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SOIL CHEMISTRY

Assessing the Quality of Dissolved Organic Matter in Forest Soils Using Ultraviolet Absorption Spectrophotometry

^a Biogéochimie des Ecosystèmes Forestiers, UPR1138, INRA, F-54280 Champenoux, France
^b current address: Biogéochimie du Sol et de la Rhizosphère, UMR1222, INRA, SupAgro, Place Viala, F-34060 Montpellier, France
^c LIMSAG, UMR5663 CNRS-Université de Bourgogne, UFR Sciences et Techniques, 9 avenue Alain Savary, BP 47870, F-21078 Dijon, France

^* Corresponding author (gerard{at}supagro.inra.fr).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Ultraviolet spectrophotometry was used to investigate the effects, 30 yr after planting, of tree species substitution on the aromatic C content and related properties of dissolved organic carbon (DOC). Precautions were taken to correct measurements for the absorbance of NO₃ and dissolved Fe. In litter leachates, a significant reduction in the aromatic content of DOC was found in the Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantation but not in the beech (Fagus sylvatica L.) plantation. The disturbance of short-term C dynamics thus revealed agreed well with field observations. Significant differences in aromatic content were also found in capillary soil solutions from the two planted stands. Overall, these modifications, produced by the substitution of trees 30 yr previously, mostly concerned the beech plantation. Soil processes, and probably adsorption, played a central role in controlling the quality of DOC in this soil and appeared to be influenced by the species planted. In low-capillary solutions, located in larger pores, changes to the aromatic content were only detectable in the surface soil of the beech plantation. We found a more pronounced effect of tree substitution in high-capillary solutions filling soil micropores, where the aromatic content of DOC might be tightly controlled by soil organic matter. It is difficult to say, however, whether the planting of Douglas-fir had actually accelerated soil recovery, or whether there will be future changes to attain a completely new equilibrium.

Abbreviations: DOC, dissolved organic carbon • SUVA, specific ultraviolet absorbance • UV, ultraviolet

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The concentration and composition (i.e., quality) of natural organic matter are variables that have a significant effect on nutrient cycling. Because trees interact markedly with their environment, the common practice of tree species substitution or shifting usually leads to ecosystem modifications (e.g., Binkley, 1995). It is well established that the chemistry of soil solutions constitutes a good indicator of current ecosystem functioning (e.g., Zabowski and Ugolini, 1990; Smethurst, 2000; Ranger et al., 2001). Although the inorganic chemistry of soil solutions has been quite extensively studied, less attention has been paid to the quality of dissolved organic carbon (DOC) (Ströbel et al., 2001).

A broad range of chemical and physical methods are available to assess the different properties of DOC (Leenheer and Croué, 2003). Most of these techniques, however, are costly, time consuming, and invasive, and require a large sample volume even for a single analysis such as measuring the concentration in humic substances (Martin-Mousset et al., 1997; Simonsson et al., 2005). These analytical constraints generally constitute a limiting factor when it comes to studying soil solutions (Dilling and Kaiser, 2002). Ultraviolet (UV) absorption spectrophotometry is commonly used to study various properties of the DOC, such as its aromaticity, hydrophobic content, and biodegradability (e.g., Hautala et al., 2000; Dilling and Kaiser, 2002; Croué et al., 2003; Uyguner et al., 2004). The method is rapid, requires little sample preparation, is noninvasive, and only a small sample is necessary (Chen et al., 2002). Ultraviolet absorption spectrophotometry has been used extensively in a number of surface environments (e.g., McKnight et al., 1997; Maurice et al., 2002; O'Loughlin and Chin, 2004), including soil solutions sampled in fen areas and temperate forest ecosystems (Kalbitz, 2001; Simonsson et al., 2005). The UV properties of DOC derive from its aromatic C content. The –* electronic transition absorbs the wavelength corresponding to the UV spectral domain. This molecular orbit is driven by the double-bond C of benzene-type material (Wieteska, 1986). Specific absorption at 254 nm (i.e., measured absorbance divided by the DOC concentration), referred to as specific ultraviolet absorbance (SUVA), has been used for many years to assess the aromaticity of DOC. Specific UV absorbance is used routinely as a criterion for water quality by the drinking water industry (see, e.g., Weishaar et al., 2003). Specific absorbance value, or absorptivity, measured at different key wavelengths has also been used to assess a variety of properties, such as aromaticity, hydrophobic content, apparent molecular weight and size, and biodegradability (e.g., Chin et al., 1994; Peuravuori and Pihlaja, 1997; Kalbitz et al., 2003; Simonsson et al., 2005). A review of the literature allowed us to select the most commonly used wavelengths (Table 1 ). Empirical relationships are usually established between absorptivity and various DOC properties using standard methods. Because of the heterogeneous nature of DOC and the expected spatial and temporal variability of its properties, however, its applicability to different field sites may be uncertain and could be misleading (Marschner and Kalbitz, 2003). We have therefore chosen to restrict our study to relative variations in specific absorbance that are indicative of relative differences in the aromatic C content and related properties of DOC (see Table 1).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Materials
Soil solutions were sampled in the Breuil-Chenue forest (Morvan, France, 47°18'10''N and 4°4'44''E). This site is located on a south-tilted granite plateau with a mean elevation of approximately 650 m above sea level. The mean annual temperature is roughly 9°C and mean annual precipitation and potential evapotranspiration values reach approximately 1280 and 640 mm, respectively. The soil has been classified as a Typic Dystrochrept according to Soil Survey Staff (1994), and as an Alocrisol according to the AFES (1992). The soil is well aerated. The mean pH and organic C content are pH 4.4 and 60 g kg^–1, respectively. The soil cation exchange capacity is about 5.56 cmol_c kg^–1 with a base saturation of 18% (Levrel and Ranger, 2006). Vegetation cover corresponds to plantations of different tree species planted in 1976 to replace a 150-yr-old clear-cut native forest. The native forest is dominated by beech (Fagus sylvatica L.), with sessile oak (Quercus sessiliflora Smith) being found less frequently. In addition to the samples collected in a reference plot of native forest, we focused our study on soil solutions collected in two planted stands: beech, a deciduous species, and Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco], a coniferous species. The difference in the thickness of the holorganic horizon (hereinafter referred to as the O horizon) in the three forest stands was marked. We measured an O horizon of 6 to 8 cm in the two broad-leaved plots (native forest and beech plantation) vs. only about 1 cm in the Douglas-fir stand.

Methods
Ceramic suction-cup lysimeters maintained at a constant suction (50 kPa) were used to collect solutions from the mineral soil. We also extracted solutions from the mineral soil by centrifuging fresh samples in the laboratory. The use of these two techniques enabled us to consider two types of solutions collected from the mineral soil, hereinafter referred to as low-capillary and high-capillary solutions, respectively. We also sampled litter leachates by means of zero-tension plate lysimeters installed immediately below the O horizon.

Low-capillary soil solutions are located in larger interconnected pores; given the suction applied, they correspond to a matric potential <50 kPa. As suction is continuously maintained, some gravitational flow is nonetheless collected after rain events. This type of capillary water is therefore composed of both mobile and weakly fixed capillary solutions. For reasons of convenience, we will use the term low-capillary solutions for this admixture of both mobile and capillary solutions. Conversely, high-capillary solutions constitute a good proxy for a solution that reacts with soil material, as they are retained by stronger capillary forces in narrower, interconnected pores (Zabowski and Ugolini, 1990; Giesler et al., 1996; Ranger et al., 2001; Gérard et al., 2003). Dissolved organic C properties in this type of soil solution may be more closely related to those of soil organic matter than those in low-capillary soil solutions, which are closer to DOC transferred from the O horizon as litter leachates (Stevenson, 1994; Kaiser and Guggenberger, 2000).

We used the centrifuge drainage method to collect capillary solutions corresponding to a high matric potential (Giesler and Lundström, 1993; Ranger et al., 1993; Giesler et al., 1996; Gérard et al., 2003). A JOUAN KR4 22 centrifuge (Saint-Herblain, France) was used for this purpose. This apparatus enables the centrifugation of six soil samples of approximately 1600 cm³ at a fixed temperature (20°C). The samples are placed in a double-bottomed polycarbonate tube consisting of an upper, soil-holding cylindrical part with a perforated base, and a lower solution-collecting cup. A stepwise procedure was adopted to extract that portion of the capillary solutions corresponding to the highest matric potential. A first extraction was performed at 1000 rpm for 20 min, yielding capillary solutions corresponding to a matric potential of <180 kPa. These solutions were discarded. The second step consisted in centrifuging the same samples for a further 20-min period at 3000 rpm. Capillary solutions corresponding to a matric potential within the range of 180 to 1600 kPa were thereby obtained.

Two replicates of the litter leachates were available for each forest stand. Suction cup lysimeters were located at depths of 15 and 30 cm (five replicates). These two types of soil solution were routinely collected during monthly sampling campaigns. For the present investigation, we took samples collected during five sampling campaigns unevenly distributed between June 2003 and late January 2004. More precisely, our database of litter leachates and low-capillary solutions corresponded to two set of samples collected in the autumn, two in the winter, and only one in the spring (calendar basis). We thus obtained a total of 30 samples of litter leachates and 98 samples of low-capillary soil solutions. Fresh soil samples used in the centrifugation method were collected at two soil depth intervals, 0 to 15 and 15 to 30 cm, which broadly corresponded to the A and B soil horizons, respectively. A total of 155 high-capillary solution samples were collected during six field campaigns, unevenly distributed during the winter, spring, and autumn of 2003. During the summer, drought conditions did not supply sufficient solution to perform chemical characterizations.

The samples were filtered at 0.45 µm (Pall Corp., East Hills, NY, Metricel membrane filter) in the laboratory and stored at 4°C in the dark before analysis, which was usually performed within 2 d. The Fe content was determined by inductively coupled plasma–optical emission spectroscopy (Jobin-Yvon Instruments, Longjumeau, France, Model JY180). Nitrate and DOC concentrations were measured by ionic chromatography (Dionex Corp., Sunnyvale, CA, Model X320) and using a C analyzer (Shimadzu Biotech, Manchester, UK, TOC 5050), respectively. A rod electrode was used to measure the pH (Sentron Europe BV, Roden, the Netherlands). Ultraviolet absorption spectra were measured at a constant temperature (25°C) with either a Cary 50 or Cary 500 UV-visible spectrophotometer (Varian Scientific Instruments, Palo Alto, CA), both instruments producing identical results. As suggested in the literature (Nowicka-Jankowska, 1986), we complied with the warm-up procedure recommended by the manufacturer. Spectra were acquired between 200 and 500 nm at a scan rate of 150 nm min^–1 and with a uniform data point interval of 0.5 nm. Samples were placed in a quartz cell (Suprasil 300, Hellma, Mullheim, Germany) with a 1-cm optical path length. The cell holder was connected to a RE106 (Lauda, Lauda-Königshofen, Germany) water circulator (25°C), while each sample was equilibrated in a water bath for at least 15 min before measurement. Double-deionized high-purity water (18.2 M cm), obtained using a Maxima (USF Elga, High Wycombe, UK) cartridge system designed for trace analysis, was used to collect baseline data and as a reference. Some measurements were duplicated randomly to ensure the validity and repeatability of the scans thus performed. The pH of the samples was neither adjusted nor buffered.

Statistical analysis of the results was performed using the commercial Unistat (Unistat Ltd., London) and Sigmastat (Systat Software, San Jose, CA) packages. Analyses of variance were run under the General Linear Model (Lane, 2002). Full interactions between factors were considered. We tested the effects of tree substitution and season in litter leachates using a two-way ANOVA, whereas in capillary solutions, the influence of plantations, soil depth, and time was tested by means of a three-way ANOVA. The normality and homoscedasticity postulates had been checked before variance analysis. Multiple comparisons were performed with the Holm Sidack test ( = 0.05), as suggested by Glantz (2002).

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Inorganic Absorbing Solutes
A commonly acknowledged problem using UV spectroscopy to evaluate DOC is that dissolved Fe and NO₃ also absorb light in the selected UV wavelength range (e.g., Simonsson et al., 2005). With respect to NO₃, this artifact can be corrected using the specific absorbance, or absorptivity, of the individual species and the additivity property of the Beer–Lambert law. It should be noted that NO₃ concentrations exhibited variations that depended on both the nature of the planted tree species and soil depth (Tables 2, 3, and 4). First of all, we corrected the absorbance measurements for the influence of NO₃ by making use of the additivity properties of the Beer–Lambert law (Wieteska, 1986; Wang and Hsieh, 2001). The extinction coefficients for NO₃ as a function of temperature were calculated from the work of Simeon et al. (2003). For all samples, we found that the maximum contribution of NO₃ absorption reached 5% of the total absorbance at 300 nm and was negligible at around 260 nm. Measured absorbance can also be corrected for the absorbance of dissolved Fe, based on the work of Weishaar et al. (2003) concerning the absorbance of Fe³⁺ in the presence of aromatic DOC. Knowledge of the absorptivity of Fe³⁺ is limited to the wavelength of 254 nm, however, which is commonly used in studies addressing the quality of natural organic matter (e.g., Korshin et al., 1996). Using the concentrations of dissolved Fe (see Tables 2, 3, and 4), we calculated that Fe³⁺ absorbance was slight, as it ranged from 2% of the measured absorbance in litter leachates to 4% in high-capillary solutions. Such a small contribution of Fe³⁺ to the measured absorbance was expected, given the relatively low Fe concentrations encountered in this soil, which are typical of well-aerated soils (Weishaar et al., 2003).

Variations in the pH can also act on the absorption properties of aromatic C (e.g., Lawrence, 1980; De Haan et al., 1982; Baes and Bloom, 1990). Buffering the solution to a specific pH has even been recommended to enable an unbiased comparison between studies (e.g., Chen et al., 1977). Piccolo et al. (1999) suggested that this pH dependency of the UV absorption properties of DOC might be caused by the disruption of weak hydrophobic forces and subsequent conformational changes. During the present study, however, we decided to ignore the potential influence of pH on absorptivity. We believe that a clearer and more broad-based understanding is required before accounting for or correcting such pH effects, since several researchers found no influence across broad wavelength and pH ranges, such as Dilling and Kaiser (2002) and Wang and Hsieh (2001) in solutions containing humic substances only (from 200–600 nm and pH 2–9.5). More recently, Weishaar et al. (2003) investigated natural water samples and also observed a lack of pH effect across similar pH ranges. In our case, the mean pH of different types of soil solutions ranged from pH 4 to 5.3 (see Tables 2, 3, and 4). This narrow range reinforced the assumption that the influence of pH on the specific absorbance of DOC could be neglected in our case.

Quality of Dissolved Organic Matter
Based on the aforementioned theoretical relationship between UV absorbance properties and the aromatic content of DOC, the terms specific absorbance and aromaticity have been used to denote the quality of the DOC.

In all samples, we measured a relatively constant reduction in the specific absorbance of DOC as the wavelength increased. This trend was accompanied by a less marked influence of tree substitution. The greatest influence of tree substitution was observed in litter leachates at low wavelengths (around 254 nm). Variance analysis further revealed that the influence of tree substitution varied according to the type of soil solution and soil depth. Furthermore, in each solution type and at each soil depth, the effects of tree substitution on the quality (as well as on the quantity) of DOC were found to be unaffected by the sampling period.

In litter leachates, we detected a lack of significant influence of tree substitution on DOC concentrations, while we found that this factor significantly influenced specific absorbance, and thus the aromatic content, of the DOC released from the O horizon (Table 2 ). The lowest values were obtained in solutions collected beneath the O horizon in the Douglas-fir plantation (Table 2). The influence of this coniferous plantation on DOC quality in litter leachates was marked, as significant differences were observed throughout the wavelength range (see Fig. 1 ). Higher specific absorbance values were exhibited by litter leachates from the natural forest. Intermediate values were found in the beech plantation, between 272 and 400 nm.

View larger version (8K): [in this window] [in a new window]   Fig. 1. Mean specific absorbance values for dissolved organic C in solutions collected beneath the O horizon in beech, Douglas-fir, and native forest stands. Error bars correspond to the standard error.

View larger version (11K): [in this window] [in a new window]   Fig. 2. Mean specific absorbance values for dissolved organic C in solutions collected by means of tension lysimeters (i.e., low-capillary solutions) in beech, Douglas-fir, and native forest stands at two depths. Error bars represent the standard error.

View larger version (11K): [in this window] [in a new window]   Fig. 3. Mean specific absorbance values for dissolved organic C in solutions collected by centrifugation (i.e., high-capillary solutions) in beech, Douglas-fir, and native forest stands at two depths. Error bars correspond to the standard error.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

In what follows, we focus on the effect of tree substitution and soil depth in the different types of soil solutions by considering the specific absorbance at 254 nm (hereafter referred to as SUVA_254nm). As noted above, the most marked effects were observed at this key wavelength, and SUVA_254nm has been used widely in the literature for a variety of purposes relative to DOC quality. Moreover, the correction of measured absorbance due to the bias arising from the occurrence of dissolved Fe was limited to this wavelength, as explained above.

Overall, the significant variations observed in SUVA_254nm values indicated that the substitution of natural forest by Douglas-fir or beech had an effect on organic C dynamics 30 yr after planting. In addition, the extent to which the aromaticity and related properties of DOC had been modified (e.g., hydrophobic C content, apparent molecular weight and size) depended on the nature of the planted tree species, soil depth, and type of soil solution.

More specifically, our results showed that, 30 yr after substitution, the O horizon of the beech stand released DOC that exhibited the same aromaticity and related properties as the DOC released by the native forest stand (see Table 2, Fig. 1). This showed that short-term DOC dynamics were unaffected by the planting of beech. This result tended to be in line with the composition of the native forest, which was mostly composed of beech. Interestingly, we also observed an influence of the beech plantation on C dynamics, from examination of the results concerning low-capillary solutions (see Table 3, Fig. 2). A significant difference was found in the topsoil (15-cm depth) of the beech plot, showing a lower SUVA_254nm (i.e., less aromatic DOC). This difference in DOC quality disappeared when the solutions came from a depth of 30 cm. Even though no difference in terms of DOC aromaticity was observed in litter leachates, a significant effect "suddenly" appeared in this type of capillary solution collected from topsoil. As observed in high-capillary solutions, increasing the residence time not only reinforced the effect of the beech plantation on aromatic content but also altered it, as the DOC collected from the two soil depth layers became more aromatic (see Table 4, Fig. 3). These findings revealed the complex and considerable influence of soil processes on control of the aromatic content of DOC. Soil processes also reflected the long-term disturbance engendered by clear-cutting the natural beech-dominated forest and planting the same species. Such a change in the quality of DOC as a function of the residence time of solutions in soil suggested that the nature of these soil processes was controlled by the pore size network (Kaiser and Guggenberger, 2007).

The O horizon of the Douglas-fir stand released much less aromatic DOC than either planted or native deciduous stands, suggesting that its biodegradability might also be higher. This was consistent with the thinness of the O horizon in this plantation (about 1 cm as opposed to 6–8 cm under broadleaved species). Higher decomposition rates in the O horizon have been observed elsewhere under Douglas-fir when compared with beech (Hobbie et al., 2006; Moukoumi, 2006) Therefore, our results showed that short-term DOC dynamics were still modified by the Douglas-fir plantation 30 yr after substitution, as could be expected given the change from the broad-leaved litter of the native forest to a coniferous litter. The influence of soil processes was also marked in the Douglas-fir plantation, as no effect of substitution on SUVA_254nm was observed in low-capillary solutions at depths of either 15 or 30 cm. Any difference in the DOC quality observed with respect to Douglas-fir litter leachates thus rapidly disappeared before the solutions passed through the first 15 cm of mineral soil. Nevertheless, a less aromatic DOC was found in high-capillary solutions collected from the topsoil, and this effect disappeared as soil depth increased.

Results in capillary solutions suggested that soil processes acted differently on DOC quality in the Douglas-fir and beech plantations. This suggests that the intensity or nature of the controlling soil processes responsible for drastic changes in DOC quality from litter leachates to soil solutions were notably affected by the nature of the planted species. Based on the close relationship between DOC in micropore solutions and soil organic matter (Stevenson, 1994; Kaiser and Guggenberger, 2000), it is very likely that the aforementioned differences concerning high-capillary waters better reflected the effects of substitution on the quality of soil organic matter (SOM). Thus SOM quality would also be changed by the type of tree species planted, and such changes varied with soil depth (0–15 and 15–30 cm).

Overall, capillary solutions exhibited more marked disturbances resulting from the beech plantation in terms of the aromatic content of DOC. This could suggest that the planting of beech 30 yr previously more markedly modified present-day, and thus long-term, C dynamics. It is difficult to say, however, whether the planting of Douglas-fir has actually accelerated soil recovery, or whether there will be further changes to attain a completely new equilibrium. This contrasted markedly with the DOC in litter leachates, indicative of present-day and shorter term C dynamics, where SUVA_254nm values did not differ significantly between the beech plantation and the native beech forest. This contrast may have stemmed from the source of aromatic DOC, which should differ as a function of solution type, that is, decomposition of fresh organic residues in litter leachates and close control by reactions with SOM in high-capillary solutions. Figure 4 illustrates the relationships between SUVA_254nm and DOC for the different types of soil solutions, and highlights the plausible relationship between DOC in litter leachates and in low-capillary solutions (litter influenced), and the particular quality of DOC contained in high-capillary solutions (SOM influenced).

View larger version (11K): [in this window] [in a new window]   Fig. 4. Mean dissolved organic carbon (DOC) and specific ultraviolet absorbance at 254 nm (SUVA254nm) values in the different types of solutions and as a function of forest type and soil depth. Errors bars correspond to the standard error. The lines represent the trends (linear regressions) obtained for the forest types (solid line, beech plantation; dashed line, Douglas-fir plantation; dotted line, native forest) in the light of measurements performed in litter leachates (LL), low-capillary solutions at 15- (LCS 15) and 30-cm (LCS 30) depths, and high-capillary solutions at 0- to 15- (HCS 0–15) and 15- to 30-cm (HCS 15–30) depths.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Use of the UV absorption spectrophotometry method made it possible to monitor, 30 yr after planting, the influence of two tree species (Douglas-fir and beech) on the aromatic C content and related properties of DOC, and by extension, on several aspects of C dynamics in these ecosystems by comparison with a reference native forest plot. Our major findings can be summed up as follows:

1. During this 1-yr study, we found no time dependence for the effect of tree substitution. This result may reveal the predominance over time of the same effect of tree species on the quality (and quantity) of DOC.
   
2. At present, short-term C dynamics are only influenced by the planting of coniferous species. The same aromatic-C content in DOC (SUVA254nm) was observed in litter leachates from the two deciduous stands, the beech plantation and the natural forest, which is dominated by this tree species.
   
3. Long-term C dynamics were more clearly disturbed by the planting of beech, as revealed by more marked differences in the aromatic-C content of DOC in the various types of capillary soil solutions collected from this stand. The effect was more pronounced in the DOC of solutions filling soil micropores. This result could be extended to soil organic matter. It was not clear, however, whether the planting of Douglas-fir had actually accelerated soil recovery after clear-cutting, or whether there would be future changes to attain a completely new equilibrium.
   
4. Soil processes, including adsorption, probably played a central role in controlling the aromatic C content of DOC vs. soil depth.
   
5. The degree of adsorption processes could depend on the nature of the planted tree species and could also be ascribed to variations in solid phases (quantity, nature, and accessibility of adsorbing surfaces, including soil organic matter).
   

	ACKNOWLEDGMENTS

 
We gratefully acknowledge the financial support received from the GIP-Ecofor, which enabled maintenance of the field site within the French Ministry of Research's Observatoire de Recherche pour l'Environnement network (ORE). We are also grateful to the Ministry of Research's Ecologie Quantitative program for funding this work.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
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Received for publication May 26, 2006.

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