Understory plant communities of cultivated polar groves: relationship between phytomass, species richness and disturbance

Understory plant communities of cultivated poplar groves are submitted to strong disturbances essentially due to mechanical upkeep. Upkeep consists of "disking", i.e. weeding by a metallic disk which totally destroys and buries vegetation. This upkeep differs among poplar groves: the intensity decreases with tree age and it totally disappears during the years preceding clearing. With canopy closure, these differences lead to a modification in species communities. The four poplar groves chosen for this study showed different upkeep levels. Phytomass and species richness were studied by sampling vegetation at its stage of maximal growth, just before upkeep periods. As the intensity of upkeep treatment decreases with time, communities exhibited different successional stages, from annual pioneer, meadow perennial and woodland ligneous species. The results show that species richness increases with upkeep intensity, whereas phytomass values decrease. Most of these phytomasses were between 250 and 1 000 g m(-2) with a maximum at 2 000 g m(-2) in the oldest and less maintained poplar groves. For strong disturbance levels, species richness was high and reached a maximum for moderate phytomasses. However, the steady state existing in abandoned poplar groves exhibited strong phytomass accumulation near trees. On the one hand, community species enrichment mainly depended on disturbance levels induced by upkeep. On the other hand, the flora enrichment was due to maturation of plantation. We can observe an increase of species, characteristic of a more stable site. This phenomenon exists only in the oldest plantations. (C) 1999 Editions scientifiques et medicales Elsevier SAS.