Pierre Arpin

Soil acidification under the crown of oak trees. I. Spatial distribution

Thirty oak trees (Quercus robur L. sensu lato) growing on the same site were selected on the basis of their stem diameter (Dbh ≥ 0.625 m) and their effect on soil properties was assessed. Litterfall, old litter accumulation, acidity and buffering capacity of the A1 horizon were measured at three distances from the trunk (0.4, 1.4 and 2.4 m) and in four directions (N, E, S, W). Results led to the conclusion that on average more acidification and litter accumulation occurred near the trunk base and in the north direction, but this general trend was far from being followed by every tree. The clay content of the soil was inversely related to acidification and litter accumulation. Interrelationships between soil organisms, crown leaching, bark substances and the parent rock are proposed as working hypotheses for the future of this study.

Diversity and dynamics of eco-units in the biological reserves of the Fontainebleau Forest (France): Contribution of soil biology to a functional approach

Beech integral biological reserves of the Fontainebleau forest (France) display varied site conditions due to geomorphological heterogeneity and to interactions between biological components of the ecosystem. Taking examples in shifts observed in plant communities following gap opening, the authors show that, as Oldeman viewed it, tree-fall gaps seem to be the driving force in sylvigenesis as well as a source of spatial biodiversity. Studies carried out on macromorphological features of humus profiles and on the behaviour of soil invertebrate communities (Lumbricidae and Nematoda) pointed out two key aspects of forest functioning. First, the renewal of the forest ecosystem is linked to the dynamics of humus forms and of soil animal functional groups, featuring the regeneration of trees. Second, tree-fall gaps are places where the forest ecosystem is destabilized and thereafter may renew itself or on the contrary may evolve towards another ecosystem, showing either a co-adaptation between the sylvigenetic and the edaphic cycle, or a discordance between these two cycles. These two aspects (co-adaptation and discordance), important from the point of view of fundamental ecology and forest management, suggest a need for further field research.