Soil Chemical Fertility Change Over 4 Decades In The Morvan Mountains And In � uence of Tree Species ( Burgundy , France )


 Background: Intensive silvicultural practices and the planting of monospecific forests of coniferous, more productive compared to hardwoods, may threaten over the mid to long-term the sustainability of soil chemical fertility of forest ecosystems and is a major concern for forest managers and policy.Methods: We investigated the tree species effect (Quercus sessiliflora Smith, Fagus sylvatica L., Picea abies Karst., Pseudotsuga menziesii Mirb. Franco., Abies nordmanniana Spach. and Pinus nigra Arn. ssp laricio Poiret var corsicana) on the change over time of soil chemical properties and nutrient pool sizes in the mineral and organic layers of the soil during the 45 years after the plantation of the Breuil-Chenue common garden experiment (Burgundy, France). The organic and mineral soil layers down to 70 cm depth were sampled in the different monospecific plots in 1974, 2001 and 2019. Results: The Ca and Mg exchangeable pools and soil pH increased over the entire soil profile in most stands. However, the decrease of pH and the increase of exchange acidity in the topsoil layers under conifers and the overall decrease of exchangeable K pools in most stands highlighted that soil acidification is still on-going at this site but the intensity of this process depends on the tree species. Indeed, three groups of species could be distinguished: i) Nordmann fir / Norway spruce where acidolysis and chelation occurred, resulting in the most pronounced pH decrease in the topsoil, ii) Douglas fir / Laricio pine where acidification caused by elevated nitrification rates is probably currently compensated by larger weathering and/or atmospheric depositions fluxes, iii) and oak / beech where soil acidification was less intense. Counterintuitively, soil acidification at this site resulted in an increase in soil CEC which limited the loss of nutrient cations. This change in soil CEC was most likely explained by the precipitation/dissolution dynamics of aluminium (Al) (hydr)oxides in the interfoliar space of phyllosilicates and/or the increase in soil carbon (C) content in the topsoil layers. Conclusion: Tree species greatly and fairly rapidly (<45 years) influence the soil chemical fertility and the pedogenetic processes which in turn may impact forest ecosystem functions and services.


Background
In Europe and in North America, forest land cover reached its minimum around the beginning of the 18th century due to the overexploitation of forest resources (Kaplan et al., 2009). Forest land cover has since then greatly increased helped by the afforestation policies and the use of new energy sources during the second half of the 20th century (Puech, 2009). Vast areas were planted with productive coniferous tree species that were translocated within Europe (e.g. Norway spruce, Picea abies Karst. and Scots Pine, Pinus sylvestris L.) or imported from North America (e.g. Sitka spruce, Picea sitchensis Bong. and Douglas r, Pseudostuga menziesii Mirb.). In many cases, native deciduous forest stands such as beech and oak were converted to coniferous plantations to increase forest productivity (Bouvet et al., 2019).
Forest plantation still remains a common silvicultural practice.
Tree species impact many different processes and uxes at different ecosystem levels as reported by the numerous reviews and studies (Augusto et al., , 2002De Schrijver et al., 2007). Tree species may in uence (i) the geochemical cycling (atmospheric deposition, mineral weathering, leaching) and (ii) the biological cycling (uptake, immobilization in biomass, litterfall, mineralization) as de ned by Legout et al., (2020): (i)Tree species have different capacities to scavenge atmospheric dry and occult deposition, depending on their canopy architecture, height, foliage type and stand Atmospheric deposition and concentrations of nitrogen and sulfur in throughfall are generally higher under coniferous stands compared to hardwood stands (Augusto et al., 2002;De Schrijver et al., 2007). According to Augusto et al., (2002), Norway spruce stands collect more atmospheric depositions than beech or oak (respectively +187 %, +59% et +99% deposition under canopy compared to bulk deposition) and nitrogen and sulfur deposits are higher under conifers than deciduous trees Verstraeten et al., 2012). Augusto et al., (2002) also showed that mineral weathering was enhanced under Norway spruce stands (2 to 3 times higher) compared to hardwood stands. Lastly, tree species may also in uence nutrient leaching (Augusto et al., 2002;Legout et al., 2016). Tree species exert a control over N mineralization and nitri cation processes (ii) Per unit of biomass produced, uptake and immobilization of nutrients in aerial biomass is usually higher in hardwood species than for coniferous species (Augusto et al., 2002). However, even if nutrient contents in aerial biomass are usually higher for hardwood species than for coniferous species, the coniferous species produce more biomass and their rotation lengths are shorter than hardwood species (Augusto et al., 2002;Binkley, 1995). Depending on litterfall mass and chemical composition, tree species may also in uence the pools of carbon and nutrients in the organic layer (Carnol and Bazgir, 2013;Moukoumi et al., 2006;Vesterdal et al., 2008). Slower organic matter decomposition rates in coniferous stands have been reported (Binkley, 1995;Rovira and Vallejo, 2002) which may also contribute to increase soil acidity. Gruba and Mulder (2015) showed that tree species in uence the cationic exchange capacity in the soil, with an organic cationic exchange capacity per mass of carbon greater for hardwood species compared to coniferous species.
As a result of these different processes, tree species in uence soil acidi cation and chemical fertility over the long-term. Soil acidi cation processes have been shown to be enhanced under Norway spruce stands compared to other species (oak., beech, birch, Douglas r, pine,) (Augusto et al., 2002(Augusto et al., , 2000(Augusto et al., , 1998Bergkvist and Folkenson, 1995). Several studies demonstrated that exchangeable pools of Ca, Mg and K are generally lower (Cremer and Prietzel, 2017;Ranger, 1995) and solution concentrations in Al and H + are generally higher (Brown and Iles, 1991;Legout et al., 2016) under coniferous stands compared to broadleaved stands. Despite numerous studies, our understanding of the impact of different tree species on soil chemical fertility is yet incomplete because very few studies have been able to report the long term effects of different tree species on the soil chemical fertility. Over short time periods (year to decade), the entire extent of tree species effects may not be resolvable because changes in soil fertility may be small due to slow soil processes.
Yet, understanding the effects of tree species on the long-term fertility of forest soils is of utmost importance to ensure its sustainability because (i) forests generally grow on acidic and nutrient poor soils, (ii) which in many cases have been severely impacted by the elevated acidic atmospheric inputs during the 20th century, (iii) although atmospheric deposition of sulphur has decreased since the 1980s (Boxman et al., 2008;Engardt et al., 2017;Prechtel et al., 2001), nitrogen inputs remain in many cases elevated (Boxman et al., 2008;Vuorenmaa et al., 2017) and (iv) forest endure today increased pressure due to the intensi cation of silvicultural practices and harvesting (Achat et Mareschal et al., (2010) showed that tree species had an impact on the chemical properties of the topsoil 25 years after plantation and variations of cationic exchange capacity and soil pH between the different stands were partly controlled by the carbon content of the soil. Thereafter, numerous uxes and processes have been shown to differ between the different tree species plots. Tree species strongly in uence soil carbon stocks and mineralization (Trum et al., 2011). The rate of organic matter decomposition in the different stands was Norway spruce > Native forest > Beech > Douglas r and larger amounts of dissolved organic carbon (organic acids) were released into the solution leaching the humus layer in the Norway spruce stand (Moukoumi et al., 2006). Nitri cation was shown to also be a very important factor differentiating trees species: highest under Douglas r, Laricio pine, beech and oak The goal of this study is to investigate the consequences of these different processes and uxes on the chemical fertility of the soil 45 years after the plantation of the monospeci c stands. The detailed objectives of the present study are (i) quantify the change of soil chemical fertility and the effect of tree species over the 45 years after plantation and, (ii) try to integrate these results in a more global approach to evaluate tree species effect based on several criterions (soil fertility, water quality, wood production…). 2007), we hypothesize that chemical fertility degradation occur as follow: (i) elevated in the Douglas r and Laricio pine, due mainly to acidolysis related to high nitri cation rate and nutrient cation leaching, (ii) intermediate in the Nordman r, Norway spruce and oak due to acidolysis and chelation (in relation to the release of organic acids from organic matter transformation) and (iii) less intense in the beech plot.

Experimental site
The experimental site of Breuil-Chenue is located in the Breuil-Chenue state forest in the Morvan Regional Natural Park, Burgundy, France (latitude 4°18'10', longitude 4°4'44') at 640 m elevation. The annual rainfall is on average 1180 mm and the annual temperature 9°C (over the period 2006-2012). In 1975, the native forest (coppice with standards (CwS) composed of beech and oak) located on a homogeneous soil type was clear-cut. The organic layer and harvest residues were removed with a bulldozer.
The soil is developed on the "La Pierre qui Vire" leucogranite the composition of which is poor in Mg, K and particularly Ca.  (Ranger et al., 2004). The soil is acidic and nutrient poor with a saturation rate below 10% in the A horizon in the native forest plot . The humus type in all plots is a mesomull (Brêthes et al., 1995) with a layer L and F for all species except in the native forest plot where the humus type is a moder (Moukoumi et al., 2006). The silvicultural management of the different plots followed the regional guidelines, forest planning and management practices de ned by the French National Forest O ce (ONF). No thinnings occurred during the rst 24 years. From 2000 onwards, thinnings occurred approximately every 7 years for the coniferous plots and every 9 or 10 years for hardwood plots. All above ground biomass of thinned trees was exported. The thinnings were carried out manually (i.e. no mechanization of felling, logging or skidding) as follows (Legout et al., 2016): November 2000: 1st thinning of the Norway spruce (30% removed) and Douglas r (43% removed) stands.
An additional thinning for the plots of Norway spruce (15% removed), Nordmann r (10 % removed) and Douglas r (15% removed) was carried out in the fall of 2016. In 2001, the density of ne earth was measured for each stand and horizon. The texture was measured for each stand and each depth of four pro les. The pH water and pH KCl of soil samples were measured using a Mettler pH meter TSDL25 after 12 hours of contact with respectively distilled water and a 1 mol.L -1 solution of KCl (soil/solution ratio: 1/5). The exchangeable pools of Mg 2 +, Ca 2+ , Na + , Fe 3+ , Al 3+, and Mn 2+ were determined from two sequential soil extractions using a 1 mol.L -1 KCl reagent. The exchangeable pools of K + were determined from two sequential soil extractions using a 1 mol.L -1 NH 4 Cl reagent (Espiau and Peyronel, 1976). Extracted solutions were dosed by ICP-AES (JY180 ULTRACE, Horiba Jobin Yvon, Longjumeau, France). H + ions were extracted with a solution of KCl 1 mol.L -1 and titrated with an automatic titrimeter (Mettler TS2DL25) (Rouiller et al., 1980). Concentrations of C and nitrogen (N) were determined by elemental analysis from a test sample of 0.5 g and 1 g soil, respectively (Anne method (1945) and Kjeldahl method). Poorly crystalized and amorphous aluminium (hydr)oxides were extracted by a Tamura extraction (Tamura, 1958) (0.5 g of soil in 40 mL of sodium tricitrate reagent at 1M, pH=7.3) and analyzed by ICP-AES (Jobin-Yvon Instruments, Longjumeau, France, model JY 180) (Mareschal, 2008).

Soil and organic layer sampling and analysis
In 2019, the chemical properties of soil samples were obtained by analyses at the INRAE-Arras laboratory: pH water (soil water pH -1:5 soil to water volume ratio) and pH KCl (soil KCl pH -1:5 soil to 0.1 mol L -1 KCl reagent volume ratio) (NF ISO 10390), C and N content, cationic exchange capacity and exchangeable pools (Mg, Ca, K, Na, Al, H, and Mn) extracted with a cobaltihexamine reagent (5 g of soil in a 100 mL of Co(NH 3 ) 6 3+ solution at 1 molc.L -1 ) (NF X 31-130 We estimated that the excess dry mass measured in the organic layer in 2001 was caused by a 0-5cm organo-mineral soil layer inclusion of less than one millimetre of thickness. No correction on soil stocks was applied because i) this soil thickness was very small compared to the sampled 0-5cm layer and ii) the estimated Mg, Ca and K pools in a millimetre thick layer were much smaller than the Ca, Mg and K pool correction in the organic layer.

Quanti cation limits and date outliers
We chose to replace values in the dataset that were below the analytical quanti cation limit by the quanti cation limit value. Changes in quanti cation limits can result in a considerable bias . In order to avoid such biases, i) quanti cation limit values were set to the highest value between the 2001 and 2019 campaign and ii) we replaced the values below the analytical quanti cation limit by the quanti cation limit values. These changes affected 9.4%, 0.2%, 15.4%, 55.6% and 1.4% of the exchangeable Fe, Mg, Ca, Na and H values in the initial dataset.
Potential outliers were identi ed based on the distributions of each measured variable. Outliers were identi ed if they were outside the boundaries A (Eq. 2) and B (Eq. 3) de ned as: where A x,y and B x,y are the criteria below and above which concentrations of element x are considered abnormally low or high in soil y; (25%) is the 25% quantile of element x in soil y ; (75%) is the 75% quantile of element x in soil y ; is the interquantile of element x in soil y.
No organic layer data and no 2019 mineral soil data were removed.

Pool size calculation methodology
As described previously, ne earth bulk density was measured in 1974 and 2001 but not in 2019.
An average of ne earth density measured in 1974 was calculated for each sampled soil layer.
A statistical test (ANOVA) on bulk density was carried out and no signi cant difference between the plots was observed between in 2001 (Tukey test; pvalue > 0.05). We therefore assumed that ne earth bulk

Statistical tests
In order to test differences between tree species stands for each depth and for both dates (2001 and 2019), an ANOVA and a Tukey test were performed with a 5% level of signi cance. The differences between both sampling dates for each stand were tested with a Student test when the data followed a normal distribution (Shapiro test) with a signi cance of p-value<0.05 or 0.1 or with a Wilcoxon test when the data failed the normal distribution test. Though the design of this experiment didn't include replicated blocks (simple pseudoreplication according to Hurlbert (2004)), the comparison of the different tree species plots is nevertheless valuable to understand tree species effects on soil chemical fertility (Davies and Gray, 2015). Moreover, a detailed study of the Breuil site showed that the soil under these different tree species is comparable (Ranger et al., 2004). What is more, each individual tree species plot of this common garden experiment covers a large surface (1000 m 2 ) given them a strong spatial representativeness in contrast to a "classic block experiment" where the plots may be much smaller.
Correlation between different data series were tested with Pearson (data followed a normal distribution) or Kendall (data not followed a normal distribution) tests with a signi cance of p-value<0.05.

Multi-criteria approach
Be interested in only chemical parameters is not enough to judge the tree species effect. A multi-criteria approach was established to better compare the tree species at the ecosystem scale by including soil chemical fertility criteria as well as other ecosystem compartments and functions. For this approach, the following criteria were considered: For comparison, each criteria was translated into a score ranging from 1 to 7, 7 corresponding to the "best observed condition" for each criteria (Supplementary material 4).

Pool size change in the mineral soil
Over the 1974-2001 period, the stocks of exchangeable Mg in the soil decreased strongly in all the stands: an average net loss of 148 kg.ha −1 and a maximal loss of 156 kg.ha −1 in the Nordmann r stand (Fig. 2).
The strongest decrease in exchangeable Ca and K pools (respectively -63 kg.ha −1 and -45 kg.ha −1 ) was also observed for the Nordmann r stand. For the other stands, soil exchangeable Ca pools only slightly decreased whereas exchangeable K pools increased in the Norway spruce, Douglas r and oak stands by respectively +47, +15 and +124 kg.ha −1 .
Between The change in nutrient pool sizes in the organic layer is partially explained by the decrease in organic layer dry weight observed in all stands except for the Nordmann r and CwS stands in which it increased.
The decrease in organic layer dry weight in the Douglas r stand was smaller than in the other stands and was not statistically signi cant (Supplementary material 5). Differences between tree species were larger in 2019 compared to 2001 and more pronounced in the organic layer compared to the mineral soil.

Soil chemical property change over time
Even though few differences (between tree species and between sampling dates) were signi cant at the soil layer scale (Fig. 3, Fig. 4, Fig. 5 and Fig. 6), similar behaviors were observed. The oak and the beech showed similar behaviors: the pH KCl in these two stands increased over time and over the entire soil pro le while the exchangeable acidity (EA), the sum (S) of nutrient cations (Ca+Mg+K) and the CEC decreased (Fig. 3, Fig. 4, Fig. 5 and Fig. 6). The Nordmann r, Norway spruce and CwS stands also showed similar behaviors: the CEC, EA and S increased in the topsoil layers and decreased or remained stable in the deeper soil layers. The behavior of the Douglas r and Laricio pine stands was intermediary compared to the Nordmann r/Norway spruce/CwS and oak/beech. The EA and CEC in the Douglas r and Laricio pine stands increased in the topsoil layers and soil pH KCl decreased but less remarkably than in the Nordmann r, Norway spruce and CwS stands (Fig. 3). The general behavior of the Douglas r was closer to that of the Nordmann r, Norway spruce and CwS stands whereas the behavior of the Laricio pine was closer to that of the oak and beech stands (the Laricio pine stand differed from the oak and beech stands only in the 0-5cm soil layer).
Soil carbon content increased signi cantly in the 0-10 cm for all tree species except for the oak and beech stands for which the increase was not signi cant. The increase in carbon occurs mainly in the 0-5 cm horizons and weaker over 5-10 cm (Fig. 7).

Correlations between different soil chemical properties
In the soil layers down to 15cm, the variations of S and CEC between 2001 and 2019 were positively related to the variations of soil carbon content (Fig. 8E).
The relation between the variations of S (ΔS) and the variations of soil pH KCl (ΔpH KCl ) (p-value <0.05) ( Fig. 8D) shows that, in the deeper soil layers, when pH KCl increases S slightly increases but that S is  (Fig. 8C, Fig. 9D), and negatively correlated to the variations in Tamura-extractible Al content for the topsoil layers but not for the deeper soil layers (Fig. 8B).
Observing the exchangeable cations individually (Fig. 9) showed that as the variations in exchangeable K were strongly correlated to the variations of the CEC (correlation coe cient of 0.75, p-value <0.05) whereas exchangeable Mg and Ca remained relatively stable despite the CEC decrease (except for the exchangeable Ca in the beech and oak stands). The oak and beech stands differed from the general trend in the topsoil layers (0-15cm) for both exchangeable Ca and Al.

Multi-criteria approach
For most criteria, the CwS plot was among the best graded plots: fungal biodiversity, chemical quality of draining water and temporal change in plant-available nutrient cations (Ca, Mg and K). This suggests that a certain balance between the different ecosystem functions has taken place in this stand. However, the CwS plot had poor grades for the criteria assessing the change over time of soil CEC, pH KCl and exchangeable acidity (Fig. 10). The oak and beech plots had the best grades for the criteria assessing the change over time of soil pH and exchangeable acidity in the 0-70cm soil pro le. The coniferous stands had the highest grades for the criteria assessing the change over time of soil CEC. The Douglas r and to a lesser extent the Norway spruce and Laricio pine stands displayed a much higher biomass production compared to the beech and oak stands. However, the Douglas r and Laricio pine stands had the lowest grades for the criteria assessing the chemical quality of draining waters and the fungal biodiversity ( These results illustrated the necessity to study and compare different soil indicators to correctly assess the evolution of soil chemical fertility and also showed that tree species effect must be taken into account. Even if compensation may occur at the scale of the soil pro le (0-70cm), tree species effect is more pronounced in the topsoil layers, as pointed out by other studies (Augusto et al., 2002).

Soil cationic exchange capacity change
The change in CEC likely played a major role in the temporal changes observed in the different tree species plots. Several causes may have led to these CEC changes in the different stand and soil layers. These changes cannot be explained by a change in the contribution of CEC variable charges (activation or deactivation of exchange sites) because the variations in CEC were negatively correlated to the variations in soil pH (Fig. 8C). The dynamics of the CEC may be, at least partially, explained by the dynamics of the soil carbon pool (Fig. 8E). Indeed, a large proportion of the total CEC originates from organic cationic exchange sites (Mareschal et al., 2010;van der Heijden et al., 2014), and, for the coniferous (Norway spruce, Nordmann r, Douglas r) and CwS stands, the increase in soil CEC was related to the increase in carbon content in the 0-15 cm soil layers (Fig. 8E). The dynamics of carbon cannot, however, account alone for the observed CEC changes because (i) the soil carbon content increased in the topsoil but not or very little in depth (no signi cant differences) ( Fig. 7 and Fig. 8E), (ii) for the beech and oak stands, the carbon content increased in the topsoil but the CEC decreased (Fig. 6, The behavior of exchangeable Ca and Al in the beech and oak stands differed slightly form the other stands: the decrease in soil CEC in the topsoil layers seemed to impact more strongly exchangeable Ca and less strongly exchangeable Al pools (Fig. 9A). Despite the soil pH increase, Al remained very competitive with other basic cations under beech and oak in the topsoil layers. Since aluminum has a strong a nity for organic matter and remained abundant in these two stands, Al may have substituted Ca adsorbed on organic exchange sites, but the origin of this pattern which seems speci c to the hardwood stands remains unclear.

Tree species effect on soil chemical fertility
While the tree species effect on soil chemical fertility and its change over time varied between the different tree species, similar behaviours were identi ed which led us to de ne three groups: i) Norway spruce and Nordmann r, ii) Douglas r and Laricio pine, and iii) Oak and Beech (Fig. 12). Several studies have focused on the effect of tree species on pedogenetic processes at the Breuil site (Legout et al., 2016; Mareschal, 2008….) and we discuss hereafter the consistency between these processes and our observations.

Norway spruce, Nordmann r
Organic acids are released during the decomposition of organic matter contributes to the acidi cation of soils (Dijkstra et al., 2001). In the Nordmann r stands, crypto podzolisation (i.e. acidolysis and chelation) seem predominant among the pedogenetic processes (Legout et al., 2016). Indeed, nitrate leaching is very limited in these stands and the vertical transfer of Al in the soil pro le is mainly ensured by dissolved organic compounds, as suggested by the predominance of organically chelated Al species in the soil solutions (Legout et al., 2016;Mareschal, 2008). This is also the case under Norway spruce, but nitrate leaching is far from negligible (

Laricio pine and Douglas r
In these stands, the excess of nitrate production (nitri cation) over consumption is the cause of the higher acidity and the higher Al concentrations with a predominant Al 3+ species in the soil solution as well as the higher leaching uxes of Mg, Ca and K compared to the other stands (Legout et al., 2016;Mareschal, 2008). In these stands, acidolysis is the main pedogenetic process (Legout et al., 2016). The C:N ratio decreased in the entire soil pro le of the Douglas r stand between 2001 and 2019 which concurs with elevated nitri cation and elevated nitrate leaching. However, our initial hypothesis that nutrient depletion is more intense under these two stands is only partly validated. The depletion of exchangeable K pools was not greater than in the other stands. Soil pH decreased in the topsoil but not as strongly as in the Norway spruce and Nordmann r stands (Fig. 12). What is more, the exchangeable exchangeable acidity and exchangeable nutrient cation pools. This discrepancy between the solid and liquid phase may be caused by (i) higher mineral weathering in these stands (in relation to the acidity produced by nitri cation), (ii) higher inputs of nutrient cations by atmospheric deposition and/or (iii) the CEC increase. In the Douglas r stand, Legout et al., (2016) showed that atmospheric deposition of Ca, Mg and K were higher compared to the other stands. In the Laricio pine stand, atmospheric deposition levels were similar to the other stands but it is likely that the Ca, Mg and K losses in the leaching ux were compensated by the vertical transfer of Ca, Mg and K from the organic layer to the mineral soil.

Oak and beech
The two broadleaved stands show signs of recovery from past acidi cation over the 2001-2019 period.
The soil pH increased strongly (pvalue <0.05) in the 0-10 cm and over the entire soil pro le but the CEC decreased over the entire soil pro le as well as the nutrient cation pools.This signi cant behavior stands out from other coniferous species (Fig. 12).The decrease in soil CEC is most likely due to both the precipitation of Al hydroxides in the phyllosilicate interlayer space (pH increase) and the better mineralization of the organic matter in these stands (the carbon content increase in the topsoil was the weakest for these two stands). For the same site, Trum et al., (2011) showed that organic matter decomposition was the fastest in the oak stand. The results support our initial hypothesis for the beech stand but the high demineralization in the oak stand hypothesis was not validated. According to Legout et al., (2016), the oak stand was the third stand with the highest excess of nitrate production but, contrarily to the Laricio pine and Douglas r, the excess of nitrate production in the topsoil is compensated over the entire soil pro le by nitrate consumption. This may contribute to explain why soil acidi cation in this stand was not intense despite the nitri cation dynamics.
According to Mareschal (2008), the coniferous species at the Breuil-Chenue experimental site cause a higher acidi cation and a higher depletion of the exchangeable pools of nutrient cations in the following order: Norway spruce ≥ Laricio pine = Douglas r > oak = beech. In a general way, our study agrees with this classi cation but the effect of acidolysis and chelation processes in Nordmann r stand seems to be more damaging for the solid phase than previously, with a substantial decrease of pH and an increase of exchange acidity in the topsoil layers. Our study also con rm that tree species directly in uence pedogenetic processes over short to mid-term time periods (up to 43 years) as suggested by previous studies .

Ecosystem scale multi-criteria approach
Used individually, static indicators such as soil pH, CEC, pools of exchangeable cations may be misleading and their change over time does not necessarily re ect the processes occurring in the soil or the ecosystem. For instance, an increase in soil CEC over time is generally considered to be a sign of recovery from past soil acidi cation: an increase in soil pH often increases the number of active cationic exchange reaction sites (variable charge component of the CEC) (Camberato, 2001;Helling et al., 1964). In our study, we showed that the CEC increase under certain tree species (Douglas r, Norway spruce, Nordmann r, etc.) is associated with a decrease in soil pH and thus indicates a degradation of the soil for these species rather than a recovery. Yet, this CEC increase also resulted in an increase in exchangeable nutrient cations available for tree uptake and growth. This illustrates both (i) the necessity to study and compare different static soil indicators in order to assess the intensity of on-going soil processes and their consequences in terms of chemical fertility and (ii) the importance of assessing the consequences of these processes and changes over time on the other components and functions of the ecosystem.
The multi-criteria approach that is proposed here aims at better assessing the functioning of the ecosystem and how tree species in uence its functioning as a whole. Such multi-criteria approaches (which may encompass broader criteria than those selected in this study) coupled with the knowledge of the processes and functioning of ecosystems enable to help rational forest management (Schwaiger et al., 2019).
For this purpose, tree species effects on soil chemical fertility were compared to other ecosystem functions reported in previous studies carried out at the Breuil-Chenue experimental site such as wood/biomass production, chemical quality of the draining water and soil fungal biodiversity (Fig. 10).
Depending on the criteria used the interpretation of the tree species effect on the functioning of the forest ecosystem can be very different. Our results underline the importance of taking into account the capacity of the soil to sustain over the long term the different ecosystem functions. This is best illustrated by the Douglas r case (Fig. 10). Although the biomass production for this species is remarkable, other ecosystem functions are severely degraded (chemical quality of draining water and fungal diversity) and the 'cost' of this elevated production in terms of soil chemical fertility is very high. The very strong ongoing acidi cation may currently enable to maintain su cient amounts of plant-available nutrients in the soil for tree growth but it is highly uncertain whether this tree growth rate may be sustained over the long term (more than a forest revolution). What is more, the strong modi cations induced by this species in less than 50 years questions the capacity of the soil to supply available nutrients over the long term thus ensuring all ecosystem function necessary for the growth of future stands. Our results also raise concerns for the other coniferous stands although their tree species effect is often not as strong as the Douglas r.

Conclusion
The increase in soil pH and in exchangeable Mg and Ca in most stands and soil layers between 2001 and 2019 may suggest that the soil at the Breuil-Chenue experimental site is recovering from past acidi cation. However, the decrease of pH and the increase of exchange acidity and cationic exchange capacity (CEC) in the topsoil layers under coniferous stands and the overall decrease of exchangeable K pools in most stands highlighted that soil acidi cation and/or crypto podzolisation are still on-going at this site. Tree species in uence the pedogenetic processes occurring at the Breuil Chenue site, but our initial hypothesis that these processes would be accompanied by the depletion of exchangeable nutrient cation pools was not validated since Mg and Ca pools increased between 2001 and 2019. The changes in soil chemical properties over time were mainly explained by the changes in soil CEC which was probably due to the increase in carbon content in the topsoil and/or to the dynamics of precipitation/dissolution of Al (hydr)oxides in the interfoliar space of phyllosilicates. The increase in soil CEC over time may have partly compensated for the loss of nutrient cations, especially in the coniferous stands. Our results illustrate how important the choice of the adequate indicators is to characterize and quantify the changes in soil fertility over time because different indicators may lead to opposed conclusions.
Our study demonstrate that tree species strongly in uence how nutrient pools in the soil change over time. Our hypotheses which were that the chemical degradation of fertility would be (i) high in Douglas r and laricio pine, due mainly to acidolysis linked to the high rate of nitri cation and leaching of nutrient cations, (ii) intermediate in the Nordman r, Norway spruce and oak due to acidolysis and chelation (in connection with the release of organic acids from the transformation of organic matter) and (iii) less intense in the plot beech are not completely veri ed.
Based on their behavior, we were able to distinguish three groups of species: i) Nordmann r and Norway spruce with acidolysis and chelation processes (related to the release of organic acids from organic matter transformation), resulting in the most intense changes in topsoil properties (e.g. decrease in pH, increase in exchange acidity), ii) Douglas r and Laricio pine, where the strong acidi cation occurring (related to high nitri cation rate and nutrient cation leaching) is probably compensated by a greater weathering and/or atmospheric depositions uxes, and iii) Oak and beech with a lower soil acidi cation compared to all the other stands. However, even if signs of soil acidi cation recovery occurred in these deciduous stands between 2001 and 2019, the decrease in soil CEC did not allow to improve signi cantly the exchangeable nutrient pools over the soil pro le.
Finally, our study highlights the di culties of comparing multiple indicators and shows just how necessary multi-criteria approaches are to properly understand how the ecosystem functions. Tree species selection by the forest manager should be based on such multi-criteria approaches to properly account for the different ecosystem functions and services, opportunities and threats.      Comparison of multiple criteria each assessing different functions of the ecosystem for the different tree species. Each criteria corresponds to a grade between 1 (center of the diagram) and 7 (outer ring) and corresponds to a relative classi cation of the seven tree species, 7 being the best grade.

Figure 11
Relationship between CEC and Al Tamura in 2001 (A) and 2019 (B) for the different tree species and sampled soil layers.