Richesse sp�cifique et phytomasse des sous-bois de peupleraies cultiv�es en bordure de Garonne (Sud-Ouest de la France)

Abstract

Species richness and phytomass in understory cultivated poplar groves at the margin of the Garonne (South-West France). Cultivated poplar groves occupy an important part of the Garonne alluvial lowlands. Plant communities belonging to the understory, are disturbed by upkeep (disking), i.e. weeding by a metallic disk which totally destroys and buries vegetation. This upkeep differs among poplar groves: the intensity and the frequency of which decreases with the age of the poplar grove and it totally disappears during the years preceding clearing. The four poplar groves chosen for this study showed different upkeep levels. Understory species were sampled taking into account the species phenology and upkeep periods, in order to study the dynamics of the species richness and phytomass of plant communities in the understory between March et September. At the stand scale, the species richness of plant communities of the understory increased accordingly to the upkeep level (intensity and frequency). This trend was mainly due to annual species and secondarily to biennials ones. The analyses achieved from quadrat areas located at random in the studied poplar groves showed that the plant species richness of the understory communities increased, depending on the disturbance intensity induced by the upkeep, whereas this relation was inverted for phytomass. These two distinct trends were observed in inter-poplar groves comparison as well as in intra-poplar groves comparison. Interactions were observed between the general upkeep level of a poplar grove and the exposure to this upkeep of sites within this poplar grove. The species richness of sites gradually more exposed to the upkeep increased up to a threshold value, beyond which the species richness values decreased. This threshold was reached only for the poplar grove of which the general upkeep level was the most important. The results are discussed in relation to the ecological disturbance theory. When the upkeep frequency decreases, herbaceous, rhizomeous, then woody species take place. The understory plant communities of cultivated poplar groves constitute a rich and diversified zone. Their implantation into floodplain is important for adjacent ecosystem diversity.