stability, and integration of organic matter at
selected sites with contrasting moisture conditions and plant communities. The soil macrofauna
was inventoried and soils were described using conventional field observations and parameters
as well as micromorphology.
Four forest community types were identified, dominated respectively by Alnus glutinosa, A.
incana, Fraxinus excelsior, and Pinus sylvestris. Over a period of 40 years, some sites have
changed their community types, particularly from wetter A. incana forests to drier Fraxinus
forests. A loss of diversity in hygrophilous and heliophilous species was observed. Regeneration
of the dominant tree species was either weak or absent. Owing to drying conditions, riparian
forests have lost their characteristics, with significant changes in species composition, increased
canopy density, and loss of diversity at the landscape scale.
Two distinct organic matter cycles were observed in these environments. Wetter forests (Alnus
incana and A. glutinosa) with groundwater table close to the surface were characterized by rapid
decomposition, low soil respiration, low macrofaunal biomass, and water-related humus forms.
In contrast, less humid forests (Fraxinus and Pinus) with a summer groundwater table deeper than
70 cm or absent showed slower decomposition, higher summer respiration, higher macrofaunal
biomass, and mull-type humus forms. Furthermore, organic matter accumulation and
stabilization were mainly influenced by physicochemical conditions, depending on the parent
material and moisture. However, soil texture and bioturbation played a key role in the integration
of organic matter into the soil.
In conclusion, this study emphasized the depletion of plant community characteristics as a result
of the reduction in lake-level variations and highlighted the complexity of the processes governing
organic matter dynamics in young riparian soils.
climate impact mitigation, and recreation. They also represent strong cultural and historical
heritage. In Central Europe, these environments have been extensively destroyed and degraded
during the last centuries. For example, the water levels of most large lakes in Switzerland have
been regulated. Lake Neuchâtel is one of these lakes and constitutes a perfect case study. Its
water level was lowered by approximately 3 m 150 years ago, and then adjustments were made to
reduce water level variations, promoting vegetation colonization and soil formation on previously
submerged surfaces. These new lands began to be set aside in 1967, and in 2002, they became
the nature reserves of the Grande Cariçaie, the largest lake marsh complex in Switzerland.
Despite the importance of riparian ecosystem services, the effects of the reduction of the water
variations on forest dynamics have not been extensively studied. Therefore, it is necessary to
better understand these regulated riparian forests to efficiently preserve them. The objectives of
this research are: (i) to understand the recent evolution of the plant communities; (ii) to estimate
the importance of the different compartments of organic matter and to study its dynamics through
its production, decomposition, and accumulation; and finally (iii) to describe the different forms
of organic matter and its integration in soils in different types of riparian forest habitats.
For this purpose, historical floristic surveys (1970-1980) were compared to recent surveys (2020).
Inventories were grouped into plant community types and changes in environmental conditions
were assessed using ecological indicators. The species compositions of the tree and shrub strata
were compared to predict future forest evolution. In addition, precise measurements were
performed of the production, decomposition, stability, and integration of organic matter at
selected sites with contrasting moisture conditions and plant communities. The soil macrofauna
was inventoried and soils were described using conventional field observations and parameters
as well as micromorphology.
Four forest community types were identified, dominated respectively by Alnus glutinosa, A.
incana, Fraxinus excelsior, and Pinus sylvestris. Over a period of 40 years, some sites have
changed their community types, particularly from wetter A. incana forests to drier Fraxinus
forests. A loss of diversity in hygrophilous and heliophilous species was observed. Regeneration
of the dominant tree species was either weak or absent. Owing to drying conditions, riparian
forests have lost their characteristics, with significant changes in species composition, increased
canopy density, and loss of diversity at the landscape scale.
Two distinct organic matter cycles were observed in these environments. Wetter forests (Alnus
incana and A. glutinosa) with groundwater table close to the surface were characterized by rapid
decomposition, low soil respiration, low macrofaunal biomass, and water-related humus forms.
In contrast, less humid forests (Fraxinus and Pinus) with a summer groundwater table deeper than
70 cm or absent showed slower decomposition, higher summer respiration, higher macrofaunal
biomass, and mull-type humus forms. Furthermore, organic matter accumulation and
stabilization were mainly influenced by physicochemical conditions, depending on the parent
material and moisture. However, soil texture and bioturbation played a key role in the integration
of organic matter into the soil.
In conclusion, this study emphasized the depletion of plant community characteristics as a result
of the reduction in lake-level variations and highlighted the complexity of the processes governing
organic matter dynamics in young riparian soils.
PID Serval
serval:BIB_87F965D8BA40
Date de création
2025-04-16T14:16:52.507Z
Date de création dans IRIS
2025-05-20T23:36:47Z
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Nom
these-ok-sl.pdf
Version du manuscrit
imprimatur
Licence
https://creativecommons.org/licenses/by/4.0
Taille
47.11 MB
Format
Adobe PDF
PID Serval
serval:BIB_87F965D8BA40.P002
URN
urn:nbn:ch:serval-BIB_87F965D8BA409
Somme de contrôle
(MD5):8b0bb6e6bd7725ad834a276936076392