Nassima Sadaka

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

Soil animal communities in holm oak forests: influence of horizon, altitude and year

Keywords: Soil fauna, Vertical distribution, Holm oak

Soil animals (macro-and microarthropods, annelids, nematodes) were sampled along an altitudinal gradient and over two years in holm oak (Quercus rotundifolia) forests of the Moroccan Atlas. We studied the influence of elevation and year on the vertical distribution of soil fauna. Whatever the elevation (1500, 1700 and 1900 m), the humus form was a Dysmull, with a thick litter horizon and a fine crumb A horizon. Thirty-six categories of fauna were found and classified at the group level. The influence of horizon, altitude and year was analysed by ANOVA (on seven broad zoological groups and on total fauna) and correspondence analysis (on thirty-six zoological groups).

There was a decrease in the population size of most zoological groups from organic (OL, OF) to mineral horizons (A, S), but OL and OF horizons varied as the most populated horizon according to years and animal groups. More animals and more animal groups were present at higher elevation, following an increase in food and habitat availability.

Introduction

The vertical distribution of soil animal communities has been poorly studied because of the lack of appropriate methods for recovering all animals living in organic as well as mineral horizons of the topsoil. Soil sections without soil-drying [START_REF] Anderson | Improvements in the gelatine-embedding technique for woodland soil and litter samples[END_REF][START_REF] Haarlov | Vertical distribution of mites and Collembola in relation to soil structure[END_REF][START_REF] Pande | Observations on the vertical distribution of soil Oribatei in a woodland soil[END_REF] and rhizotrons [START_REF] Gunn | The exploitation of food resources by soil meso-and macro invertebrates[END_REF][START_REF] Lussenhop | Observing soil biota in situ[END_REF] prove efficient in the observation of soil animals at the true place where they are living, but the small observable volume implies that many repeated counts are made before quantitative data can be obtained. Microstratification is used to separate the different layers of a soil profile before extracting fauna [START_REF] Ghilarov | Distribution of humus, root-systems and soil invertebrates within the soil of the walnut forests of the Ferghana Mountain Range[END_REF], but extraction methods are in general more efficient for some groups than for others [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF]. Thus, there are few studies on the vertical distribution of soil animals embracing a wide range of animal groups, from microfauna to macrofauna [START_REF] Arpin | Réactions des populations animales et microbiennes du sol à la privation des apports annuels de litière: exemple d'une rendzine forestière[END_REF][START_REF] Schaefer | The soil fauna of beech forests: comparison between a mull and a moder soil[END_REF]. Direct counts obtained by dissecting soil horizons with forceps prove more efficient than extraction procedures for some mesofauna groups which play a major role in plant litter decomposition and building of the soil structure, such as enchytraeids, insect larvae and phthiracarid oribatid mites [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF].

Changes in the vertical distribution of soil animals occur through variation in the distribution of soil organic matter, in particular in the thickness of organic compared to mineral horizons [START_REF] Arpin | Réactions des populations animales et microbiennes du sol à la privation des apports annuels de litière: exemple d'une rendzine forestière[END_REF][START_REF] Ghilarov | Abundance, biomass and vertical distribution of soil animals in different zones[END_REF][START_REF] Ponge | Interaction between soil fauna and their environment[END_REF]. Seasonal changes, in particular litter fall, winter frost and summer drought, are responsible for cyclic variations of soil animal vertical distribution [START_REF] Abrahamsen | Ecological study of Enchytraeidae (Oligochaeta) in Norwegian coniferous forest soils[END_REF][START_REF] Briones | Climate change and Cognettia sphagnetorum: effects on carbon dynamics in organic soils[END_REF][START_REF] Usher | Seasonal and vertical distribution of a population of soil arthropods: Collembola[END_REF]. In addition, year-to-year changes in the amount of litterfall, rain and temperature will exert similar effects through their influence on decomposition processes [START_REF] Witkamp | Van der Drift, Breakdown of forest litter in relation to environmental factors[END_REF]. Similarly, altitude, and climate and nutrient effects mediated by this factor, may influence the vertical distribution of soil animals through changes in the thickness of organic horizons, which increases or decreases according to vegetation types [START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF][START_REF] Frak | The influence of altitude on the distribution of subterranean organs and humus components in Vaccinium myrtillus carpets[END_REF].

The present study focused on holm oak forests, an important component of mediterranean woody landscapes [START_REF] Ibañez | Contribution à l'étude du sous-système pédologique des écosystèmes forestiers montagnards du massif de Moncayo (Chaîne Ibérique). I. Caractéristiques générales des écosystèmes compris dans le transect étudié[END_REF]. Edible holm oak (Quercus rotundifolia Lam.) is an evergreen Mediterranean oak being common in western Mediterranean countries (Morocco, Algeria, Tunisia, Central Spain), mostly in the mountains where it tolerates a dry and cold climate [START_REF] Achhal | À propos de la valeur bioclimatique et dynamique de quelques essences forestières au Maroc[END_REF][START_REF] Barbéro | Le chêne vert en région méditerranéenne[END_REF]. This tree is characterized by a persistent, spiny, stiff foliage.

Litter fall is scattered over the year but there is a maximum input of dead leaves to the ground from April to June. In holm oak stands the thickness of litter layers is determined by seasonal litter fall, decomposition rate, and biennial cycles of high and low litter input [START_REF] Poli | Produzione di littiera nella lecceta del M. Minardo (Etna) nel periodo Settembre 1970-Giugnio 1974[END_REF][START_REF] Rapp | Cycle de la matière organique et des éléments minéraux dans quelques écosystèmes méditerranéens[END_REF][START_REF] Sadaka-Laulan | Comparative leaf decomposition within the holm oak complex[END_REF].

In a previous study on humus forms of holm oak forests (Sadaka and Ponge submitted) we observed that the thickness of O (organic) and A (hemorganic) horizons was modifed by altitude and varied from year to year. Thus we may expect that the same factors which act on the distribution of topsoil horizons will act on the distribution of soil animals, and that both animal communities and humus forms are linked by feedback processes [START_REF] Lavelle | Ecological challenges for soil science[END_REF][START_REF] Ponge | Heterogeneity in soil animal communities and the development of humus forms[END_REF][START_REF] Ponge | Interaction between soil fauna and their environment[END_REF]. The purpose of the present study was to determine to what extent the distribution of topsoil horizons, and its altitudinal and annual variation, may explain the vertical distribution of soil animals.

Materials and methods

Study sites

The study was conducted in February 1999 and February 2002 in an holm oak forest (Q. rotundifolia) at Toufliht (northern slope of the High Atlas, Morocco). The climate is subhumid to semiarid mediterranean, with most precipitation from October to February followed by a long warm dry period from May to September (mean annual rainfall 840 mm; maximum summer temperature ca. 30.5°C and minimum winter temperature ca. 1°C). The parent rock consists of triassic molasses made of an alternance of red clay, sandstone and conglomerate [START_REF] Beauchamp | Reconstitution d'un milieu de sédimentation dans les molasses rouges triasiques du Haut Atlas[END_REF].

Three sites (SI, SII and SIII), where Q. rotundifolia is the dominant tree species Despite variations in litter thickness the humus form is always a Dysmull [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF], with a thick (more than 1 cm) OF horizon and an A horizon (2 to 4 cm thick) with a microcrumb structure [START_REF] Sadaka | Humus forms in holm oak forests[END_REF]. The pH (water) of the A horizon is 5.5 to 5.8. The A horizon overlays a S horizon, made of weathered parent rock.

Sampling procedure

In 1999 and 2002, at each site, and at more than 1 m from tree trunks, an unique humus block 5 x 5 cm in surface and 8.5 to 11.5cm depth was carefully excavated, according to the method devised by Ponge [START_REF] Ponge | Étude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF]. It was cut with a sharp knife, with as little disturbance as possible, and the litter and soil surrounding it were gently excavated. Layers, about 0.5 to 2.5cm thickness, were separated directly in the field from the top to the bottom of the profile on the basis of morphological differences which were visible to the nake eye, and immediately preserved in 95% ethyl alcohol (Table 1). Their thickness was noted before collection. Samples were classified into OL (entire leaves), OF (fragmented leaves with faecal pellets), A (hemorganic horizon) and S (mineral horizon), taking as a basis the classification of forest humus horizons by Brêthes et al. [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF]. When several layers were sampled in the same horizon (on the basis of visible differences) they were numbered according to their order from the top to the bottom of this horizon, for example OL1, OL2, OF1, OF2... Thus A2 (Table 1)

was not an A 2 horizon (eluvial horizon in past European classifications) but it was just the second layer sampled in the A horizon (the A 1 horizon in the old classification).

Each layer was carefully transferred to a Petri dish filled with ethyl alcohol, then a quantitative analysis of humus components was done by a point-count method [START_REF] Frak | The influence of altitude on the distribution of subterranean organs and humus components in Vaccinium myrtillus carpets[END_REF][START_REF] Peltier | Humus forms in Mediterranean scrublands with aleppo pine[END_REF][START_REF] Sadaka | Humus forms in holm oak forests[END_REF]. Afterwards, the material was thoroughly dissected with forceps under a dissecting microscope and each animal found was assigned to a group level (Table 2).

Data analysis

The population size of each group in each unit sample was subjected to correspondence analysis, a multivariate method using the chi-square distance [START_REF] Greenacre | Theory and Applications of Correspondence Analysis[END_REF].

The different zoological groups were the active variables. The nature of the corresponding horizon (OL, OF, A, S), the year (1999,2002), the site (SI, SII, SIII) and the depth at which the sample was taken were put as passive variables, i.e. they were projected on the factorial axes as if they had been involved in the analysis, without contributing to the factorial axes. They were coded as 1 or 0. All the variables (active and passive) were transformed according to the method of Ponge and Delhaye [START_REF] Ponge | The heterogeneity of humus profiles and earthworm communities in a virgin beech forest[END_REF], in order to give them the same weight and variance. The formula used was as follow: X = (x-m)/s + 20, where X is the standardized value, x the original value, m the mean of the variable and s its standard deviation. The addition to each standardized variable of a constant factor of 20 allows all values to be positive, because correspondence analysis deals only with positive numbers. Thus, factorial coordinates of variables can be interpreted directly in terms of their contribution to factorial axes. The farther a variable was projected from the origin of the axes (barycentre) along a factorial axis, the more it contributed to this axis. In order to depict gradients of bulk faunal abundance, every active variable (zoological group) was split into two symmetrical variables, the one being the original value (standardized and refocused as abovementioned), the other being created by complementing the original value to 40.

The second variable had thus the same mean [START_REF] Hassall | Vertical migration of Onychiurus subtenuis (Collembola) in relation to rainfall and microbial activity[END_REF] and the same standard deviation [START_REF] Abrahamsen | Ecological study of Enchytraeidae (Oligochaeta) in Norwegian coniferous forest soils[END_REF] as the first variable but varied in an inverse sense. Higher values of the former (original) variable corresponded to lower values of the latter (complementary) variable, and the reverse. Each zoological group was thus represented by two points, the one for higher values (the original variable), the other for lower values (the complement), and by this way the analysis was able to discern between rich (with abundant fauna) and poor samples, and to depict gradients of population size [START_REF] Loranger | Does soil acidity explain altitudinal sequences in collembolan communities?[END_REF].

Analyses of variance (3-way ANOVAs without replication, followed by SNK procedure [START_REF] Glantz | Primer of Biostatistics[END_REF]) were performed on broad zoological groups encompassing one or several of the groups listed in Table 2, as well as on total population size and zoological richness. Zoological richness was expressed by the number of zoological groups, sensu Ponge [START_REF] Ponge | Biodiversité et biomasse de la faune du sol sous climat tempéré[END_REF]. Horizon (OL, OF, A, S), altitude (1500 m, 1700 m, 1900 m) and year (1999,2002) were used as main factors. Data (counts) were log-transformed before analysis in order to ensure additivity of variances.

Results

Analysis of variance

Figure 1 is a graphical presentation of the population size of the main broad zoological groups. A strong variation exists between horizons, altitude and year and the effects of these three factors vary according to invertebrate categories. Results of variance analysis (main effects) and a posteriori comparisons among means are summarized in Table 3. Horizon, altitude and year can be classified in a decreasing order of influence on the population size of broad zoological groups, judging from the number of groups exhibiting significant variation: six groups among seven were significantly influenced by horizon, four by altitude and only two by year. Springtail densities were influenced by altitude (P = 0.03) and year (P = 0.004), but not by horizon (P = 0.35), without any significant interaction effect. Among the seven broad groups they were unique in having an even vertical distribution.

Collembola were the most abundant animal group in A and S horizons. Their population size increased with altitude, SIII (1900 m) harbouring four times more springtails than SI (1500 m) and SII (1700 m). Four times more animals were found in 2002 compared to 1999.

Mites were influenced by horizon (P = 0.0008) and year (P = 0.003), not by altitude (P = 0.18), without any significant interaction effect. There was a decrease in the population size from surface to deeper horizons, A and S (mineral horizons) harbouring less animals than OL and OF (organic horizons). Mites were the most abundant animal group in the OL horizon. Three times more animals were found in 2002 compared to 1999.

Nematodes were influenced by horizon (P = 0.01) and altitude (P = 0.03), not by year (P = 0.53), without any significant interaction effect. Like for mites, there was a decrease of the population size in the mineral soil (A and S), but the A horizon was intermediate between OF and S horizons. A strong increase in the abundance of nematodes was observed with altitude, SIII harbouring more animals than SI (x8) and SII (x3). Nematodes were the second most abundant animal group in OL and OF horizons.

Enchytraeids were influenced only by horizon (P = 0.03), without any significant interaction effect. They were more abundant in the OF horizon than in A (x7) and S (x12) horizons, the OL horizon being intermediate. Lumbricids, like enchytraeids, were influenced only by horizon (P= 0.01), without any significant interaction effect. They were more abundant in the OF horizon than in OL (x5), A (x4) and S (x9) horizons.

Insect larvae were influenced by horizon (P = 0.003) and altitude (P = 0.02), not by year (P = 0.20), without any significant interaction effect. They were more abundant in the OF horizon compared to OL (x2), A (x4) and S (x28) horizons and the S horizon harboured less animals than OL and A. Thus there was a vertical gradient of increasing (from OL to OF) then decreasing abundance (from OF to deeper horizons). Insect larvae were the most abundant invertebrate group in the OF horizon.

Miscellaneous animals (adult insects, Symphyla, spiders and pseuscorpions) were influenced by horizon (P = 0.01) and altitude (P = 0.02), but interactions between all couples of factors were significant (P < 0.05), suggesting that the effects of horizon and altitude were not additive, and depended on the year. This was not unexpected, since the miscellaneous group was strongly heterogeneous in its composition.

The size of the total invertebrate population (the total faunal abundance) was influenced by horizon (P = 0.008), altitude (P = 0.03) and year (P = 0.01), without any interaction effect. There were more animals in OL and OF horizons than in the S horizon (x4 and x5, respectively) and the A horizon was intermediate. There were more animals in SIII compared to SI (x1.6) and SII (x1.3). There were approximately 1.6 times more animals in 2002 than in 1999.

The zoological richness (number of zoological groups as classified in Table 2) was influenced by horizon (P = 0.0005) and altitude (P = 0.02), not by year (P = 0.2), but there was a significant interaction between horizon and year (P = 0.02). The number of animal groups decreased in the A horizon compared to OL (x0.7) and OF (x0.6) horizons and still decreased in the S horizon (0.6 times the number of groups found in the A horizon). The interaction between horizon and year was due to the fact that there were more animal groups in OF compared to OL in 1999, while the reverse was observed in 2002, but this did not obscure the overall decrease observed just beneath the organic horizons. The zoological richness increased with altitude, more animal groups being present at SIII than at SI (x1.6) and SII (x1.3).

Correspondence analysis

A correspondence analysis was perfomed on a data matrix crossing 40 samples with 36 zoological groups (main variables) and 23 descriptors (additional variables).

The first two factorial axes extracted 17% and 13% of the total variance, respectively, thus 30% of the total variance was extracted by the plane formed by Axes 1 and 2.

Only these two axes could be interpreted in light of the additional variables (horizon, altitude, year). The projection of zoological groups in the plane of Axes 1 and 2 (Fig. 2)

showed an overall trend of decreasing faunal abundance from organic (OL and OF) to mineral (A and S) horizons. Axis 1 displayed the most important information about the vertical distribution of soil invertebrates. Higher values of all but three zoological groups were projected on the positive side of Axis 1 (corresponding to OL and OF horizons) while most lower values were projected symetrically on the negative side (corresponding to A and S horizons). The three groups which did not follow the global trend of decreasing density with depth were Symphyla [START_REF] Gunn | The exploitation of food resources by soil meso-and macro invertebrates[END_REF], Heteroptera [START_REF] Pande | Observations on the vertical distribution of soil Oribatei in a woodland soil[END_REF] and Hymenoptera [START_REF] Meyer | The role of the fungus Cenococcum graniforme (Sow.) Ferd. et Winge in the formation of mor[END_REF]. All other groups were more abundant in organic than in mineral horizons, those being in the most superficial position (more attracted to litter) being nymph and adult oribatid mites [START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF], nematodes [START_REF] Ghilarov | Abundance, biomass and vertical distribution of soil animals in different zones[END_REF] and entomobryid springtails (3).

The vertical distribution of the different invertebrate categories can be quantified by their coordinates along Axis 1 (Table 2). Axis 1 coordinates can be considered as an index of epigeicity, varying from -0.009 (Symphyla) to 0.041 (adult and nymphal Cryptostigmata). Sites SI, SII and SIII were not projected at the same place along Axis 1, indicating that changes in the vertical distribution of Collembola occurred along the altitudinal gradient. SIII was projected on the positive side, while SI and SII were projected on the negative side. This indicated that litter-dwelling animal groups (those projected far from the origin on the positive side of Axis 1) were more abundant at higher altitude.

Axis 2 was strongly related with year, since 1999 and 2002 were projected far from the origin along this axis, the positive side corresponding to 1999 and the negative side to 2002. As ascertained from the projection of animal groups and horizon names it can be said that litter-dwelling fauna occupied mainly the OF horizon in 1999 and mainly the OL horizon in 2002 (see also Fig. 1) and that different faunal groups were involved. For instance animal groups far from the origin on the positive side of both Axis 1 and Axis 2, such as chironomid larvae [START_REF] Hassall | Vertical migration of Onychiurus subtenuis (Collembola) in relation to rainfall and microbial activity[END_REF], sciarid larvae [START_REF] Haarlov | Vertical distribution of mites and Collembola in relation to soil structure[END_REF], empidid larvae [START_REF] Joosse | Ecological strategies and population regulation of Collembola in heterogeneous environments[END_REF], spiders [START_REF] Frak | The influence of altitude on the distribution of subterranean organs and humus components in Vaccinium myrtillus carpets[END_REF] and enchytraeids [START_REF] Glantz | Primer of Biostatistics[END_REF], were typically found in the OF horizon in 1999 (see Fig. 1 for enchytraeids). Conversely, animal groups far from the origin on the positive side of Axis 1 and on the negative side of Axis 2, such as neanurid springtails (4), astigmatid mites [START_REF] Belotti | Variability in space and time and redundancy as stabilizing principles of forest humus profiles[END_REF][START_REF] Briones | Climate change and Cognettia sphagnetorum: effects on carbon dynamics in organic soils[END_REF] and larvae of mesostigmatid and oribatid mites (9, 12), were typically found in the OL horizon in 2002. Coordinates along Axis 2 could be taken as a measure of their affinity with one or the other of the two years 1999 and 2002 (Table 2), positive values indicating a higher abundance in 1999, negative values a higher abundance in 2002.

Correspondence analysis can be used also to discern changes in faunal communities according to depth levels rather than to horizons. Putting as additional variables the different depth levels (from 0-1 cm to 11-12 cm), these can be projected in the plane of the first two factorial axes and linked by running segments (Fig. 3). The faunal composition typical of the OL horizon encompasses the first three centimetres, with positive values along Axis 1 (indicating an epigeic fauna) and negative values along Axis 2 (indicating that fauna of this horizon is mostly represented in 2002). The faunal composition typical of the OF horizon is observed between 3 and 5 cm depth, and is mostly represented in 1999. Beneath 5 cm the faunal composition does not change to a great extent. These trends are mean trends which do not account for site (altitude) and year effects. Such effects can be displayed by coding separately the different depth levels according to the three sites or the two years.

The influence of altitude on the vertical distribution of soil animals is shown by Figure 4. Site III, at the highest elevation (1900 m) shows profound changes in faunal abundance and composition according to depth, while these changes are smoother in SII and even less pronounced in SI. While animal communities do not differ to a great extent in the first top cm, most differentiation between the three sites occurs in the two centimetres beneath. Between 1 and 3 cm depth the fauna remains typically that of an OL horizon at SIII, while it rather turns to that of an OF horizon at SII and to that of an A horizon at SI. The influence of the year is shown by Figure 5. Differences according to year exist both in organic horizons (OL and OF are clearly separated along Axis 2), but also in the mineral soil, even though year differences in faunal abundance and composition are less so pronounced in A and S horizons than in OL and OF horizons. In 1999 (positive side of Axis 2) the fauna of the first top cm is poorly differentiated (the 0-1 cm of 1999 is projected not far from the origin), while strong differentiation occurs from 1 to 5 cm, with a faunal community more abundant and typical of an OF horizon. At 5 cm depth the community passes abruptly to that of the mineral soil. In 2002 (negative side of Axis 2), the fauna of the first top cm (as well as that of the first three cm) is typical of an OL horizon, then shifts to one typical of the mineral soil under 5 cm, the faunal composition being intermediate between 3 and 5 cm depth.

Discussion

In the three sites investigated the humus form was a Dysmull [START_REF] Sadaka | Humus forms in holm oak forests[END_REF]. It was characterized by a thick litter layer (OL and OF horizons) overlying an organo-mineral A horizon with a crumb structure [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF]. Two compartments were thus present in the topsoil: a multi-layered organic compartment, made of plant debris at different stages of decomposition and intervened by organic-and mineral-dominant animal faeces, and a mineral compartment, made of mineral-dominant faeces and weathering mineral particles, permeated by living and dead roots (rhizosphere). This constituted habitat and food resources of the animal communities we studied. Faeces of dominant animal groups were retrieved during micromorphological investigations, in particular those of enchytraeids, earthworms (small epigeic species) and insect larvae [START_REF] Sadaka | Humus forms in holm oak forests[END_REF]. Faeces of springtails and mites were probably ingested by other animal groups, such as enchytraeids and earthworms [START_REF] Ponge | Food resources and diets of soil animals in a small area of Scots pine litter[END_REF][START_REF] Ponge | Heterogeneity in soil animal communities and the development of humus forms[END_REF], since they were not retrieved in noticeable amounts, although springtails and mites were abundant.

The present study showed that, although most faeces were found in OF and A horizons (Sadaka and Ponge submitted), most animals were living in OL and OF horizons (depending on year), at least at the time of sampling (February) and if we except springtails which exhibited an even distribution at the group level. Thus it may be hypothesized that OL and OF horizons was the zone of most saprophagous activity (litter decomposition and mixing of organic matter with mineral matter), while underlying horizons resulted from the accumulation of end-products of faunal activity [START_REF] Ponge | Horizons and humus forms in beech forests of the Belgian Ardennes[END_REF]. The root system in A and S horizons is a source of nutrients, both direct through growth and death of plant tissues and indirect through its action on soil microflora [START_REF] Norton | Carbon flow in the rhizosphere of ponderosa pine seedlings[END_REF][START_REF] Ponge | Food resources and diets of soil animals in a small area of Scots pine litter[END_REF][START_REF] Rovira | Plant root exudates and their influence upon soil microorganisms[END_REF]. In particular, we noted that most of the microbial biomass was produced by the black mycorrhizal ascomycete Cenococcum geophilum Fr. [START_REF] Sadaka | Humus forms in holm oak forests[END_REF]. Surprisingly, these subterranean resources, which were abundant at the time of sampling, seemed to be neglected, most animals being found rather in the litter compartment. Several reasons could be invoked to explain this phenomenon. First, the mycelium of C. geophilum is mechanically and chemically resistant, hard to digest and is hardly consumed by soil animals [START_REF] Lussenhop | Observing soil biota in situ[END_REF][START_REF] Meyer | The role of the fungus Cenococcum graniforme (Sow.) Ferd. et Winge in the formation of mor[END_REF][START_REF] Ponge | Food resources and diets of soil animals in a small area of Scots pine litter[END_REF]. The stiff fungal sheath surrounding the roots colonized by C. geophilum protects them against desiccation [START_REF] Mexal | The growth of selected mycorrhizal fungi in response to induced water stress[END_REF] and pathogenic infection [START_REF] Marx | The influence of ectotrophic mycorrhizal fungi on the resistance of pine roots to pathogenic infections. IV. Resistance of naturally occurring mycorrhizae to infections by Phytophthora cinnamomi[END_REF] and also against consumption by soil fauna [START_REF] Sutherland | Effect of the nematode Aphelencus avenae on some ectotrophic mycorrhizal fungi and on a red pine mycorrhizal relationship[END_REF]. Second, saprophagous animals could be attracted by fungi living in the litter layer. For instance, it has been demonstrated that the onychiurid springtail Onychiurus folsomi Schäffer (syn. O. sinensis Stach), was attracted to the white-rot basidiomycete Marasmius quercophilus Pouzar and to several other fungal strains isolated from decaying holm oak leaves [START_REF] Sadaka-Laulan | Influence of holm oak leaf decomposition stage on the biology of Onychiurus sinensis Stach (Collembola: Onychiuridae)[END_REF][START_REF] Sadaka-Laulan | Feeding preferences of the collembolan Onychiurus sinensis for fungi colonizing holm oak litter (Quercus rotundifolia Lam.)[END_REF]. These fungi were highly palatable to O. folsomi and ensured optimum growth and reproduction. Third, soil animals may move between several food sources [START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF][START_REF] Verhoef | Relative importance of fungi and algae in the diet and nitrogen nutrition of Orchesella cincta (L.) and Tomocerus minor (Lubbock) (Collembola)[END_REF] or between a place for feeding and another place for reproduction, moulting or protection from adverse events [START_REF] Joosse | Ecological strategies and population regulation of Collembola in heterogeneous environments[END_REF], and faeces can be deposited in the meantime [START_REF] Ponge | Horizons and humus forms in beech forests of the Belgian Ardennes[END_REF]. The third reason can be invoked to explain the deposition of organo-mineral faeces in OF and even OL horizons of the humus profiles [START_REF] Sadaka | Humus forms in holm oak forests[END_REF].

As an exception among the soil populations we studied, Collembola exhibited a regular vertical distribution and were by far the most abundant animal group in the S horizon, especially during the year 2002. The same phenomenon has been observed in mull [START_REF] Marinissen | Earthworm-amended soil structure: its influence on Collembola populations in grassland[END_REF] and is particularly pronounced under Mediterranean climate [START_REF] Poinsot-Balaguer | Étude d'un écosystème forestier méditerranéen. II. Les Collemboles d'une forêt mixte (Chênes verts, Quercus ilex L. -Chênes blancs, Quercus pubescens W.) de la région provençale[END_REF]. This can be explained by the abundance of roots and their mycorrhizal symbionts, a food source for soil-dwelling springtails [START_REF] Gunn | The exploitation of food resources by soil meso-and macro invertebrates[END_REF][START_REF] Ponge | Vertical distribution of Collembola (Hexapoda) and their food resources in organic horizons of beech forests[END_REF][START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF], in mineral horizons of Mediterranean mull profiles [START_REF] Peltier | Humus forms in Mediterranean scrublands with aleppo pine[END_REF]. Note that dead roots were particularly abundant in 2002, i.e. when the density of Collembola was at its highest in the S horizon [START_REF] Sadaka | Humus forms in holm oak forests[END_REF].

The main effect of altitude was probably to increase the thickness of horizons where most animals were living, more especially an increase in the thickness of the OL horizon was observed at 1900 m altitude, compared to the two lower sites, and an increase in the thickness of the OF horizon was observed from 1500 to 1700 m [START_REF] Sadaka | Humus forms in holm oak forests[END_REF].

More habitat, more food and a better protection against harmful climate events (drought and frost) were thus provided to fauna, the abundance of which increased from 1500 to 1700 m and even more from 1700 to 1900 m. This reinforces the view that the litter compartment, both as a food source and as a habitat, governs the abundance of most forest soil fauna [START_REF] David | Reactions of the macrofauna of a forest mull to experimental perturbations of litter supply[END_REF][START_REF] Judas | The development of earthworm populations following manipulation of the canopy leaf litter in a beechwood on limestone[END_REF]. Reasons for the increased abundance of organic matter at higher elevation [START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF][START_REF] Ibañez | Contribution à l'étude du sous-système pédologique des écosystèmes forestiers montagnards du massif de Moncayo (Chaîne Ibérique). I. Caractéristiques générales des écosystèmes compris dans le transect étudié[END_REF][START_REF] Stanko | Sulfur transformations in forest soils collected along an elevational gradient[END_REF] can be found in a decreased rate of microbial decomposition due lower temperature [START_REF] Insam | Are the soil microbial biomass and basal repiration governed by the climatic regime?[END_REF][START_REF] Van Cleve | Response of black spruce (Picea mariana) ecosystems to soil temperature modification in interior Alaska[END_REF][START_REF] Wardle | Interactions between microclimatic variables and the soil microbial biomass[END_REF]. This influences the balance between microbial and animal contribution to plant litter decomposition [START_REF] Stachurski | Methods of studying forest ecosystems: microorganism and saprophage consumption in the litter[END_REF]. In a previous paper it had been postulated that the observed increase in litter thickness was probably due to a decrease in the activity of litter-feeding animals [START_REF] Sadaka | Humus forms in holm oak forests[END_REF]. This hypothesis can be refuted to the light of the present results.

At increasing altitude the thickness of the A horizon did not decrease while more litter accumulated on the ground. This horizon was made of enchytraeid and earthworm organo-mineral faeces, the abundance of these animals remaining nearly unchanged. Thus we may think that along the studied altitudinal gradient the threshold of tolerance of mull animal communities was not reached [START_REF] Belotti | Variability in space and time and redundancy as stabilizing principles of forest humus profiles[END_REF][START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF][START_REF] Ponge | Interaction between soil fauna and their environment[END_REF] and that, despite an increase in litter thickness, no shift towards less active humus forms such as moder and mor was prone to occur, contrary to what was observed under colder and moister climates [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][START_REF] Ponge | Changes in humus form and forest dynamics in the French Northern Alps[END_REF]. We can now reconstruct the chain of processes which may explain the observed patterns. Since no corresponding increase was observed in animal groups able to mix plant debris with mineral matter (here not only earthworms but also enchytraeids), the slower decay of litter at higher elevation was not compensated by a more rapid incorporation to the mineral soil. A similar phenomenon has been observed in a litter-doubling experiment with sessile oak litter, where an increase in litter thickness had been observed, without any corresponding increase in soil animal populations [START_REF] David | Reactions of the macrofauna of a forest mull to experimental perturbations of litter supply[END_REF][START_REF] Ponge | Collembolan response to experimental perturbations of litter supply in a temperate forest ecosystem[END_REF]. This can be attributed to the existence of two superposed but relatively independent functional domains, litter and rhizosphere [START_REF] Lavelle | Ecological challenges for soil science[END_REF], which is particularly well-represented in nutrient-poor (acid) mull profiles with thick litter [START_REF] Michalet | Modes de fonctionnement d'humus des forêts sempervirentes des Alpes internes françaises[END_REF].

Changes from year to year, in particular the strong increase in springtail and mite densities from 1999 to 2002, cannot be explained by a corresponding increase in litter thickness [START_REF] Takeda | Dynamics and maintenance of collembolan community structure in a forest soil system[END_REF], since less litter was present in 2002 at the top of soil profiles [START_REF] Sadaka | Humus forms in holm oak forests[END_REF]. This increase in faunal abundance was accompanied by a shift of litter-dwelling animals from OF to OL horizons. The most probable reason was that the OL horizon was moistened by a recent rain. This was the case in 2002, since 17 mm rain fell during the week previous to sampling. The attraction of litter-dwelling animals to the more superficial litter and even to tree trunks has been observed after a rainfall [START_REF] Bowden | Climbing Collembola[END_REF][START_REF] Hassall | Vertical migration of Onychiurus subtenuis (Collembola) in relation to rainfall and microbial activity[END_REF]. This phenomenon could be due to a sudden increase in food resources, more especially for microbial feeders such as cryptostigmatid (oribatid) mites and springtails [START_REF] Lussenhop | Mechanisms of microarthropod-microbial interactions in soil[END_REF][START_REF] Ponge | Succession of fungi and fauna during decomposition of needles in a small area of Scots pine litter[END_REF]. 2). Horizons (OL, OF, A, S), altitudes (SI, SII, SIII) and years (1999,2002) have been projected as additional (passive) variables

( 3 -

 3 7 m height, 85-95 % cover), were chosen according to an altitudinal gradient from 1500 to 1900 m above sea level. At site SI (1500 m, N-NE aspect), the shrub layer consists of Juniperus oxycedrus L. and Cistus monspeliensis L. At SII (1700 m, N-NE aspect), the shrub layer consists of J. oxycedrus, C. monspeliensis, Cistus salvifolius L. and Nerium oleander L. At SIII (1900 m, E-SE aspect), the evergreen oak is associated with J. oxycedrus, Pinus halepensis L., Cistus laurifolius L. and Chamaerops humilis L.

Fig. 1 .Fig. 2 .

 12 Fig. 1. Number of individuals of the broad zoological groups in each horizon of the six

Fig. 3 .Fig. 4 .Fig. 5 .

 345 Fig. 3. Correspondence analysis. Projection of bulk depth indicators (depth values in

Fig. 1

 1 Fig. 1

Fig. 2

 2 Fig.2

Table 1 .

 1 List of layers (with their thickness in cm) sampled in the three study sites

	5	1	1

Table 2 .

 2 Zoological groups found in the whole set of samples and coordinates along the first two axes of correspondence analysis

		OL	OF	A	S	1500 1700 1900	1999 2002
	Springtails	20.5	33.2	24.8	27.3	12.8 b 13.6 b 53.0 a	10.1 b 42.8 a
	Mites	49.3 a 38.5 a 6.7 b	2.8 b	15.9	23.5	33.6	11.7 b 37.0 a
	Nematodes	47.2 a 43.7 a 11.2 a,b 3.0 b	7.8 b	18.0 b 53.0 a	20.4	32.1
	Enchytraeids	2.5 a,b a	1.2 b	0.7 b	2.8	1.8	5.0	3.5	2.8
	Lumbricids	3.3 b	15.8 a 4.2 b	1.7 b	2.5	8.1	8.1	6.8	5.8
	Insect larvae	21.0 b 50.3 a 13.2 b 1.8 c	6.4 b	20.1 a,b 38.2 a	29.0	14.2
	Miscellaneous	3.3 a	4.7 a	3.7 a	1.5 b	3.8 a	2.3 b	3.9 a	3.8	2.8
	Total	147.2 a,b 194.5 a 64.8 b,c 38.8 c	51.8 b 87.4 a,b 194.9 a	85.2

b 137.5 a Zoological richness 15.7 a 18.3 a 10.3 b 6.2 c 10.1 b 12.0 b 15.8 a 12.7 12.6 Horizon Altitude (m) YearTable 3 .

 3 Mean densities (number of individuals per 25 cm2) of the main zoological groups and zoological richness according to horizon, altitude and year. Subscript letters refer to homogeneous groups (3-way ANOVA followed by SNK procedure)

Acknowledgements

This study was undergone by help of a grant given by the Museum National d'Histoire Naturelle de Paris to the junior author.

OL


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

Humus forms in terrestrial ecosystems: a framework to biodiversity

Keywords: Humus forms, Biodiversity, Soil organisms, Communities, Biogenic structures, Feed-back processes

Humus forms are the seat of most biological transformations taking place in terrestrial ecosystems, being at the interface between plants, animals and microbes. The diversity of terrestrial humus forms (mor, moder and mull) can be attributed to the existence of different patterns (strategies) for the capture and use of resources by ecosystems, in ascending order of biodiversity and bioavailability. Arguments are found in the parallel development of humus forms and terrestrial ecosystems, in exclusion mechanisms between organisms living in different humus forms, and in palaeontological studies. The diversification of terrestrial life forms in the course of Earth history, concomitant with an improvement in resource availability due to the development of sedimentary layers at the surface of continents, may explain the successive appearance of more active humus forms working the co-existence of an increasing number of organisms. Contradictory reports about the relationships between biodiversity and stability of ecosystems can be explained by the existence of different belowground pathways making ecosystems more stable.

Introduction

Humus forms can be defined as morphological patterns observed in the association of organic matter with mineral matter at the top of soil profiles. To the light of recent developments in soil science and ecosystem dynamics [START_REF] Bardgett | Linking above-ground and below-ground interactions: how plant responses to foliar herbivory influence soil organisms[END_REF][START_REF] Ponge | The forest regeneration puzzle[END_REF][START_REF] Schwartz | Linking biodiversity to ecosystem function: implications for conservation ecology[END_REF][START_REF] Hooper | Interactions between aboveground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks[END_REF][START_REF] Lavelle | Ecological challenges for soil science[END_REF][START_REF] Landeweert | Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals[END_REF][START_REF] Klironomos | Animal nitrogen swap for plant carbon[END_REF] it can be suggested that humus forms play a central role in the functional biodiversity of terrestrial ecosystems. They are the stable, visible result of most animal and microbial life in the soil and, in a feed-back process, they condition the development of terrestrial plant, animal and microbial communities [START_REF] Perry | Bootstrapping in ecosystems[END_REF][START_REF] Wardle | The influence of island area on ecosystem properties[END_REF][START_REF] Ponge | Interaction between soil fauna and their environment[END_REF][START_REF] Ponsard | Assessing top-down and bottom-up control in a litter-based soil macroinvertebrate food chain[END_REF]. We may wonder whether the observed variation in terrestrial humus forms just indicates the diversity of parent rocks and climates or, better, if it has something to tell us about biodiversity. The present paper is a revisitation of the original work of [START_REF] Müller | Recherches sur les formes naturelles de l'humus et leur influence sur la végétation et le sol[END_REF], who described humus forms on the basis of ecosystem properties and provided avenues for an ecological development of soil science. After a description of the three main humus forms, mull, moder and mor, mostly focussed on ecosystem processes and biodiversity levels, arguments will be presented proposing humus forms as hubs for the self-conservation of terrestrial ecosystems. Past history of the Earth will be examined to illustrate the increasing diversification of humus forms and their parental reltionships. The present paper offers arguments in favour of an evolutionary classification of humus forms and discusses their possible use as tools for the assessment of biodiversity. It will be focused on temperate, mountain and boreal ecosystems, given the experience of the author.

Mull: the fast use of nutrients

Mull can be characterized by the rapid disappearance of leaf litter under the influence of burrowing animals [START_REF] Staaf | Foliage turnover and earthworm populations in three beech forests of contrasting soil and vegetation types[END_REF]) and/or white rots [START_REF] Hintikka | Studies on white-rot humus formed by higher fungi in forest soils[END_REF], and by the homogenization of humified organic matter with mineral particles within macro-aggregates [START_REF] Bernier | Earthworm feeding activity and development of the humus profile[END_REF]. The hemorganic A horizon is the place where most soil organisms are living, plant roots included [START_REF] Bornebusch | The fauna of forest soil[END_REF]. Fungi are present, both as saprophytic and mycorrhizal (VAM zygomycetes) species, but bacteria abound, due to the numerous mineral particles at the surface of which they adhere [START_REF] Marshall | Clay mineralogy in relation to survival of soil bacteria[END_REF] and primer effects from animal mucus [START_REF] Barois | Transformation of the soil structure through Pontoscolex corethrurus (Oligochaeta) intestinal tract[END_REF] and root exudates [START_REF] Cheng | Effect of living roots on soil organic matter decomposition[END_REF].

Mull fauna exhibit a high biomass and a high species richness [START_REF] Petersen | A comparative analysis of soil fauna populations and their role in decomposition processes[END_REF], including megafauna (moles, small rodents), macrofauna (earthworms, large arthropods, molluscs), mesofauna (mites, springtails, enchytraeids) and microfauna (nematodes, protozoa). Large animals typical of mull have more nutrient requirements (per unit of mass produced) than smaller-sized animals [START_REF] Teuben | Direct contribution by soil arthropods to nutrient availability through body and faecal nutrient content[END_REF].

Mull is associated with the most fertile soils [START_REF] Brady | The Nature and Properties of Soils[END_REF] and supports an abundant and diversified herb layer which plays a significant role in the maintenance of a rich soil community [START_REF] Wolters | Allium ursinum litter triggering decomposition on a beech forest floor. The effect of earthworms[END_REF][START_REF] Hooper | Interactions between aboveground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks[END_REF]. It is typical of grassland and deciduous forest ecosystems under mild climate [START_REF] Green | Towards a taxonomic classification of humus forms[END_REF]. In woodlands, the understory is characterized by a luxuriant, nutrient-rich vegetation [START_REF] Bary-Lenger | La Forêt. Écologie, Gestion, Économie, Conservation[END_REF], with a low nutrient use efficiency [START_REF] Vitousek | Nutrient cycling and nutrient use efficiency[END_REF].

Availability of nutrients is thus a prerequisite for the build-up of mull, but in turn mull is favourable to soil fertility through high nutrient input and turnover of nutrients [START_REF] Schaefer | The soil fauna of a beech forest on limestone: trophic structure and energy budget[END_REF]. Mull is characterized by a rapid cycling of nutrients effected by a variety of organisms (plant roots included) living together in the topsoil [START_REF] Vedder | Impact of faunal complexity on microbial biomass and N turnover in field mesocosms from a spruce forest soil[END_REF]. Table 1 summarizes the main features of mull humus forms at the ecosystem level.

Buffering effects of earthworm mucus [START_REF] Schrader | Influence of earthworms on the pH conditions of their environment by cutaneous mucus secretion[END_REF][START_REF] Barois | Transformation of the soil structure through Pontoscolex corethrurus (Oligochaeta) intestinal tract[END_REF] and easily weathered minerals [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF] have been demonstrated. The availability of nutrients and the productivity of the ecosystem remain high even over a wide range of soil acidity providing that no disruption of the nutrient cycle occurs through uncontrolled man activities [START_REF] Kimmins | Evaluation of the consequences for future tree productivity of the loss of nutrients in whole-tree harvesting[END_REF][START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF].

Moder: the conservation of nutrients through fungal and animal activity

In moder humus forms macrofauna are smaller and reduced in abundance and diversity compared to mull [START_REF] Schaefer | Fauna of the European temperate deciduous forest[END_REF], thus organic matter accumulates in the form of three holorganic horizons OL (entire leaves or needles), OF (fragmented litter) and OH (humified litter). The cementation of organic matter by mineral particles is nil or poor, due to the scarcity of adhesive substances such as mucoproteins or bacterial and root polysaccharides [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF].

Most microbial biomass is fungal, due to more acid conditions than in mull [START_REF] Nagel-De-Boois | Hyphal activity in mull and mor of an oak forest[END_REF]. Fungi produce antibiotics [START_REF] Marx | The influence of ectotrophic mycorrhizal fungi on the resistance of pine roots to pathogenic infections. I. Antagonism of mycorrhizal fungi to root pathogenic fungi and soil bacteria[END_REF] which contribute to the collapse of bacterial populations, and they further acidify the soil by excreting organic acids [START_REF] Takao | Organic acid production by basidiomycetes. I. Screening of acid-producing strains[END_REF].

Nutrients in moder are sequestered in decaying plant debris, faeces of epigeic fauna and fungi, which form the bulk of the OF horizon where most organisms are living (Webb, 1977;[START_REF] Cromack | Soil microorganism-arthropod interactions: fungi as major calcium and sodium sources[END_REF][START_REF] Wolters | Effects of Mesenchytraeus glandulosus (Oligochaeta, Enchytraeidae) on decomposition processes[END_REF]. The colonization of OF horizons by fine tree roots and their associated mycelia (mostly ectomycorrhizal basisiomycetes) allows the vegetation to take up nutrients at the place where they have been released by detrital fungi and animals [START_REF] Ponge | Ecological study of a forest humus by observing a small volume. I. Penetration of pine litter by mycorrhizal fungi[END_REF]. Due to the conservation of organic matter in the form of accumulated animal faeces and fungal biomass, nitrogen is abundant but mainly present as recalcitrant protein-N (dead fungal walls, animal cuticles, tanninprotein complexes).

The acidity of moder is high (low pH), due to the conjunction of cation preferences by plants [START_REF] Nilsson | Forest growth as a possible cause of soil and water acidification: an examination of the concepts[END_REF], excretion of organic acids by fungi [START_REF] Devêvre | Release of complexing organic acids by rhizosphere fungi as a factor in Norway spruce yellowing in acidic soils[END_REF] and humification of organic matter [START_REF] Stevenson | Humus Chemistry. Genesis, Composition, Reactions[END_REF]. The chemical environment of moder-living organisms is characterized, not only by low pH, but also by a concentration of heavy metals [START_REF] Laskowski | of some mineral nutrients and heavy metals in decomposing forest litter[END_REF] and a high phenolic and terpenic content [START_REF] White | Monoterpenes: their effects on ecosystem nutrient cycling[END_REF][START_REF] Gallet | Evolution of phenolic patterns in plants and associated litters and humus of a mountain forest ecosystem[END_REF]. The soil atmosphere, too, is characterized by pockets of high carbon dioxide and methane content [START_REF] Verdier | Étude de l'atmosphère du sol. Éléments de comparaison et signification écologique de l'atmosphère d'un sol brun calcaire et d'un sol lessivé podzolique[END_REF][START_REF] Sexstone | Production of methane and ethylene in organic horizons of spruce forest soils[END_REF], due to the absence of carbonate buffering and poor redox values [START_REF] Lafond | Oxidation-reduction potential as a characteristic of forest humus types[END_REF][START_REF] Lee | The calcicole-calcifuge problem revisited[END_REF]. Since some important functions, such as burying of organic matter and creation of a network of large pores, are ensured only by macrofaunal groups (earthworms, millipedes, bibionid larvae), a decrease in functional biodiversity stems from the decrease in zoological richness observed between mull and moder (Table 1, Fig. 1, see also [START_REF] Schaefer | The soil fauna of beech forests: comparison between a mull and a moder soil[END_REF].

Moder humus forms are mainly found in deciduous (oak, beech) and coniferous forests (Table 1), with depauperate ground flora and nutrient-poor litter [START_REF] Howard | Titratable acids and bases in tree and shrub leaf litter[END_REF]. This process of impoverishment of the ground flora is further reinforced by the production of allelopathic substances [START_REF] Jarvis | Interference by Deschampsia flexuosa (L.) Trin[END_REF].

Mor: the immobilization of nutrients in plant organs

Mor, often confused with dysmoder (moder with a thick OH horizon) under the vague designation of raw humus [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF], is characterized by the scarcity of animal and white-rot activity. Undecomposed plant debris accumulate, forming a matted OM horizon with relatively few animal faeces [START_REF] Barratt | A classification of humus forms and micro-fabrics of temperate grasslands[END_REF][START_REF] Ponge | Comment distinguer dysmoder et mor? L'exemple de la forêt domaniale de Perche-Trappe (Orne)[END_REF]. The matted OM horizon shows a sharp transition with the underlying mineral horizon impoverished in microaggregates (E horizon), contrary to moder where the transition with the mineral horizon is gradual [START_REF] Nielsen | Effects on soils of an oak invasion of a Calluna heath, Denmark. I. Morphology and chemistry[END_REF]. The poor nutritional value of mor restricts the production of the vegetation it supports and litter input is low [START_REF] Aerts | The advantages of being evergreen[END_REF].

Fire is the main agent of litter disappearance [START_REF] Gimingham | European heathlands[END_REF]. Typically the vegetation is made of lichens, mosses and ericaceous shrubs [START_REF] Bonan | Environmental factors and ecological processes in boreal forests[END_REF], but cases occur where mor is present under coniferous trees such as pine and spruce, more especially under harsh environmental conditions [START_REF] Northup | Polyphenols as regulators of plant-litter-soil interactions in northern California's pygmy forest: a positive feedback?[END_REF] or when they have been introduced as exotic species [START_REF] Nihlgård | Pedological influence of spruce planted on former beech forest soils in Scania, South Sweden[END_REF].

In this environment with a very poor biological activity, the most frequent associations of plants with fungi are the ascomycetous endomycorrhizae of ericales [START_REF] Read | The status and function of ericoid mycorrhizal systems[END_REF] and the association of coniferous trees (and less often deciduous trees) with the ascomycete Cenococcum geophilum, which is also present in dysmoder humus [START_REF] Ponge | Ecological study of a forest humus by observing a small volume. I. Penetration of pine litter by mycorrhizal fungi[END_REF]. All these ascomycetous species are known for their efficiency in the use of organic nitrogen [START_REF] Read | The status and function of ericoid mycorrhizal systems[END_REF], which makes them able to transfer nutrients to the plants despite poor mineralization rates [START_REF] Van Vuuren | Nitrogen mineralization in heathland ecosystem dominated by different plant species[END_REF].

Mor originates from harsh (mostly cold) climate conditions or very poor parent rocks, and strong allelopathic properties of the associated vegetation [START_REF] Gallet | Phenolic metabolites of ecological significance in Empetrum hermaphroditum leaves and associated humus[END_REF]. The production of acidic substances by mor vegetation and the recalcitrant nature of the litter [START_REF] Northup | Polyphenol control of nitrogen release from pine litter[END_REF] contribute to further impoverish the soil by a positive feed-back process [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF].

In mor humus forms vegetation takes a prominent place to the detriment of animals and microbes, which are at their lowest level of abundance and diversity [START_REF] Davis | Structure and of two antarctic terrestrial moss communities[END_REF]. Dead and senescent parts of plants are the only seat of nutrient recapture, without resorting to humification/mineralization by saprophagous organisms [START_REF] Aerts | The advantages of being evergreen[END_REF]. Compared to mull and moder, the conservation of organic matter in mor is at its optimum and the release of nutrients at its minimum [START_REF] Aerts | The advantages of being evergreen[END_REF]. Although it has been claimed that the fitness of mor vegetation to environmental conditions it creates is good [START_REF] Northup | Polyphenols as regulators of plant-litter-soil interactions in northern California's pygmy forest: a positive feedback?[END_REF], it appears more realistic to think that severe disturbance events such as fires and avalanches and/or cyclic successions are involved in the longterm sustainability of present-day mor ecosystems [START_REF] Heilman | Change in distribution and availability of nitrogen with forest succession on north slopes in Interior Alaska[END_REF]Miles, 1985, Kaagman and[START_REF] Kaagman | Cyclic succession in heathland under enhanced nitrogen deposition: a case study from the Netherlands[END_REF].

Humus forms as ecosystem strategies: three arguments

The term strategy is used here to designate patterns involving plants, animals, microbes, and their common habitat, the humus form. It does not stem from a theoretical work. Rather, I will present several arguments which all aim at shaping a realistic, process-based view of the self-organization of ecosystems, which evolved and diversified in the course of Earth history. It can be hypothesized that a limited number of humus forms have evolved together with the development of terrestrial life as a response to scarcity or availability of nutrients and other environmental influences. The argumentation presented below lies on the observation of changes in humus forms, the knowledge of mechanisms by which humus forms are transformed or not, and ends by evolutionary aspects.

Argument 1: Changes in humus forms occur during the development of terrestrial ecosystems

A succession of humus forms at the scale of a few decades has been demonstrated by chronosequence analyses in temperate, mountain and boreal late-successional forests [START_REF] Page | Effects of forest cover on the properties of some Newfoundland forest soils[END_REF][START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF][START_REF] Ponge | The heterogeneity of humus profiles and earthworm communities in a virgin beech forest[END_REF]. Mull is generally associated with early developmental stages (regeneration) of forest stands, moder with phases of intense growth of trees before they reach maturity, and mull recovers during maturity and senescence of the trees. This is consistent with the preparation of the regeneration niche in the mature forest, a step which is necessary for the renewal of late-sucessional ecosystems [START_REF] Ponge | The forest regeneration puzzle[END_REF]. During the phase of intense growth of trees [aggradation phase sensu [START_REF] Oldeman | Forests: Elements of Silvology[END_REF]], more nutrients are used for the build-up of wood than are released through litter decomposition and mineral weathering [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF].

During maturity of the forest stand the internal recycling of nutrients and the slower growth of trees yield more nutrients to the decomposer system [START_REF] Nilsson | Forest growth as a possible cause of soil and water acidification: an examination of the concepts[END_REF]. Earthworms are particularly sensitive to environmental changes occurring during tree stand development (Fig. 2), especially in coniferous stands but also in deciduous stands [START_REF] Ponge | The heterogeneity of humus profiles and earthworm communities in a virgin beech forest[END_REF]. There is a global trend of decreasing abundance of earthworms during the phase of intense growth of trees (except a short spell following thinning operations), followed by progressive recovering as trees reach maturity then senesce [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF]. This process, which is probably controlled by tree physiology, can be considered as the driving force for the observed changes in humus forms. In old-growth forests, composed of a patchwork of small-sized eco-units [START_REF] Oldeman | Forests: Elements of Silvology[END_REF], functional diversity may recover during maturity through colonization by herbs and mull-forming organisms which locally disappear during the phase of intense nutrient uptake by trees [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF]. Studies on chronosequences within late-successional forest ecosystems always concluded to a reversal under older trees in the process of soil acidification and organic matter accumulation [START_REF] Page | Some effects of conifer crops on soil properties[END_REF][START_REF] Vitousek | Ecosystem succession and nutrient retention: a hypothesis[END_REF][START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF].

Argument 2: Exclusions exist between mull, moder and mor organisms

Among soil animal groups, annelids always form a major part of soil animal biomass [START_REF] Huhta | Numbers, biomass and community respiration of soil invertebrates in spruce forests at two latitudes in Finland[END_REF][START_REF] Standen | Production and diversity of enchytraeids, earthworms and plants in fertilized hay meadow plots[END_REF][START_REF] Schaefer | Fauna of the European temperate deciduous forest[END_REF]), but a clear distinction must be made between enchytraeids and earthworms. By compiling field data on mull and moder from several European authors who used reliable methods for extracting and counting both groups in grassland and woodland sites, it appears a clear discrimination between soils populated with earthworms and soils populated with enchytraeids (Fig. 3). The second-order hyperbolic equation fitted to the data indicates that soils with medium to high densities of both lumbricids and enchytraeids never occur in Europe. The strong departure from linearity of the curve (non-additivity of environmental responses) suggests that the relationship between earthworm and enchytraeid population sizes cannot be explained entirely by differential responses to environmental factors. A spatial exclusion between these two groups has been experimentally demonstrated by introducing earthworms in compost heaps colonized by enchytraeids [START_REF] Haukka | Growth and survival of Eisenia fetida (Sav.) (Oligochaeta: Lumbricidae) in relation to temperature, moisture and presence of Enchytraeus albidus (Henle) (Enchytraeidae)[END_REF] but further experimental support is awaited before antagonism between enchytraeids and earthworms can be considered to play an active role in the differentiation between mull and moder.

Other interferences, such as those between fungi and bacteria [START_REF] Dickinson | Interactions between bacteria, streptomycetes and fungi from Picea sitchensis litter[END_REF], and between plants [START_REF] Wardle | An ecosystem-level perspective of allelopathy[END_REF]) may as well be considered as mechanisms by which the development of humus form follows preferentially the mull, moder or mor pathway. Positive feedback loops (synergistic processes) involving plants, animals and microbes [START_REF] Beare | A hierarchical approach to evaluating the significance of soil biodiversity to biogeochemical cycling[END_REF] could then be considered as amplifiers by which the whole ecosystem is transformed following some local or regional disturbance [START_REF] Wardle | The influence of island area on ecosystem properties[END_REF]. Among several examples, the long-term transformation of moder into mull still visible twenty years after the application of fertilizers should be highlighted [START_REF] Deleporte | Long-term effects of mineral amendments on soil fauna and humus in an acid beech forest floor[END_REF]. On the contrary negative feedback loops (buffering systems) contribute to stabilize the ecosystems around equilibrium positions [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF] which we expect to correspond to the three humus forms mull, moder and mor. The buffering effect of functional redundancy, i.e. the replacement of organisms ensuring the same function [START_REF] Wolters | Functional aspects of animal diversity in soil. Introduction and overview[END_REF], has been already suggested to explain the absence of visible change in humus form under present-day atmospheric pollution [START_REF] Belotti | Variability in space and time and redundancy as stabilizing principles of forest humus profiles[END_REF]. Personal observations that changes in humus form occur abruptly along linear environmental gradients point on the same combination of exclusion and stabilization processes [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF].

Argument 3: Humus forms evolved together with the diversification of life in terrestrial habitats

Fossil records of soil biological activity are scarce, more especially concerning palaeozoic times when most of the evolution of terrestrial invertebrates took place [START_REF] Retallack | Fossil soils as grounds for interpreting the advent of large plants and animals on land[END_REF]. Nevertheless Rhynian fossils (early Devonian, -400 million years) show a variegated fauna with most actual arthropod groups already present [START_REF] Kevan | Interrelationships of early terrestrial arthropods and plants[END_REF]. More information can be gained from the knowledge of past vegetation, if we consider present day representatives of the different plant phyla and their known relationships with humus forms. Given that the three main humus forms, namely mull, moder and mor, can be classified in a decreasing order of biodiversity, we can see them as steps in the evolution of terrestrial ecosystems.

Mor humus forms, with depauperate fauna and microflora, are associated with ericales, mosses, lichens, bryophytes, pteridophytes and, often, gymnosperms. If we discard ericales as recently evolved spermaphytes [START_REF] Meyen | Fundamentals of Palaeobotany[END_REF], the rest of mor vegetation is typically made of plants evolved during palaeozoic times, mostly living at that time on Gondwana and Old Red Sandstone continents [START_REF] Meyen | Fundamentals of Palaeobotany[END_REF]. We may suspect that mor was the dominant humus form before the appearance of the first spermaphytes during the Carboniferous age (-360 to -300 million years B.P.), and thereafter moder, at least in primitive forests. This is consistent with the present knowledge about the evolution of arthropods, with most radiation occurring during the Devonian age, i.e. from -410 to -360 million years B.P. [START_REF] Dunger | Some remarks on the role of the soil in the evolution of early Antennata[END_REF]. Fossil records of tunnelling in vascular plants by oribatid mites (a typical moder process) are known from the Carboniferous age only [START_REF] Labandeira | Oribatid mites and the decomposition of plant tissues in palaeozoic coal-swamp forests[END_REF]. Given what we know of present environments colonized by lichen, moss, fern and gymnosperm vegetation, we can see palaeozoic terrestrial habitats as an acid world. Data about the composition of the Earth's atmosphere corroborates this idea, acid rains reinforcing the acidifying effects of primitive vegetation [START_REF] Budyko | History of the Earth's Atmosphere[END_REF].

Mull humus forms appeared probably after the explosion of flowering plants during the Cretaceous age (-135 to -65 million years B.P.), i.e. when litter reached a better nutrient quality and was thus able to sustain abundant and diverse macroinvertebrate populations. This coincided with the massive emergence of limestone following coalescence of continental plates during mesozoic times [START_REF] Budyko | History of the Earth's Atmosphere[END_REF]. Eathworm burrows were unknown from the primary era, the oldest records being dated from the beginning of the secondary era [START_REF] Retallack | The fossil record of soils[END_REF]. Given the absence of fossils it is hard to trace the evolution of earthworms, but the geographic segregation between the European Lumbricidae (now still invading North America from the first European settlements) and more evolved Circumpacific and Pantropical families such as Megascolecidae and allies [START_REF] Dindal | Soil Biology Guide[END_REF] is in favour of the late addition of earthworms to already highly diversified terrestrial invertebrate fauna. These animals played probably a decisive role in the appearance of mull [START_REF] Hayes | Darwin's 'vegetable mould' and some modern concepts of humus structure and soil aggregation[END_REF], thus it may be argued that mull (with mesozoic molluscs, annelids and crustaceans) was probably much more recent than moder (with palaeozoic arthropods) and moder much more recent than mor (without fauna).

Variations in species composition of soil invertebrates have been noted in different humus forms [START_REF] Wauthy | Synecology of forest soil oribatid mites of Belgium (Acari, Oribatida)[END_REF][START_REF] Arpin | Mise au point sur la caractérisation biologique des humus forestiers par l'étude des Nématodes Mononchida[END_REF][START_REF] Chagnon | Community structures of Collembola in sugar maple forests: relations to humus type and seasonal trends[END_REF]. The threshold of pH 5 was found as a limit between acidophilic and acido-intolerant springtail communities [START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF], i.e. the same threshold as for vegetation [START_REF] Roem | Soil acidity and nutrient supply ratio as possible factors determining changes in plant species diversity in grassland and heathland communities[END_REF]. The examination of morphological characters and the knowledge of phyletic relationships between species and genera of Collembola revealed that, within each lineage, species and genera considered as near from the ancestor were acidophilic [START_REF] Ponge | Acidophilic Collembola: living fossils? Contributions from the Biological Laboratory[END_REF]. While still limited to a single invertebrate group, this examination of phyletic data is an additional argument for considering moder and mor as primitive humus forms compared to mull.

The view that ecosystems evolved towards a greater diversification of life forms, with an increasing complexity of functional relationships and a higher productivity, in the order mor-modermull, is in agreement with the application of thermodynamic laws to the successional development of ecosystems [START_REF] Odum | The strategy of ecosystem development[END_REF]. [START_REF] Rusek | Pedozootische Sukzessionen während der Entwicklung von Ökosystemen[END_REF], studying soil animal communities along a successional gradient, observed an increase in the diversity of both plant and animal communities, earthworms and other macro-invertebrates being present only in late-successional stages. [START_REF] Price | An overview organismal interactions in ecosystems in evolutionary and ecological time[END_REF] expressed the view that evolution selected the development of food webs of increasing complexity. Nevertheless it must be added that primitive assemblages were not totally replaced by more evolved ones, relicts of palaeozoic times such as mor and moder humus forms being now restricted to nutrient-poor sites where mull organisms cannot live.

Biodiversity and ecosystem processes: humus forms may help to clarify the debate

Following intense theoretical speculations [START_REF] Hutchinson | Homage to Santa Rosalia, or why are there so many kinds of animals?[END_REF][START_REF] Odum | The strategy of ecosystem development[END_REF][START_REF] Menge | Community regulation: variation in disturbance, competition, and predation in relation to environmental stress and recruitment[END_REF], the effects of biodiversity on ecosystem processes have been tested experimentally [START_REF] Vedder | Impact of faunal complexity on microbial biomass and N turnover in field mesocosms from a spruce forest soil[END_REF][START_REF] Tilman | Biodiversity: population versus ecosystem stability[END_REF][START_REF] Tilman | The influence of functional diversity and composition on ecosystem processes[END_REF] and by comparing species-rich and species-poor communities [START_REF] Wardle | The influence of island area on ecosystem properties[END_REF]. Contradiction between expected and observed results [START_REF] Coates | Fungal population and community development in cut beech logs. III. Spatial dynamics, interactions and strategies[END_REF]Grime et al., 1987;[START_REF] Wardle | The influence of island area on ecosystem properties[END_REF] prompted [START_REF] Grime | La biodiversité n'est pas une fin en soi, l'urgence est d'identifier les espèces irremplaçables[END_REF][START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF] to recall that biological properties of organisms (as opposed to theoretical speculations) were fundamental to the knowledge of the interrelationships between ecosystem properties and biodiversity.

The recognition of key organisms [START_REF] Wolters | Functional aspects of animal diversity in soil. Introduction and overview[END_REF], also called dominants [START_REF] Grime | Dominant and subordinate components of plant communities: implications for succession, stability and diversity[END_REF] or ecosystem engineers [START_REF] Lavelle | Soil function in a changing world: the role of invertebrate ecosystem engineers[END_REF], may help to better understand the fate of ecosystems under the influence of man or other disturbances [START_REF] Sprugel | Disturbance, equilibrium, and environmental variability: what is 'natural' vegetation in a changing environment[END_REF], and in the course of successions [START_REF] Mccook | Understanding ecological community succession: causal models and theories, a review[END_REF]. Key organisms shape their immediate environment in such a way that other organisms (called subordinates) can live only in the conditions thus created [START_REF] Lavelle | Ecological challenges for soil science[END_REF][START_REF] Hooper | Interactions between aboveground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks[END_REF]. This concept is based on a hierarchical (holistic) consideration of causes and effects [START_REF] Beare | A hierarchical approach to evaluating the significance of soil biodiversity to biogeochemical cycling[END_REF] which contradict what we know of the feed-back between organisms and their environment [START_REF] Clarholm | Possible roles for roots, bacteria, protozoa and fungi in supplying nitrogen to plants[END_REF][START_REF] Scheu | Links between the detritivore and the herbivore system: effects of earthworms and Collembola on plant growth and aphid development[END_REF][START_REF] Ponge | Interactions between earthworms, litter and trees in an old-growth beech forest[END_REF]. The mutualistic concept [START_REF] Wall | Interactions underground. Soil biodiversity, mutualism, and ecosystem processes[END_REF] and the processing chain concept [START_REF] Heard | Processing chain ecology: resource condition and interspecific interactions[END_REF] are steps toward a more realistic approach, since they take into account interactions which give an advantage to one or both partners according to circumstances. Now let us examine how the concept of humus forms as ecosystem strategies can better explain the links between biodiversity and the stability and productivity of ecosystems.

The high plant, animal and microbial biodiversity and productivity observed in mull ecosystems can be explained first by more resources available to organisms (milder climate, higher richness of the parent rock), and second by the permanent disturbance created by the activity of organisms themselves. The coexistence of a number of different organisms creates more heterogeneity, thus it creates in turn conditions for the maintenance of a high variety of organisms [START_REF] Hansen | Effects of habitat complexity and composition on a diverse litter-microarthropod assemblage[END_REF][START_REF] Hooper | Interactions between aboveground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks[END_REF]. This occurs not only through mutualistic associations or facilitation processes, but also through switches in the occupation of space or in the use of nutrients when more species are living together [START_REF] Anderson | Competition between two unrelated species of soil Cryptostigmata (Acari) in experimental microcosms[END_REF][START_REF] Capowiez | Differences in burrowing behaviour and spatial interaction between the two earthworm species Aporrectodea nocturna and Allolobophora chlorotica[END_REF]. More heterogeneity in the distribution of resources has been also demonstrated to increase plant productivity [START_REF] Hutchings | Patchy habitats, division of labour and growth dividends in clonal plants[END_REF]. Such a high level of biodiversity and productivity can be achieved only through a rapid cycling of nutrients which allows (and forces) organisms to adapt themselves to a constantly changing environment and to a high level of competition. Mull organisms have a low nutrient use efficiency, which means that they use more carbon per unit nutrient taken-up [START_REF] Vitousek | Nutrient cycling and nutrient use efficiency[END_REF], because they have high energy costs for capturing space and nutrients at a high level of competition [START_REF] Grime | Botanical contributions to contemporary ecological theory[END_REF]. This is both cause and effect of the high nutrient requirements of mull organisms [START_REF] Piearce | The calcium relations of selected Lumbricidae[END_REF]. The stability of mull ecosystems when faced to pollution, climate change or soil impoverishment, i.e. their capacity to resist or recover after disturbance [START_REF] Grime | Dominant and subordinate components of plant communities: implications for succession, stability and diversity[END_REF], is exemplified by the high level of acid input and heavy metal deposition needed before observing a shift from mull to moder or mor humus forms [START_REF] Belotti | Variability in space and time and redundancy as stabilizing principles of forest humus profiles[END_REF][START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. In the case of mull, its stability lies on a high level of functional redundancy, which is the positive side of competition [START_REF] Belotti | Variability in space and time and redundancy as stabilizing principles of forest humus profiles[END_REF][START_REF] Tilman | Biodiversity and stability in grasslands[END_REF]. Such a system, by its high biodiversity, generates the conditions of its own stability, according to the bootstrapping effect described by [START_REF] Perry | Bootstrapping in ecosystems[END_REF]. Nevertheless it should be stressed that initial conditions for its build-up are a wealth of nutrients and a mild climate, as shown by Figure 1, and that severe limits appear when an important function such as burying of organic matter is ensured by only one and the same organism, for instance the earthworm Lumbricus terrestris [START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF].

At the opposite, dearth of nutrient resources and harsh climate conditions prevailing in mor ecosystems impose a stricter use of energy and nutrients. Fewer species are present and functions fulfilled by the most exacting organisms (for instance burying of organic matter) are absent. Mor plants exhibit an intense production of recalcitrant and sometimes toxic secondary metabolites [START_REF] Northup | Polyphenols as regulators of plant-litter-soil interactions in northern California's pygmy forest: a positive feedback?[END_REF], which further decreases biodiversity [START_REF] Wardle | The influence of island area on ecosystem properties[END_REF][START_REF] Wardle | An ecosystem-level perspective of allelopathy[END_REF]. The low productivity of mor ecosystems and the absence of internal disturbance (due to the poor number of competitors) allows a better nutrient use efficiency than in the case of mull [START_REF] Aerts | The advantages of being evergreen[END_REF][START_REF] Northup | Polyphenols as regulators of plant-litter-soil interactions in northern California's pygmy forest: a positive feedback?[END_REF]. The stability of such ecosystems is not ensured by the fast adaptation of a variety of organisms to constantly changing conditions, but rather to the durability of environmental conditions created by a few morforming organisms, more especially through the accumulation of recalcitrant organic matter [START_REF] Meyer | The role of the fungus Cenococcum graniforme (Sow.) Ferd. et Winge in the formation of mor[END_REF][START_REF] Read | Mycorrhizas in ecosystems[END_REF][START_REF] Näsholm | Boreal forest plants take up organic nitrogen[END_REF] which isolates the ecosystem from outer harsh and poor environmental conditions. Between these two extreme cases, moder ecosystems (or moder phases of the development of ecosystems) share advantages and disadvantages of both mull and mor strategies, by restricting the number of functional processes (compared to mull) while authorizing a still high level of biological activity. It is harder to understand how moder ecosystems are stabilized, and more knowledge about negative feed-back loops prevailing in moder is needed before approaching the reasons why this intermediary humus form is so frequent in deciduous and coniferous ecosystems, at least within the temperate zone. The existence of cyclic processes in moder ecosystems, with mull prevailing during the regeneration phase [START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF], might suggest a solution to this enigma. Moder being associated with forest ecosytems (deciduous as well as coniferous), the existence of moder could well be explained by tree physiology. Since trees immobilize nutrients in their woody parts, the resulting impoverishment of the soil [START_REF] Nilsson | Forest growth as a possible cause of soil and water acidification: an examination of the concepts[END_REF] necessitates an adaptation of the ecosystem, by restricting the number of competing organisms and improving nutrient use efficiency at the ecosystem level, at least during the phase of intense growth of trees, also called the autotrophic phase [START_REF] Ponge | The forest regeneration puzzle[END_REF]. The need for woody parts to be decayed rapidly for the renewal of the ecosystem necessitates the preservation of enough biological activity and diversity during the heterotrophic phase [START_REF] Ponge | The forest regeneration puzzle[END_REF], also called the disintegration phase [START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF]. Thus the stability of the ecosystem lies on alternating phases of high (mull) and moderately low (moder) levels of biodiversity.

The recognition of three distinct not exclusive) strategies may thus help to understand why authors claimed that more species give more stability and productivity to ecosystems (which refers to the mull strategy) while others claimed the contrary (which refers to moder and mor strategies). It must be pointed out that no general model of stability and productivity can be constructed [START_REF] Hooper | Interactions between aboveground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks[END_REF], rather, a limited number of particular cases have to be identified, each of them having its own coherence within a regional set of ecological and historical factors [START_REF] Wardle | The influence of island area on ecosystem properties[END_REF][START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF]. That each terrestrial ecosystem falls into one of the three above mentioned strategies is still a matter of conjecture, but this could be considered as the most realistic hypothesis explaining the limited number of observed solutions to the complex interplay between organisms and ecological factors. Although unachieved by lack of more experimental studies on interactions between soil organisms, the concept of humus forms as ecosytem strategies may help us to better understand the short-term and long-term effects of ecological crises such as land use changes, invading species, pollution, hurricanes or fires. All sudden environmental changes are amenable to the local extinction of species and, thus, above some given threshold of tolerance, of functions [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]. Local changes in humus forms and important soil functional processes have been repeatedly described under the influence of pesticide and fertilizer use [START_REF] Barratt | Differences in humus forms and their microfabrics induced by long-term topdressings in hayfields[END_REF][START_REF] Topoliantz | Earthworm and enchytraeid activity under different arable farming systems, as exemplified by biogenic structures[END_REF], heavy metal deposition [START_REF] Coughtrey | Litter accumulation in woodlands contaminated by Pb, Zn, Cd and Cu[END_REF][START_REF] Gillet | Humus forms and metal pollution in soil[END_REF], plantation [START_REF] Nihlgård | Pedological influence of spruce planted on former beech forest soils in Scania, South Sweden[END_REF][START_REF] Muys | Ecological changes following afforestation with different tree species on a sandy loam in Flanders, Belgium[END_REF], fire [START_REF] Majer | Short-term responses of soil and litter invertebrates to a cool autumn burn in jarrah (Eucalyptus marginata) forest in Western Australia[END_REF][START_REF] Zackrisson | Key ecological function of charcoal from wildfire in the boreal forest[END_REF] and liming [START_REF] Deleporte | Long-term effects of mineral amendments on soil fauna and humus in an acid beech forest floor[END_REF], among others. Some of these changes are irreversible, i.e. they persist a long time after original causes have disappeared [START_REF] Koerner | Influence of past land use on the vegetation and soils of present day forest in the Vosges mountains, France[END_REF]. To the light of present knowledge, we postulate that humus forms (shaped and stabilized by interactions between plants, microbes, animals and their physical environment) could well be the driving force which explains most of the observed variation in terrestrial ecosystems.

Figure captions

Fig. 1. The decrease in soil zoological richness (number of higher invertebrate taxa belonging to macro-and mesofauna) observed along an altitudinal gradient in 13 beech forest stands from the Belgian Ardennes. Black circles = moder. White circles = mull. See [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF] for soil and litter chemical properties associated with altitude Fig. 2. Changes in earthworm densities along a time sequence of spruce in a late-successional ecosystem at 1750 m altitude (Savoy, France). Eight even-aged clumps of trees (eco-units) have been selected in order to represent the age variability found within a 0.5 ha surface showing constant soil and aspect conditions. Data from [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF] Fig. 3. Densities of enchytraeid and lumbricid worms compiled according to [START_REF] Huhta | Numbers, biomass and community respiration of soil invertebrates in spruce forests at two latitudes in Finland[END_REF], [START_REF] Standen | Production and diversity of enchytraeids, earthworms and plants in fertilized hay meadow plots[END_REF], [START_REF] Römbke | Population dynamics of Oligochaeta of a beech forest in the Northern Black Forest (FRG)[END_REF][START_REF] Römbke | Lebensraum Buchenwaldboden. IX. Die Enchytraeen und Regenwürmer[END_REF], [START_REF] Graefe | Untersuchungen zum Einfluss von Kompensationskalkung und Bodenarbeitung auf die Zersetzerfauna in einem bodensauren Buchenwald-und Fichtenforst-Ökosystem[END_REF], [START_REF] Schaefer | Fauna of the European temperate deciduous forest[END_REF] and [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF]. Data were adjusted to a second-order hyperbolic equation after log (x+1) transformation of earthworm densities followed by correlation analysis with enchytraeid densities [START_REF] Sokal | Biometry. The principles and practice of statistics in biological research[END_REF] y = 9887.6x -1 R 2 = 0.6501 Fig. 3

  Fig.1