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Chablis et diversité des coléoptères en forêt feuillue de plaine : impact à court terme de la trouée, de sa surface et de son contexte paysager

Abstract : Windstorm is a major natural disturbance in temperate forests. This thesis deals with the impact of the Lothar storm (december 1999) on insect diversity in managed deciduous lowland forests in the Brie region (France, Seine-et-Marne). Are there short-term gap effects and gap size effects on beetle assemblages in uncleared gaps in 2001 ? Moreover, does the response of beetle assemblages also depend on local conditions within gaps and on the landscape pattern around gaps ? I used two interception trapping methods in order to sample beetles at ground (pitfall traps) and air (window flight traps) levels, and I sampled 24 windthrow gaps (ranging from 0.1 to 3.3 ha in area), 14 closed-forest controls and 12 open stands (seed cuts and sapling stands). I studied all the beetles sampled by window traps at the family level, and the following families at the species level : epigaeic and aerial Carabidae, Scolytinae and other saproxylic families. 125 000 beetles were sampled, that belong to 66 families, 93 carabid species and 185 saproxylic species. Gap effect. In the second year after the windstorm, in response to variations in micro-habitats and resources (dead wood, root plates, flower mats, preys~), the beetle assemblages clearly changed in gaps for all the study groups. At the family level, the abundance of aquatic, flower-visiting and zoophagous beetles increased in gaps, probably due to the local quantity of temporary ponds below root plates, of flowers (Rubus, etc.) and to the number of preys, respectively. The phytophagous terrestrial beetles and the opophagous beetles did not respond positively to the development of the vegetation in the overstorey and to sap flows from wounded trees, respectively. The abundance of pioneer and secondary xylophagous beetles did not vary in gaps compared to controls. However, at the species level, the composition of saproxylic assemblages in gaps and controls was significantly different. The cumulative richness of saproxylic species (except scolytids) increased in gaps. Various individual responses were detected : about 15 species were characteristic of closed forest and strongly decreased in gaps, whereas more than 30 species were typical of gaps. The total carabid richness and the richness of open-land species were greater in gaps than in controls. The group of forest species decreased in abundance and local richness in pitfall traps in gaps, and several species underwent severe local reductions. In contrast, some forest taxa were not affected or were even favoured, and no species disappeared in the whole studied gaps. Inside gaps, some key elements, such as down tree crowns, herbs and dead wood contributed to species distribution. Gap size effect. The gap area triggers some variations in beetle richness and composition, by changing the micro-climatic conditions and the quality and quantity of micro-habitats. Many carabid and saproxylic taxa are characteristic of mid-size and large gaps, but none in small gaps. The higher cumulative richness occurred in large gaps for Carabidae, and in mid-size gaps for saproxylic beetles. Effect of landscape pattern. The gap isolation (i.e. a low cover of open-land areas around gaps) reduced the colonisation of gaps by open-land carabid species. The cover of cutting areas around gaps influenced the scolytid assemblages in gaps. The other results are difficult to interpret, as they depend on the study scale. The community composition in gaps was significantly different from controls and open stands, particularly for saproxylic beetles ; their richness was higher in natural gaps thanks to the great number of species related to fresh and sun-exposed dead wood. The gaps act as open areas rich in dead wood, and they take part to the increase of the gamma diversity in a forest. The maintenance of unlogged gaps and the half-salvaging of others are discussed as management opportunities. The results related to gap size and gap context, are discussed in terms of forest landscape planning (size and spatial distribution of cuts). Obviously, traditional fellings and uncleared gaps significantly differ, especially in terms of volume and diversity of dead wood. Because of the high number of species characteristic of mid-size and large gaps, it would be interesting to carry out some mid-size fellings (about 0.5 ha in area) and to maintain a high volume and a high diversity of dead-wood within these fellings. This management option has not been tested yet.
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  • HAL Id : tel-02582792, version 1
  • IRSTEA : PUB00013375

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Christophe Bouget. Chablis et diversité des coléoptères en forêt feuillue de plaine : impact à court terme de la trouée, de sa surface et de son contexte paysager. Sciences de l'environnement. Thèse pour obtenir le grade de Docteur du Museum National d'Histoire Naturelle spécialité "Ecologie", MNHN Paris, 2004. Français. ⟨tel-02582792⟩

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