Shrub encroachment modifies soil properties through plant resource economic traits

doi:10.21203/rs.3.rs-4938772/v1

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Shrub encroachment modifies soil properties through plant resource economic traits

https://doi.org/10.21203/rs.3.rs-4938772/v1

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Background and Aims

Shrub encroachment alters ecosystem functions. Yet, changes in plant community traits and soil properties along succession from grassland to shrubland in European mountains are poorly understood.

Methods

We investigated the cascading effects of shrubs on community weighted means (CWM) of plant traits and onto soil properties along a gradient of encroachment in subalpine grasslands in two sites in the Alps using a trait-based approach. We hypothesized that increasing shrub density shifts plant communities towards more conservative traits, which non-linearly increases carbon sequestration and impacts nutrient cycling. We tested our hypothesized model of cascading effects using structural equation models. This model accounted for biomass allocation to leaves and stems in CWM calculations.

Results

Consistent with expectations, CWM dry matter content (DMC) increased and CWM of nitrogen (N) and phosphorus (P) decreased with increasing shrub biomass. Increasing CWM DMC resulted in increasing soil C:N ratio and soil organic matter (SOM) concentration, and decreasing pH. Decreasing CWM P was coupled with decreasing soil available P, but changes in CWM N had no effect on available N. There was however no indication of nonlinear changes.

Conclusion

This study demonstrates that with shrub encroachment plant communities gradually become more conservative with tougher and nutrient-poor tissues, which leads to soil acidification, SOM accumulation and lower P availability. We also demonstrate that DMC, an easy measurable trait, is a sufficient indicator for plant tissue quality in shrub encroached subalpine grasslands and could be used in future trait-based models, allowing projections under climate change scenarios.

Shrub encroachment

Plant functional traits

Plant economics spectrum

Plant-soil interactions

Structural equation models

Shrub encroachment is a global process that profoundly impacts ecosystem functioning (Myers-Smith et al. 2011; Naito & Cairns 2011; Stevens et al. 2017; Van Auken 2009; Wookey et al. 2009). Increased shrub cover has been linked to multiple drivers, including climate warming in arctic and subarctic tundra (Cannone and Malfasi, 2024; Elmendorf et al. 2012; Myers-Smith and Hik 2018), increased rainfall in savanna (García Criado et al. 2020; Venter et al. 2018), or recovery from disturbances like fires or anthropogenic pressures (D’Odorico et al. 2012; Eldridge et al. 2011). In European mountains, increasing temperature and land use change (i.e. grazing cessation) since the 1950s have facilitated the colonization by shrub species in grass-dominated ecosystems (Ameztegui et al. 2016; Anthelme et al. 2007), including in the Pyrenees (Gartzia et al. 2014) and the Alps (Cannone et al. 2007). Subalpine grasslands are key providers of multiple ecosystem services such as fodder production, and regulation of climate, water quality or soil fertility (Lavorel et al. 2017), even far beyond mountain boundaries (Schirpke et al. 2019). Shrub encroachment is expected to alter the biogeochemical functioning of these grasslands through cascading effects from plant communities to soil properties, and thereby to impact the provision of important ecosystem services.

Recent studies have increased our understanding of the functional impacts of shrub encroachment. Increased shrub density modifies plant community structure (Illa et al. 2017), plant-soil stoichiometry (Urbina et al. 2020), soil properties like soil bulk density, soil compressive strength and soil shear strength (Li et al. 2016; Liu et al. 2023) and water balance (Liu et al. 2022). These changes in plant and soil properties have also been associated with changed soil microbial communities (Broadbent et al. 2022; Grau et al. 2019; Li et al. 2017; Urbina et al. 2020). However, the complexity, site-specificity and the multivariate nature of these multiple processes makes it difficult to generate comprehensive understanding. For example, while some studies have showed that shrub encroachment decreases nutrient availability (Urbina et al. 2020), others have found increased soil fertility with leguminous shrub presence (Yang et al. 2024b) or in semi-arid savanna (Ward et al. 2018). Eldridge et al. (2011) showed that in arid ecosystems shrub height and shape could be linked to microclimate and soil fertility, also consistent with expectations of shrub effects on microclimate in tundra ecosystems (Myers-Smith et al. 2019). Moreover, few studies have investigated the underlying mechanisms of aboveground-belowground linkages in response to shrub expansion (Broadbent et al. 2022; Grau et al. 2019; Urbina et al. 2020). These studies have shown that shrub expansion results in stochiometric changes in aboveground plant tissues which are coupled with changes in soil nutrients, possibly linked to nutrient allocation (e.g. nutrient accumulation in shrub species due to higher biomass) and shifts in soil microbial communities (e.g. increase of ericoid mycorrhiza fungi which might lead to nitrogen immobilization).

Trait-based ecology has made it possible to mechanistically link vegetation changes to ecosystem services by quantifying links between community-level plant traits and ecosystem functions (Lavorel 2013). Shrub encroachment is considered a ‘Plant Functional Type transformation’ (Wookey et al. 2009), with a cascading set of mechanisms underpinning effects on nutrient cycling processes based on the response-effect trait framework (Lavorel and Garnier 2002). The plant economics framework links shifts from acquisitive plants, with traits such as high tissue nitrogen content and associated fast growth, to conservative plants with high tissue density and associated slow growth (Reich 2014). Studies of subalpine grasslands have confirmed these global patterns (Grigulis et al. 2013; Laliberté and Tylianakis 2012, Moretti et al. 2013), however its relevance to shrub ecosystems has received little attention (de Bello et al. 2010). Specifically, while subalpine shrub encroachment has established effects on soil carbon and nutrient pools (Seeber et al. 2022; Tasser et al. 2005; Urbina et al. 2020) these effects have not been linked to community-level shifts in plant traits. The relevance of the plant economics framework to cascading effects from shrub encroachment to nutrient cycling thus stands as a critical knowledge gap.

Instead, effects of shrubs on ecosystem functioning could be nonlinear reflecting a qualitative change in tissue quality, and hence decomposability in woody plants (Hattenschwiler and Vitousek 2000; Wookey et al. 2009). One of the major changes linked to shrub growth is increasing biomass, and consequently lignin amounts in ecosystems. Lignin is a structural and highly polymerised compound whose concentration, and abundance relative to highly decomposable nitrogen, has been shown to drive decomposition of leaf and stem litter (Cornwell et al. 2008; Yang et al., 2024a). According to the biomass ratio hypothesis (Grime 1998), shrub species traits could then be expected to have the greatest effect on ecosystem functions due to their higher biomass contribution. Furthermore, given the qualitative and quantitative differences in lignin between stems and leaves (Lourenço and Pereira 2018), effects of shrub encroachment on soil processes requires considering traits of entire plants, rather than just leaf traits as done previously (Myers-Smith et al. 2019; Zuo et al. 2018). Given that litter decomposition is influenced by the distribution of biomass across woody plant organs (Guo et al. 2022a) the consideration of biomass allocation along with traits of leaves and stems thus appears warranted in community-level trait calculations (Garnier et al. 2004).

This paper investigates the mechanisms through which shrub expansion modifies grassland functioning. The aim of our research was to test whether the plant economics spectrum linearly explains cascading effects of shrub encroachment on plant communities and soil properties, as opposed to a nonlinear qualitative shift reflecting a functional switch to prevalent woody tissues. For this, we analysed changes in soil parameters along a gradient of increasing shrub density. Indeed, most studies have analysed the functional effect of woody species based on their presence or absence, but to our knowledge, quantitative studies along gradients of shrub density are rare (Blaser et al. 2014; Pornaro et al. 2017; Zhou et al. 2017), or in some cases limited to the inclusion of one or two intermediate successional stages from grassland to shrubland (Crofts et al. 2018; Ding et al. 2019; Liu et al. 2023; Urbina et al. 2020).

We used a novel trait-based approach, incorporating plant economics traits scaled according to plant allometries. Our hypothesized model (Fig. 1) suggests that increasing shrub biomass in an ecosystem has direct effects on plant trait community weighted means (CWM) and indirect effects on soil properties. Encroached grasslands store higher amounts of aboveground phytomass compared to managed grasslands (Seeber et al. 2022), which results in higher aboveground stocks of carbon (C), nitrogen (N) and phosphorus (P) (Urbina et al. 2020). The conservative nature of shrub species, reflected in their plant traits (i.e. lower leaf nitrogen concentration (LNC) and higher dry matter content (DMC)) (Díaz et al. 2016), and their different biochemical composition, rich in phenolic compounds such as lignin, results in changes in plant-soil stoichiometry and nutrient cycling (Hattenschwiler and Vitousek 2000). Lignin-rich litter forms recalcitrant compounds, which slow down the decomposition of soil organic matter (SOM) (Kraus et al., 2003). These changes in decomposability can affect soil pH, soil nutrient availability and stoichiometry. While these mechanisms have been studied individually, we tested them as a complex network, using structural equation modelling (SEM), a hypothesis confirmatory approach (Shipley 2018), as shown in Fig. 1. Using plant trait and soil data collected along gradients of shrub encroachment at two subalpine grassland sites in the European Alps, we tested cascading effects of shrub encroachment on ecosystem functions by quantifying direct and indirect effects of increased shrub biomass on plant traits and soil parameters. We hypothesized that: 1) Changes in plant community traits and soil properties along the gradient are not linear; 2) Shrub encroachment results in a shift towards more conservative, nutrient- poor plant communities; 3) These community-level plant functional changes feed forward to soil properties characteristic of slow nutrient cycling ecosystems.

Study sites

We conducted our study in two locations in the European Alps: Lautaret (France) and Stubai Valley (Austria) where changes in management over the past decades have resulted in a mosaic of plant communities (Lavorel et al. 2017).

The Lautaret site is located in the Central French Alps on the southern slopes of the Guisane valley (45.03 °N, 6.24 °E), at an average altitude of 1900 meters above sea level (m.a.s.l.) on flysch sediment. The climate is sub-alpine with a pronounced continental influence and summer drought. Mean annual precipitation is 956 mm and mean monthly temperatures range between − 7.4°C in February and 19°C in July (1991–2020). Significant shrub encroachment has taken place due to the extensification of livestock production, with initial conversion from mowing to grazing (1970s) and current summer grazing by cattle (Lavorel et al. 2017). The site is divided in two grassland areas: one dominated by the grass species Bromus erectus and the other by Patzkea paniculata. B. erectus grassland is colonised by the evergreen shrub Juniperus communis and the deciduous Rosa spinosa. P. paniculata grassland is colonised by J. communis or by the deciduous Vaccinium myrtillus and Vaccinium uliginosum (Fig. 2).

The Stubai site is located in Austria on the southern slopes of the Stubai valley (47.11 °N, 11.30 °E) at approximately 2000 m.a.s.l. on a siliceous substrate. The climate is humid continental. Mean annual precipitation is 1097 mm and mean monthly temperatures range between − 9°C in January and 12°C in August (1991–2020). The grassland abandoned 25 years ago (Lavorel et al. 2017) is divided into two areas: one dominated by the grass species Nardus stricta, and other dominated by Festuca rubra and Sesleria albicans. N. stricta grassland is colonised by shrub deciduous species, V. myrtillus and V. uliginosum. F. rubra - S. albicans grassland is colonised by evergreen species, Rhododendron ferrugineum, Calluna vulgaris and Vaccinium vitis-idaea. In both grasslands shrub have a diffuse distribution within the herbaceous matrix (Fig. 2).

Sampling design

At each site, we selected homogenous and relatively flat (slope < 10 °) 10 x 10 m plots with increasing shrub cover. We collected data on plant traits and soil parameters from a total of 54 plots, each with a size of 10 m x 10 m.

The Lautaret site comprised 26 plots. Within each grassland community we sampled a gradient of increasing shrub cover on plots encroached by J. communis (from 0–76% − 7 plots within each grassland type), by R. spinosa (from 0–45% − 7 plots) and by Vaccinium species (from 0–90% − 5 plots). The Stubai site comprised a total of 28 plots. Within each grassland community we sampled along a gradient of increasing shrub cover on plots encroached by Vaccinium species (from 9–76% − 15 plots), by R. ferrugineum (from 8–76%, 9 plots) and C.vulgaris (from 1–11%, 4 plots). In total, the 54 plots covered a continuous gradient of increasing shrub cover from 0–90% (Fig S1).

All soil samples and biomass estimates were taken at peak biomass during summer 2021. At Lautaret, herbaceous plant traits were measured during summer 2021 for F. paniculata plots and during summer 2022 for B. erectus plots, considering them as functionally distinct plant communities (Quétier et al. 2007). In Stubai, herbaceous trait measurements were spread distributed across the two grasslands in 2021 (13 plots) and 2022 (15 plots). Interannual variation was not significant in the Stubai site (Table S1).

Biomass estimation

Estimates were made of shrub species and herbaceous standing biomass in each of the 10 by 10 m experimental plots. We used allometric techniques to estimate biomass, which are often used as a more rapid and non-destructive method opposed to the more labour-intensive harvesting ones (Lavorel et al. 2008).

Shrub cover was estimated using the point method (Levy and Madden 1933). One hundred points were recorded every meter along ten transects laid out at one-meter intervals. At each point the presence or otherwise of shrub species or herbaceous vegetation was noted in a 10 cm diameter circle around the measurement point. The percentage cover of each component of the vegetation was expressed as the number of points at which it was present over the 100 points.

Table height (height of vegetation canopy closure) of each shrub species (if any) and/or total herbaceous vegetation was also measured at each of the measured 100 points in each plot. Plot mean canopy height was then calculated for each shrub species and the herbaceous vegetation component. These measurements then allowed us to estimate the volume occupied by each shrub species in each plot (total plot area in m² multiplied by the proportion area covered by each shrub, multiplied by the mean canopy height of that shrub in the plot). Standing biomass of leaves and stems of each shrub species was then estimated using allometric equations developed for each shrub species relating shrub volume or cover to shrub standing biomass of leaves and stems. For the herbaceous component, total standing biomass was estimated using allometric equations relating the mean plot herbaceous canopy height to biomass in tones/hectare (then converted to kg based on the plot area of 100 m²). Methods used to develop these equations and their fit and precision are presented in supplementary materials.

Plant functional traits

Shrub traits

For each shrub species we measured leaf and stem dry matter content (LDMC, StDMC), leaf and stem carbon concentration (LCC, StCC), leaf and stem N concentration (LNC, StNC), leaf and stem P concentration (LPC, StPC), and stem lignin concentration. Leaf and stem lignin concentration in herbaceous species was considered negligible in comparison to shrubs, and approximated to zero for further calculations. For each species, 10 samples of a single leaf and adjoining stem were taken from ten well developed individuals from the immediate area of the plots. C. vulgaris was taken as a combined sample due to unfeasibility of leaf-stem separation. Methods used for the measurement of DMC (at full hydration), C, N and P concentrations and lignin content followed standard trait measurement protocols (Cornelissen et al. 2003). C and N concentrations were measured using a standard elemental analyser. P concentrations were quantified after acid extraction using an inductively coupled plasma - optical emission spectrometer following Europen standard ISO 11885. For lignin analyses, 1 g of ground (1 mm) plant material was subjected to a dissolution in 100 ml acidic detergent, followed by a second attack by H₂SO₄ 72%. After washing until neutralisation, the dry organic residue (ADL-lignin) was determined as the difference between the dry residue (oven drying at 103°C) and the residue after mineralisation (oven at 550°C) (Soest 1963).

Herbaceous traits

Herbaceous traits were measured at plot level using a variation of the taxonomic-independent trait transect method (Lavorel et al. 2008). This method seeks to directly measure plant traits on a representative sample of herbaceous community biomass sampled directly in a plot, rather than by estimating species relative abundances and using this to weight species-specific trait values. Samples were taken at 50 cm intervals along the two 14.4 m diagonals of each plot (56 points per plot). At each point, a sample of one well developed mature leaf was taken for each well-grown herbaceous species within a 10 cm diameter cylinder. The sampled leaves from each point were transported to the laboratory in a cool box (ca. 4°C), pooled and measured using standard trait methods (Cornelissen et al. 2003) for LDMC (at full hydration), LCC, LNC and LPC concentrations. Leaves from each point were considered as an aggregate sample for LDMC, then averaged across the 56 points to calculate a plot value. Dry leaves were pooled to three aggregate samples per plot for analyses of C, N and P concentrations then averaged to calculate a plot value.

Calculation of community weighted plant traits

Estimates of the standing biomass of leaves and stems of each shrub species and of the herbaceous vegetation component in each plot were used to calculate the proportional representation of total plot community biomass represented by leaves and stems of each shrub species and by herbaceous vegetation (Fig. 3). CWM of each plant trait for each plot was then estimated by extending the biomass weighted mean calculation (Garnier et al. 2004) to integrate allometric distribution of plant material across leaves and stems. For this, trait values estimated for leaves and stems of each shrub species and mean plot-based values for herbaceous vegetation were weighted by their relative biomass representation in the plot. Such CWM values were calculated for each plot for DMC, C, N, P concentrations and lignin content as follows (equ. 1):

where %Biom_i is the relative biomass of species i to the total biomass in the community; %Leaf_i and %Stem_i are the relative leaf and stem abundances, respectively, of species i to the its total biomass; Leaftrait_i and Stemtrait_i is the trait value for the species i and n the total number of species.

Soil physical-chemical properties

Soil sampling and processing methods followed Grigulis et al. (2013). Nine soil cores of 5 cm diameter and 15 cm depth were taken from each experimental plot, pooled to a composite soil sample and taken back to the lab in cool boxes (ca. 4°C) for the measurement of soil physical-chemical properties. Sampling was stratified based on the proportion of shrub cover in plots (i.e. in plots with > 80% shrub cover all cores were taken under shrubs, in plots with 40–60% shrub cover 2–3 cores were taken under shrubs, and in plots with 0–20% shrub cover 0 to 1 cores were taken under shrubs, with the balance of cores being taken from under herbaceous vegetation). Composite soil samples were sieved at 2-mm and then analysed for pH (with CaCl₂), C and N concentration (colorimetric analysis after KCl extraction in Lautaret; elemental analyser in Stubai), P Olsen and SOM (loss on ignition).

Plant available N and P

Plant available N and P ions were measured using cation- and anion-exchange resin membranes that adsorb ammonium (NH₄⁺), and nitrate (NO₃⁻) and phosphate (PO₄ ^− 3) ions, respectively, from the soil solution following Hovenden et al. (2017). These sticks simulate plant roots and when left in the soil for four weeks provide a way of estimating the concentrations of ions to which a plant root would be exposed in situ and therefore provide an integrated view of inorganic ion availability across this time window.

Briefly, during the period of peak biomass production, ten charged (using a 0.5 M NaHCO₃ solution) 5cm by 1 cm anion resin strips (AXM-100S Anion Exchange Membrane Sheet, Membranes International Incorporated - https://membranesinternational.com/) and five charged (using a 0.5 M NaCl solution) 5cm by 1cm cation resin strips (CXM-200S Cation Exchange Membrane Sheet, Membranes International Incorporated) were placed vertically into the soil in each plot, with a small plastic tag attached to the resin allowing for the relocation of the resin after incubation. The incubated resins were removed from the soil after four weeks and rinsed clean with distilled water. Anion sticks from each plot for the measurement of PO₄³⁻ were extracted using a 0.5 M HCl solution; anion sticks for the measurement of NO₃⁻ were extracted using a 2.0 M KCl solution, and cation sticks for the measurement of NH₄⁺ were extracted using a 2.0 M KCl solution. Extractant solutions were then analysed for NH₄⁺, NO₃⁻ and PO₄ ^− 3 using colorimetric analysis. The ion concentrations measured from the five resins per plot destined for the measurement of each ion were averaged to provide mean plot values of available NH₄⁺, NO₃⁻ and PO₄ ^− 3 and standardized per time in the field and resin seize (units: µg L^− 1 week^− 1 cm^− 2).

Statistical analysis

We used principal component analysis (PCA) to visualize general patterns of variation. Highly correlated variables were removed prior to the SEM analysis. We added individual PCAs for plots encroached by ericaceous species to confirm the similarity of Vaccinium communities dominated by common species in both sites at a 95% confidence interval (Fig S2). Therefore, all Vaccinium deciduous dominated plots from Stubai and Lautaret site were grouped into the same shrub type ericaceous deciduous (“Ericaceous D”). We classified the plots into four categories: “Ericaceous E” (ericaceous evergreen dominated plots), “Ericaceous D” (ericaceous deciduous dominated plots), “J. communis” (Juniperus communis dominated plots), “Rosa sp.” (Rosa spinosa dominated plots) (Table 1). We used R packages FactoMineR (Le et al. 2008) and factoextra (Kassambara and Mundt 2020) for the PCA analysis.

Table 1

Categories for grouping shrub species: Ericaceous deciduous (Ericaceous D), Ericaceous Evergreen (Ericaceous E), *J. communis* and *Rosa sp*. Site: Stubai (S) or Lautaret (L).
Category	Dominant encroaching shrub species	Site	Grass dominant species
Ericaceous D	Vaccinium myrtillus, Vaccinium uliginosum	S	Festuca rubra and Sesleria albicans
Ericaceous D	Vaccinium myrtillus, Vaccinium uliginosum	L	Patzkea paniculata
Ericaceous E	Rhododendron ferrugineum Calluna vulgaris, Vaccinium vitis-idaea	S	Nardus stricta
J. communis	Juniperus communis	L	Patzkea paniculatea or Bromus erectus
Rosa sp.	Rosa spinosa	L	Bromus erectus

We used graphical representation to analyse the linearity of changes along shrub encroachment using ggplot2 package in R and simple linear models to test the effects of shrub encroachment on measured CWM traits, soil properties using the stats R package. All assumptions for normality, independence and homoscedasticity were checked. We performed a joint analysis for plots Stubai and Lautaret as exploratory analysis showed no site differences.

We used SEM to test our hypothesis of cascading effects of shrub encroachment on soil properties. SEM are increasingly used in ecology as they are a powerful tool to represent complex natural systems (Lefcheck 2016) and they can be used for testing the strength of the relationships between variables based on a priori information (Eldridge et al. 2017; Lavorel and Grigulis 2012). SEMs are usually represented using path diagrams, in which arrows indicate the causal relationship between two variables (Lefcheck 2016). We used piecewise SEM to test our hypothetical causal model, which allows for the specification of a wide variety of model types between model links and overcomes some of the shortcomings of traditional SEM (Shipley 2000).

A preliminary analysis pointed out an identity effect of J. communis in plant-soil relationships (Fig S3). Therefore, we included J. communis presence/absence as a random effect for links between community plant traits and soil properties by using mixed effect linear models (lme) in the SEM. We used simple linear models (lm) for links between shrub encroachment and CWM of plant traits. All assumptions for the linear and mixed effects models were checked. Variables were transformed when necessary to meet the models assumptions (i.e. log(PO₄ ^− 3)). We tested two hypothetical models: a first model including all significant and non-significant links (model 1 hereafter), and a second model with only significant links (model 2 hereafter). All SEM analyses were performed using R packages piecewiseSEM (Lefcheck 2016) and nlme (Pinheiro and Bater, 2023). All statistical and graphical analyses were conducted using the open-source R Statistical Software v. 4.2.2 (RStudio Team 2022).

General patterns

The PCA suggested that plots varied along two main independent dimensions (Fig. 4). The first axis (Dim 1) explained 45.1% of the variance, while the second axis (Dim 2) explained 19.1%. The variables that contributed more to Dim 1 were CWM Lignin:N (14% contribution), CWM DMC (13% contribution), CWM CN (11% contribution) (Fig S4). Higher values of these traits were associated with higher shrub biomass. Soil properties and nutrients had a lower contribution to Dim 1 compared to CWM traits. Amongst soil properties, SOM and soil C contributed most, with their higher values being associated with higher values of CWM CN and CWM DMC at higher shrub biomass.

The second axis (Dim 2) differentiated plots by nutrients in plant tissues and in the soil. Soil NO₃⁻ and CWM P were the two most contributing variables to Dim 2 (23% and 20% respectively), followed by pH (10%) (Fig S4). Ericaceous colonised plots had higher CWM P, lower pH and NO₃⁻ compared to J. communis colonised plots. These differences were more pronounced at higher shrub biomass.

Linearity of changes along the gradient of increasing shrub biomass

We found no evidence of nonlinear patterns for changes in plant community traits and soil parameters with increasing shrub biomass (Fig S3). Simple linear models of CWM traits with increasing shrub biomass were significant for all measured traits (Table 2). By contrast, we found fewer significant regressions for simple linear models of soil properties along the gradient (Table 2).

CWM C (R² = 0.87), CWM DMC (R² = 0.78) and CWM Lignin:N ratio (R² = 0.89) increased significantly with increasing shrub biomass (Table 2). CWM DMC increased by 60% from low to high density plots (Fig. 5a), reflecting increasing prevalence of dense shrub tissue and especially stems. Traits related to nutritional quality of plant communities such as CWM N and CWM P decreased significantly with increasing shrub biomass (Table 2) and, while also following linear trends, showed a higher variability at lower shrub biomass (Fig. 5b-c).

Plant-soil parameter relationships also suggested linear changes in soil parameters and nutrients as a response to changes in plant traits (Fig. 5d-i). pH decreased with increasing CWM DMC, while SOM and soil CN increased with increasing CWM DMC (Fig. 5d-f). Available soil P (PO₄ ^− 3) increased significantly with increasing CWM P (Fig. 5g), while available soil N nutrients (NO₃⁻ and NH₄⁺) showed no significant changes with increasing CWM N (Fig. 5h-i). Plant-soil relationships were influenced by shrub type (Fig. 5d-f), reflecting the presence of J. communis. We also identified this species effect in linear regressions of soil parameters with shrub biomass (Fig. S5B). The shrub identity effect was particularly evident for pH, with decreasing values from 7 to 5 with increasing CWM DMC for Ericaceous E, Ericaceous D and Rosa sp plots, in contrast to a constant value for J. communis plots (Fig. 5d).

Table 2. Effects of increasing shrub density on plant community traits and soil properties. All models are simple linear regressions with shrub biomass (Shrub Mass) as independent variable and measured Plant community traits and Soil properties as dependent variable (Dependent variable ~ Shrub Mass). Adjusted R-square (R²), F-statistic and significance of the regressions are given. Level of significance according to p-values: not significant (n.s.), < 0.1 (.), < 0.05 (*), < 0.01 (**), < 0.001 (***), n=54.

Cascading effects of shrub encroachment on plant communities and soil

The SEM analyses showed that increasing shrub biomass had direct effects on plant community traits with cascading effects onto soil properties. To illustrate these effects, we present the two piecewise SEMs.

Model 1 (Fig. 6a) including all causal relationships hypothesized in our conceptual model (Fig. 1) differed significantly from observed data structure. These results indicate that the correlation structure of the real data deviated significantly from that of the hypothesised model (p < 0.05). Thus, the model cannot be supported as a reasonable representation of the data. However, all links in this hypothetical model were significant, other than those between CWM N and available N ions (NH₄⁺ and NO₃⁻) (Supplementary Material).

Model 2 (Fig. 6b), which was a sub-set of Model 1 where we removed the non-significant links between CWM N and available NO₃⁻ and NH₄⁺ did not differ significantly from observed data structure. Model 2 shows how shrub biomass had direct effects on CWM of plant traits and indirect effects on soil properties. First, increasing shrub biomass led to significant decreases in CWM N and CWM P, as well as a significant increase in CWM DMC, with shrub biomass explaining a 87% of the variance of the DMC in the plant community (Fig. 6b). These effects were followed by significant effects of plant community traits onto soil properties. Increasing CWM DMC was significantly related to increasing CNs, increasing SOM, and decreasing soil pH. Thus increasing shrub biomass led to more acidic soils, increased SOM, which then led to increases in soil CNs. Decreasing tissue P content (CWM P) with increasing shrub biomass led to decreasing soil available P (PO₄ ^− 3). In contrast, increasing shrub biomass led to decreasing N in plant tissues (CWM N) but with no significant effects on soil N availability.

Including J. communis as a random effect in the analysis of plant-soil linkages increased the proportion of variance explained by the model (Table 3). This was evidenced by a higher conditional R² (including random effects) vs. marginal R² (excluding random effects) for all dependent variables, with the exception of PO₄ ^− 3 which remained constant. In particular, the amount of explained variance in soil pH increased from 8–39% when accounting for the presence of J. communis.

Table 3

Variance of soil properties and soil available nutrients (Dependent variables) explained by the simple linear models (lm) and mixed effects model (lme) included in the SEM Model 2. Dependent variables: phosphate (PO₄^3-), pH, soil organic matter (SOM) and soil carbon/nitrogen (CNs). Variance in lm without random variable (Marginal R²) and in lme with random variable (Conditional R²). Random variable as presence or absence of *J. communis* (JUNIPER). Independent variables: CWM phosphorus concentration (CWM P), CWM dry matter content (CWM DMC) and soil organic matter content (SOM).
Dependent variables	Marginal R²	Conditional R² (random variable = J. communis)	Independent variables
PO₄³⁻	0.20	0.20	CWM P
pH	0.08	0.39	CWM DMC
SOM	0.19	0.35	CWM DMC
CNs	0.38	0.44	CWM DMC, SOM

General patterns of trait variation with increasing shrub density

This study provides the first trait-based approach of the effects of shrub encroachment in subalpine grasslands, testing the relevance of the plant economics spectrum (Reich 2014) to ecosystems comprising significant proportions of these woody species. Our results support general hypotheses on the effects of shrubs on community-level plant strategies and their implications for carbon and nutrient cycling (Myers-Smith et al. 2019; Wookey et al. 2009), thereby complementing evidence from arctic and sub-arctic (Crofts et al. 2018; Kemppinen et al. 2021), arid and semi-arid (Ambrosino et al. 2023; Guo et al. 2022b), subhumid (Vogel et al. 2022), alpine (Collins et al. 2020), and mediterranean (Garnier et al. 2004; Nunes et al. 2019) ecosystems.

The PCA (Fig. 3) showed that most of the variation in plant and soil parameters is explained by shrub biomass (Dim 1), and associated with a gradient of plant resource economics. Grassland plots were located at the more acquisitive side of the spectrum with softer tissues and higher nutrient concentrations (lower CWM DMC, lower CWM CN). Communities became more conservative along succession towards shrubland, with lower N concentration and greater investment in tougher tissues (higher CWM CN and CWM DMC). DMC and C concentration were higher in shrub stems compared with herbaceous leaves, which on the contrary had higher N concentration. We acknowledge that as biomass fractions of plant organs were used to calculate CWM of traits, correlation of plant traits with increasing shrub density were expected, especially for CWM DMC and CWM CN. These results are consistent with findings for shrub-encroached subalpine grasslands in the Pyrenees, where C concentration in shrub leaves was higher than in herbaceous leaves, and negatively correlated to N concentration (Urbina et al. 2020). They also match patterns of secondary succession elsewhere including Mediterranean shrublands (Navas et al. 2010). Contrary to the Pyrenees, we however did not find a high contribution of aboveground P to Dim 1, which suggests that although plant P concentrations might decrease with shrub encroachment (Urbina et al. 2020) and along some successional gradients (Tsujii et al. 2023), it may not be a core trait of the resource economics spectrum in our communities.

Soil properties showed a less strong association with the shrub density gradient than aboveground traits. The second axis of variation in the PCA (Dim 2) reflected shrub identity effects on CWM P, available nitrate and pH. Despite the fact that aboveground traits analysed here may be less strongly associated to soil parameters than belowground traits, which interact more directly with soil microenvironment (Spitzer et al. 2023), multiple studies have shown the effect of aboveground traits on belowground processes (Kahl et al. 2017; Sokol et al. 2022). Nevertheless, our analyses for the Stubai site showed that Specific Root Length and Root Tissue Density decreased with shrub encroachment (Tello-García et al. forthcoming). Patterns of association between above- and belowground traits as part of the plant economics spectrum are not consistent or fully understood, but overall coordination has been confirmed in floras relevant to our sites (de la Riva et al. 2016; Freschet et al. 2010; Pérez-Ramos et al. 2012), suggesting that aboveground plant traits are relevant indicators for quantifying ecosystem-level functional responses during shrub encroachment processes.

Linear changes of plant traits and soil properties in shrub encroached subalpine grasslands

Contrary to our expectations, we found no indication of nonlinear trends in community-level plant traits or soil processes with increasing shrub density. Instead, we observed linear responses of plant community traits to increasing shrub biomass. All our linear models for plant traits were significant with a high R², with the exception of P concentration which, while still significant, showed a higher variance. These results suggest the functional response of plant communities to increasing shrub biomass in subalpine grasslands in the Alps might be gradual along gradients of encroachment, at least for low shrubs such as those considered here. Although soil properties showed a higher variance, we did not detect abrupt changes along the density gradient either. Soil C, C:N and SOM increased significantly and linearly with increasing shrub biomass. However, we measured concentrations of total soil SOM and C soil, as opposed to specific carbon fractions or phenolic compounds. While some studies have found nonlinear changes of soil organic C and lignin in shrub encroached temperate grasslands (Zhou et al. 2017), they included a narrower range of density (0 to 30%) and different functional groups (i.e. leguminous shrub C. microphylla) compared with our study. Instead, our results are consistent with linear increases of soil C found in subalpine grasslands with increasing cover of R. ferrugineum in the Eastern Italian Alps (Pornaro et al. 2017).

Nevertheless, our analysis across the two sites’ plots with deciduous Vaccinium species showed a qualitative shift in fungal communities taxonomic and functional composition at around 35% shrub cover (Mouhamadou et al. forthcoming), suggesting abrupt changes in soil microbial communities. Likewise, nonlinear decreases in the fungi:bacteria and Gram-:Gram + bacteria ratios were observed at levels as low as 10% shrub cover in inner Mongolia (Zhou et al. 2017). In contrast, no change in the fungi:bacteria ratio was detected in response to shrub removal in an Austrian subalpine grasslands (Broadbent et al. 2021). Lastly, we did not detect nonlinear relationships between soil properties and plant community traits, which suggests that qualitative shifts in microbial communities along a gradient of aboveground plant community traits might not translate to changed total soil carbon or nutrient availability.

Cascading effects of shrub encroachment on plant traits and soil properties

Our SEM supports our initial hypothesis that shrub encroachment has cascading effects on soil properties via plant traits (Fig. 6b). We found that the succession toward shrubland leads to plant communities with tougher and nutrient-poorer tissues (Fig. 7). These in turn resulted in increased SOM and soil CN, and decreased pH and P availability (Fig. 7), confirming our hypothesis that shrub encroachment has indirect effects on soil properties. Contrary to our hypothesis, these cascading effects did however not apply to soil N availability.

Plant trait responses to shrub encroachment

Our results support previous evidence of shrub encroachment in subalpine grasslands in the European mountains resulting in plant communities with tougher tissues and lower nutrient content (Grau et al. 2019; Seeber et al. 2022; Urbina et al. 2020). While some previous studies have used CN as a proxy for tissue quality, we found that CWM DMC as a weighted sum of LDMC and StDMC was also a good indicator. CWM DMC was highly and positively correlated not only with CN in the PCA, but also with CWM Lignin:N. Indeed, in a test of alternative models incorporating CWM Lignin:N, we found that these had a lower level of support compared with models including CWM DMC as an indicator of tissue quality (Fig S5).

This suggests that aboveground CWM DMC, an easily measurable trait, can be used as an indicator of the amount of recalcitrant compounds (especially lignin) in shrub encroached grassland communities, and thus of decomposability (Bumb et al. 2018; Freschet et al. 2012; Pietsch et al. 2014) and integration into soil organic matter (Cheng et al. 2023). Our findings add to previous research that focused on leaf traits only (Grau et al. 2019; Seeber et al. 2022; Urbina et al. 2020) by demonstrating the relevance of integrating stem traits (Kobayashi et al., 2024; Zuo et al., 2018) with approaches that consider biomass allocation to leaves vs. stems in woody plants (Guo et al., 2022a). Lastly, these findings confirm that considering allocation is important in community trait-based studies (Garnier et al. 2004) in which not only species dominance but also structural components are relevant to understand ecosystem functioning.

We also found that increasing shrub biomass led to plant communities with lower N and P concentration. Likewise, Urbina et al. (2020) found that plant N and P concentrations decreased along succession to shrubland. However, they showed that these were combined with an increase in their aboveground stocks, given that high shrub biomass per se overrides the dilution of nutrients in their aboveground biomass. Accordingly, despite the decrease of nutrient concentrations in the aboveground biomass, we also found and increase of total aboveground stocks of N and P with increasing shrub biomass (Fig S6 c-f). This increase in stocks was mainly driven by the stem biomass of shrub species, which emphasises once again the importance of biomass allocation to understand nutrient cycling and storage in these ecosystems.

Cascading effects on soil properties

We found that the increase of woody material, which has high CN, high DMC and high lignin (and lignin:N), led to increased SOM and soil CN and to decreased pH. These results are consistent with well-known trait effects on decomposition (Cornwell et al. 2008; Rosenfield et al. 2020, Zuo et al. 2018) and hence on soil carbon accumulation (De Deyn et al. 2008; Huys et al. 2022; Sokol et al. 2022). Recalcitrance of woody tissues results from their high fibre density (Bani et al. 2018) and their high concentration in polyphenols (i.e. tannins, lignin) (Kahl et al. 2017), compared to leaves. Accordingly, leaf and wood litter traits (i.e. lignin) have been shown to be independent within woody tree species, resulting in different effects on decomposition (Kobayashi et al. 2024). Lignin can impact decomposition and nutrient cycling in multiple ways. First, it can have direct effects on decomposer communities such as saprotrophic fungi or soil macrofauna (Sanghaw et al. 2023; Yang et al., 2024a; Zuo et al. 2014). Second, lignin can interact with soil minerals and be protected from microbial access, therefore influencing decomposition (Liao et al. 2022). Our model also showed decreasing soil pH with shrub encroachment, which was also reported in other studies (Collins et al. 2020; Eldridge et al. 2011), and might be linked to the leaching of organic acids into de soil (Swift et al. 1979; Whitford 1992).

Shrub encroachment decreased soil P availability as a result of the lower P concentration in plant communities, and despite the increase in total aboveground P stocks and soil P concentrations (Fig S6 c-d). These results highlight the importance of quality, and not only quantity, for nutrient release in the case of P, thus validating observed effects of plant functional traits on nutrient cycling in shrub encroached grasslands (Yang et al. 2022; Zhang et al., 2008a). Moreover, P cycling depends on other mechanisms such as fungal communities and their ability transform organic P into forms of P available for plants (Ferrol et al. 2019; L. Wang et al. 2023). These mechanisms can be affected by soil pH. It is well known that low soil pH suppresses microbial activity and can hence lower the nutrient use efficiency (Zhang et al., 2008b). We suspect that the immobilization of P in aboveground and belowground compartments, together with decreased pH, cause a decline in available P in shrub encroached plant communities.

Surprisingly, we found that decreases of N in plant communities did not cascade to decreases of soil N availability. We expected that the increase of woody biomass with high concentrations of lignin and tannins, which are known to inhibit N release (Rahman et al. 2013), would result in decreasing soil available N. Broadbent et al. (2022) observed decreased soil N availability in Austrian subalpine grasslands encroached by ericaceous species. However, seasonality, soil biota or microclimate might offset this effect. For instance, Crofts et al. (2018) found no differences in N availability in tundra ecosystems with increasing shrub density, potentially linked to a lower pulse of nutrients in summer (vs. spring), when the sampling was done. The symbiosis of shrubs with ericaceous mycorrhizae and ectomycorrhiza may also impact N cycling by immobilizing soil nutrients (Broadbent et al. 2022). Yet, we did not find an increase in the abundance of ericoid and ectomycorrhiza in the soil in the same experimental plots encroached by Vaccinium species (Mouhamadou et al., forthcoming). Lastly, microclimatic factors such soil moisture or temperature due to rainfall interception or shading by shrubs can affect N cycling (Aguirre et al. 2021). It is likely that the multifactorial determinism of N cycling, including microbial communities, dominant functional shrub species, and abiotic factors such as precipitation regimes and soil water content, made it difficult to observe decreasing trends of available N in this study.

At our sites, land use history with varying duration since mowing (Lautaret) and more intensive grazing (Stubai), along with idiosyncratic colonisation into initially homogenous grasslands (Lavorel et al. 2017) have resulted in varying shrub density. Our results based on our directional structural equation model show that this shrub density gradient then influences soil properties through plant-soil feedbacks (Bennett et al. 2017; Wardle et al. 2004). Furthermore, we cannot exclude that changed soil conditions (greater amounts of more recalcitrant organic matter, lower pH and P availability) may result in a feedforward loop further favouring shrub colonization through changes in microclimate and/or soil microbial communities (Myers-Smith et al. 2019; Wookey et al. 2009). For example, the presence of ericoid mycorrhiza fungi present in ericaceous species can outcompete the ubiquitous arbuscular mycorrhiza fungi in the obtention of nutrients from recalcitrant litter (Wurzburger and Hendrick, 2009) and limit N availability to non-ericoid mycorrhizal plants, including the herbaceous matrix (Northup et al. 1998; Ward et al. 2021). Such effects require experimental testing including long-term shrub removal experiments (Aguirre et al. 2024; Broadbent et al. 2021; Fanin et al. 2022; Urcelay et al. 2009).

Effects of shrub species identity

The functional type of the dominant species has been found to be one of the main drivers of the functional structure of alpine subshrub communities (Illa et al. 2017), and of functional effects of shrub encroachment globally (Eldridge et al. 2017). The effect of ericaceous species on soil properties has been well documented (Hattenschwiler 2000), and their negative effect on N availability shown in various ecosystems, including boreal forests (Fanin et al. 2022) and subalpine grassland (Broadbent et al. 2022). In this study, we found consistent changes in responses of soil properties and nutrient availability to shrub density for ericaceous species, whether deciduous or evergreen, across the two sites. In contrast, our second SEM incorporating J. communis as a source of variation confirmed that the effects of shrub encroachment on soil properties were species dependant (Fig. 4). Plots with high abundance of J. communis had soils with higher pH, lower concentrations of PO₄ ^− 3 and NH₄⁺ and higher concentrations of NO₃⁻ compared to plots dominated by ericaceous species or R. spinosa, while grassland plots did not significantly differ across corresponding communities. Responses to shrub density also differed in J. communis encroached communities, with limited changes in pH, SOM or soil CN. The higher litter persistence of J. communis (Illa et al. 2017) might explain the lower incorporation of plant material into soil organic matter and corresponding stability in C:N stoichiometry. Grau et al. (2019) found similar results in subalpine grasslands encroached by Juniperus sabina in south facing slopes of the Pyrenees, with no change in pH between young and mature shrublands. Consistent with these results, in our study J. communis dominated communities had lower P concentrations in plant tissues than other, and especially ericaceous dominated communities, reflecting lower P concentration in this species than in grassland or other shrub species. In the Pyrenees, this resulted in decreases in total aboveground and belowground P stocks in plots encroached by J. communis or J. sabina (Urbina et al. 2020). Sequestration of nutrients in the abundant biomass of shrubs and especially Juniper species (Illa et al. 2017) might explain the observed decrease of available P. Despite the non-significant change of N availability with increasing shrub density, we found higher concentrations of NO₃⁻ in plots encroached by J. communis compared with plots encroached by ericaceous species. The later also showed lower values of pH at high shrub densities (Fig. S3b), which can have negative impacts on N availability (Wang and Kuzyakov 2024), and thus explain the lower NO₃⁻ concentrations compared with plots encroached by J. communis.

Implications for the quantification of ecosystem services in shrub-encroached land

Shrub encroachment is expected to significantly modify societal values of grasslands through changes in ecosystem services (Bardgett et al. 2021). Quantification of these changes with trait-based models is required to assess their magnitude and project the effects of future climate and land use change scenarios (Barros et al. 2017; Kambach et al. 2024). Previous models were developed for herbaceous communities (Grigulis et al. 2013; Lavorel et al. 2011), and have been applied to project changes in ecosystem services in response to subalpine grassland management scenarios (Lamarque et al. 2014; Lavorel 2018; Schirpke et al. 2017). These models based on the plant economics spectrum and especially on traits reflecting plant size (height) and aboveground tissue composition (dry matter content, nutrient concentrations) could however not be applied to shrub-encroached communities until demonstrating the validity of these traits beyond grasslands. With this study, we show that this extrapolation is warranted.

Grazing extensification or abandonment and climate warming have fostered shrub encroachment in European subalpine grasslands. The succession from grassland to shrubland is expected to have strong impacts on ecosystem functions, but their specific mechanisms are not fully understood. In this study at two sites in the Alps, we tested the hypothesis that increasing shrub biomass cascades onto soil properties via community-level plant functional traits. First, our investigation of community-level traits and soil properties along a unique continuous gradient of encroachment showed that, contrary to expectations based on a shift to prevalence of woody tissues, their changes were linear with shrub density. Secondly, our structural equation model identified the direct and indirect effects of increasing shrub biomass on soil properties via the continuous shift to more conservative communities with lower nutrient concentrations and tougher tissues. The production of these more recalcitrant tissues led to soil organic matter accumulation, more acid and phosphorus-poor soils. These effects were moderated by shrub identity, where colonisation by Juniperus communis qualitatively modified community traits (especially CWM P) and soils (especially pH). Our results also suggest that weighted means of a single trait like tissue density (DMC), assessed at whole community level with contributions of leaf and stem biomass, could be a good indicator for cascading impacts of shrub encroachment on soil properties. Further research will need to explore associated mechanisms via litter decomposition, changed soil carbon fractions, changes in root traits and impacts on soil microbial communities and their activities. Lastly, our confirmation that the plant resource economics spectrum is applicable to shrubland succession indicates that models of ecosystem services developed for grasslands are also applicable to communities dominated by low shrubs. This will enable scenario-based projections of ecosystem services of subalpine landscapes under land use and climate change scenarios.

Competing Interests

The authors have no competing interests.

Funding

This study was conducted as part of LUCSES project, funded by the L'Agence nationale de la recherche (ANR) and the Österreichischer Wissenschaftfonds (FWF), (FWF-I 4969-B and ANR-20-CE91-0009). It contributes to Zone Atelier Alpes and the Lautaret eLTER platform. The Stubai site is part of the LTSER platform Tyrolean Alps, belonging to the national and international long-term ecological research network (LTER-Austria, LTER Europe and ILTER).

Author Contributions

Sandra Lavorel, Karl Grigulis and Georg Leitinger conceptualized and designed this study. The data collection was done by Lucía Laorden-Camacho, Elena Tello- García, Marie-Pascale Colace and Blandine Lyonnard. Lucía Laorden-Camacho and Karl Grigulis performed the data analysis. The first draft of the manuscript and data visualization was done by Lucía Laorden-Camacho, in close consultation with Karl Grigulis, Elena Tello-García and Georg Leitinger. Sandra Lavorel made substantial contribution to the final draft. All authors contributed to the review and editing of the manuscript.

Acknowledgements

We acknowledge Elia Barnier, Amanda Casanovas, Philippine Magnin and Morgane Storey for field help at Lautaret, and Jardin du Lautaret for logistic support. Erich Tasser, Anna-Lena Neunteufel, Katrin Müller, Carmen Hassler, Vanessa Meier, Hannah Chessell, Martin Tasser, Emma Egger, Joachim Hochrainer for field help at Stubai.

Data Availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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Shrub encroachment modifies soil properties through plant resource economic traits

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Abstract

Background and Aims

Methods

Results

Conclusion

Figures

Introduction

Materials and Methods

Study sites

Sampling design

Biomass estimation

Plant functional traits

Shrub traits

Herbaceous traits

Calculation of community weighted plant traits

Soil physical-chemical properties

Plant available N and P

Statistical analysis

Results

General patterns

Linearity of changes along the gradient of increasing shrub biomass

Cascading effects of shrub encroachment on plant communities and soil

Discussion

General patterns of trait variation with increasing shrub density

Linear changes of plant traits and soil properties in shrub encroached subalpine grasslands

Cascading effects of shrub encroachment on plant traits and soil properties

Plant trait responses to shrub encroachment

Cascading effects on soil properties

Effects of shrub species identity

Implications for the quantification of ecosystem services in shrub-encroached land

Conclusion

Declarations

Competing Interests

Funding

Author Contributions

Acknowledgements

Data Availability

References

Supplementary Files

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