Dynamique des populations et mosaïque forestière en hêtraie-sapinière naturelle préservée. Une comparaison entre Vosges et Carpates

A comparison of population dynamics and forest architecture has been carried in two well preserved beech-fir forests of Europe, in the Carpates and in the Vosges. The two reserves present marked differences in climate, soils and local environment. We show that beech dominates all the strata. In a situation of valley bottom (Carpates), its shady canopy explains growth patterns of understorey. Beech and fir present alternating phases of suppression and re-growth. Seedlings and saplings grow preferentially at the edges of adult canopies and in gaps. In the Vosges, slope induces imbricated crowns, and lateral light inputs are higher than in the Carpates. However regeneration is less dense because of less favourable soils and climate, and located preferentially in the neighbourhood of gaps. Both forests present a fine-grained architecture of eco-units, which increase resistance to disturbances.Une recherche comparative de sylvigénèse en hêtraie-sapinière a été entreprise dans deux réserves forestières intégrales, présentant des différences écologiques marquées dans les Hautes Vosges et les Carpates orientales. Les données recueillies concernent la structure des populations de ligneux (hêtre, sapin, épicéa, érable sycomore) et l’organisation architecturale des mosaiques forestières, par deux profils architecturaux, qui illustrent un aspect des situations sylvigénétiques rencontrées dans ces forêts de montagnes. Les principaux résultats démontrent que le hêtre domine les strates des deux forêts. En situation de fond de vallon (Carpates), l’ombrage qui résulte d’une canopée dense impose aux arbustes dominés (jeunes hêtres, mais surtout sapins) une croissance en palier parfois très longue, qui explique les situations préférentielles des semis en bordure des éco-unités matures ou dans les trouées. En situation de pente (Vosges), l’architecture en pile d’assiette des dominants, qui résulte de l’importance des apports lumineux latéraux, raccourcit cette attente. Mais les conditions plus rudes rencontrées dans cette réserve diminuent les densités à tous les étages. L’architecture des deux réserves montre une mosaïque fine d’ éco-unités à différentes hauteurs, composées de plusieurs espèces ligneuses atteignant parfois des dimensions impressionnantes. L’ensemble est très stable, en dépit des aléas climatiques.Schnitzler A., Closset-Kopp Deborah, Gafta D., Cristea Victoria, Schwoehrer C. Dynamique des populations et mosaïque forestière en hêtraie-sapinière naturelle préservée. Une comparaison entre Vosges et Carpates . In: Revue d'Écologie (La Terre et La Vie), tome 59, n°1-2, 2004. pp. 213-228

Silvicultural disturbance has little impact on tree species diversity in a Central African moist forest

Timber production is an important economic sector in most forested countries of Central Africa, where about 14 million hectares of lowland moist forests are now planned for management. This production is expected to be sustainable, but the actual impact of logging on biodiversity is still questioned. To answer this question, we used a unique long-term controlled experiment implemented more than 20 years ago in an old-growth semi-deciduous moist forest of the Central African Republic (CAR). We tested whether (i) anthropogenic disturbances associated with silvicultural operations had an effect on the composition and diversity of tree communities, and (ii) there is a relationship between diversity and disturbance intensity in those forests. For this, we botanically identified all treesP10 cm DBH in 28 1-ha plots where no treatment (controls), logging and logging + thinning operations were implemented 24 years ago and created a strong gradient of disturbance. We investigated the relationships between five diversity indices and a disturbance index calculated for each 1-ha plot, for all species and separately for three regeneration guilds. We found a strong positive monotonic relationship between the intensity of disturbance and the percentage of pioneer species in the tree communities, which proved to be equally detrimental, in terms of relative abundance, to the non-pioneer light-demanding and the shade-bearing species. Overall, disturbance appeared to have a weak monotonous negative effect on diversity, irrespective to the index considered. The diversity of shade-bearers slightly decreased along the disturbance gradient without significant decrease in species density; disturbance had no effect on non-pioneer light demanders, but a clear significant negative effect on the diversity of pioneers, with a significant decrease in species density. This negative effect was associated with the massive recruitment of the early-successional, fast-growing Musanga cecropioides R. Br. (Urticaceae), which rapidly preempted space and resources in the most disturbed plots. Despite this effect, disturbance did not significantly affect the local heterogeneity in species distribution. These results suggest that the semi-deciduous moist forests of CAR are locally resilient to small-scale disturbances associated with silvicultural operations. This may be a consequence of the past anthropogenic and/or climatic disturbances, which have been stronger and more long-lasting than elsewhere within the tropical forest biome, and would have removed the most vulnerable species. Because logging intensity in these forests is usually low, we do not expect any direct major impact on tree species diversity, at least after the first felling cycle

Biodiversité, évolution et fonctionnement des écosystèmes: Prospectives de l’Institut Ecologie et Environnement du CNRS,

International audienc

Biodiversité, évolution et fonctionnement des écosystèmes: Prospectives de l’Institut Ecologie et Environnement du CNRS,

International audienc