Jean-François Ponge
email: jean-francois.ponge@wanadoo.fr

Richard Chevalier

Humus Index as an indicator of forest stand and soil properties

Keywords: Humus forms, Stand development, Management practices, Soil types, Oak stands, Coppices-with-standards, Even-aged high forests

The Humus Index, based on the visual assessment of topsoil horizons and a classification of humus forms, is a numerical score which can be used as a correlate of stand and soil properties. In oak stands from the Montargis forest (Loiret, France) we observed a good linear relationship of the Humus Index with most parameters describing stand development (age, basal area, height and diameter at breast height of dominants) and soil type (depth of clay horizon). The relationship with parameters describing nutrient availability (exchangeable bases, base saturation) was similarly good but non-linear. In the studied forest the Humus Index was affected first by stand age and second by soil type. When corrected for age and soil type, data (96 pooled estimates) indicated a slight decrease in the Humus Index (shift towards more active humus forms) in stands converted from old coppices-with-standards when compared with even-aged high forest.

Introduction

There is a growing need for synthetic indicators to be used for the wide scale monitoring of terrestrial ecosystems, in particular when threatened by pollution, climate change and human pratices [START_REF] Moore | Assessment of disturbance on soil ecosystems[END_REF][START_REF] Lindenmayer | Indicators of biodiversity for ecologically sustainable forest management[END_REF][START_REF] Duelli | Biodiversity indicators: the choice of values and measures[END_REF]. This goal can be achieved through field measurements if (i) they can be done by untrained people and (ii) field data are correlated with a lot of ecosystem parameters indicative of woodland well-being.

Humus forms (Mull, Moder, Mor) indicate the rate at which nutrients are circulating within terrestrial ecosystems [START_REF] Ovington | Organic production, turnover and mineral cycling in woodlands[END_REF][START_REF] Chapin | The nature of nutrient limitation in plant communities[END_REF][START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF]. They vary according to climate and parent rock [START_REF] Vitousek | Litter decomposition of the Mauna Loa environmental metrix, Hawai'i: patterns, mechanisms and models[END_REF][START_REF] Ponge | The heterogeneity of humus profiles and earthworm communities in a virgin beech forest[END_REF][START_REF] Sadaka | Climatic effects on soil trophic networks and the resulting humus profiles in holm oak (Quercus rotundifolia) forests in the High Atlas of Morocco as revealed by correspondence analysis[END_REF], but also to canopy and understory vegetation [START_REF] Beniamino | Soil acidification under the crown of oak trees. I. Spatial distribution[END_REF]Muys et al., 1992;[START_REF] Aubert | Effect of tree mixture on the humic epipedon and vegetation diversity in managed beech forests (Normandy, France)[END_REF], stand age [START_REF] Emmer | Temporal and vertical changes in the humus form profile during a primary succession of Pinus sylvestris[END_REF][START_REF] Sagot | Earthworm distribution and humus forms in the development of a semi-natural alpine spruce forest[END_REF][START_REF] Aubert | Effect of tree mixture on the humic epipedon and vegetation diversity in managed beech forests (Normandy, France)[END_REF], management [START_REF] Aber | Predicting the effects of different harvesting regimes on forest floor dynamics in northern hardwoods[END_REF][START_REF] Terlinden | Effets de l'intensité d'éclaircie sur les horizons organiques et hémiorganiques du sol en futaie équienne de Picea abies[END_REF][START_REF] Covington | Changes in forest floor organic matter and nutrient content following clear cutting in northern hardwoods[END_REF], fertilization [START_REF] Toutain | Possibilités de modification du type d'humus et d'amélioration de la fertilité des sols à moyen terme en hêtraie par apport d'éléments minéraux[END_REF][START_REF] Deleporte | Long-term effects of mineral amendments on soil fauna and humus in an acid beech forest floor[END_REF], irrigation [START_REF] Vavoulidou-Theodorou | Ein Bewässerungsversuch zur Dynamik von Humusprofilen in Nadelholsbeständen mit Wuchsstörung[END_REF] and pollution [START_REF] Coughtrey | Litter accumulation in woodlands contaminated by Pb, Zn, Cd and Cu[END_REF][START_REF] Kuperman | Relationships between soil properties and community structure of soil macroinvertebrates in oak-kickory forests along an acidic deposition gradient[END_REF][START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. In turn, humus forms, by their focus position within biogeochemical cycles, influence many ecosystem compartments and processes such as ground flora [START_REF] Le Tacon | Valeurs indicatrices des principales espèces végétales des hêtraies du Nord-Est de la France vis-à-vis des types d'humus[END_REF][START_REF] Klinka | Relationships among humus forms, forest floor nutrient properties, and understory vegetation[END_REF][START_REF] Bartoli | ECOFLORE, un logiciel simple de diagnostic écologique[END_REF], regeneration of forest canopy species [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF][START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF][START_REF] Ponge | The forest regeneration puzzle[END_REF], forest productivity [START_REF] Delecour | Facteurs édaphiques et productivité forestière[END_REF] and litter quality [START_REF] Davies | Polyphenols in plant, humus, and soil. IV. Factors leading to increase in biosynthesis of polyphenol in leaves and their relationship to mull and mor formation[END_REF][START_REF] Toutain | Étude comparée des bilans biologiques de certains sols de hêtraie[END_REF]. They are considered, together with ground vegetation, as an indicator of the soil nutrient regime [START_REF] Wilson | The use of ground vegetation and humus type as indicators of soil nutrient regime for an ecological site classification of British forests[END_REF] and we expect them to be the best predictor of stability domains within ecosystems [START_REF] Odum | The strategy of ecosystem development[END_REF][START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF][START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF]. However, the wider use of humus forms as a site factor is limited by subjectivity in the identification of forest floor and topsoil horizons [START_REF] Federer | Subjectivity in the separation of organic horizons of the forest floor[END_REF] and by the existence of small-scale variation [START_REF] Riha | Spatial variability of soil pH and organic matter in forest plantation[END_REF][START_REF] Carter | Lateral variability of forest floor properties under secondgrowth Douglas-fir stands and the usefulness of composite sampling techniques[END_REF][START_REF] Torgersen | The spatial heterogeneity of soil invertebrates and edaphic properties in an old growth forest stand in western Oregon[END_REF].

The Humus Index has been designed for the transformation of a scale of discrete humus forms in a numerical parameter, which could be manipulated statistically (Ponge et al., 2002;[START_REF] Ponge | Collembolan communities as bioindicators of land use intensification[END_REF][START_REF] Fédoroff | Small-scale response of plant species to land use intensification[END_REF]. In the abovementioned studies the Humus Index proved to be significantly correlated with some important ecological parameters of forest ecosystems such as topsoil physical and chemical properties and plant and soil animal communities.

In forests, stand and soil properties are of paramount importance for the management and choice of target tree species [START_REF] Carmean | Forest site quality evaluation in the United States[END_REF][START_REF] Miller | Forest fertilization: some guiding concepts[END_REF]Muys and Lust, 1992) and for the assessment of health and productivity of the ecosystem [START_REF] Christie | A comparison of forest productivity in Britain and Europe in relation to climatic factors[END_REF][START_REF] Ulrich | Process hierarchy in forest ecosystems: an integrative ecosystem theory[END_REF][START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF]. The present study was intended to correlate the Humus Index with parameters of stand and soil development under varying management regimes of the same canopy species.

Study sites

The Montargis forest (Loiret, France) is a state forest (4090 ha) located in the northern half of France (Fig; 1), in the rainwater basin of river Seine. The general aspect is fairly level, with a slight westward declivity, the altitude varying between 95 and 132 m. The climate is oceanic, with a weak continental influence. The mean annual precipitation, calculated over the last thirty years, is 650 mm, 50% of which falling as rain during the growing season, from early April to late September. The mean temperature, calculated over the same thirty-year period, is 10.9°C, with a minimum monthly mean of 3.7°C in January and a maximum monthly mean of 19.0°C in July. The parent rock is Senonian chalk (late Cretaceous), covered with postglacial (Holocene) deposits of variegated textural properties, sand being dominant in the western part and silt in the eastern part. This is at the origin of a variety of soil types, weakly acidic to acidic, with a depth of 40 to 70 cm, generally welldrained year-round or at worst with weak temporary water-logging during Winter. Most variation occurs through changes in the vertical distribution of particle size, in particular the depth at which clay becomes dominant varies to a great extent, ranging from 30 to 80 cm in our data set.

The Montargis forest exhibits a compact shape, extending around the Paucourt village (Fig. 1), without any change in surface area and tree composition since the 12e century [START_REF] Garnier | La forêt de Montargis. Excursion du 4 avril 1965[END_REF]. Coppice-with-standards, with sessile oak [Quercus petraea (Mattus.)

Liebl.] standards and hornbeam (Carpinus betulus L.) coppices, was the dominant management type from 1670 on. The cutting period for coppices was first fixed to 70 years then to 25, 40 or 50 years (according to site conditions) from 1783 on. Since 1857, coppiceswith-standards were partly converted to oak-dominated stands, with some admixture of beech (Fagus sylvatica L.) and hornbeam according to the sites. The conversion was total from 1872 on. Even-aged oak stands were issued from seed from the original mixed stands, oldest ones being 99 years-old, without any agricultural past nor plantation.

Ninety-six stands were selected, in order to embrace the variety of oak stands growing on medium acidic, well-drained sandy loam with level aspect, with one sampling area in each stand. All soils are luvisols according to FAO classification, varying according to the depth of the argillic horizon. The sampling area was selected in homogeneous vegetation and stand structure, beyond 50 m of stand limit. The choice of a restricted array of site conditions was aimed at testing the influence of stand properties. The sampling design was balanced according to 8 forest types, either even-aged high forest or conversion from previous coppice-with-standards:  FG15: even-aged high forest 15 years-old (12 stands)  FG35: even-aged high forest 35 years-old (12 stands)  FG50: even-aged high forest 50 years-old (12 stands)  FG90: even-aged high forest 90 years-old (12 stands)  CS1: coppice-with-standards converted to medium-diameter regular stand (19 stands)  CS2: coppice-with-standards converted to large-diameter regular stand (14 stands)  CS3: coppice-with-standards converted to irregular stand (15 stands)

Methods

In each sampling area the Humus Index was visually assessed in triplicate at four plots, located at 14 m from the central post in the four main directions. At each plot three replicated estimates of the Humus Index were made at angles of a one-meter side equilateral triangle. The twelve measurements were averaged, giving a composite value for the sampling area, which could smooth out two scales of the local variation not directly related to stand properties [START_REF] Riha | Spatial variability of soil pH and organic matter in forest plantation[END_REF][START_REF] Ponge | Humus Index: an integrated tool for the assessment of forest floor and topsoil properties[END_REF]. The Humus Index was based on the classification of humus forms by [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF], modified by [START_REF] Jabiol | Typologie des formes d'humus peu actives. Validation par des critères macro-et micromorphologiques, biologiques et chimiques[END_REF]: Eumull = 1, Mesomull = 2, Oligomull = 3, Dysmull = 4, Hemimoder = 5, Eumoder = 6, Dysmoder = 7.

At each of the four plots within the same sampling area, a probe was used to measure the depth at which clay enrichment was found for the first time and the depth at which clay was dominant. The four values were averaged for each sampling area. At each of the four plots a soil sample was taken at 15-20 cm depth, then the four samples were pooled then air-dried for laboratory analyses on the fraction less than 2 mm: particle size distribution (clay, silt, sand), pH water , pH KCl , cation exchange capacity, main exchangeable bases (extracted at soil pH using cobaltihexamine), and base saturation. Analytical methods followed ISO standards (Anonymous, 1999).

At each sampling area we noted the time elapsed from the last thinning operation (except for clear-cuts), the age of the stand (only for even-aged high forest), we measured the height and diameter at breast height of three dominant trees distant from less than 14 m from the central post, and we estimated the wood standing crop, using production tables by [START_REF] Dagnélie | Tables de Cubage des Arbres et des Peuplements Forestiers[END_REF]. At each plot we measured the basal area (BA), the percent basal area occupied by beech, the percent basal area occupied by hornbeam. These four values were averaged for each sampling area.

The statistical treatment of the data involved regression analysis, using the Humus Index as a predicted (dependent) variable and several stand and soil properties as explanatory (independent) variables [START_REF] Sokal | Biometry. The Principles and Practice of Statistics in Biological Research[END_REF]. The analyses were performed with the StatBox® software. Residuals were tested for normality previous to analysis.

Results

The Humus Index exhibited a significant (P<0.05) to highly significant (P<0.001) correlation with 17 out of 21 stand and soil properties in oak stands of the Montargis forest (Table 1). Other stand and soil parameters did not reach such a high level of indication, as measured by the number of significant coefficients. For instance pH water was significantly correlated with only 4 out of 8 stand measurements (against 7 our of 8 for Humus Index) and with only 8 out of 12 soil measurements (against 10 out of 13 for Humus Index). The best predicted variable was the age of even-aged high forest (r=0.73, P=5.10 -9 ). Among soil parameters, Humus Index predicted the best base saturation (r=-0.61, P=6.10 -11 ).

The Humus Index increased in value with the age of stands, indicating a shift from Mull to Moder in the course of time (Fig. 2), but it was not seemingly influenced by thinning operations (r=0.10, P=0.18, Table 1). It should be noted that pH water , on the contrary, was negatively correlated with the time elapsed from the last thinning operation (r=-0.27, P=0.006). In short, after each thinning operation, soil acidity decreased then increased again but without any concomitant change in the humus form.

These global trends were depicted by the whole set of sampling areas, without any account to possible effects of management pratices. If we separate even-aged high forest stands from stands converted from old coppices-with-standards, a more variegated landscape appears (Fig. 3).

The positive correlation between Humus Index and dominant height was better depicted by even-aged high forest (Fig. 3b) than by the whole set of oak stands (Fig. 3a).

Coppices-with-standards did not exhibit any such trend, all of them falling within the range of even-aged high forest stands with tallest trees as dominants. A comparison by paired t-test between actual and calculated values of Humus Index for coppices-with-standards (using equation 1) did not reveal any departure from the trend exhibited by even-aged high forest, provided trees are of the same height (t=0.89, P=0.19). Thus the relationship between Humus Index and dominant height was not affected by management practices.

A quite different picture was exhibited by dominant diameter. Similar to dominant height, the correlation between Humus Index and dominant diameter was positive, better depicted by even-aged high forest than by the whole set of stands, and null for coppiceswith-standards (Figs. 3c and3d). However, Humus Indices measured in coppices-withstandards differed by more than one unit from values calculated using equation 1 derived for even-aged high forest (t=7.48, P=2.10 -9 ). Thus, provided they had the same dominant diameter, coppices-with-standards seemed to exhibit more active humus forms than evenaged high forest. However, when comparing Figures 2b and2d it appears that coppices-withstandards, the dominants of which have the same height than tallest trees of even-aged high forest (90 years-old, see Fig. 2), exhibit larger diameters at breast height, which flaws any comparison based on diameter. Coppices-with-standards, the mean diameter of dominant trees is 45 cm, should compare with even-aged high forest stands with a mean diameter of 35 cm for dominants.

Wood standing crop was estimated using both diameter and height of trees. If we consider wood volume and Humus Index, most coppices-with-standards fell within the range of tallest even-aged high forest stands (also oldest, r=0.98, P=2.10 -32 ), but some coppiceswith-standards exhibited higher volumes of wood and lower Humus Indices than expected on the base of even-aged high forest (Figs. 3e and3f). However, a comparison between observed and calculated values of the Humus Index for coppices-with-standards did not reveal any significant shift (t=1.17, P=0.12). Here too, there was no effect of management practices.

Basal area did not vary to a great extent among oak stands, although this parameter displayed a positive correlation with the Humus Index (Fig. 3g). Most coppices-withstandards fell within the range of variation of even-aged high forest stands (Fig. 3h) but they exhibited a higher Humus Index than expected from their basal area (t=-4.96, P=5.10 -6 ).

Beech and hornbeam (in percent of the total basal area) were positively correlated with the Humus Index, but at a lower level of significance than age, height, diameter and basal area (Table 1).

There was a positive relationship between the Humus Index and the depth at which clay becomes dominant (Fig. 4a): the shallower was the clay horizon, the lower was the Humus Index (Mull). Both even-aged high forest and coppices-with-standards exhibited the same relationship (Fig. 4b), with a similar slope of the regression line (t=1.52, P=0.13). This figure may also help to verify that even-aged high forest and coppices-with-standards grew on the same range of soil conditions, thus comparisons between management pratices were not biased by a possible influence of the soil type. When the combined effect of stand age and soil type (expressed by depth of clay horizon) on the Humus Index of full-gown stands was analysed by multiple regression, the mixed model explained 70.1% of the total variation, shared between 52.9% for age and 17.2% for soil type.

For the same depth of clay horizon, coppices-with-standards exhibited a higher Humus Index than the even-aged high forest (t=7.13, P=5.10 -9 ), the difference being ca. 1 unit. However, since the group of even-aged high forest stands included young stands with more active humus forms (lower Humus Index), a possible bias due to aging was questioned.

If a Humus Index could be extrapolated for young stands supposed at the age of 90-years, then more valid comparisons between coppices-with-standards and even-aged high forest would be made. We used the equation shown in Figure 2 to extrapolate Humus Indices at 90 years (HI 90 ) from actual values of young stands (HI), according to the formula HI 90 =HI+0.03(90-age), with the age of the stand expressed in years. The Humus Index thus calculated for theoretical 90-years-old even-aged high forest remained significantly (and positively) explained by the depth of clay horizon (R 2 =0.37, P=10 -4 ). When even-aged high forest stands were thus corrected for aging, coppices-with-standards exhibited a lower Humus Index (ca. 0.5 unit less) than even-aged high forest (t=3.8, P=2.10 -4 ), for the same soil conditions (expressed by depth of clay horizon).

The Humus Index showed a negative relationship with exchangeable Ca (fig. 4c), but, contrary to above mentioned parameters, a better fitness was obtained with logarithmic values of calcium concentrations (R 2 =0.53 against 0.26 for linear regression). Even-aged high forest and coppices-with-standards exhibited the same relationship (Fig. 3d), and for the same concentration of exchangeable Ca the Humus Index did not differ between them (t=0.06, P=0.48).

Similarly, the relationship between Humus Index and base saturation was non-linear (Fig. 4e) and was depicted both by even-aged high forest and coppices-with-standards (Fig. 4f), which did not differ between them at a given level of base saturation (t=0.017, P=0.49).

Discussion

First, it should be highlighted that our Humus Index differs to a great extent from the same notation recently used by other authors to describe humus quality [START_REF] Godefroid | Variation in the herb species response and the humus quality across a 200-year chronosequence of beech and oak plantations in Belgium[END_REF]. The Humus Index they used was based on floristic composition, by averaging scores of different plant species pertaining to the same plant community. The scores were calculated on the model of [START_REF] Ellenberg | Zeigerwerte der Gefasspflanzen Mitteleuropas[END_REF] indices, by noting the presence of plant species along a scale of humus forms, which were given a number as in our own method. In the present study, as in previously published papers [START_REF] Ponge | Humus Index: an integrated tool for the assessment of forest floor and topsoil properties[END_REF][START_REF] Ponge | Collembolan communities as bioindicators of land use intensification[END_REF][START_REF] Fédoroff | Small-scale response of plant species to land use intensification[END_REF], the Humus Index was directly derived from the observation of humus forms, not of flora. Several authors noted that Ellenberg indices should be used with caution, given the existence of regional and temporal changes in ecological requirements of plant species [START_REF] Parrish | Ontogenetic niche shifts in old-field annuals[END_REF][START_REF] Hill | Ellenberg's Indicator Values for British Plants[END_REF][START_REF] Diekmann | Shifts in ecological behaviour of herbaceous forest species along a transect from northern Central to North Europe[END_REF]. We suggest that the identification of the humus form [START_REF] Green | Towards a taxonomic classification of humus forms[END_REF][START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF], which can be used directly on the field for building a Humus Index, should be preferred to a list of plant species.

The positive correlation between the Humus Index and the age of oak stands (Fig. 2) can be attributed to changes in humus forms and associated parameters (soil acidification, organic matter accumulation) which have been repeatedly observed to occur during crop rotation [START_REF] Adam | Nutrient fluctuations in Sitka spruce (Picea sitchensis) plantations: the implications for future forest management practice[END_REF][START_REF] Aubert | Effect of tree mixture on the humic epipedon and vegetation diversity in managed beech forests (Normandy, France)[END_REF][START_REF] Godefroid | Variation in the herb species response and the humus quality across a 200-year chronosequence of beech and oak plantations in Belgium[END_REF]. The passage from Mull (Humus Index 1-4) to Moder (Humus Index 5-7) accompanies the growth of trees and their increasing influence on the soil, more especially when their litter is poor in nutrients and rich in secondary metabolites [START_REF] Nicolai | Phenolic and mineral content of leaves influences decomposition in European forest ecosystems[END_REF][START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF][START_REF] Ponge | The forest regeneration puzzle[END_REF]. Studies on old-growth forests reveal that soil acidification under the influence of tree growth is temporary and may reverse if environmental conditions and spatial configurations of habitats are proper for the re-establishment of adapted decomposer communities [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF][START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF][START_REF] Aubert | Effect of tree mixture on the humic epipedon and vegetation diversity in managed beech forests (Normandy, France)[END_REF]. This occurs when nutrient requirements of the aboveground compartment of the forest ecosystem decrease after cessation of stem elongation [START_REF] Nilsson | Forest growth as a possible cause of soil and water acidification: an examination of the concepts[END_REF]Miller, 1984a;[START_REF] Chapin | The nature of nutrient limitation in plant communities[END_REF]). Here we did not show such reversal of the Humus Index in ageing stands, because our stands were probably too young and occupied too large surfaces, in an otherwise intensively managed forest. The fact that the relationship between age and Humus Index was linear (Fig. 2) indicates that the humus form changed steadily during stand development, at least during the first 90 years of crop rotation. The linear relationship between the Humus Index and the age of trees contradicts the hypothesis of stability domains within soil communities [START_REF] Bengtsson | Disturbance and resilience in soil animal communities[END_REF][START_REF] Graefe | Spatial variety of soil biota: diversity of types vs. diversity of species[END_REF][START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF]. According to this hypothesis, changes in soil communities would occur by jumping from a species distribution to another, better adapted distribution, when the original community has been disrupted by an environmental stressor, such as for instance changes in environmental conditions and resource availability which occur during stand development. This should result in a discrete response of the Humus Index to tree growth, which was not depicted by our series of even-aged high forest stands.

The negative influence of beech upon soil biological activity was reflected in the increase in Humus Index when the percent basal area occupied by beech increased (Table 1). [START_REF] Muys | Evaluation of conversion of tree species and liming as measures to decrease soil compaction in a beech forest on loamy soil[END_REF] observed an increase in humus quality (expressed by an increase in earthworm biomass) and a decrease in soil compaction when beech was replaced by oak in a Belgian forest. Similar results, using herb species as indicators of humus quality, were obtained by [START_REF] Godefroid | Variation in the herb species response and the humus quality across a 200-year chronosequence of beech and oak plantations in Belgium[END_REF] in the same country. This phenomenon could be explained by a higher increment in wood standing crop and basal area and a lower decomposition rate of litter in beech compared to oak [START_REF] Lemée | Recherches sur les écosystèmes des réserves biologiques de la forêt de Fontainebleau. II. Décomposition de la litière de feuilles des arbres et libération des bioéléments[END_REF][START_REF] Monserud | A basal area increment model for individual trees growing in even-and uneven-aged forest stands in Austria[END_REF], with concomitant soil impoverishment [START_REF] Nilsson | Forest growth as a possible cause of soil and water acidification: an examination of the concepts[END_REF][START_REF] Chapin | The nature of nutrient limitation in plant communities[END_REF].

The relationship between Humus Index and parameters of stand development (height, diameter at breast height, wood standing crop) can be mostly explained by stand age, as these parameters increase steadily during stand development (Miller, 1984b;[START_REF] Chapin | The nature of nutrient limitation in plant communities[END_REF][START_REF] Ulrich | Process hierarchy in forest ecosystems: an integrative ecosystem theory[END_REF]. Stands resulting from the conversion of old coppices-withstandards compare well with 90-years-old even-aged high forest stands, the dominants of which are of the same height (25 m, see Fig. 3b), except that they reached a larger diameter at breast height (Fig. 3d), due to higher annual increments [START_REF] Guilley | The influence of site quality, silviculture and region on wood density mixed model in Quercus petraea Liebl[END_REF] and probably older age. Both stand types exhibit a Humus Index averaging 4 (Dysmull). Howvever, coppices-with-standards show a high degree of variation in their stand characteristics, which are not correlated with the Humus Index (Figs. 3b,3d,3f,3h). A more clear picture appears when soil types are taken into account. They explain most of the variation which remained unexplained by stand characteristics: both coppices-with-standards and even-aged high forest show a positive relationship between Humus Index and depth of clay horizon (Fig. 4b).

When even-aged high forest trees are corrected for stand age, this relationship remains positive and significant, thus is not age-dependent, and differences between both forest types can be clearly perceived. For a given soil type, coppices-with-standards have a lower Humus Index (minus 0.5 unit) than oldest even-aged high forest stands. This means that coppices-with-standards exhibit less litter accumulation even though they have a higher standing crop and the same basal area than even-aged high forest stands (Figs. 3f and3h).

Given our knowledge of the relationships between humus forms (and closely related processes such as litter decomposition) and functional biodiversity of forest soils [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF][START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF][START_REF] Heemsbergen | Biodiversity effects on soil processes explained by interspecific functional dissimilarity[END_REF], we hypothesize that stands issuing from coppices-with-standards exhibit slightly more diversified animal and microbial communities than even-aged high forest. At first sight, this improvement of soil condition in converted coppices-with-standards could be explained by a higher diversity of woody vegetation, in particular to the presence of hornbeam in mixture with sessile oak, especially in the understory [START_REF] Aubert | Effect of tree mixture on the humic epipedon and vegetation diversity in managed beech forests (Normandy, France)[END_REF]. However, in our study site we did not register any positive influence of hornbeam upon the humus form (Table 1). Similarly, [START_REF] Bonneau | Effect of an oak forest on a silty acid soil. Changes in humus form and exchangeable cations[END_REF] observed a shift from Mull or Mull-Moder to Moder and a decrease in exchangeable cations when even-aged high forest stands were compared to coppices-with-standards in the Marchenoir forest, which is located not far from our study site. They attributed this shift to increased nutrient uptake and immobilization in the woody biomass of even-aged high forest, which impoverished the soil. Awaiting further studies, this interpretation could be questioned, because we observed that stands converted from coppices-with-standards neither exhibited a smaller basal area nor a smaller standing crop than even-aged high forest (Fig. 3). An alternative hypothesis could be that even-aged high forest trees were still too young to depict the improvement in soil biological activity (and thus the decrease in Humus Index) which is typically observed under older trees in natural forests [START_REF] Page | Effects of forest cover on the properties of some Newfoundland forest soils[END_REF][START_REF] Ponge | The forest regeneration puzzle[END_REF].

Other comparisons with literature data, especially when climate conditions and tree composition are different, should be made with caution. For instance, [START_REF] Hölscher | Effects of coppicing in temperate deciduous forests on ecosystem nutrient pools and soil fertility[END_REF] concluded that soils from oak coppices exhibited less acidity and higher mineral pools than those from even-aged high forests, but the former group was made of oak while the latter was made of beech, which flawed the comparison.

The non-linear relationship between Humus Index and base availability (Fig. 4c-f) possibly indicates a trend towards a saturation of the ecosystem in exchangeable bases, in particular in the below-ground compartment which is chiefly responsible for the building of humus forms [START_REF] Chapin | The nature of nutrient limitation in plant communities[END_REF]. Some studies have shown that an increase in nutrient availability maybe ineffective in changing humus quality, if adapted decomposer communities and efficient foodwebs are not present or cannot build-up rapidly [START_REF] Von Törne | Experimenteller Nachweis zootischer Einflüsse auf den Stoffumsatz in einem Kiefernforst[END_REF][START_REF] Graefe | Untersuchungen zum Einfluß von Kompensationskalkung und Bodenbearbeitung auf die Zersetzerfauna in einem bodensauren Buchenwald-und Fichtenforst-Ökosystem[END_REF]Muys and Lust, 1992).

The observed stability of the Humus Index against thinning operations (Table 1), despite a significant change in soil pH (see Results), can be ascribed to a redundancy phenomenon within the humus profile, which has been explained in detail by [START_REF] Belotti | Variability in space and time and redundancy as stabilizing principles of forest humus profiles[END_REF]. Each time a function (for instance the building of a horizon) is ensured by a variety of organisms, no pronounced change appears until the least sensitive species disappears [START_REF] Heemsbergen | Biodiversity effects on soil processes explained by interspecific functional dissimilarity[END_REF]. We hypothesize that the time from selection cutting to crown recovery is too short for destabilizing humus profiles, because of biological inertia, but also of the time required for building or disappearance of a horizon [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF]. A previous study on a spruce chronosequence showed that the increase in earthworm population size which accompanied thinning operations was only temporary and did not reverse the observed shift from Mull to Moder [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF].

We are aware that, although exhibiting a number of significant trends when correlated with stand and soil variables, the Humus Index does not explain the whole variation of these conditions. Roughly, the Humus Index explains at best half the total variation of stand and soil parameters (Figs. 2 to 4). This could be explained by (i) the wide range of stand and soil types covered by our study, (ii) the existence of other, not accounted for, factors which may have influenced the building of humus forms. Among these factors, the past history of the stands is probably responsible for a significant part of the unexplained variation. Fire places for charcoal production, agricultural past, human settlements, among others, are known to affect the distribution of plant species, which is probably true of soil organisms, too [START_REF] Koerner | Influence of past land use on the vegetation and soils of present day forest in the Vosges mountains, France[END_REF]. Even though we can discard agricultural past in the case of the studied forest, other human influences should not be neglected.

Conclusion

We showed that the Humus Index can be correlated with several important parameters of stand development and soil type, pointing on its possible use in the assessment of site quality and the long-term survey of ecosystems. Awaiting further theoretical and experimental developments, the Humus Index should be considered as a synthetic measurement of the complexity of soil communities [START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF], which could be used as an early tool to predict changes at the ecosystem level, due to tree growth, management pratices, climate change and pollution. Practicability of the method cannot be questioned, since it does not need any other measurement than the estimate by eye of horizons and structures. The only point which deserves further elaboration is a possible shift from person to person in the estimate of horizon thickness, which has been highlighted by [START_REF] Federer | Subjectivity in the separation of organic horizons of the forest floor[END_REF]. A standardization of the method would alleviate such possible biases.

Further studies should also take into account between-forest variation and the time required fro reaching an equilibrium in the humus type [START_REF] Wilson | The use of ground vegetation and humus type as indicators of soil nutrient regime for an ecological site classification of British forests[END_REF], before reaching firm conclusions about the use of the Humus Index for ecological site classification [START_REF] Ray | An Ecological Site Classification for Forestry in Great Britain[END_REF].

Figure captions

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .

 1234 Fig. 1. Location map of the Montargis forest (France)

  Fig. 1

Code number Forest type % Quercus % Fagus % Carpinus Mean humus form Last time thinning

	1	even-aged high forest	67	25	8	Dysmull	3
	2	even-aged high forest	88	7	5	Dysmull	3
	3	even-aged high forest	69	20	12	Eumoder	3
	10	even-aged high forest	98	2	0	Oligomull	6
	12	even-aged high forest	84	5	11	Mesomull	6
	13	even-aged high forest	99	0	1	Mesomull	6
	17	even-aged high forest	91	6	3	Oligomull	1
	18	even-aged high forest	82	2	16	Mesomull	6
	19	even-aged high forest	96	0	4	Oligomull	2
	20	even-aged high forest	99	0	1	Mesomull	2
	21	even-aged high forest	100	0	0	Mesomull	2
	23	even-aged high forest	82	0	17	Oligomull	3
	25	even-aged high forest	99	0	1	Oligomull	3
	26	even-aged high forest	98	0	2	Oligomull	8
	27	even-aged high forest	90	0	9	Oligomull	8
	28	even-aged high forest	51	12	37	Hemimoder	8
	29	even-aged high forest	81	0	19	Hemimoder	8
	34	even-aged high forest	90	4	6	Dysmull	2
	36	even-aged high forest	97	0	3	Oligomull	2
	37	even-aged high forest	82	1	17	Oligomull	3
	38	even-aged high forest	90	0	10	Oligomull	3
	39	even-aged high forest	85	15	0	Oligomull	5.5
	40	even-aged high forest	88	11	0	Dysmull	5.5
	41	even-aged high forest	80	15	5	Eumoder	7
	42	even-aged high forest	81	15	4	Eumoder	7
	43	even-aged high forest	78	20	2	Dysmull	6
	44	even-aged high forest	70	6	25	Dysmull	6
	47	even-aged high forest	73	23	5	Eumoder	6
	48	even-aged high forest	80	11	10	Eumoder	6
	49	even-aged high forest	68	17	15	Eumoder	6
	50	even-aged high forest	69	17	14	Eumoder	6
	51	even-aged high forest	77	22	1	Oligomull	3.5
	54	converted coppice-with-standards 69	4	27	Dysmull	3
	55	converted coppice-with-standards 83	9	7	Hemimoder	2
	57	converted coppice-with-standards 82	6	12	Eumoder	2
	58	converted coppice-with-standards 78	7	15	Hemimoder	2
	59	converted coppice-with-standards 80	2	18	Hemimoder	12
	60	converted coppice-with-standards 34	2	64	Oligomull	12
	62	converted coppice-with-standards 77	10	13	Dysmull	7
	63	converted coppice-with-standards 85	3	13	Hemimoder	7
	64	converted coppice-with-standards 82	0	18	Oligomull	7
	65	converted coppice-with-standards 58	4	38	Dysmull	1
	66	converted coppice-with-standards 51	0	49	Hemimoder	7
	68	converted coppice-with-standards 80	9	11	Hemimoder	7
	69	converted coppice-with-standards 38	0	62	Dysmull	7
	75	converted coppice-with-standards 79	11	10	Dysmull	13
	76	converted coppice-with-standards 70	26	4	Oligomull	13
	77	converted coppice-with-standards 65	16	19	Oligomull	12
	79	converted coppice-with-standards 72	21	7	Dysmull	12
	83	converted coppice-with-standards 69	28	3	Oligomull	12
	84	converted coppice-with-standards 86	9	6	Dysmull	12
	86	converted coppice-with-standards 65	3	32	Oligomull	12
	87	converted coppice-with-standards 62	10	28	Oligomull	12
	90	converted coppice-with-standards 88	6	7	Oligomull	3
	91	converted coppice-with-standards 68	23	9	Dysmull	2
	92	converted coppice-with-standards 71	1	28	Hemimoder	12
	93	converted coppice-with-standards 50	1	48	Eumoder	12
	96	converted coppice-with-standards 71	8	21	Dysmull	6
	98	converted coppice-with-standards 52	8	41	Hemimoder	6
	99	converted coppice-with-standards 22	11	67	Dysmull	6
	100	converted coppice-with-standards 45	3	53	Dysmull	6
	101	converted coppice-with-standards 76	0	24	Hemimoder	1
	102	converted coppice-with-standards 65	1	34	Hemimoder	2
	103	converted coppice-with-standards 63	7	30	Hemimoder	1.5
	105	converted coppice-with-standards 71	11	18	Dysmoder	12
	106	converted coppice-with-standards 54	4	42	Eumoder	8
	109	converted coppice-with-standards 39	2	59	Hemimoder	8
	114	converted coppice-with-standards 41	20	39	Dysmull	3
	115	converted coppice-with-standards 56	23	21	Dysmull	2
	116	converted coppice-with-standards 63	30	7	Dysmull	3
	117	converted coppice-with-standards 61	5	34	Hemimoder	7
	119	converted coppice-with-standards 76	6	18	Hemimoder	8
	128	converted coppice-with-standards 55	26	18	Mesomull	6
	129	converted coppice-with-standards 65	35	0	Eumoder	7
	130	converted coppice-with-standards 59	4	37	Hemimoder	2
	131	converted coppice-with-standards 32	0	68	Dysmull	12
	132	even-aged high forest	100	0	0	Mesomull	99
	133	even-aged high forest	100	0	0	Mesomull	99
	135	even-aged high forest	98	0	2	Hemimoder	99
	136	even-aged high forest	100	0	0	Mesomull	99
	138	even-aged high forest	94	0	6	Mesomull	99
	140	even-aged high forest	89	1	11	Mesomull	99
	141	even-aged high forest	62	0	38	Mesomull	99
	143	even-aged high forest	20	0	80	Mesomull	99
	144	even-aged high forest	94	0	6	Mesomull	99
	145	even-aged high forest	95	0	4	Oligomull	99
	147	even-aged high forest	99	0	1	Oligomull	99
	148	even-aged high forest	99	1	0	Mesomull	99
	152	even-aged high forest	76	0	24	Dysmull	8
	153	even-aged high forest	100	0	0	Oligomull	5
	155	even-aged high forest	91	3	6	Oligomull	5
	156	even-aged high forest	89	0	11	Dysmull	8
	158	converted coppice-with-standards 76	8	16	Eumoder	12
	159	converted coppice-with-standards 55	23	22	Mesomull	10
	160	converted coppice-with-standards 40	6	53	Hemimoder	10
	167	converted coppice-with-standards 76	3	21	Dysmull	2

Table 1 .

 1 Main features of the 96 selected stands. The percent presence of the three main tree species refers to the total basal area. The time since the last thinning operation is expressed in years

	1

Acknowledgements

The authors acknowledge the Centre d'Études du Machinisme Agricole et du Génie Rural des Eaux et Forêts (Cemagref) for financial support and the Office National des Forêts (ONF) for field facilities.

Humus Index pH water


Jean-François Ponge
email: jean-francois.ponge@wanadoo.fr

Stéphanie Topoliantz

Sylvain Ballof

Jean-Pierre Rossi

Patrick Lavelle

Jean-Marie Betsch

Philippe Gaucher

Ingestion of charcoal by the Amazonian earthworm Pontoscolex corethrurus: a potential for tropical soil fertility

Keywords: Tropical earthworms, Tropical soil fertility, Slash-and-burn agriculture, Charcoal Pontoscolex corethrurus

It is now attested that a large part of the Amazonian rain forest has been cultivated during Pre-Colombian times, using charcoal as an amendment. The incorporation of charcoal to the soil is a starting point for the formation of fertile Amazonian Dark Earths, still selected by Indian people for shifting cultivation. We showed that finely separated charcoal was commonly incorporated in the topsoil by Pontoscolex corethrurus, a tropical earthworm which thrives after burning and clearing of the rain forest, and that this natural process could be used to improve tropical soil fertility. Our paper is a contribution to the present debate about (i) the origin of black carbon in fertile Dark Earths, (ii) the detrimental vs favourable role of Pontoscolex corethrurus in tropical agriculture, (iii) natural processes which might be used to increase tropical soil fertility

terrestrial earthworm which commonly inhabits rain forest soils over the whole Amazonian basin [START_REF] Römbke | Earthworm densities in central Amazonian primary and secondary forests and a polyculture forestry plantation[END_REF]. Its important burrowing activity through the topsoil, associated with its efficient digestive system [START_REF] Zhang | Activity and origin of digestion complexes in gut of the tropical earthworm Pontoscolex corethrurus[END_REF][START_REF] Barros | The role of macrofauna in the transformation and reversibility of soil structure of an Oxisol in the process of forest to pasture conversion[END_REF], allows it to thrive in soils poor in organic matter, such as those found in areas now deforested for the need of agriculture. In the absence of organic input to the soil, excessive casting activity of this species may cause damage to permanent pastures through the coalescence of earthworm casts, leading to appearance of a thick compact surface crust [START_REF] Chauvel | Pasture damage by an Amazonian earthworm[END_REF]. However, the detrimental influence of this species may be questioned when the agricultural use of the land is only temporary, as in slash-and-burn shifting agriculture, or when available carbon is regularly added to the soil. We hypothesized that P. corethrurus could be responsible for the observed increase in soil fertility which has been reported to occur in Amazonian Dark Earths formed during Pre-Colombian times [START_REF] Myers | Historical perspectives on Amazonian Dark Earths[END_REF][START_REF] Steiner | Slash and char: an alternative to slash and burn practiced in the Amazonian Basin[END_REF].

Using a quantitative optical method [START_REF] Topoliantz | Earthworm and enchytraeid activity under different arable farming systems, as exemplified by biogenic structures[END_REF], we investigated the distribution of humus components in soils under shifting cultivation, still practised by Wayana and Aluku people settled along the Maroni river, French Guiana (Topoliantz et al., 2005b). This method allowed us to estimate the relative volume of components of the soil matrix, including plant tissues at varying stages of decomposition, mineral particles of varying size and nature, aggregates of varying colour, size and shape.

The untouched old forest exhibited low contents of both charcoal and charred material, representing 2% and 7% of the volume of the soil matrix, respectively. We showed that six months after burning of the same forest for cultivation, the contents of charcoal and charred material increased to 10% and 20% of the soil matrix, respectively, in the top 3 cm. After three years of cultivation these amounts decreased to 6% and 15%, respectively. During the cultivation period the amount of dark humus (mixture of charcoal and mineral soil in varying proportion) increased steadily. The examination of dark humus revealed that it was mainly comprised of faecal pellets of P. corethrurus, which contained a multitude of small charcoal fragments of 10-100 µm admixed in a mineral paste.

Charcoal, ingested together with soil particles, is mixed with mucus secreted in the oesophagus then finely ground in the muscular gizzard of earthworms. It is excreted as a muddy paste which is further stabilized by Van der Wals forces after drying, thus forming dark humus [START_REF] Hayes | Darwin"s "vegetable mould" and some modern concepts of humus structure and soil aggregation[END_REF]. We also showed by laboratory experiments that P. corethrurus did not ingest charcoal alone but rather added it to mineral soil. A mixture of charcoal and soil was preferred to either pure charcoal or pure soil (Topoliantz andPonge, 2003, 2005a). This points to a positive feed-back which improves the habitat of P. corethrurus by increasing the carbon content of the soil.

It has been demonstrated that finely divided charcoal (also called black carbon) was a source of stable humus [START_REF] Tryon | Effect of charcoal on certain physical, chemical, and biological properties of forest soils[END_REF][START_REF] Chan | The oxidative reactivity of coal chars in relation to their structure[END_REF]. Slow oxidation and hydroxylation increase donor/acceptor charges, giving the soil strong exchangeable properties. The positive impact of charcoal in ameliorating the physical and chemical properties of tropical soils has been reported in various situations [START_REF] Glaser | Ameliorating physical and chemical properties of hughly weathered soils in the tropics with charcoal. A review[END_REF]. To the light of our results we expect the peregrine earthworm P. corethrurus to be the main agent for the incorporation of charcoal to the topsoil in the form of fine particles of silt size, which favoured the formation of stable humus in Amazonian Dark Earths or "Terra Preta" during Pre-Colombian times [START_REF] Glaser | Black carbon in density fractions of anthropogenic soils of the Brazilian Amazon region[END_REF].

The natural development of P. corethrurus, able to feed and reproduce in tropical soils poor in organic matter, opens avenues for new agricultural practices better adapted to permanent settlements, using charcoal in mixture with nutrient-rich amendment [START_REF] Steiner | Slash and char: an alternative to slash and burn practiced in the Amazonian Basin[END_REF]. In situ experiments were conducted with the help of a local agriculturist at Maripasoula (French Guiana), using waste products of slash-and-burn agriculture (charcoal and manioc peels) as an amendment. We demonstrated that the addition of charcoal together with manioc peels, known to be rich in phosphorus (a limiting nutrient in tropical soils), increased yardlong bean production at the natural population size of P. corethrurus, thus allowing diversification of family agriculture without any additional cost [START_REF] Topoliantz | Effect of organic manure and the endogeic earthworm Pontoscolex corethrurus (Oligochaeta: Glosscoscolecidae) on soil fertility and bean production[END_REF];Topoliantz et al., 2005).

Acknowledgements

This work was supported by the French Ministry of Ecology and Sustainable Development (SOFT program) and by a grant from the Commission for the Creation of the Guiana Natural Park. The authors warmly acknowledge Dr. C. Kerdelhué (INRA, Pierroton, France) and Dr.

L. Greenfield (Canterbury University, Christchurch, New Zealand) for language editing.


Servane Gillet

Jean-François Ponge

An optical analysis of the organic soil over an old petroleum tar deposit

Keywords:

), located at Merkwiller-Pechelbronn (Alsace, France).

Comparisons were done with a nearby unpolluted control plot under similar vegetation.

The humus form over the tar deposit was described as an Hemimoder. It was characterised by fine fragmentation of litter, darkening of tree leaves with depth and a dense mycelial mat associated with an ectomycorrhizal root system. Faunal activity was dominated by enchytraeids. The Mesomull described at the control plot was characterized by fast recycling of litter and earthworm activity.

Introduction

Studies on metal-polluted soils showed that the decomposition of organic matter was affected by a high rate of contamination by trace elements, so that plant debris  Fax: +33-1-60465009. E-mail address: jean-francois.ponge@wanadoo.fr accumulate undecayed over the mineral soil, forming Mor [START_REF] Coughtrey | Litter accumulation in woodlands contaminated by Pb, Zn, Cd and Cu[END_REF][START_REF] Balabane | Mutual effects of soil organic matter dynaics and heavy metals fate in a metallophyte grassland[END_REF][START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. This change in humus form was attributed to the impact of heavy metals on soil organisms, in particular earthworms [START_REF] Nahmani | Effects of heavy metal pollution on soil macrofauna in a grassland of northern France[END_REF][START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. We may wonder whether a similar phenomenon occurs when a site polluted by hydrocarbons is abandoned then colonised by forest vegetation, providing an organic matter input to the soil system. Similar changes in humus forms are expected, given detrimental effects of hydrocarbons on soil animal communities and decomposition processes [START_REF] Erstfeld | Effects of chronic low-level PAH concentration on soil invertebrate communities[END_REF][START_REF] Blakely | Soil invertebrate and microbial communities and decomposition as indicators of polycyclic aromatic hydrocarbon contamination[END_REF][START_REF] Hanna | Earthworm survival in oil contaminated soil[END_REF].

To analyse humus forms, we used a micromorphological method, based on the optical characterization and quantification of biological debris and biogenic structures [START_REF] Peltier | Humus forms in mediterranean scrublands with aleppo pine[END_REF][START_REF] Sadaka | Climatic effects on soil trophic networks and the resulting humus profiles in holm oak (Quercus rotundifolia) forests in the High Atlas of Morocco as revealed by correspondence analysis[END_REF]. The same method has been used to study the effects of heavy metal contamination [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. Our work hypothesis was that disturbance of soil microbial and animal activity resulting from hydrocarbon pollution will affect the distribution of organic matter in the upper soil profile, thus in the humus form. As a corollary, the humus form could be used for the early detection of pollution, which will be the purpose of a further study.

Materials and methods

Study site

The study was conducted at the site of the old petroleum refinery of Merkwiller-Pechelbronn, about 50 km north of Strasbourg (Alsace, France). The refinery had an intensive activity until 1964, afterwards it was progressively dismantled then totally abandoned. The site is located on the Pechelbronn oil field (bituminous sand) on the western edge of the Rhine rift valley. The surface soil material (5 m deep) is composed of recent fluvial deposits overlaying 1400 m thick sediments above the granitic substratum [START_REF] Sittler | Les hydrocarbures d'Alsace dans le contexte historique et géodynamique du fossé Rhénan[END_REF].

Today, the site area (20 ha) is characterised by the presence of old buildings and vegetation including a great variety of semi-natural ecosystems (woodland, grassland, ponds) with zones polluted by hydrocarbons, in particular tar deposits. A polluted plot was selected by visual inspection of the deciduous woodland in October

Soil micromorphology

Topsoil profiles (litter included) were sampled and analysed by the micromorphological method of small volumes [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF]. A block 5x5 cm section, with variable depth, was collected at each plot with as little disturbance as possible. A thorough visual inspection of the two compared plots confirmed that sampled humus profiles were representative of the biological soil functioning at both plots. Different layers were distinguished in the block by eye and were directly separated then fixed in 95% (v/v) ethyl alcohol. Layers were identified according to the nomenclature of soil horizons by [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF] as OL (entire leaves), OF (fragmented leaves with faecal pellets) and A (hemorganic horizon). When several layers where collected in the same horizon, they were sub-labelled as OL1, OL2,…

The thickness of each layer was measured to the next mm.

In the laboratory, each layer was spread gently in a Petri dish, then covered with 95% ethanol, taking care not to break the aggregates. The different components were identified under a dissecting microscope at 40x magnification, with a reticle in the eye piece and quantified by a point-count method. A transparent film with a 200-point grid was placed over the preparation. At each grid node, using the reticule as an aid for fixing the position, the litter/soil component beneath it was identified and classified according to vegetation type, organ, decomposition stage and colour for plant organic matter and according to zoological group, colour and degree of organo-mineral mixing for animal faeces. The various kinds of plant debris were identified visually by comparison with a collection of main plant species growing in the vicinity of the sampled topsoil profiles. Animal faeces were classified by the size, the shape, the degree of mixing with organic matter and colour according to animal groups when possible [START_REF] Ponge | Food resources and diets of soil animals in a small area of Scots pine litter[END_REF][START_REF] Topoliantz | Earthworm and enchytraeid activity under different arable farming systems, as exemplified by biogenic structures[END_REF]. Afterwards, the relative volume percentage of each component was estimated by summing the corresponding counts then dividing this total by the number of points inspected.

Chemical analyses

Samples were collected in April 2003 at the Merkwiller-Pechelbronn site to determine the amount of extractable polycyclic aromatic hydrocarbons (EPA list of 16 PAHs) in (1) the 10 top cm of the control soil (2) the tar deposit at 10-20 cm depth and

(3) the organic soil overlying the tar deposit. Five samples were taken randomly at each plot then bulked for analysis. They were kept in glass jars then rapidly transported to the laboratory. At the laboratory, soil samples were homogenised and sieved at 1 cm then kept in glass jars at -18C° until analysis. Each sample was defrozen, dried, then sieved at 2 mm. To check the validity of our control plot, samples were collected in May 2003 in the park of the laboratory, in the 10 top cm of a similar soil (rich earthworm mull at neutral pH) under deciduous woody vegetation.

PAHs were extracted from each bulk sample with the automatic system ASE 200 (Accelerated Solvent Extraction) DIONEX, using a mixture of dichloromethane and acetone (50/50) for soil or acetonitrile for tar. The extract was concentrated under forced air with a TuroVap LV (Zymark), then PAHs were separated by HPLC (High Power Liquid Chromatography) with UV detection (alliance 2690 chain, PDA 996 detector, column LC-PAH Supelco). Unfortunately, after sifting the contaminated soil, which was mainly made of badly decomposed tree litter, only a few amount of fine matter was available for analysis, thus only 7 major PAHs could be analysed on this material: naphthalene, phenanthrene, fluoranthrene, pyrene, benzo(a)anthracene, chrysene and benzo(ghi)perylene, the others being under detection levels. All measurements were done in triplicate.

Five soil samples were collected in the same time at both plots from the Pechelbronn site, then they were air-dried and stored in plastic bags for pH measurement. Soil pH was measured in water and in potassium chloride suspensions according to ISO 10390 [START_REF] Afnor | Qualité des Sols[END_REF]. The soil was suspended in deionized H 2 0 and 1M KCl (1:5 soil:water v:v) for pH H 2 0 and pH KCl, respectively. Each suspension was shaken for five minutes, then pH was measured in the supernatant after sedimentation for 2 h with a glass electrode. The difference between these two measurements (pH) was taken as a rough estimate of exchange acidity.

Results and discussion

The distribution of the 16 PAHs of the EPA list and the total amount of PAHs were quite similar in the control mineral soil from the Pechelbronn site and the park of the laboratory (Table 1). As a consequence we estimated that the plot chosen as a control at Pechelbronn was valid, even though it was selected within an ancient industrial site. The total amount of the seven PAHs analysed in the soil over the tar deposit at Pechelbronn (5.95 mg/kg) was six times higher than the corresponding amount at the control plot (0.93 mg/kg). At the species level, the concentration of benzo(ghi)perylene was eleven times higher than the control, while the amount of fluoranthrene was only three times higher.

The pH measured in water was nearly neutral, while the pH measured in potassium chloride was one unit lower in both Pechelbronn sites (Table 1). Exchange acidity, expressed by pH, was higher on the unpolluted site.

One hundred and sixty one categories of litter/soil components were identified in the soil matrix. They were bulked into forty one gross categories for further treatment of the data (Appendix). The humus form at the unpolluted (control) plot was a Mesomull [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF]. Six layers were sampled, which were grouped in two horizons, OL (0-2.7 cm) and A (2.7-6.5 cm). The humus form at the polluted plot was a thin Hemimoder overlying the tar deposit. Seven layers were sampled in three horizons, OL (0-0.7 cm), OF (0.7-1.6 cm) and A (1.6-3.2 cm).

The diagrammatic presentation of the data showed marked differences between plots (Figure 1). In the polluted zone, leaf decomposition was mainly characterised by fine fragmentation and by darkening of limbs (from white to dark black), both processes increasing with depth. In the unpolluted zone, light-brown entire leaves were dominant in the upper part of the OL horizon then leaves were skeletonized and fragmented in the lower part of this horizon, before being totally incorporated into hemorganic assemblages in the A horizon.

The animal activity was dominated by enchytraeids at the polluted plot, where their dark-coloured faeces abounded in OF and A horizons. On the contrary, earthworm activity was prominent in the A horizon from the unpolluted plot, where the hemorganic matter was made of earthworm faeces (of the same colour as the surrounding soil), which became incorporated in a hemorganic mass of similar colour in the lower part of this horizon.

At the polluted plot a dense mycelial system was present in OF and A horizons, which was concomitant to the development of an ectomycorrhizal root system. The unpolluted plot was characterised by non-mycorrhizal root systems (without any visible ectomycorrhizae) in OL and A horizons (OF being absent) and a little mycelial network.

At the polluted plot the lower part of the A horizon was characterised by tar masses and assemblages of tar and mineral matter, which we considered as a probable artifact of the sampling method.

In spite of a slight distance between plots, a dissimilarity in humus forms was observed. The decomposition of organic matter was reduced at the plot polluted by hydrocarbons. This was confirmed by a lower exchangeable acidity, indicating incomplete humification of the accumulated organic matter. However, faunal activity was shown by the presence of springtail and enchytraeid faeces, and skeletonization of leaf litter by the latter group [START_REF] Ponge | Humus forms and horizons in beech forests of the Belgian Ardennes[END_REF]. The finer size of foliar debris at greater depth could be attributed to mechanical fragmentation by frost-and-thaw events, since few earthworm activity was observed at this plot. The absence of an earthworm structure could be attributed to the shallow soil overlying the tar deposit, which prevented animals from burying during winter frost and summer drought. Toxicity of the environment, known to severely affect soil decomposer activity [START_REF] Blakely | Soil invertebrate and microbial communities and decomposition as indicators of polycyclic aromatic hydrocarbon contamination[END_REF], could not explain this local collapse in earthworm populations, since at our PAHs concentrations no toxic effect was observed by authors [START_REF] Dorn | Assessment of the acute toxicity of crude oils in soils using earthworms, Microtox®, and plants[END_REF][START_REF] Erstfeld | Effects of chronic low-level PAH concentration on soil invertebrate communities[END_REF].

Over the tar deposit we observed a pronounced darkening of leaves, which was reflected in the dark colour of enchytraeid faeces, while on the control, organic matter decomposition was characterised only by early fragmentation and skeletonization of leaves. No bleaching of oak leaves was observed, which could be explained by fast incorporation of litter by earthworms into the mineral part of the soil. On the contrary, at the polluted plot, where white rot activity was similarly absent, fragmented leaves accumulate and come into contact with the pollution source (tar), making possible a transfer of hydrocarbons to decaying leaves, acting as a sink, which could explain their darkening. Strong affinity between organic matter and hydrocarbons has been demonstrated [START_REF] Xing | Sorption of naphthalene and phenanthrene by soil humic acids[END_REF].

We showed that roots were ectomycorrhizal at the polluted plot whereas at the control plot no sign of ectomycorrhizal development was observed. [START_REF] Leyval | Effect of polyaromatic hydrocarbons in soil on arbuscular mycorrhizal plants[END_REF] demonstrated the advantage of mycorrhizal over non-mycorrhizal plants in the presence of a pollution of the soil by hydrocarbons. They showed that mycorrhizal fungi contributed to the establishment and maintenance of plants in PAH-polluted soils.

Unlike [START_REF] Blakely | Soil invertebrate and microbial communities and decomposition as indicators of polycyclic aromatic hydrocarbon contamination[END_REF] working on a creosote-polluted soil, we observed at the tarpolluted plot the development of a dense mycelial network. According to observations done by [START_REF] Ponge | Ecological study of a forest humus by observing a small volume. I. Penetration of pine litter by mycorrhizal fungi[END_REF] on forest litter, and in the absence of fungal decomposition of oak litter on our polluted plot, we suspect that the mycelial mat was mainly formed by mycorrhizal fungi. It is noteworthy that, in addition to facilitate the establishment of vegetation, ectomycorrhizal fungi are able to degrade PAHs with three to five benzene rings [START_REF] Gramss | Conversion rates of five polycyclic aromatic hydrocarbons in liquid cultures of fifty-eight fungi and the concomitant production of oxidative enzymes[END_REF].

Conclusion

Although limited by lack of replication our study showed that strong changes in soil foodwebs could be observed under the influence of pollution by hydrocarbons, forty years after abandonment of industrial activity. Despite a decrease in PAH concentration in the soil accumulated over tar deposits, the inability of earthworm populations to colonize pollution spots, and the development of a superficial ectomycorrhizal root mat caused profound changes in the environment of animal as well as microbial communities, as suggested by [START_REF] Blakely | Soil invertebrate and microbial communities and decomposition as indicators of polycyclic aromatic hydrocarbon contamination[END_REF]. According to the integrated model by [START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF], changes in humus forms can reveal a damage to the ecosystem, and thus could be used for the detection of diffuse as well as spot pollution. 2).

Appendix

List of the 41 classes of litter/soil components identified in our study

  2002. It was considered to be representative of tar spots in the study site, as ascertained by the visual inspection of numerous soil trenches in the course of a threeday peer-around of the whole site. Only woodland vegetation was considered, since only woody areas remained untouched after cessation of industrial activity. The polluted plot (ca 25 m 2 ) was characterised by a 3-5 cm thick organic layer overlying a shallow (30-50 cm) pasty petroleum tar deposit. Vegetation was characterized by a discontinuous field layer, composed of Hedera helix, Geranium robertianum, Carex pilosa, Solidago canadensis and Taraxacum officinale, a shrub layer composed of Fraxinus excelsior, Rubus fruticosus and Salix capraea and a tree layer composed of Acer campestre, Betula pendula and Quercus robur. A nearby, control plot without any visible sign of pollution by hydrocarbons but with similar vegetation, was selected 15 m south of the polluted plot, for the sake of comparison. It was characterised by a much greater plant biodiversity and an earthworm Mull humus form. The field layer was composed of Hedera helix, Arum maculatum, Carex pilosa, Fragaria vesca, Geranium robertianum, Geum urbanum, Potentilla reptans, Stachys sylvatica, Solidago canadensis and Taraxacum officinale, the shrub layer was composed of Acer pseudoplatanus, Carpinus betulus, Cornus mas, Cornus sanguinea, Crataegus monogyna, Fraxinus excelsior, Ligustrum vulgare, Prunus avium, Rosa canina and Rubus fruticosus, and the tree layer was composed of Acer campestre, Prunus avium and Quercus robur.

Figure captions Fig 1 .

 captions1 Figure captions

  Chemical features of tar, organic matter accumulated over tar, control soil from Pechelbronn site and laboratory soil. Concentration of PAH (Polycyclic Aromatic Hydrocarbons) is expressed in μg/g dry soil.Means of three replicated measures on pooled samples (PAHs) or of five replicate samples (pHs) are follow ed by standard errors. Significant differences among groups at 0.05 level (for pH only) are indicated by different superscript letters (Mann-Whitney test). N.D. = not determined

																-15--16--17-
	1	Table 1
		N°Class										N°Class
		1	Fauna										22	White entire leaf
		2	M o ss											23	Light-bro wn entire leaf
		3	Lichen										24	Dark-bro wn entire leaf
		4 5	Tar M ycelium							Tar		25 Soil over tar (P) Control soil (C) Laboratory soil Light-bro wn leaf fragment > 1 cm 26 Light-bro wn leaf fragment < 1 cm
		pH (w ater) 6 M yco rrhizal ro o t						6.7±0.3 a 27	7±0.4 a Dark-bro wn leaf fragment > 1 cm
		pH (potassium chloride)  pH 7 Nerve Naphthalene 8 Ro o t Acenaphthene 9 Twig > 1 cm length			52.7±1.0 N.D.	6.3±0.3 a 0.4±0.04 b 28 Dark-bro wn leaf fragment < 1 cm 6.2±0.4 a 0.8±0.05 a 0.493±0.022 0.096±0.003 29 B ro wn-black leaf fragment > 1 cm N.D. 0.038±0.002 30 B ro wn-black leaf fragment < 1 cm	0.064±0.005 0.023±0.000
		Phenanthrene 10 Twig < 1 cm length		151.1±2.8	0.776±0.048 31 B lack leaf fragment > 1 cm 0.102±0.006	0.119±0.019
		Anthracene 11 Light-bro wn hemo rganic mass		N.D.		N.D. 32	0.025±0.001 B lack leaf fragment < 1 cm	0.024±0.008
		Fluoranthrene 12 Hemo rganic mass with tar		N.D.		0.655±0.012 33 Skeleto nized leaf fragment 0.238±0.020	0.230±0.024
		Pyrene 13 M ineral matter				N.D.		1.551±0.136 34 Skeleto nized entire leaf 0.178±0.020	0.152±0.014
		Benzo(a)anthracene 14 B ud scale and seed			N.D.		0.633±0.033 35 B ro wn black entire leaf 0.107±0.011	0.080±0.009
		Chrysene 15 B ro wn-black hemo rganic faeces	35.7±0.4	0.694±0.051 36 Leaf co vered by mycelium 0.108±0.005	0.105±0.013
		Benzo(k)fluoranthrene Benzo(a)pyrene 16 Hemo rganic enchytraeid faeces	N.D. N.D.		N.D. N.D. 37	0.057±0.004 0.157±0.019 Leaf co ated with hemo rganic faeces 0.048±0.004 0.147±0.011
		Dibenzo(a,h)anthracene 17 B ro wn-black ho lo rganic enchytraeid faeces N.D.	N.D. 38	0.023±0.003 Flaky mass (dead hyphae+bacteria+o rganic matter) 0.016±0.001
		Benzo(ghi)perylene 18 Hemo rganic millipede faeces		27.2±2.0	1.146±0.031 39 Unidentified o rganic matter 0.103±0.012	0.074±0.003
		Indeno(123cd)pyrene 19 Hemo rganic earthwo rm faeces	N.D.		N.D. 40	P etio le	0.130±0.008	0.083±0.006
		Σ 13 PAHs analysed in C soil 20 Light-bro wn ho lo rganic earthwo rm faeces		41	Stem	1.363±0.102	1.165±0.094
	1	Σ 7 PAHs analysed in P soil 21 Light-bro wn mineral earthwo rm faeces		5.950±0.290	0.934±0.070	0.824±0.083
	2
		4	100%
			OL1	OL2	OL3	OF1	OF2		A1	A2		OL1	OL2	OL3	A1	A2	A3
			cm	-0,2	-0,6	-0,7	-1	-1,6	-2,6		-3,2	0 cm	-1	-1,7	-2,7	-4	-4,8	-6,5
	1								Organic	soil over	tar					soil Control
	2	Fig. 1

Acknowledgements

We thank the Agence de l'Environnement et de la Maîtrise de l'Energie (ADEME) for financial support and Vasilica HAMMADE and Laurence BELKESSAM from the Centre National de Recherche sur les Sites et Sols Pollués (CNRSSP, Douai, France) for PAH analyses.


Jean-François Ponge
email: jean-francois.ponge@wanadoo.fr

Florence Dubs

Servane Gillet

Jose Paulo Sousa

Patrick Lavelle

Decreased biodiversity in soil springtail communities: the importance of dispersal and landuse history in heterogeneous landscapes

Keywords: Landuse intensification, Springtails, Landuse diversity, Dispersal rate

In previously published papers it had been demonstrated that at the local level the species richness of soil springtail communities was negatively influenced by landuse diversity. When the dispersal rate of soil animals was taken into account, quite opposite trends were displayed by species having poor or high dispersal capabilities. At the local level, species with short legs, non functional jumping aparatus and reduction of visual organs were distinguished against by landuse diversity, while species with long legs, functional jumping apparatus (furcula) and complete eyes, thus able to disperse at the soil surface, were not. It was verified, through aerial photographs taken fifty years ago, that landuse changes, expected to be more frequent in heterogeneous landscapes, may contribute to explain this phenomenon.

activities. Soil and climate conditions, as well as landuses, were described in two previously published papers [START_REF] Ponge | Collembolan communities as bioindicators of landuse intensification[END_REF][START_REF] Fédoroff | Small-scale response of plant species to land-use intensification[END_REF]. The distribution of landuse types in the six LUUs is shown in Table 1. In each LUU the diversity of landuse types was expressed by the Shannon Index.

Using aerial photographs, a grid of 16 regularly spaced plots (200 m) was identified in each of the six LUUs. Sampling of Collembola took place in June 2001. Methods used for sampling, extraction, sorting and identification of Collembola at the species level were detailed in [START_REF] Ponge | Collembolan communities as bioindicators of landuse intensification[END_REF]. One sample was discarded for extraction, because of waterlogging at the time of sampling.

Collembolan species were classified in two groups, according to their ability to disperse actively or not (Table 2). Species with long legs and antenna, a developed jumping apparatus (furcula) and complete visual apparatus (8 ocella per eye spot) were considered able to disperse rapidly by their own means [START_REF] Hopkin | Biology of the Springtails[END_REF]. All other species, because of a reduction in motion or vision organs, were considered as having poor dispersal capabilities. It has been demonstrated that fully functional visual organs allow springtails to move directionally over long distances [START_REF] Hågvar | Long distance, directional migration on snow in a forest collembolan, Hypogastrura socialis (Uzel)[END_REF]. Conversely, springtails showing a reduction in their eye number, even when fully motile, exhibit negative phototaxis and thus cannot disperse easily [START_REF] Salmon | Responses to light in a soil-dwelling springtail[END_REF].

Ecological requirements of Collembolan species were derived from the distribution of species over all samples (n = 95), which was studied by multivariate analysis [START_REF] Ponge | Collembolan communities as bioindicators of landuse intensification[END_REF].

Axis 1 coordinates of correspondence analysis (CA) in [START_REF] Ponge | Collembolan communities as bioindicators of landuse intensification[END_REF] were used to separate forest from agricultural species (Table 2). It should be noted that the environmental gradient depicted by the first axis of CA was a combination of all factors which contrasted woodland and agricultural land, humus type included.

Ancient aerial photographs (1948 IGN campaign) were examined for each LUU, in order to identify the landuse type which prevailed at the place where sampling took place in 2001. Black and white photographs easily distinguished woodland, agricultural land, and hedgerows, but could not be used for finer resolution. Thus landuse types were gathered into two gross categories, namely woodland (deciduous and coniferous forests) and agricultural land (hay meadows, pastures, arable crops, recent fallows). Hedgerows (one sample) and clear-cuts (two samples) were not taken into account in the census. Afforested land was comprised of 10-to 50-yr-old coniferous plantations (Douglas fir, Norway spruce) and old fields (wooded fallows).

When landuse units were ordered according to increasing landuse diversity (Table 1), the balance between the two springtail categories changed markedly (Fig. 1). Slow-dispersal species were largely dominant in forested areas, decreasing from LUU 1 to LUU 4, while the fast-dispersal species increased. In the most diverse landscape (LUU 4) fast-dispersal and slow-dispersal species were at the same richness level. In areas dominated by agriculture (LUU 5 and LUU 6), slowdispersal species were also dominant, but to a lower extent than in areas dominated by forests (LUU1 and LUU2). The mean species richness of slow-dispersal Collembola was negatively correlated with landuse diversity (r = -0.93, P = 0.003), while fast-dispersal species were positively, but poorly correlated with landuse diversity (r = 0.68, P = 0.07).

Examination of past landuse revealed that some changes took place over the last half century. Eight samples were taken in places where there was a shift from agricultural land to woodland (afforestation), while two samples were taken in places where woodland was replaced by agricultural land (deforestation). Calculation of the impact of landuse change on local species richness of springtail communities was only possible in afforested sites. It revealed a deficit of species richness in sites where agricultural land was afforested (8.4±1.1), compared to stable woodland (13.2±0.6, Mann-Whitney U = 54.5, P = 0.001). When springtail species were separated in two dispersal groups, still clearer features appeared. In afforested land, the richness of slow-dispersal species was equal to that of the original agricultural land (i.e. near half that of woodland), while the richness of fast-dispersal species decreased to the level typical of woodland, i.e. near half that of agricultural land. While dominance of slow-over fast-dispersal species was prominent in woodland as well as agricultural land, afforested land displayed the reverse phenomenon. The examination of ecological requirements of species (forest vs agricultural species) showed that in afforested land fast-dispersal species were shared between forest (57%) and agricultural species (43%), while nearly all slow-dispersal species were still those typical of agricultural soils (88%).

We found that (i) the heterogeneity of the landscape exerted a negative influence on slowdispersal collembolan species, (ii) at least part of these effects could be explained by time-related processes, acting differentially on organisms with distinct life habits. Soil collembolan communities are negatively affected by deforestation as well as afforestation and this impact was shown to be durable [START_REF] Jordana | Effect of reafforestation by conifers in natural biotopes of middle and South Navarra (Northern Spain)[END_REF][START_REF] Deharveng | Soil Collembola diversity, endemism, and reforestation: a case study in the Pyrenees (France)[END_REF]. The contrast between closed and open habitats is one of the chief complex of factors which govern the species composition of most soil animal groups [START_REF] Nordström | Environmental factors and lumbricid associations in southern Sweden[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic grass-woodland ecosystems[END_REF].

Soil and climate factors are in play in the influence of landuse change on soil animal communities, more especially when agricultural land shifts to woodland, or the reverse. However, in the present state of our knowledge of ecological requirements and dispersal abilities of Collembola, only cursory explanation can be found for the observed changes in species composition and diversity, which are summarized below.

In the present study, the passage from agricultural land to woodland was accompanied by soil acidification [START_REF] Ponge | Collembolan communities as bioindicators of landuse intensification[END_REF]. In mixed landscapes, the higher acidity of the soil in woody areas is not solely due to the acidifying influence of forest growth [START_REF] Nilsson | Forest growth as a possible cause of soil and water acidification: an examination of the concepts[END_REF] but also to (i) the choice of more fertile (thus less acidic) soils for agriculture and (ii) the fertilization associated with the permanent use of land for agriculture [START_REF] Brady | The Nature and Properties of Soils, 12th edn[END_REF]. Soil acidity and associated factors influence the species composition of most soil animal communities [START_REF] Nordström | Environmental factors and lumbricid associations in southern Sweden[END_REF][START_REF] Wauthy | Synecology of forest soil oribatid mites of Belgium (Acari, Oribatida). III. Ecological groups[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic grass-woodland ecosystems[END_REF], but they affect primarily species in permanent contact with humified organic matter [START_REF] Ponge | Biocenoses of Collembola in atlantic grass-woodland ecosystems[END_REF].

Micro-climate changes affect primarily soil animal species living at the soil surface or not far from it, with agricultural land exhibiting more contrasting thermic and hygric conditions than woodland at the ground surface [START_REF] Coffin | Implications of natural history traits to system-level dynamics: comparisons of a grassland and a forest[END_REF]. In springtails, it has been demonstrated that the first instar, i.e. the first stage of development following egg hatching, was more sensitive to desiccation that further instars and that this phenomenon was more pronounced in woodland than in agricultural land species [START_REF] Betsch | Caractérisation de deux phases juvéniles d'Allacma fusca (Collembole, Symphypleona) par leur morphologie et leur écophysiologie[END_REF].

Slow-dispersal Collembola, contrary to fast-dispersal species, are favoured by forest environments. Most of them need a protection towards desiccation, because they are badly equipped for jumping rapidly from a micro-site to another in a changing environment [START_REF] Bauer | Habitat dependent differences in the flight behaviour of Collembola[END_REF]. Most species classified in the slow-dispersal category are soil-dwelling species, to the exception of small surface species, such as Xenylla spp. and Brachystomella parvula, which live under the protection of mosses and lichens or exhibit anhydrobiosis [START_REF] André | Introduction à l'étude écologique des communautés de microarthropodes corticoles soumises à la pollution atmosphérique[END_REF][START_REF] Poinsot-Balaguer | Dynamique des communautés de Collemboles en milieu xérique méditerranéen[END_REF].

In a complex landscape where forest and agricultural land are intimately mixed, two categories of underground biodiversity should be considered apart: those organisms which are able to disperse at the scale of landscape heterogeneity (these communities will be positively or not affected by heterogeneity, here fast-dispersal springtails), and those unable to do that, which will be negatively affected except those tolerant of landuse changes. Examination of literature shows that most springtail species which are considered of particular interest, because they are endemic or rare, fall into the slow-dispersal category, thus are threatened by landuse changes [START_REF] Barrocas | Impact of reafforestation with Eucalyptus globulus Labill. on the edaphic collembolan fauna of Serra de Monchique (Algarve, Portugal)[END_REF][START_REF] Deharveng | Soil Collembola diversity, endemism, and reforestation: a case study in the Pyrenees (France)[END_REF][START_REF] Lauga-Reyrel | Diversity within the Collembola community in fragmented coppice forests in south-western France[END_REF].

Figure captions Fig. 1 .

 captions1 Figure captions

  D ist rib u t io n o f lan d u se t y p es am o n g t h e six lan d u se u n it s (LU U s), o rd ered ac c o rd in g t o in c reasin g lan d u se d iv ersit y . S ix t een sam p les w ere t ak en in eac h LU U , ex c ep t LU U 4 w it h f if t een Lis t o f c o llem b o lan s p ec ies f o u n d in t h e s t u d y s it es , c las s if ied ac c o rd in g t o t h eir ab ilit y o t d is p ers e (es t im at ed b y ap p en d ag e an d ey e d ev elo p m en t ).

		T able 1 . sam p les o n ly T able 2 .
		D ec id u o u s f o rest S low -dispersal	LU U 1 16	LU U 2 1 Fast-dispersal LU U 6 0	LU U 3 8	LU U 5 3	LU U 4 0
		C o n if ero u s f o rest A A n u rid a g ran aria	0	14	F	0 A llac m a f u s c a	2	2	3
		C lear-c u t A A n u rid a u n if o rm is	0	1	A	0 D eu t ero s m in t h u ru s s u lp h u reu s 0	0	1
		H ed g ero w F A rrh o p alit es b if id u s 0	0	A	0 D ic y rt o m a f u s c a	0	1	0
		H ay m ead o w F A rrh o p alit es s p .	0	0	A	0 D ic y rt o m in a m in u t a 4	4	4
		Past u re A Brac h y s t o m ella p arv u la 0	0	A	2 D ic y rt o m in a o rn at a 1	6	3
		Fallo w F C erat o p h y s ella arm at a 0	0	A	5 En t o m o b ry a m u lt if as c iat a 0	0	1
		A rab le c ro p A C erat o p h y s ella d en t ic u lat a 0	0	F	9 En t o m o b ry a n iv alis 1	0	3
		N o t sam p led F C erat o p h y s ella lu t eo s p in a 0	0	A	0 Fas c io s m in t h u ru s q u in q u ef as c iat u s 0 0	1
	1	S h an n o n In d ex A C y p h o d eru s alb in u s 0	0 . 6 7	A	1 . 3 7 Is o t o m a an t en n alis 1 . 8 8	2 . 1 1	2 . 4 2
		A	Fo ls o m ia c an d id a			A	Is o t o m a t ig rin a
	2	A	Fo ls o m ia f im et aria		A	Is o t o m a v irid is
		F	Fo ls o m ia q u ad rio c u lat a		A	Is o t o m u ru s p alu s t ris
	3	F F	Fries ea c lav is et a Fries ea m irab ilis			A F	Lep id o c y rt u s c y an eu s Lep id o c y rt u s lan u g in o s u s
		F	Fries ea t ru n c at a			A	Lep id o c y rt u s lig n o ru m
		A	H et ero m u ru s n it id u s		F	Lip o t h rix lu b b o c k i
		F	Is o t o m iella m in o r			F	O rc h es ella c in c t a
		F	Is o t o m o d es p ro d u c t u s		A	O rc h es ella q u in q u ef as c iat a
		A	K alap h o ru ra b u rm eis t eri		A	O rc h es ella v illo s a
		F	M eg alo t h o rax m in im u s		F	Po g o n o g n at h ellu s f lav es c en s
		F	M es ap h o ru ra b et s c h i		F	Ps eu d is o t o m a s en s ib ilis
		F	M es ap h o ru ra jev an ic a		A	S m in t h u rid es p arv u lu s
		F	M es ap h o ru ra leit zaen s is		A	S m in t h u rid es s c h o et t i
		F	M es ap h o ru ra m ac ro c h aet a		A	S m in t h u rin u s au reu s
		F	M es ap h o ru ra y o s ii		A	S m in t h u rin u s n ig er
		F	M ic ran u rid a p y g m aea		F	S m in t h u rin u s s ig n at u s
		F	M ic ran u rid a s en s illat a		A	S m in t h u ru s n ig ro m ac u lat u s
		F	M ic rap h o ru ra ab s o lo n i		A	S m in t h u ru s v irid is
		F	N ean u ra m u s c o ru m		A	S p h aerid ia p u m ilis
		F	N eelid es m in u t u s			A	S t en ac id ia v io lac ea
		A	N eo t u llb erg ia ram ic u s p is		F	T o m o c eru s m in o r
		F	O n c o p o d u ra c ras s ic o rn is		F	V ert ag o p u s arb o reu s
		F	O n y c h iu ro id es p s eu d o g ran u lo s u s	A	W illo w s ia n ig ro m ac u lat a
		F	O n y c h iu ru s c eb en n ariu s
		A	O n y c h iu ru s ju b ilariu s
		F	Parat u llb erg ia c allip y g o s
		A	Paris o t o m a n o t ab ilis
		F	Pro is o t o m a m in im a
		A	Pro t ap h o ru ra arm at a
		A	Pro t ap h o ru ra m erid iat a
		A	Pro t ap h o ru ra p ro lat a
		F	Ps eu d ac h o ru t es p arv u lu s
		A	Ps eu d an u ro p h o ru s b in o c u lat u s
		A	Ps eu d o s in ella alb a
		A	Ps eu d o s in ella illic ien s
		F	Ps eu d o s in ella m au li
		A	S p in o n y c h iu ru s ed in en s is
		A	S t en ap h o ru ra d en is i
		A	S t en ap h o ru ra q u ad ris p in a
		F	W illem ia an o p h t h alm a
		F	W illem ia d en is i
		F	W illem ia in t erm ed ia
		F	X en y lla g ris ea
		F	X en y lla t u llb erg i
		F	X en y llo d es arm at u s

Acknowledgements

The authors thank the staff of the Morvan Regional Nature Park and numerous private owners for facilities during choice of sites and sampling operations. This study was part of the European Community program BioAssess EVK2-CT-1999-00041, coordinated by A. Watt (UK), who is gratefully acknowledged for financial support and fruitful exchange of ideas between participants.