Spiegelberger

Pierre Mariotte
email: pierre.mariotte@epfl.ch

Alexandre Buttler

Florian Kohler

Anna K Gilgen

Thomas Spiegelberger

Vegetationsaufnahmen 11 Naturnaher Wiesen

How do subordinate and dominant species in semi-natural mountain grasslands relate to productivity and land-use change?

Keywords: die Aufgabe der Beweidung. Die Ergebnisse bestätigen den glockenförmigen Zusammenhang zwischen Artenreichtum und Unsere Ergebnisse haben direkte Auswirkungen auf die Art und Weise, wie Grünland bewirtschaftet werden sollte. Während die Intensivierung und die Aufgabe beweideter Flächen seit Jahrzehnten schneller vorangehen, bestätigen die Ergebnisse Analyse die Bedeutung einer mittleren Beweidungsintensität für den Erhalt der Artenvielfalt diversity-productivity relationship, grazing, land-use management, plant community structure, species coexistence

How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change?

Introduction

Semi-natural mountain grasslands are widespread components of north-temperate landscapes and play an important role by providing fodder for livestock and acting as reservoirs for both carbon [START_REF] Follett | Soil Carbon Sequestration in Grazing Lands: Societal Benefits and Policy Implications[END_REF] and biodiversity (Mariotte et al. 2013a). These ecosystems host species-rich communities with up to 40 plant species per square meter but are among the most endangered ecosystems in Europe, threatened by both land-use [START_REF] Silva | LIFE and Europe's grasslands: restoring a forgotten habitat[END_REF][START_REF] Fava | Fine-scale assessment of hay meadow productivity and plant diversity in the European Alps using field spectrometric data[END_REF]) and climate change [START_REF] Jentsch | Beyond gradual warming: extreme weather events alter flower phenology of European grassland and heath species[END_REF][START_REF] Mariotte | Subordinate species enhance community resistance against drought in semi-natural grasslands[END_REF]). Indeed, changes in agricultural practices, which have been accelerated since the 1950 in Central Europe are leading to either agricultural intensification or abandonment (Gillet & Galandat 1996, Buttler et al. 2012). While plant community structure (species diversity and composition) is largely the result of management (regular grazing) of semi-natural grasslands, land-use changes are likely to modify this structure and, as a consequence, perturb community stability and ecosystem functioning. Therefore, it seems essential to understand which structural components of plant communities contribute most to species diversity and how they are affected by land-use changes.

The easiest way to classify components of floristic biodiversity is to order species according to their relative abundance and to define their functional role [START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF]. This approach was further developed by using dominance-diversity profiles, which allowed to distinguish three components: dominant, subordinate and transient species [START_REF] Grime | Dominant and subordinate components of plant communitiesimplications for succession, stability and diversity[END_REF][START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF]. Those elements may vary in species richness, functional traits and functional groups.

According to Grime, dominant species are few in number, tall and more expansive in morphology and produce high quantities of biomass. Therefore, immediate influence of plant communities on the ecosystem properties is expected to be primarily determined by traits of dominant species ("mass ratio theory", [START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF]. Subordinate species are generally more numerous, but smaller in stature and form a low proportion of the total community biomass.

By contrast, transient species, which generally do not persist after years and appear only briefly as seedlings, may replace the other species or colonize new areas after disturbance ("founder effects"). Species-rich ecosystems, such as semi-natural grasslands, are driven by coexisting species in a strict dominance hierarchy where superior competitors (dominants) can displace inferior competitors (subordinates) from occupied patches whereas inferior competitors cannot displace superior competitors [START_REF] Amarasekare | Competitive coexistence in spatially structured environments: a synthesis[END_REF]. Subordinate species have no access to patches occupied by dominants, but their better resource efficiency ("filter effects", Grime 1998) allows them to establish in patches that dominants are not able to colonize. This mechanism leads towards spatial niche differentiation between the two species-groups and resource complementarity instead of resource competition. Species coexistence, through niche partitioning, is mainly the result of differences in light availability, soil fertility and disturbance which all contribute to ecosystem productivity. Productivity has been demonstrated to be a function of species diversity in artificially created grasslands [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF]) but a meta-analysis suggested that, in natural grasslands, diversity is a function of productivity [START_REF] Grace | Does species diversity limit productivity in natural grassland communities[END_REF]). Diversity-productivity relationships of species-rich communities follow in general a humpback model [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF] where species diversity is higher at intermediate levels of productivity, fertility and disturbance (Odum 1963[START_REF] Hughes | Reciprocal relationships and potential feedbacks between biodiversity and disturbance[END_REF]). However, the humpbacked pattern has generated strong debate in ecology and recent meta-analytical syntheses concluded that various patterns can occur [START_REF] Gillman | The influence of productivity on the species richness of plants: A critical assessment[END_REF]. Indeed, based on standardized samples from 48 herb-dominated plant communities on five continents, the productivity-diversity relationship shows different patterns depending on land-use history and management of the sites (Adler et al. 2011). Nevertheless, evidence of a humpback relationship was found when including restored prairies, pastures and old fields in the analysis (Adler et al. 2011), confirming findings of [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF] for species-rich Mariotte, P. ;Buttler, A. ;Kohler, F. ;Gilgen, A.K. ;Spiegelberger, T. (2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology,vol. 14,n° 3; communities. According to the theory, productive grasslands with high soil fertility and/or light availability are sites of competitive exclusion, which allows dominance of some species, while low productive grasslands with low soil fertility and/or light availability are stressful environments for plant species [START_REF] Al-Mufti | A quantitative analysis of shoot phenology and dominance in herbaceous vegetation[END_REF]. Therefore, higher species richness should be observed at intermediate productivity levels, which represent the equilibrium between disturbance and dominance. Concerning the role of subordinate species it is further suggested [START_REF] Grime | Dominant and subordinate components of plant communitiesimplications for succession, stability and diversity[END_REF]) that subordinate species represent a higher proportion of species within the hump of the diversity-productivity model but this statement has not been confirmed by field experiments and calls for verification.

In semi-natural mountain grasslands, grazing by large herbivores is an important driver for creating and maintaining biodiversity and many studies showed that species diversity decreases after abandonment of pasturing (Kohler et al. 2006a,b, Sebastià et al. 2008[START_REF] Marion | How much does grazing-induced heterogeneity impact plant diversity in wet grasslands?[END_REF][START_REF] Parolo | Large-scale heterogeneous cattle grazing affects plant diversity and forage value of Alpine species-rich Nardus pastures[END_REF]. In these ecosystems, the persistence of subordinate species seems to be closely related to the activity of large herbivores, which reduce competition from dominant species through gap creation (Mariotte et al. 2012a[START_REF] Kohler | Effect of cattle activities on gap colonization in mountain pastures[END_REF] or grazing [START_REF] Tahmasebi Kohyani | Differential herbivory tolerance of dominant and subordinate plant species along gradients of nutrient availability and competition[END_REF][START_REF] Gillet | Effect of dung deposition on small-scale patch structure and seasonal vegetation dynamics in mountain pastures[END_REF]. Inversely, the exclusion of cattle can lead to the dominance of competitive tall-growing species [START_REF] Tasser | Impact of land use changes on mountain vegetation[END_REF][START_REF] Mayer | Effects of grazing exclusion on species composition in high altitude grasslands of the Central Alps[END_REF], which reduces light availability for small-stature ground layer species. These findings confirm the hypothesis that intermediate disturbance through cattle activity may over-proportionally benefit small-stature subordinate species while cattle abandonment is expected to reduce the abundance of these species.

In this multi-site study, we aim at describing the structure of plant communities in semi-natural mountain grasslands along a productivity gradient. The study includes eleven sites of the Swiss Jura and the Swiss and European Alps, which differ in their aboveground biomass production. The abundance of dominant, subordinate and transient species (species-groups) is determined and related to species diversity. We then compare the abundance of these species-groups in paired grazed and abandoned plots from nine sites to test whether the abundance of subordinate species decreases after abandonment and how this impacts plant species diversity.

Materials and methods

Study sites

In order to have a large productivity gradient, we brought together results of eleven similar studies carried out in semi-natural grasslands of the Jura and the Alps (Kohler et al. 2006a,b, Spiegelberger et al. 2006[START_REF] Vandenberghe | The influence of competition from herbaceous vegetation and shade on simulated browsing tolerance of coniferous and deciduous saplings[END_REF][START_REF] Gilgen | Response of temperate grasslands at different altitudes to simulated summer drought differed but scaled with annual precipitation[END_REF], Mariotte et al. 2013a, see Appendix A: Table 1). The sites, situated between the montane and the subalpine belt, were floristically similar and dominated by Festuca nigrescens, Agrostis capillaris, Dactylis glomerata, Trifolium repens and Taraxacum officinale. The eleven sites were extensively grazed by cattle (summer grazing, see Appendix A: Table 1) and nine of them included also 3-to 5-years-old abandoned plots. Grazed and abandoned communities were situated either in the same place (sites 1, 4, 6, 7 & 10) or with a maximum of 5 km between them but in similar conditions of exposition and inclination (sites 2, 5, 8 & 9, see [START_REF] Spiegelberger | Scale-dependent effects of land use on plant species richness of mountain grassland in the European Alps[END_REF] for a more detailed description). Each site included five very homogeneous plots except for sites 1, 4 and 6 where three plots were available. Aboveground community biomass production was measured at peak biomass by harvesting standing crop above 5-7 cm in each plot, drying at 60 °C for 72 hours and weighing. Vegetation surveys were done in mid-summer using different methods: Braun-Blanquet index (site 3), biomass sorting (sites 7, 10 & 11), point quadrats (sites 1, 4 & 6) or estimated to the nearest per cent (sites 2, 5, 8 & 9). The sampled surface corresponded to 1 m 2 , except in site 11 where it was 0.2 m 2 . Smaller quadrats were justified here because no further species were expected to be included by a 1 m² sample, and the species number was used without correcting for quadrat size. Proportion of species assessed in the field as Braun-Blanquet index, cover or biomass was transformed in relative proportion (%) to allow comparison between sites.

Plant community structure (analysis A)

For each site, species diversity was calculated as the mean species number in grazed plots (n = 3 or 5). We averaged relative species proportion observed in replicated plots (n = 3 or 5) to obtain the mean relative species proportion per site. Species were classified in competitive hierarchical groups [START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF], Mariotte et al. 2013a,b) for each site according to a frequency-proportion curve (see Appendix A. Figure 1) based on species frequency and cumulative proportion. Species were ranked by proportion in ascending order and cumulated to obtain cumulative proportion value for each species. 100% frequency means that the species is present in all plots and 100% cumulative proportion corresponds to the most abundant species. A species was classified as dominant if its frequency was greater than 50% and its cumulative relative proportion greater than 12%. A species was classified as subordinate if its frequency was greater than 50% and its cumulative relative proportion between 2 and 12% (adapted from [START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF]. The other species were classified as transient plants. Vegetation surveys were done during several years (except in sites 2, 5, 8 & 9) and we confirmed that our species-groups selection was consistent over these years. Moreover, further analysis on plant functional traits in sites 7 and 10 showed that selected species-groups differed (Mariotte et al. 2013a), with dominant species linked to high specific leaf area (SLA) and plant height whereas subordinate species are associated to low SLA and small-stature traits. We determined the relative contribution (%) of the three species-groups to the diversity along a productivity gradient containing the 11 sites. The relative contributions of dominant, subordinate and transient species correspond to the number of species per species-group divided by the total number of species in the respective plot.

Abandonment of pasturing (analysis B)

Mariotte, P. ; Buttler, A. ; Kohler, F. ; Gilgen, A.K. ; Spiegelberger, T. ( 2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology,vol. 14,n° 3; In a second analysis, we analysed the effect of cattle abandonment on the structure of plant communities and put this in relation to changes in species diversity. Species diversity was calculated as the mean species number in abandoned plots (n = 3 or 5) and compared with the productivity of abandoned communities and with the species diversity of nearby grazed communities at the same location. As previously, dominant, subordinate and transient species were determined in abandoned communities and changes in the relative contribution of the three species-groups (plant community structure) in abandoned plots were plotted against the diversity in grazed communities. For each site, proportional changes in dominant species (subordinate or transient) were calculated as the difference in log between the number of dominant species (respectively subordinate or transient) in abandoned plots and in grazed plots. The use of log allows to account for the effect of initial species diversity and highlights the negative, neutral or positive effects of abandonment. In addition, to test whether abandonment of pasturing benefits tall-growing species at the expense of small-stature species, we classified all species from vegetation surveys in one of those two groups and calculated their relative proportion in grazed and abandoned communities. Species growing taller than 15-20 cm were classified as tall-growing species while the others were classified as small-stature species. Data on canopy height was taken from "Flora Helvetica" [START_REF] Lauber | Flora Helvetica[END_REF]) and the "LEDA trait database" [START_REF] Kleyer | The LEDA Traitbase: A database of life-history traits of Northwest European flora[END_REF].

Statistical analysis

In the analysis (A), we first focused on the relationship between species diversity and productivity and then on the relative importance of species-groups in this relationship. Species diversity, productivity and abundance of species-groups were summarized into mean values per site. In the second analysis (B), we explored the relationship between the species diversity in abandoned and grazed communities and observed the effect of abandonment on speciesgroups contribution at 9 sites (see Appendix A: Table 1). Polynomial models (degree 2) were Mariotte, P. ; Buttler, A. ; Kohler, F. ; Gilgen, A.K. ; Spiegelberger, T. ( 2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology, vol. 14, n° 3; pp. 217-224 calculated with the orthogonal polynomial function (poly {stats}) in R version 2.14.1 (R Development Core Team, 2011). Statistical significance of models (p-value) and multiple R 2 values were obtained using ANOVAs to estimate the goodness-of-fit. The differences in the relative contribution of tall-growing versus small-stature species between grazed and abandoned communities were analysed using linear mixed-effects models specifying plot (n = 3 or 5) nested into site (n = 9) as random factors.

Results

Diversity-productivity relationship in grazed plots

The mean species diversity per site ranged from 13.2 ± 0.6 species to 36.8 ± 1.1 species along the productivity gradient, which ranged from 99 ± 7 to 590 ± 77 g dwt.m -2 . Species diversity was clearly linked to productivity (Fig. 1A) and best explained by a polynomial function (F 2,8 = 11.525, P < 0.01, R 2 = 0.75). Dominant species represented between 41 and 61% of species but their contribution to the total diversity remained similar along the productivity gradient (Fig. 1B). By contrast, subordinate species, representing 15 to 36% of species (Fig. 1C), followed a polynomial distribution along the productivity gradient (F 2,8 = 10.065, P < 0.01, R 2 = 0.72). The contribution of subordinate species to total species diversity was less important at low and high productive sites but higher at intermediate productivity. The transient species, which contributed from 9 to 41% (Fig. 1D), followed an inverse pattern, fitted by a significant polynomial function (F 2,8 = 6.671, P < 0.05, R 2 = 0.63). Soil dissolved inorganic nitrogen, which had been measured in four sites (see Appendix A: Table 1), was positively correlated to productivity (F 1,2 = 119.792, P < 0.01, R 2 = 0.98).

Abandonment of pasturing

In abandoned plots, productivity did not affect species diversity (see Appendix A: Figure 2; Polynomial: F 2,6 = 0.332, P = 0.73, R 2 = 0.09; Linear: F 1,7 = 0.766, P = 0.41, R 2 = 0.10).

However, species diversity of abandoned communities was linked to the species diversity of paired grazed communities (Fig. 2A), which was assumed to represent the initial species diversity before pasturing of the sites was abandoned. Mean species diversity in abandoned plots was either similar or lower compared to mean species diversity in grazed plots (Fig. 2A).

The loss of species due to abandonment was remarkably pronounced in initially low (e.g. 10 species less in abandoned site 9 compared to the grazed site 9) and highly diverse sites (e.g. 24 species less in site 5) resulting in a significant polynomial relationship between initial species diversity and current diversity (F 2,6 = 62.606, P < 0.001, R² = 0.95).

At the species-group level, the differences in number of dominant (F 2,6 = 12.616, P < 0.01, R 2 = 0.81) and subordinate species (F 2,6 = 11.501, P < 0.01, R 2 = 0.79) showed the similar polynomial pattern as observed for species diversity whereas the difference in number of transient species (Fig. 2D) was similar in grazed and abandoned communities (-12% to +19%). The difference in number of dominant species (Fig. 2B) was similar in four sites, higher in one site (+14%) and lower in four sites (respectively -58% and -45% in lower and higher initial species diversity). In abandoned communities, the difference in number of subordinates (Fig. 2C) was high in eight of the nine plots (-6% to -94%) and much higher than the difference in number of dominants, especially in sites with high species diversity. The analysis of vegetation surveys showed that the identity of dominant species was almost similar in grazed and abandoned communities, while the identity of subordinate and transient species showed more variation between both communities. Indeed, some transient species observed in grazed plots became subordinates in abandoned plots and vice versa.

The relative contribution of tall-growing species was significantly higher (F 1,38 = 31.788, P < 0.001) in abandoned plant communities (81%) compared to grazed communities (63%) at the expense of small-stature species. Mariotte, P. ;Buttler, A. ;Kohler, F. ;Gilgen, A.K. ;Spiegelberger, T. (2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology,vol. 14,n° 3; with Grime's work, which included a wide range of herbaceous vegetation, we based our analysis only on semi-natural mountain grasslands. In these ecosystems, cattle activities such as grazing and trampling, even at low intensity, tend to prevent potentially competitive species from attaining maximum size at high productive sites [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF][START_REF] Augustine | Ungulate effects in the functional species composition of plant communities: herbivore selectivity and plant tolerance[END_REF]. Therefore, we suggest that large herbivores may stabilize the proportion of dominant species along the productivity gradient, whereas subordinate species prevent low competitive transient species to appear at intermediate productivity.

Abandonment of pasturing in semi-natural grasslands

In the second analysis (B), we showed that, in 7 out of 9 cases, species diversity was lower in abandoned communities compared to grazed communities which confirms previous observations in species-rich pastures [START_REF] Sebastià | Grazing as a factor structuring grasslands in the Pyrenees[END_REF][START_REF] Marion | How much does grazing-induced heterogeneity impact plant diversity in wet grasslands?[END_REF][START_REF] Parolo | Large-scale heterogeneous cattle grazing affects plant diversity and forage value of Alpine species-rich Nardus pastures[END_REF]. Interestingly, the difference of species diversity associated with land abandonment was a function of the initial species diversity of the site (species diversity when plots were still grazed) with high and low diverse sites being more affected by abandonment. Plant productivity differed in abandoned communities but we did not find a clear relationship between the loss of species diversity and/or the differences in plant community structure.

Plant community structure differed in abandoned plots, with a loss of species diversity, and these differences can be interpreted here as a function of initial species diversity (e.g. diversity of the grazed plots). The loss of subordinate species was higher than the loss of dominants, showing that abandonment of pasturing over-proportionally affects subordinates.

The mechanisms underlying this pattern are difficult to explain as various components of cattle activity, such as grazing, trampling and dunging, may influence plant community [START_REF] Kohler | Effect of cattle activities on gap colonization in mountain pastures[END_REF]). Selective grazing is especially high at low cattle density [START_REF] Klapp | Wiesen und Weiden, eine Grünlandlehre[END_REF] as was the case in the sites included in our study. Selective grazing is expected to increase species diversity when dominant species are grazed (reduction of dominance), and to decrease species diversity when subordinates are grazed (increase of dominance). In our study, species diversity was lower in the absence of grazing (abandoned plots), suggesting that selective herbivory on dominants occurred in grazed communities. A major response to grazing is the promotion of ground-layer species due to the vertical defoliation [START_REF] Dupré | Differences in species richness and life-history traits between grazed and abandoned grasslands in southern Sweden[END_REF]. This adaptation favours small subordinate species, which are expected to be less affected, whereas dominant plants, which are often characterised by higher nutrient requirements and growth rate, are vulnerable to foliage losses [START_REF] Augustine | Ungulate effects in the functional species composition of plant communities: herbivore selectivity and plant tolerance[END_REF]. Subordinate species have shown to regrow better after grazing than dominants due to their physiological plasticity or because they benefit from reduced competition from grazed dominants [START_REF] Tahmasebi Kohyani | Differential herbivory tolerance of dominant and subordinate plant species along gradients of nutrient availability and competition[END_REF], Mariotte et al. 2012a,b). However, although grazing significantly favours subordinate species, increasing nutrient levels may counteract the negative effect of grazing on dominant species [START_REF] Tahmasebi Kohyani | Differential herbivory tolerance of dominant and subordinate plant species along gradients of nutrient availability and competition[END_REF]). Indeed, the fertilizing effect of dung favours competitive dominants (nutrient demanding tall species) at the expense of stress-tolerant or perturbation-resistant subordinates [START_REF] Gillet | Effect of dung deposition on small-scale patch structure and seasonal vegetation dynamics in mountain pastures[END_REF].

Trampling by cattle must also be considered as it provides spatial heterogeneity (gaps), which may benefit subordinate species [START_REF] Kohler | Effect of cattle activities on gap colonization in mountain pastures[END_REF], Mariotte et al. 2012a). Moreover, while the number of dominant species was lower, our observations on canopy height showed that the proportion of tall-growing species was higher in abandoned plots at the expense of smallstature species (less light availability or nutrient resources), which confirms that dominance increased after abandonment of pasturing. Therefore, we suggest that the higher loss of subordinate species may be due to the absence of both grazing and trampling but also to the increase of dominance, which impedes their development, while dominant species are only reduced by the absence of dunging. By contrast, transient species were not affected by abandonment of pasturing but they are generally colonizer species ("founder effect", Grime 1998) and very resistant to disturbance. Mariotte, P. ;Buttler, A. ;Kohler, F. ;Gilgen, A.K. ;Spiegelberger, T. (2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology,vol. 14,n° 3; In this study, we showed that subordinate species are an important component of plant species diversity, which is favoured by intermediate productivity and the presence of cattle at low density (extensive management). Considering that subordinate species may have a larger influence on ecosystem processes (plant productivity, nutrient cycling, decomposition) than their relative abundance would suggest (Mariotte et al. 2013a,b), favouring these species seems to be very important for ecosystem functioning in semi-natural grasslands. Our results have direct implications for the way grasslands should be managed, when management aims at maintenance of plant biodiversity value. Indeed, while intensification and abandonment have been accelerated since the 1950s in Central Europe, findings of this study confirm the necessity to maintain intermediate levels of pasturing to preserve biodiversity and ecosystem integrity. Mariotte, P. ; Buttler, A. ;Kohler, F. ;Gilgen, A.K. ;Spiegelberger, T. (2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology,vol. 14,n° 3; Fig. 2

FiguresFig. 1 .

 1 Figures

Fig. 2 .

 2 Fig. 2. Relationship between species diversity in grazed plots and (A) species diversity (mean

Mariotte, P. ; Buttler, A. ; Kohler, F. ; Gilgen, A.K. ; Spiegelberger, T. (2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change?Basic and Applied Ecology, vol. 14, n° 3;

Acknowledgments

We acknowledge funding by the Swiss National Science Foundation (PM: project n°31003A-114139, FK: project n°3100-064116, and TS: project n°4048-064424), the NCCR Climate (AKG) and the Competence Centre in Environmental Science (CCES) of the ETH Domain through the CCES-project Mountland (AB and TS). We thank also Charlotte Vandenberghe and Aurélie Thébault for their help in this study.

Discussion

Plant community structure and the diversity-productivity relationship

This multi-sites analysis, including 11 semi-natural mountain grasslands, determined plant community structure along a productivity gradient. In analysis A, we highlighted a diversityproductivity relationship, which was best fit by a polynomial model. These results confirm the diversity-productivity dependency suggested by various authors [START_REF] Al-Mufti | A quantitative analysis of shoot phenology and dominance in herbaceous vegetation[END_REF][START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF][START_REF] Grace | Does species diversity limit productivity in natural grassland communities[END_REF], Adler et al. 2011). In pastures, plant communities are continuously disturbed by cattle activities [START_REF] Gillet | Effect of dung deposition on small-scale patch structure and seasonal vegetation dynamics in mountain pastures[END_REF], Kohler et al. 2006a,b), and disturbance due to grazing was constant and similar for the eleven sites (see Appendix A: Table 1). Here, we suggest that the productivity gradient is more related to soil fertility as concentrations of dissolved inorganic nitrogen in soil increase along this gradient. In our study, the maximum diversity (30-35 species) occured at intermediate productivity (between 180 and 420 g dwt.m - 2 ) and supports the humpback model of [START_REF] Grime | Competitive exclusion in herbaceous vegetation[END_REF][START_REF] Grime | Dominant and subordinate components of plant communitiesimplications for succession, stability and diversity[END_REF], where species diversity is predicted to be higher at intermediate productivity and fertility (Odum 1963). Our findings therefore confirm that intermediate productivity favours species coexistence in semi-natural grasslands.

Interestingly, the contribution of species-groups differed along the diversityproductivity relationship. [START_REF] Grime | Dominant and subordinate components of plant communitiesimplications for succession, stability and diversity[END_REF] proposed that low productivity favours transient species, intermediate productivity favours subordinate species and high productivity favours dominant species. Indeed, in our analysis, the proportion of subordinates peaked at intermediate productive sites and we confirm that intermediate productivity favours the presence of subordinate species through coexistence, and thus increases species diversity. In contrast to Grime's hypothesis (1987), the proportion of transient species increased in our multi-sites study at low and high productivity sites, whereas dominant species did not change along the productivity gradient, representing about 50% of the species diversity. In contrast Basic and Applied Ecology,vol. 14,n° 3; hierarchical groups. Dominant and subordinate species are always present in the community and was distinguished from transient species (3), which do not persist in the vegetation, by a frequency higher than 50%. The distinction between dominant (1, grey area) and subordinate species (2, darkgrey area) is based upon cumulative proportion (%) and has been set at an arbitrary value of 12%. Mariotte, P. ;Buttler, A. ;Kohler, F. ;Gilgen, A.K. ;Spiegelberger, T. (2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology,vol. 14,n° 3; the productivity (mean biomass production per plot) of abandoned plots in 9 sites of seminatural grasslands. No significant linear (F 1,7 = 0.766, P = 0.41, R 2 = 0.10) or polynomial model (F 2,6 = 0.332, P = 0.73, R 2 = 0.09) was found to link diversity and productivity. Mariotte, P. ;Buttler, A. ;Kohler, F. ;Gilgen, A.K. ;Spiegelberger, T. (2013) How do subordinate and dominant species in semi-natural grasslands relate to productivity and land-use change? Basic and Applied Ecology,vol. 14,n° 3;


Benoît D'angelo

Ary Bruand
email: ary.bruand@univ-orleans.fr

Jiangtao Qin

Xinhua Peng

Christian Hartmann

Sun Bo

Hao Hongtao

Olivier Rozenbaum

Fabrice Muller

Origin of the high sensitivity of Chinese red clay soils to drought: significance of the clay characteristics

Keywords: clay mineralogy, porosity, shrinkage, water retention, aggregate, external surface area

The red clay soils which are widespread in China are known to be highly sensitive to drought during the dry season but the origin of this high sensitivity to drought remains unclear.

Several red clay soils were selected in the Hunan province for study. We studied their basic physico-chemical properties and clay mineralogy, their structure and shrinkage properties, as well as their water retention properties. Results show that the amount of water available between -330 and -15 000 hPa water potential is consistent with that recorded in many other clay soils from different parts of the world and thus cannot explain the high sensitivity of the red clay soils to drought. This high sensitivity to drought might be related to the high proportion of poorly available water which was characterized by the amount of available water between -3300 and -15 000 hPa water potential. Comparison with clay soils located in different parts of the world and for which the sensitivity to drought was not identified, showed that this proportion of poorly available water is indeed much higher in the red clay soils studied than in clay soils representing a large range of both clay content and mineralogy. This specific behaviour of the red clay soils studied is thought to be related to the history of their parent materials: these materials are continental sediments which may have been submitted to great hydric stress, thus leading to strongly consolidated soils with consequences such as a high proportion of poorly available water, strong aggregation and weak shrinkage properties.

Introduction

Soils called red soils are widespread in China. They cover 102 million ha, including ten provinces [START_REF] Cao | Aspects of red soils properties and water management in China[END_REF][START_REF] He | Challenges in the management and utilization of red soils[END_REF]Wilson et al., 2004a;[START_REF] Zheng | Impacts of reforestation approaches on runoff control in the hilly red soil region of southern China[END_REF]. They develop mainly in alluvial Quaternary sediments which reorganize materials resulting from continental alteration or in continental sediments from the Cretaceous or Eocene [START_REF] Bgmrhn | Bureau of Geology and Mineral Resources of Hunan province[END_REF]Wilson et al., 2004a and b;[START_REF] Hu | Regional distribution of the Quaternary red clay with aeolian characteristics in subtropical China and its paleoclimatic implications[END_REF][START_REF] Shi | Cross reference for relating Genetic Soil Classification of China with WRB at different scales[END_REF]. The Chinese red soils are either Ferrallisols (Latosolic red soils or Red soil groups) or Semi-Alfisols (Torrid red soil group) of the Genetic Soil Classification of China [START_REF] Shi | Cross reference for relating Genetic Soil Classification of China with WRB at different scales[END_REF]. In the International Reference Base System (ISSS Working Group WRB, 2007), Chinese red soils belong to the Alisol, Acrisol or Cambisol group. Most Chinese red soils are Ultisols in the Soil Taxonomy (Soil Survey Staff, 1998), some being Alfisols or Inceptisols (Wilson et al., 2004b). Even when the Chinese red soils have a clay or loamy-clay texture, they are known to be highly sensitive to drought during the dry season [START_REF] Wang | Study on Seasonal Drought in Southern China[END_REF], thus explaining why the red soil region is called the "red desert of southern China" [START_REF] Zhao | Red soil material cycling and its regulation[END_REF]. The reason for this sensitivity to drought has not yet been elucidated, however. According to [START_REF] Zhang | Cause of Seasonal Draught formation and Strategy of the eastern hills in Southern China[END_REF], the marked development of micro-aggregation in these red soils may facilitate downward water transfer without enough water storage in the different horizons, thus explaining the poor water storage efficiency of these soils. They also argue that the small amount of water stored in the micro-aggregates is poorly available for biological activity.

More recently, [START_REF] Fang | The Hydraulic Characteristics and Soil Reservoir of Typical Farmland in Red Soil Region[END_REF] suggested that because of the characteristics of the clays present in the red soils, water retained by these clays is not available enough for biological activity, thus explaining the high sensitivity of the red soils to drought. The clay mineralogy of Chinese red soils has often been described as dominated by kaolinite and oxi-hydroxide minerals (Wilson et al., 2004b). However, several studies have shown a wide variation in the clay mineralogy. [START_REF] Vogel | People's Republic of China: Red reference soils of the subtropical Yunnan Province[END_REF] studied red reference soils from the subtropical Yunnan Province and showed that smectite could be the main clay mineral or present in significant amounts as well as chlorite and illite in addition to kaolinite as the main mineral. [START_REF] Zhang | Mineralogy of red soils in southern China in relation to their development and charge characteristics[END_REF] studied red soils in southern China and showed that the clay mineralogy varied according to the parent material, latitude, elevation and topographic position. Kaolinite and halloysite were the most common clays in these soils, the proportion of kaolinite increasing with the degree of weathering. Vermiculite, illite, chlorite and smectite were also identified as secondary clay minerals. Goethite and hematite were the only Fe oxides occurring in significant quantities in the soils studied by [START_REF] Zhang | Mineralogy of red soils in southern China in relation to their development and charge characteristics[END_REF]. This wide variety of clay mineralogy encountered in the Chinese red soils probably indicates that the origin of their high drought sensitivity is not closely related to the clay mineralogy. The objective of the present work is to analyse the water retention properties of a few Chinese red clay soils, to relate these properties to the characteristics of the clays and to compare them to the water retention properties of clay soils from different regions of the world. The results will enable the discussion of the clay characteristics which are responsible for the small amount of available water and hence potentially partially responsible for the high sensitivity of Chinese red clay soils to drought.

Materials and methods

The soils studied

The red clay soils of the Taoyuan experimental station (28°55'47"N, 111°26'33"W) in the Hunan province, 200km west of Changsha were selected for study. This experimental station belongs to the Chinese Ecosystem Research Network. According to the Köppen classification, the climate of the region is Humid Subtropical (Cfa). It is characterized by a dry winter (medium temperature of the coldest month ranging from -3°C to 18 °C) and maximum rains in summer. The mean annual rainfall in Taoyuan is 1440 mm, 80% of rainfall occurring between March and August. The mean annual temperature is 16.5°C. The range between winter and summer (difference between the average temperature during winter and summer) is 21°C (Huang et al. 2004). Soils were selected at three locations along a slope (Figure 1): a soil at the top of the slope (TS) under a vegetation mainly composed of tea-oil trees (Camelia oleifera) and secondarily by camphor laurels (Cinnamomum camphora), chestnut trees (Castanea sp.) and sandalwood (Santalum sp.); a soil at the middle of the slope (MS) in an orchard of orange trees; and a soil on a ledge of the slope (LS) in a cultivated plot after a maize crop, the soil being left bare at the time of soil sampling. The soils are derived from Quaternary red clays. They are Ferric Acrisols in the International Reference Base System (ISSS Working Group WRB, 2007), Udic Ferralosols in the Chinese Soil Taxonomy (CRG- CST, 2001) and Typic Hapludult in the Soil Taxonomy (Soil Survey Staff, 1998). They were sampled in March 2011 after rewetting with 39 and 204 mm of rainfall in February and March, respectively. The soils were thus close to field capacity. A pit 1m in depth was dug and the different horizons were described. Disturbed samples were collected in every horizon as well as undisturbed samples of different volumes.

Methods

The bulk density (D b in g cm -3 ) and field water content of the horizons at sampling date were measured by using cylinders 1236 cm 3 in volume. The bulk density of undisturbed millimetric clods 20 to 30 mm 3 in total volume was measured using the kerosene method [START_REF] Monnier | Une méthode de mesure de la densité apparente de petits agglomérats terreux: application à l'analyse des systèmes de porosité du sol[END_REF]. It was measured in triplicate with 5 to 10 millimetric clods at a water content corresponding to sampling conditions and after air-drying in the laboratory [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF]. The particle size distribution was measured using the pipette method after pretreatment of samples with hydrogen peroxide and sodium hexametaphosphate [START_REF] Robert | Méthodes de preparation des argiles des sols pour les études minéralogiques[END_REF]. The cation exchange capacity (CEC, in cmol + kg -1 of oven-dried soil) and the exchangeable cations were measured using the cobalt-hexamine trichloride method [START_REF] Ciesielski | Determination of cation exchange capacity and exchangeable cations in soils by means of cobalt hexamine trichloride. Effects of experimental conditions[END_REF] and organic carbon content (OC) by oxidation using excess potassium bichromate in sulphuric acid at 135°C [START_REF] Baize | Guide des analyses en pédologie[END_REF]. The gravimetric water content was determined at -60, -100, -330, -1000, and -3300 hPa water potential by using in triplicate soil cores 100 cm 3 in volume and at -15000 hPa water potential by using undisturbed clods (10-15 cm 3 in volume) collected when the soil was near to field capacity [START_REF] Bruand | Water retention properties of the clay in soils developed on clayey sediments : significance of parent material and soil history[END_REF]. The soil cores were thoroughly saturated for one week before they were placed inside a pressure plate chamber (Soil Moisture Equipment Corp, Santa Barbara, USA) to drain at the sequence of pressure from -60 to -3300 hPa [START_REF] Jing | Anisotropy of soil hydraulic properties along arable slopes[END_REF]. The mineralogical composition of the <2µm material of the horizons B1 and C of each soil was determined by X-ray diffraction (XRD). Samples were air-dried and manually ground to a powder in a mortar. The clay fraction was collected using the sedimentation method at 20°C after mechanical dispersion. Oriented clay deposits were prepared on glass slides and studied using an X-ray diffractometer equipped with a Si(Li) solid detector to filter the Cu Kα radiation (λCu Kα = 1,5418 Å) of a standard European type X-ray tube (40kV, 40mA). The divergence, the incident beam scatter, the diffracted beam scatter and the receiving slits were 2.00, 4.00, 0.50 and 0.22 mm wide, respectively. The XRD patterns were collected from 2° to 24° 2θ at a scan rate of 0.3° 2θ/min per step of 0.05° 2θ, on natural, glycoled and heated oriented slides [START_REF] Robert | Méthodes de preparation des argiles des sols pour les études minéralogiques[END_REF][START_REF] Bruand | Analyse minéralogique quantitative d'un échantillon de sol : utilisation des données concernant la composition chimique de l'échantillon[END_REF]. Nitrogen adsorption isotherms were conducted on about 150 mg of < 2 mm soil samples dried in a 105ºC oven for 24 hours and then dried again at 105°C under a pressure of 10 2 Pa. The specific surface area (SSA) of the material was determined using the BET equation [START_REF] Brunauer | Adsorption of gases in multimolecular layers[END_REF]. Experiments were performed with a surface analyser (Model Nova 2200e, Quantachrome Instrument Company, USA).

Results and discussion

Basic physico-chemical characteristics

The clay content (CC in g per kg of oven-dried soil at 105°C) increased roughly with depth in each soil and ranged from 373 to 474 g kg -1 in soil TS, from 359 to 461 g kg -1 in soil MS and from 255 to 378 g kg -1 in soil LS (Table 1). These clay contents are similar to those reported for red soils by [START_REF] Baligar | Nature and properties of red soils of the world[END_REF]. The fine silt (2-20 µm) content ranged from 382 to 505 g kg -1 (Table 1). A high fine silt content was also recorded in other red soils located in southern China [START_REF] Hong | Red soils with net-like veins and their climate significance in south China[END_REF]. The pH ranged from 4.2 to 4.7 and increased with depth in the three soils studied (Table 1). These values are consistent with those earlier recorded in southern China for red soils under upland tree vegetation and crops [START_REF] Xu | Acidity of red soils in a subtropical region of southern China under field conditions[END_REF]. The cation exchange capacity (CEC in cmol + per g of oven-dried soil) was very small considering the clay content and poorly saturated by the alkaline cations (about 10-20 %); exchangeable aluminium (not measured here) was the main exchangeable cation, as pointed out by [START_REF] Xu | Acidity of red soils in a subtropical region of southern China under field conditions[END_REF].

Because the subsoil horizons contained little organic carbon (Table 1), we assumed that the contribution of the organic matter to the cation exchange capacity was negligible compared with the cation exchange capacity of the clay. Thus we calculated for the subsoil horizons the cation exchange capacity of the clay (CEC cl , in cmol + per g of oven-dried clay at 105°C) as follows [START_REF] Bruand | Water retention properties of the clay in soils developed on clayey sediments : significance of parent material and soil history[END_REF]:

CEC cl = (CEC / CC)×1000
Results showed that CEC cl ranged from 9.58 to 14.11 cmol + g 1 which corresponded to a clay fraction mainly made up of non active clays (Table 1). XRD patterns of the clay fraction extracted from the horizons C show the presence of:

-1:1 clays (d001 = 0.713 nm) which are in all likelihood kaolinite, as the peak at 0.713 nm is similar at 20°C, 170°C and after treatment with ethylene glycol (Figure 2). The much smaller peak at 0.713 nm recorded after heating at 500°C is thought to be related to the remaining presence of kaolinite which is not dehydroxylated at such a temperature [START_REF] Robert | Méthodes de preparation des argiles des sols pour les études minéralogiques[END_REF][START_REF] Bruand | Apport des méthodes d'enrichissement sélectives et des spectrométries à l'identification des constituants minéraux d'un échantillon de sol[END_REF]; -non swelling 2:1 clays (d001 = 1,004 nm) which are in all likelihood particles of micas < 2 µm or large particles of illite because of the very small CEC cl recorded, the peak at 1,004 nm being similar at 20°C, 170°C and after treatment with ethylene glycol (Figure 2) [START_REF] Robert | Méthodes de preparation des argiles des sols pour les études minéralogiques[END_REF]; -and non swelling 2:1/1 clays (d001 = 1.400 nm) which are probably chlorites or hydroxy-vermiculites, the peak at 1.400 nm being similar at 20°C and after treatment with ethylene glycol (Figure 2) [START_REF] Robert | Méthodes de preparation des argiles des sols pour les études minéralogiques[END_REF]. At 170°C, the X-ray diagrams show a decrease in the intensity of the peak at 1.400 nm probably because of the partial destruction of the octahedra in the interlamellar space. At 500°C, the peak at 1.400 nm was either very small (Figures 2c andf) or absent (Figures 2a, b, d ande), thus indicating the nearly total or total destruction of the octahedra in the interlamellar space.

The clay mineralogy appears therefore to be characterized by the presence of non swelling or poorly swelling clays. A similar clay composition with some illite/smectite interstratified clays was described by [START_REF] Hong | Red soils with net-like veins and their climate significance in south China[END_REF] in red soils located in southern China. Because of the lack of a swelling test using glycol or ethylene glycol in their study, the illite/smectite interstratified clays identified by the presence of a peak at 1.464 nm might in fact be chlorites or hydroxy-vermiculites such as for the soils selected for this study [START_REF] Robert | Méthodes de preparation des argiles des sols pour les études minéralogiques[END_REF].

The specific surface area (SSA in m 2 per g of oven-dried soil) ranged from 9.9 to 34.1 m 2 g -1 and the clay content accounts for 84 % of its variance. As contributions to SSA from silt and sand are negligible for clay soils, the major contribution is likely to be made by clay-size materials. We thus calculated the SSA of the clay fraction (SSA cl , in m 2 per g of oven-dried clay) for horizons B1, B2 and C as follows:

SSA cl = (SSA / CC) × 1000.
The results showed that SSA cl was between 41.5 and 77.3 m 2 g -1 (Table 1) thus indicating a small size of the <2 µm fraction in these horizons (e.g. [START_REF] Feller | N 2 -BET specific surface area of some low activity clay soils and their relationships with secondary constituents and organic matter contents[END_REF][START_REF] Brito Galvão | Mineralogical properties of a collapsible lateritic soil from Minas Gerais, Brazil[END_REF][START_REF] Balbino | Changes in porosity and microaggregation in clayey Ferralsols of the Brazilian Cerrado on clearing for pasture[END_REF]. [START_REF] Reatto | Shrinkage of microaggregates in Brazilian Latosols during drying: significance of the clay content, mineralogy and hydric stress history[END_REF] studied Brazilian clayey Acrisols (ISS Working Group WRB, 2007) and found that SSA cl was between 24.1 and 41.1 m 2 g -1 for a clay fraction made of kaolinite, gibbsite, goethite and hematite particles between 0.01 and 0.3 µm. [START_REF] Bruand | Water retention properties of the clay in soils developed on clayey sediments : significance of parent material and soil history[END_REF] discussed the water retention properties of French clayey soils developed on a large range of clayey sediments and found that SSA cl was between 50 and 130 m 2 g -1 for a clay fraction made of clay particles with a small number of layers [START_REF] Robert | Crystallochemistry, properties and organization of soil clays derived from major sedimentary rocks in France[END_REF].

Structure and porosity

The red soils studied show essentially a moderate to strong subangular blocky structure 5 to 10 mm in size (Table 2), thus making it easy to separate millimetric aggregates as defined by [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF]. Such a strong structure is consistent with the characteristics described by [START_REF] Zhang | Cause of Seasonal Draught formation and Strategy of the eastern hills in Southern China[END_REF] for red soils. Roots were mainly located in the A and B1 horizons in TS and MS where the soils were under a secondary forest vegetation and an orchard of orange trees, respectively. In soil LS, roots remaining from the maize crop were not observed.

In each soil, the bulk density was lower in the horizons A compared to its value in the subsoil horizons because of the lesser development of biological pores in the latter (Table 3). It ranged from 1.27 to 1.32 g cm 3 in soil TS, from 1.34 to 1.42 g cm 3 in soil MS and from 1.45 to 1.46 g cm 3 in soil LS. These values are close to those reported by [START_REF] Vogel | People's Republic of China: Red reference soils of the subtropical Yunnan Province[END_REF] for red reference clay soils of the Yunnan Province. The bulk densities recorded for the top horizon in soils TS, MS and LS are similar to those recorded for many topsoils of red soils under different soil managements in southern China (e.g. Huang et al., 2004;[START_REF] Zheng | Impacts of reforestation approaches on runoff control in the hilly red soil region of southern China[END_REF][START_REF] Huang | Response of runoff and soil loss to reforestation and rainfall type in red soil region of southern China[END_REF] but the clay content was often less than 30 % in the soils studied.

These bulk densities are also close to those recorded by [START_REF] Bruand | Water retention properties of the clay in soils developed on clayey sediments : significance of parent material and soil history[END_REF] for French clay soils with both a large clay content and mineralogy range.

The volume of macropores and micropores in the soil at the sampling date (i.e. close to field capacity), was computed by using the bulk density of the horizon and the bulk density of the millimetric aggregates [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF]. Thus, the specific volume of macropores in conditions close to field capacity (

) was obtained by subtracting the specific volume of the millimetric aggregates (reciprocal of their bulk density) from the specific volume of the horizons (reciprocal of their bulk density) (Table 3). The results showed that decreases strongly with depth in the three soils studied (Table 3). Such results are consistent with the few or very few roots recorded in horizons B1, B2 and C of these soils (Table 2). The volume of micropores at field capacity and after air drying was computed by subtracting the specific volume of the solid phase which is equal to the reciprocal of the particle density, the latter being considered as equal to 2.65 g cm -3 (1/2.65 g cm -3 = 0.377 cm 3 g -1 ) [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF], from the specific volume of the millimetric aggregates (Table 2). The results

showed that is smaller in the subsoil (horizons B1, B2 and C) than in horizon A for each soil. In the subsoil, ranges from 0.239 to 0.297 cm 3 g -1 and from 0.208 to 0.264 cm 3 g -1 . In clay soils the volumes of micropores result mainly from the assemblages of the clay particles in the soil material [START_REF] Fiès | Textural porosity analysis of a silty clay soil using pore volume balance estimation, mercury porosimetry and quantified backscattered electron scanning image (BESI)[END_REF]1998). We therefore computed the volume of micropores developed by the clay phase close to field capacity ( in cm 3 per g of clay) and after air-drying ( in cm 3 per g of clay) as follows:

, .

The average and computed with the data presented in Tables 1 and3 for horizons B and C with CC>300 g kg -1 are 0.663 and 0.584 cm 3 per g of clay, whereas the corresponding values for a horizon B from a French red clay soil are 0.488 and 0.327 cm 3 per g of clay, respectively [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF]. This corresponds to an 11% decrease in the average micropore volume of the clay phase between field capacity and air-drying for the Chinese red clay soils studied, compared to 33% for the clay soil studied by [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF]. Thus the clay fabric which corresponds to both the volume of clay particles and associated pore volume resulting from their assemblage shrinks much less between field capacity and air-drying in the Chinese red clay soils studied than in the French clay soil studied by [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF].

Water retention properties

The amounts of water retained at -60, -100, -330, -1000, -3300 and -15 000 hPa water potential for the different horizons studied are given in Table 4. The water content at the sampling date corresponded to a water potential between -100 and -330 hPa, thus confirming that the soil was close to field capacity during sampling (Al Majou et al., 2008a). The variance in water retained at -60, -100, -330 hPa water potential is explained mainly by the specific volume of the horizon (reciprocal of the bulk density) with R 2 = 0.863, 0.826 and 0.646, respectively. At -15 000 hPa water potential, the maximum variance in water retained is explained by the clay content (R 2 = 0.851), while the specific volume of the horizon explained only a small proportion of the variance. Such a high proportion of variance explained by the specific volume and the clay content of the horizon at high and low water potential, respectively, was earlier recorded by Bruand et al. (1988) and [START_REF] Bruand | Improved prediction of water-retention properties of clayey soils by pedological stratification[END_REF] for clayey soils.

For the horizon with a clay content > 300 g kg -1 , we computed the available water which corresponds to the amount of water available between -330 and -15 000 hPa water potential (Table 5). It ranged from 0.0091to 0.166 cm 3 cm -3 in the soils studied here. These values are consistent with those recorded in many other clay horizons originating from soils located in different countries (Table 5). The average available water measured for the Chinese red clay soils of this study is however slightly greater (0.121 cm 3 cm -3 ) than for the 1203 clayey soils from the literature (0.113 cm 3 cm -3 ) (Table 5). The available water between -3300 and -15 000 hPa reported in the literature for Chinese red clay soils is much greater (average of 0.222 cm 3 cm -3 ) but the soils selected were mainly topsoils (Table 5).

Although the available water between -3300 and -15 000 hPa of the Chinese red clay soils studied appears to be slightly greater than clay soils from other countries, a much greater proportion of water available between -3300 and -15 000 hPa compared to the water available between -330 and -15 000 hPa was recorded for these Chinese red clay soils than for clay soils originating from other countries (Table 5). This proportion ranged from 0.520 to 0.753 in the Chinese red clay soils of this study (average of 0.620), from 0.535 to 0.620 in the Chinese red clay soils from the literature (average of 0.591), and from 0.285 to 0.425 in the 1203 clay soils from other countries (average of 0.366) (Table 5). This indicates that for a given available water amount, the proportion of water weakly available is much greater in Chinese red clay soils than in clay soils originating from other countries, the total available water being just slightly greater for the former. This might explain the high sensitivity to drought of the Chinese red clay soils during the dry season as pointed out by [START_REF] Wang | Study on Seasonal Drought in Southern China[END_REF] and [START_REF] Zhao | Red soil material cycling and its regulation[END_REF].

In most clay soils, a proportion of water is released at low water potential by the clay phase concomitantly to its shrinkage [START_REF] Bruand | Effect of water content on the fabric of a soil material: an experimental approach[END_REF]. The millimetric aggregates originating from the red clay soils studied showed little shrinkage between field capacity and air-drying (Table 2). Such a small shrinkage which reflects a stiffness of the microstructure is likely related to the high proportion of non active clays as revealed by the cation exchange capacity and the X-ray diffraction patterns (Table 1 and Figure 2) and the presence of aluminum and iron oxy-hydroxides as revealed by the soil color but not studied here. This small shrinkage and the high proportion of poorly available water might also be related to the long hydric stress history of the Chinese red clay soils as discussed by [START_REF] Bruand | Water retention properties of the clay in soils developed on clayey sediments : significance of parent material and soil history[END_REF] for a large set of French clayey soils developed on clayey sediments. According to their results and those recorded earlier by [START_REF] Tessier | Mineralogical characterization of 2:1 clays in soils: Importance of the clay texture[END_REF] on pure clays, the water retention properties of clayey soils appear to be mainly affected by the greatest effective stress recorded by the soil, the proportion of poorly available water compared to the total amount of available water increasing with the effective stress recorded. The parent materials of the Chinese red clay are mainly Quaternary continental sediments which reorganize material resulting from continental alteration from the Cretaceous or Eocene [START_REF] Bgmrhn | Bureau of Geology and Mineral Resources of Hunan province[END_REF]Wilson et al., 2004a and b;[START_REF] Hu | Regional distribution of the Quaternary red clay with aeolian characteristics in subtropical China and its paleoclimatic implications[END_REF][START_REF] Shi | Cross reference for relating Genetic Soil Classification of China with WRB at different scales[END_REF]. Because of this history, they were in all likelihood submitted to great hydric stresses, thus leading to strongly consolidated clayey parent materials with consequences for the soils such as a high proportion of poorly available water, strong aggregation, and weak shrinkage properties [START_REF] Bruand | Water retention properties of the clay in soils developed on clayey sediments : significance of parent material and soil history[END_REF].

Conclusion

Our results show that the amount of water available between -330 and -15 000 hPa water potential for the Chinese red clay soils selected in the Taoyuan experimental station is consistent with that recorded in many other clay soils and thus cannot explain the high sensitivity of the Chinese red clay soils to drought. Although many soil and plant characteristics can potentially be involved in the high sensitivity of Chinese red clay soils to drought, the high proportion of poorly available water which was characterized by the amount of water available between -3300 and -15 000 hPa water potential compared to the available water between -330 and -15 000 hPa might explain this sensitivity to drought. Comparison with clay soils located in different regions of the world and for which a particular sensitivity to drought was not identified, showed that this proportion of poorly available water is indeed much greater in the Chinese red clay soils studied. Finally, our results which were recorded on a small number of soils need to be confirmed by studies on a much larger range of Chinese red clay soils coming from different areas. By providing more accurate knowledge of parent and soil history, this would enable discussion of the amount of water available between -3300 and -15 000 hPa water potential.

.

Figure 1

 1 Figure 1 Location of the soils studied in the Taoyuan experimental station (soil at the top of slope: TS, middle of the slope: MS and on a ledge of the slope: LS)

Table 1

 1 Physico-chemical characteristics of the soils studied at the top (TS), the middle (MS) and on a ledge of the slope (LS)

	Soil	Horizon Particle size distribution (µm)		Organic	pH	Cation exchange capacity (CEC) and		CEC cl	Specific	SSA cl
								carbon		exchangeable cations				surface
																area
																(SSA)
		<2		2 -	20 -	50-	200 -			CEC	Ca 2+	Mg 2+	Na +	K +
				20	50	200	2000
					g kg -1			g kg -1				cmol + kg -1			cmol + kg -1 m 2 kg -1	m 2 kg -1
	TS	A	373	470	109	21	27	35.40	4.3	6.37	0.71	0.24	0.18	0.02	-	16.0	-
		B1	441	433	93	12	21	5.44	4.4	4.52	0.41	0.08	0.17	0.02	10.25	26.2	59.4
		B2	497	382	85	12	24	2.13	4.5	6.56	0.32	0.08	0.09	0.03	13.20	36.8	74.0
		C	474	398	86	16	26	2.05	4.7	6.69	0.42	0.12	0.09	0.03	14.11	34.7	73.2
	MS	A	359	493	111	13	24	17.51	4.2	4.36	0.18	0.10	0.16	0.10	-	11.8	-
		B1	380	478	107	12	23	4.4	4.3	3.64	0.12	0.06	0.09	0.01	9.58	24.1	63.4
		B2	441	433	92	11	23	2.73	4.7	5.55	0.46	0.18	0.09	0.02	12.59	34.1	77.3
		C	461	413	92	11	23	2.44	4.7	6.34	0.45	0.15	0.10	0.02	13.75	32.3	70.1
	LS	A	255	490	142	37	76	15.23	4.4	3.68	1.55	0.39	0.14	0.02	-	9.9	-
		B1	237	505	151	35	72	7.63	4.3	3.16	0.43	0.21	0.11	0.01	13.33	8.8	37.1
		B2	322	485	118	21	54	5.24	4.6	3.30	1.10	0.18	0.08	0.02	10.25	16.6	51.6
		C	378	449	99	25	49	6.15	4.4	4.35	0.79	0.16	0.11	0.02	11.51	15.7	41.5

Table 4

 4 Water content at sampling date and water retention properties of the soils studied.

	Soil	Horizon Water content		Water retained at a water potential of (hPa) :
			at sampling	-60	-100	-330	-1000	-3300	-15 000
			date
						g g -1
	TS	A	0.345	0.394	0.375	0.340	0.315	0.286	0.210
		B1	0.273	0.289	0.278	0.252	0.234	0.215	0.175
		B2	0.295	0.318	0.307	0.288	0.275	0.255	0.220
		C	0.286	0.305	0.297	0.284	0.272	0.253	0.212
	MS	A	0.315	0.378	0.355	0.310	0.275	0.246	0.155
		B1	0.265	0.281	0.275	0.261	0.249	0.234	0.172
		B2	0.284	0.299	0.296	0.283	0.272	0.256	0.213
		C	0.289	0.299	0.297	0.287	0.279	0.264	0.223
	LS	A	0.330	0.341	0.331	0.303	0.278	0.247	0.111
		B1	0.258	0.265	0.261	0.246	0.233	0.212	0.106
		B2	0.259	0.269	0.266	0.255	0.242	0.227	0.141
		C	0.270	0.279	0.276	0.266	0.254	0.240	0.176

  Quaternary red clay with a clay content  0.362 g kg -1 ; b : estimated from soils I.1, I.2 and I.3 in figure 1; c : silty clay; d : clay; e : silty clay;

	Brazil	Reatto et al. (2007)	1	Subsoil		52	0.267	0.228	0.205	0.062	0.365
	Table 5		1 1	Subsoil Subsoil		61 75	0.263 0.265	0.226 0.232	0.208 0.216	0.049 0.049	0.328 0.339
	1 Available water between -330 hPa ( 330 ) and -15000 hPa ( 15000 ) and proportion of this available water between -3300 hPa Subsoil 55 0.277 0.233 0.206 0.071 0.371 1 Subsoil 78 0.287 0.256 0.239 0.048 0.352
	( 3300 ) and -15000 hPa ( 15000 )
	China	Qu et al. (2009)	1	Subsoil		32	0.338	0.244	0.202	0.136	0.308
	Origin	Reference	Number 1	Type of Subsoil		Clay 32	 330 0.429	 3300 0.372	 15000 0.343	( 330 - 15000 ) 0.087	( 3300 - 15000 ) 0.343
	of soils		of 1	horizon Subsoil		content 32	0.356	0.297	0.268	0.087	/ ( 330 - 15000 ) 0.325
			horizons
	Europe	Wösten et al. (1999)	21	Topsoil		% >60	_______________________ cm 3 cm 3 _____________________ _ 0.489 0.390 0.336 0.153	0.358
			217	Topsoil	This study 35 -60	0.404	0.324	0.279	0.125	0.357
	TS	This study	1 596	Topsoil (A) Subsoil	37 >60	0.354 0.407	0.297 0.338	0.218 0.298	0.136 0.108	0.588 0.368
			1 132	Subsoil (B1) Subsoil	44 35 -60	0.333 0.471	0.284 0.404	0.231 0.363	0.102 0.109	0.520 0.378
			1	Subsoil (B2)	50	0.400	0.355	0.306	0.094	0.521
	France	Al Majou et al. (2008b)	1 2	Subsoil (C) Topsoil	47 >60	0.409 0.402	0.364 0.332	0.305 0.293	0.104 0.109	0.567 0.358
			17	Topsoil		35 -60	0.367	0.304	0.272	0.095	0.337
	MS	This study	1 18	Topsoil (A) Subsoil	36 >60	0.332 0.405	0.263 0.354	0.166 0.330	0.166 0.075	0.584 0.320
			1 85	Subsoil (B1) Subsoil	38 35 -60	0.350 0.348	0.314 0.298	0.230 0.261	0.120 0.087	0.700 0.425
			1	Subsoil (B2)	44	0.393	0.356	0.296	0.097	0.619
	Portugal	Goncalves et al. (1997)	1 18	Subsoil (C) Topsoil and subsoil	46 >40	0.409 0.372	0.375 0.302	0.317 0.263	0.091 0.110	0.637 0.359
			38	Topsoil and subsoil	40 -60	0.426	0.322	0.269	0.158	0.335
	LS	This study	1	Subsoil (B2)	32	0.372	0.331	0.206	0.166	0.753
	USA	Schaap and Leij (1998)	1 34	Subsoil (C) Topsoil and subsoil	38 >40	0.386 0.320	0.348 0.206	0.255 0.160	0.131 0.161	0.710 0.287
			12	Topsoil and subsoil	35 -55	0.376	0.241	0.181	0.195	0.306
	Average (this study) Pucket et al. (1985)	1	Subsoil		33	0.321	0.287	0.272	0.121 0.049	0.620 0.316
			1	Subsoil		34	0.351	0.325	0.313	0.038	0.308
			1	Subsoil	Chinese red clay soils 35	0.356	0.319	0.297	0.060	0.368
	China	Cao and Zhu (1999)	1	n.d.		n.d. a	0.251	0.198	0.137	0.114	0.535
	Lu et al. (2004) b Weighted average (clay soils from the literature) 1	Topsoil		n.d. c	0.443	0.340	0.172	0.271 0.113	0.620 0.366
			1	Topsoil		n.d. d	0.447	0.331	0.151	0.296	0.608
			1	Topsoil		n.d. e	0.372	0.289	0.165	0.207	0.600
	Average (Chinese red clay soils)								0.222	0.591
					Clay soils from literature
	Belgium	Vereecken et al. (1989)	3	n.d.		>35	0.439	0.340	0.293	0.109	0.285

a

: soil derived from a

ACKNOWLEDGMENT

We thank Wenxue Wei for help in soil sampling. This work was supported by the National Basic Research Program of China (2011CB100506) and the Labex Voltaire (ANR-10-LABX-100-01).

(1) Grade: 1=weak; 2=moderate; 3=strong. (2) Root frequency: 0=very few (<1 root in each horizontal 100cm 2 surface area); 1= few (<1 to 5 roots in each horizontal 100cm 2 surface area); 2 = moderate (5 to 10 roots in each horizontal 100cm 2 surface area); 3=numerous (10 to 50 roots in each horizontal 100cm 2 surface area); 4=highly numerous (>50 roots in each horizontal 100cm 2 surface area).