Jean-Luc Dupouey

IRREVERSIBLE IMPACT OF PAST LAND USE ON FOREST SOILS AND BIODIVERSITY

In western Europe, forest area has been expanding rapidly since the 19th century, mainly on former agricultural land. Previous studies show that plant diversity differs between these recent forests and ancient forests that were already forested at the time of first national cadastral surveys, around 1800. Here, we investigated the duration of such agricultural aftereffects. In northeastern France, large areas were deforested during the Roman occupation and thereafter abandoned to forest. In one such forest that was farmed during the period AD 50-250, we show that species richness and plant communities vary according to the intensity of former agriculture. These variations are linked to long- term changes of chemical and structural soil properties. Hence, we suggest that such effects of past agricultural land use on forest biodiversity may be irreversible on an historical time scale.

PRESENT FOREST BIODIVERSITY PATTERNS IN FRANCE RELATED TO FORMER ROMAN AGRICULTURE

Combined archaeological and ecological investigations in a large ancient oak forest in Central France have revealed a dense network of ancient human settlements dating from the Roman period. We demonstrate a strong correlation between present-day forest plant diversity patterns and the location of Roman farm buildings. Plant species richness strongly increases toward the center of the settlements, and the frequency of neutrophilous and nitrogen-demanding species is higher. This pattern is paralleled by an increase in soil pH, available P, and delta(15)N, indicating the long-term impact of former agricultural practices on forest biogeochemical cycles. These extensive observations in a forested region on acid soils complement and confirm previous results from a single Roman settlement on limestone. Ancient Roman agricultural systems are increasingly being identified in contemporary French forests; the broad extent and long-lasting effects of previous cultivation shown in this study require that land-use history be considered as a primary control over biodiversity variations in many forest landscapes, even after millennia of abandonment.

Floristic changes in the herb-layer vegetation of a deciduous forest in the Lorraine Plain under the influence of atmospheric deposition

Abstract In 1990, 221 floristic surveys dating from 1971 and 1972 were re-sampled at the intersections of a systematic grid covering a mixed hardwood forest in the Lorraine Plain (northeast of France). The dynamics of the herb-layer vegetation were studied at various levels: first, the individual behaviour of species (modification of their frequency of occurrence in the forest) was analysed, and then, the overall evolution of the vegetation was studied based on Ellenbergs ecological values and chemical richness indices derived from automatic data processing. Three main features were noted: (i) an increase in the frequency of nitrogen-demanding species; (ii) an increase in the Ellenbergs ecological value N (nitrogen); (iii) an enrichment of the site revealed by trophic level indices. These observations highlight a process of eutrophication in the forest. The spatial representation of the floristic changes shows a distinct structure of eutrophication, which mainly appears to concern the edge of the forest. Apart from Nancy city itself, the arable land next to the forest has been a large source of nitrogen, notably since the beginning of the 1970s due to a change in agricultural practices (a substantial increase in fertilisation). With regard to these observations, the eutrophication in this forest is likely to be ascribed to air pollutants, the deposition of which is enhanced through the forest edge effect.

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.

A Retrospective Isotopic Study of Spruce Decline in the Vosges Mountains (France)

The objective of this study was to assess the time variation of mineral and water stress levels across the life of a declining, Mg-deficient, spruce stand, in order to clarify the factors that caused the decline. Since 1985, strong soil acidification linked to a large leaching of nitrate and base cations was measured at the study site. In 1994, 5 trees were felled and tree rings were measured and analysed for Ca, Mg, K, Sr, 13C12C and 87Sr/86Sr isotopic ratios. Strontium pools and fluxes as well as root Sr isotope ratio in relation to depth were also measured. Wood chemical concentrations and isotope ratios were strongly related to the dominance status of each tree. On average during the study period, the 87Sr/86Sr ratio of spruce wood decreased. Using a mechanistic model computing long term variations of 87Sr/86Sr ratio in trees and soils, we reproduced the observed trend by simulating soil acidification – increasing Sr drainage from the whole profile, and particularly from the organic horizon –, and root uptake becoming more superficial with time. Between 1952 and 1976, tree ring Δ 13C decreased strongly and continuously, which, in addition to other factors, might be related to an increase in water stress. Thus, a decrease in rooting depth, possibly related to soil acidification, appeared as a possible cause for the long term increase in water stress. The extreme drought event of 1976 appears to have revealed and triggered the decline.

Forest seed banks along an intensity gradient of ancient agriculture.

Recently, forest seed banks were proven to not only reflect former (decades-old) but also ancient (centuries-old) land use. Yet, as land-use intensity determines the magnitude of seed-bank changes in recent forests, this study aims to identify whether an ancient land-use gradient would also be reflected in the seed bank. On a forested 1600-year-old archaeological site, five different land-use intensities were mapped and sampled. Apart from seed density, species richness and composition, functional seed-bank types, defined by nine seed-bank-related plant traits, were related to the land-use intensity gradient. The land-use gradient from gardens to undisturbed sites was still clearly reflected in the soil seed bank. Six emergent functional seed-bank types, characterized by specific plant traits, changed significantly in abundance, parallel to the land-use gradient. In particular, dispersal agent (and related traits) proved an important explanatory trait of present (functional) seed-bank patterns. Poor dispersers (large and heavy seeds) were not found in the intensively used areas, contrary to animal-dispersed species. Wind-dispersers may have been inhibited in the extension of their distribution by recruitment bottlenecks (low seed production) and/or competitive exclusion. Additionally, the agricultural land-use probably introduced ruderal species into the seed bank of the most intensively used areas, yielding a simultaneous increase in vegetation–seed-bank dissimilarity with land-use intensity, eliminating present vegetation as a driver behind the differences over the seed-bank gradient. We conclude by arguing how coppice-with-standards management possibly maintained the seed-bank gradient.

Fertility indicators in temperate forest soils : issues, approaches and prospects

The specifics of forest ecosystems require a strategy of soil fertility assessment distinct from that commonly used in agro-ecosystems. Existing indicators fall into three main approaches. The first is to estimate the availability of resources (water, minerals) considering the size of the pools and the dynamics of supply. A second is based on the quantification of the response of communities of organisms other than arboreal plants in terms of presence / absence, abundance and / or activity. The third assesses fertility by analyzing the response of the forest stand / tree itself, in terms of productivity or foliar nutrient contents. For each approach, a series of indicators are presented, stating: their principles and indications, how to acquire them, their interests and limits, their links with other indicators and few prospects for improvement.

Interactive effects of land-use change and distance-to-edge on the distribution of species in plant communities at the forest-grassland interface

Questions What are the patterns of plant communities in terms of richness and floristic composition along stable forest–grassland interfaces? Are there specific species at the edge and/or specialist species transgressions between adjacent habitats? How does edge displacement following land-use change alter distance-to-edge plant diversity patterns? What are the relative influences of the edge effect and the edge displacement in forests and grasslands? Location Forests and grasslands in northeast France. Methods Floristic surveys were conducted in three types of forest–grassland interface, which can be stable or the result of edge displacements due to afforestation or deforestation. The sample comprised a total of 132 plots in 22 forest–grassland interfaces. Point-biserial correlation coefficients were used to classify the species into several groups in stable interfaces: forest, edge or grassland specialists and generalists. Using LMMs and Tukeys HSD tests, the total species richness and that of each species group were compared between the plot positions (i.e. distance-to-edge) and between the stable interfaces and cases of afforestation and deforestation. To evaluate the relative influence of the edge effect (plot position) and edge displacement (afforestation/deforestation) on plant community composition, CCAs and variation partitionings were performed. Results The largest difference in both plant community richness and composition was found between the forest and grassland edges. The highest species richness was found at the grassland edges. In addition to a high number of generalist species, a mixture of forest and grassland specialist species were predominant at the edges, and the forest specialists were almost the only transgressive species. Afforestation interfaces showed an extinction debt of grassland and edge specialist species and a colonization credit of forest specialists in recent forests. The effects of edge displacement were larger than those of the edge effect in forests, and the opposite was observed in the grasslands. Conclusions Forest and grassland plant communities differ strongly even over very short distances. However, the edges host a large number of forest and grassland specialist species, which explain higher species richness than in adjacent habitats. This results from interactive effects of distance-to-edge and edge displacement following land-use change.

Network connectivity value

In order to unveil the value of network connectivity, we formalize the construction of ecological networks in forest environments as an optimal control dynamic graph-theoretic problem. The network is based on a set of bioreserves and patches linked by ecological corridors. The node dynamics, built upon the consensus protocol, form a time evolutive Mahalanobis distance weighted by the opportunity costs of timber production. We consider a case of complete graph, where the ecological network is fully connected, and a case of incomplete graph, where the ecological network is partially connected. The results show that the network equilibrium depends on the size of the reception zone, while the network connectivity depends on the environmental compatibility between the ecological areas. Through shadow prices, we find that securing connectivity in partially connected networks is more expensive than in fully connected networks, but should be undertaken when the opportunity costs are significant.