Jean-François Picard

Potential contribution of the seed bank in coniferous plantations to the restoration of native deciduous forest vegetation

In this study, we compared the soil seed bank and current vegetation under coniferous plantations and adjacent native deciduous forests. The objective was to assess how much of the initial plant diversity is retained in such plantations, and the potential to restore this initial plant community from seed bank in case of reversion to broadleave stands. Four stands growing side by side and with different dominant species were selected at two locations (site of Haye: Quercus petraea, Pseudotsuga menziesii, Pinus sylvestris and Picea abies; site of La Petite-Pierre: Quercus petraea, Fagus sylvatica, Pinus sylvestris and Picea abies). In each stand, ground vegetation was surveyed and soil seed bank was sampled. Composition of ground flora and seed bank of stands were quite different: only 11 to 30 % of the species were in both the ground flora and the seed bank. Composition of the seed bank was mainly influenced by site location and sylvicultural practices such as the type of afforestation or the tree cover. Species richness of seed banks and vegetation were higher in the site of Haye than in the site of La Petite-Pierre. Seedling density strongly decreased with stand age. Whereas between 65 and 86 % of species found in the ground vegetation of native deciduous stand were also present in the understory or the seed bank of mature coniferous stands, this was only about 50 % in young coniferous stands. Species of deciduous stands which were absent from coniferous stands were typical of old forests. In contrast, species mainly found in the coniferous stands were often ruderal. In the studied areas, it would be possible to restore up to 86 % of the native deciduous forest vegetation, but some plant species typical of ancient forests may have disappeared during the coniferous stage.

Long-term Changes in Forest Productivity in Northeastern France: the Dendroecological Approach

Since the early 1980s, the question of “global change” has become a matter of increasing concern in Europe as well as in North America. This global change is expected as a result of the increase of so-called “greenhouse” gases in the atmosphere, of which carbon dioxide (CO2) is the most important. If the predicted doubling of the present CO2 concentration is realized by the year 2050, it is expected that the Earth will experience a global warming of between 1.5 and 4°C (e.g. Hansen et al. 1984; Washington and Meehl 1984; Wetherald and Manabe 1986; Wilson and Mitchell 1987; Schlesinger and Zhao 1987; Wigley and Raper 1992). The expected increase in global precipitation is between 7 and 15%. The regional details of expected temperature and precipitation changes are, however, very uncertain.

Long-Term Growth Trends of Trees: Ten Years of Dendrochronological Studies in France

The study of long-term growth trends in French forests began 10 years ago at the Phytoecological Laboratory of the National Agronomic Research Institute (INRA). Very large surveys have been carried out in several regions and for many species, allowing the study of changes in radial growth of individual trees during the past 150 years. In all cases, a significant increasing growth trend appeared. It varied between +50% and +160% depending on species and location. Careful analysis of possible bias has been made. Beside these biases, possible causes of such trends are discussed. This chapter summarizes the main methods and results of our laboratory. For additional information, our readers may refer to the cited articles.

Effects of the clear-cutting of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantation on chemical soil fertility

Abstract• Stand harvesting and regeneration were usually considered to be a critical phase for the sustainability of forest soils. The present study concerned the effects on soil chemical fertility of the clear-cutting of a highly productive Douglas-fir stand aged 67 years that was clear-cut with no disturbance.• Results showed that soil changes were rapid in the three-year period following the cutting. The forest floor mass considerably decreased and the mineral soil showed a limited but real acidification. Soil losses represented 4% of the available nutrients over a depth of 60 cm for N, 22% for K, 25% for Ca and 32% for Mg. Only P increased by 11%. Due to the spatial variability of forest soils, and despite regular re-sampling, confidence intervals were large and difficult to reduce.• The reversibility of the effects of the clear-cutting and its consequences on soil functions depend on the element: it should not be a problem for C, N and K, which would recover when the biological cycle is re-established once again in the young stand. Phosphorus is not at issue since it changes form in the soil. The depletion of Ca, and to a lesser extent of Mg, is of some concern as a result of limited soil reserves, the limited flux of cations from the mineral changes in the soil, the relatively large part of Ca and Mg in the forest floor, and the negative input-output budgets for those elements.• The duration of the impact of the clear-cutting on soil requires medium-term observations because it cannot be deduced from the current knowledge of this ecosystem.Résumé• La récolte des peuplements est une phase critique pour la durabilité des sols forestiers. Cette étude concerne les effets de la coupe à blanc d’un peuplement de Douglas de 67 ans, réalisée sans perturbation, sur la fertilité organo-minérale du sol.• Les résultats montrent que l’évolution du sol est rapide dans la période de trois ans après la coupe. La couche organique décroît fortement et le sol minéral se désature et s’acidifie. Les pertes représentent 4 % des éléments disponibles du sol sur 60 cm pour N, 22 % pour K, 25 % pour Ca et 32 % pour Mg. Seul P augmente de 11 %. Cependant, la variabilité spatiale des sols forestiers et la technique de l’observatoire, par échantillonnages successifs, conduisent à des intervalles de confiance larges autour de ces valeurs, intervalles qui s’avèrent difficile à réduire.• La réversibilité des modifications du sol dépend des éléments : elle ne devrait pas poser de problème pour C, N et K. P n’est pas en cause puisqu’il change de forme dans le sol mais aucune perte n’est notée. Les pertes de Ca voire de Mg sont les plus préoccupantes en raison des faibles réserves du sol, du flux limité de cations issu de l’altération des minéraux du sol, de la part relativement élevée des ces éléments dans les couches organiques, et de leur bilan entrées-sorties négatif.• La durée de l’effet de la coupe à blanc nécessite des observations sur le moyen terme car elle ne peut pas être déduite des connaissances actuelles sur cet écosystème.

Influence de l’amendement sur la composition et la dynamique du cortège floristique pour des peuplements situés sur sols acides

Soil conditioners are an effective tool for recapitalising fertility in acidic soils. Three experiments were conducted to quantify these effects on the diversity and dynamics of the floristic succession of the treated populations. The effect of browsing by the fauna was also assessed. The results show an enhancement in neutro-nitrophilous and heliophilous species that tend to be ubiquitous following application of amendments. This enrichment continues and has sometimes intensified 8 years after the treatment. In most cases, the species found prior to the amendment have not disappeared, but the abundance of acidiphilous species (e.g. Calluna vulgaris, Leucobryum glaucum) significantly decreased. The results of fencing in plots also underscore the strong pressure the fauna brings to bear on the biomass and the floristic composition. Considered in combination, these results provide a good illustration of the impact of amendments on the flora, which is often used as an indicator for variations in soil fertility.