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 fir, 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 fluxes at different ecosystem levels as reported by the numerous reviews and studies (Augusto et al., 2015, 2002; De Schrijver et al., 2007). Tree species may influence (i) the geochemical cycling (atmospheric deposition, mineral weathering, leaching) and (ii) the biological cycling (uptake, immobilization in biomass, litterfall, mineralization) as defined 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 (Rothe et al., 2002; 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 influence nutrient leaching (Augusto et al., 2002; Legout et al., 2016). Tree species exert a control over N mineralization and nitrification processes (Andrianarisoa et al., 2010; Zeller et al., 2019) and soil acidity may increase when nitrate production exceeds nitrate consumption (Legout et al., 2016).
(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 influence 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 influence 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 influence soil acidification and chemical fertility over the long-term. Soil acidification processes have been shown to be enhanced under Norway spruce stands compared to other species (oak., beech, birch, Douglas fir, pine,) (Augusto et al., 2002, 2000, 1998; Bergkvist 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 intensification of silvicultural practices and harvesting (Achat et al., 2015; Bouvet et al., 2019; De Turckheim, 1990; Paré and Thiffault, 2016). The current context of global and climate change may encourage tree species conversions in order to adapt forest stands to the changing climate and/or increase forest productivity (Bolte et al., 2009; Thurm et al., 2018).
The common garden experiment of the Breuil-Chenue site (Burgundy, France), installed in 1976 and monitored since 2001 (see details in material and methods section), is a fairly unique experiment that compares 6 monospecific plots: oak (Quercus sessiliflora Smith.), beech (Fagus sylvatica L.), Norway spruce (Picea abies Karst.), Douglas fir (Pseudotsuga menziesii Mirb. Franco.), Nordmann fir (Abies nordmanniana Spach.) and Laricio pine (Pinus nigra Arn. ssp laricio Poiret var corsicana) Based upon soil sampled in 2001, 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 fluxes and processes have been shown to differ between the different tree species plots. Tree species strongly influence 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 fir 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). Nitrification was shown to also be a very important factor differentiating trees species: highest under Douglas fir, Laricio pine, beech and oak (Andrianarisoa et al., 2010; Legout et al., 2016; Moukoumi et al., 2006; Trum et al., 2011; Zeller et al., 2007). For Douglas fir and Laricio pine, nitrate concentrations dominated the total anion charge in soil solutions and led to greater leaching fluxes of Al, Ca, Mg and K (Legout et al., 2016).
The goal of this study is to investigate the consequences of these different processes and fluxes on the chemical fertility of the soil 45 years after the plantation of the monospecific 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…). For this, we studied the humus layer, mineral soil layers down to 70cm depth from samples collected in 1974, 2001 and 2019 in the 6 monospecific stands of the Breuil-Chenue experimental site. Based on the previously cited studies (Legout et al., 2016; Mareschal et al., 2010; Moukoumi et al., 2006; Zeller et al., 2007), we hypothesize that chemical fertility degradation occur as follow: (i) elevated in the Douglas fir and Laricio pine, due mainly to acidolysis related to high nitrification rate and nutrient cation leaching, (ii) intermediate in the Nordman fir, 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.