Direct and indirect effects of terrain, snow, and shrubs on the structure of an alpine herbaceous community in the Canadian Rocky Mountains

doi:10.21203/rs.3.rs-8486553/v1

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Direct and indirect effects of terrain, snow, and shrubs on the structure of an alpine herbaceous community in the Canadian Rocky Mountains

https://doi.org/10.21203/rs.3.rs-8486553/v1

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Alpine ecosystems have extreme environments that limit plant growth, favoring stress-tolerant species. Climate change has resulted in the expansion of shrubs at the expense of poor competitors, including herbaceous species. Here, we explore the pathways linking local terrain features, snowmelt dynamics, and shrub cover on herbaceous plant cover and species richness at Cardinal Divide in the Canadian Rocky Mountain Front Ranges using a series of structural equation models (SEMs) that isolate direct from indirect effects among factors. We measured the biotic and abiotic conditions within 338 quadrats (0.25-m²) across a gradient of environments typical of the open and expanding krummholz alpine sites on the divide. Herbaceous cover and richness were negatively related to shrub cover but positively associated with late-snow persistence and terrain features that supported higher winter snow accumulation and greater soil depth. Dwarf deciduous shrubs were minimally affected by differences in the timing of snowmelt. Tall evergreen shrubs were uncommon but abundant where present, substantially reducing herbaceous cover and richness. The indirect effects of shrub cover tempered the positive association of snow on herbaceous cover and richness. Concave terrain surfaces and cooler-facing slopes increased herbaceous composition, while soil depth supported taller shrubs and herbaceous communities. This study underscores the complex relationships, both direct and indirect, between the abiotic and biotic environments of the alpine plant community.

Alpine Terrain

Snowmelt Dynamics

Shrub–Herb Interactions

Diversity

Structural Equation Modelling

Arctic and alpine ecosystems are rapidly changing, including alterations in plant composition and diversity (Post et al. 2009, Myers-Smith and Hik 2018). Recent expansions of shrubs are associated with decades-long increases in temperatures across the Arctic and alpine tundra (Myers-Smith et al. 2019, Zong et al. 2022). Shrubs may benefit from recent warming due to extended growing seasons, particularly with warmer temperatures early in the growing season (Bjorkman et al. 2020). The expansion of shrubs has resulted in substantial shifts in the remaining plant community (Scharnagl et al. 2019). This shift is facilitated mainly by earlier snowmelt, a key phenological driver of seasonal environments (Hallinger et al. 2010, Zong et al. 2022). The implications of earlier snowmelt on alpine plant communities are essential for understanding past and future changes, yet unclear at local scales, since snowmelt patterns are themselves influenced by local terrain features that affect snow accumulation and solar exposure (Abudu et al. 2016).

Temporal patterns in snow cover and snowmelt in the alpine often vary less with altitude and more with local variations in slope-aspect and surface roughness, which influence snow accumulation or loss during winter and spring (Anderton, White, and Alvera 2002, DeBeer and Pomeroy 2010). At larger scales, south-facing ridges in the northern hemisphere experience faster snowmelt, especially on steeper slopes (Abudu et al. 2016). On northern slopes, snowbanks tend to persist longer, particularly in depressions that accumulate more snow (DeBeer and Pomeroy 2010). These terrain characteristics create consistent patterns of late snow disappearance that are decoupled from the typical seasonal timing of snowmelt, thus structuring local patterns in plant community composition (Erickson et al. 2005). Snow cover protects plants from exposed freezing temperatures in winter, regulating the length of the growing season, and serving as a primary source of soil moisture in summer (Körner 2003a, Inouye 2008, DeBeer and Pomeroy 2017).

Shrubs in the alpine grow best in areas with moderate to deep snow depths, where winter protection is provided and available summer moisture is extended (Kudo and Ito 1992). However, the relationship between snow and shrubs varies between different functional groups (Elmendorf et al. 2012). For instance, dwarf shrubs are more sensitive to growing-season length, having to wait until complete snow melt for phenological cues to start growth (Bell and Bliss 1977), whereas taller shrubs rely on more moisture provided by deep snow and slower melt rates (Hallinger et al. 2010, Conner et al. 2016). Shrub leaf habits also vary with solar exposure and snow bed characteristics (Saarinen et al. 2016). Light limitation is tolerated by evergreen shrubs, which can begin photosynthesis while still under snow (Starr and Oberbauer 2003, Christiansen et al. 2018, Dobbert et al. 2021). However, the ability of deciduous shrubs to tolerate light limitations depends more on their height (Dobbert et al. 2021). In alpine zones, shrub size is limited, in part due to a short growing season, but also temperature and moisture stressors (Boscutti et al. 2018). As such, instead of growing upwards, many alpine shrubs branch outward.

The relationship between shrubs and snow has influenced how shrubs respond to early snowmelt, thus changes in early snowmelt in recent decades have altered local patterns in shrub abundance (Martin et al. 2017, Scharnagl et al. 2019, Dobbert et al. 2021). Some shrub species have expanded their range, particularly into areas that were formerly snow-covered year-round or had snow cover until late in the growing season (Tape et al. 2012, Gentili et al. 2020). However, other shrub species are responding poorly to warming, with drought leading to early senescence, and in some cases, increased mortality (Gamm et al. 2018, Dobbert et al. 2021). Many shrubs are susceptible to frost damage in early spring (Wheeler et al. 2014). Some evergreen shrubs have expanded into the deepest snow beds due to warming temperatures and earlier snow loss, but are performing poorly reproductively (Christiansen et al. 2018). Overall, there has been a trend in shrub expansion within arctic-alpine communities, which, even if limited to a few species, can have substantial effects on the overall plant community (McManus et al. 2012, Myers-Smith and Hik 2018, Zong et al. 2022), including reducing herbaceous species (Ballantyne and Pickering 2015, Sholto-Douglas et al. 2017). However, the effect of shrub expansion on herbaceous composition in alpine areas remains uncertain, as some shrub species may enhance (facilitate) herbaceous diversity, while others might reduce it through competition (Ballantyne & Pickering 2015, Sholto-Douglas et al. 2017).

Not only are herbaceous species sensitive to competition with shrubs, but also to accelerated snowmelt. Early snowmelt, particularly in spring when frost events still occur, can cause frost damage to species that are active during that period (Inouye 2008, CaraDonna and Bain 2016, Pardee et al. 2018). Consequently, some species have shifted to later-melting snow beds to avoid frost and maintain adequate moisture (Verrall and Pickering 2020, Verrall et al. 2023). However, late snow beds generally have lower species richness and plant cover, demonstrating the presence of trade-offs (Alatalo et al. 2014, Liu et al. 2022, Cheng et al. 2023). Given recent changes in snow dynamics and shrub expansion, herbaceous species may be the most sensitive to change (Sholto-Douglas et al. 2017, Scharnagl et al. 2019). More work is needed to understand the interplay between snow and shrubs on herbaceous species, particularly the potential synergistic interactions (Hallinger et al. 2010, Moura et al. 2016, Boscutti et al. 2018). In this study, we examine how variation in terrain and snow cover influences the cover of different growth forms in shrubs, and, in turn, how this affects herbaceous plant cover and richness. We examine this at an alpine site along the eastern slopes of the Canadian Rocky Mountains using pathway diagrams with Structural Equation Models (SEMs) that link and quantify the direct and indirect relationships among terrain, snow, shrubs, and herbaceous plants.

Study Area

We conducted the study during the summers of 2022 and 2023 at Cardinal Divide (Latitude: 52.90797° N, Longitude: -117.2085° W), located 22 km south of Cadomin, Alberta, Canada, in the Nikannassin Range of the Canadian Rocky Mountains (Fig. 1). Elevations of study plots ranged from 1941 to 2189 meters. The site is adjacent to the Boreal Foothills ecoregion, where boreal and sub-alpine tree species are expanding upslope due to climate warming (Harsch et al. 2009, Myers-Smith et al. 2011).

We sampled the open and semi-open/krummholz habitats across three distinct areas of the divide, each with unique slope and aspect characteristics. The "Upper Divide" (Fig. 1a) is a large southwest-facing dry, rocky slope near the top of the divide. Patches of krummholz tree cover occur up to its summit, intermixed with open alpine. The "Lower Divide," adjacent to the Upper Divide, features a distinct ridged slope (divide) having a northeast/southwest orientation (Fig. 1b). On the south side of this ridge, the open alpine of the ridgetop transitions rapidly to krummholz and then to dense forests. In contrast, the north side has a lower tree line and a rocky slope with a nearly systematic spacing of narrow dry ravine swales that are perpendicular to the ridge, reaching down to the tree line. These swales are characterized as having krummholz along the west side (east slope) of the swale, but are otherwise open, and capture substantial amounts of snow in winter and melt later in the summer. The third area of the divide is a northeast-facing open alpine basin located below Tripoli Mountain. This basin is distinguished by its deeper soils and lower concentration of bare rock, with swales and ravines running down its slope that collect winter snow (Fig. 1c). The combination of these areas spans the environmental gradients of the alpine community.

Study Design

Sampling Methods

We used random (2022) and stratified random (2023) sampling of non-forested alpine sites across the three parts of the divide. In 2022, we sampled 205 random sites, and in 2023, we used an initial remote sensing-based model of snow persistence to target the full range of snow persistence, sampling 133 additional sites (Fig. 1). This included 33 random plots around a large swale that retained the latest snow on the Cardinal Divide (Fig. 1c). At each selected site, we used a 0.25 m² quadrat (0.5 m × 0.5 m) to quantify alpine plant cover for all vascular plants during peak growing conditions between mid-July and early August. Plant cover was recorded within the quadrat up to a height of 50 cm, at 1% intervals for 0–10% cover and at 5% intervals for 10–100% cover. We then grouped plant species into herbaceous and shrub species, further defining shrub species by growth form based on leaf size and leaf habit. Locations of each quadrat were recorded in NAD83 UTM coordinates using a GNSS GPS (SX Blue II or Geode), achieving horizontal accuracy of less than 0.6 m.

Snow Persistence

We quantified snow persistence in each quadrat using a series of 0.6 m pan-sharpened SkySat satellite images obtained from Planet Labs, which were tasked with acquiring weekly images under clear-sky conditions (note that some weeks had no clear-sky conditions). We analyzed imagery from three ‘spring-to-summer’ periods: June 8 - July 18, 2022 (late-year snow melt); March 20 - June 12, 2023 (early-year snow melt); and April 1 - July 17, 2024 (late-year snow melt). Images were classified into binary rasters classified as snow (1) and no snow (0) cover for each acquisition date using the terra and sf packages in R (Hijmans et al. 2023, Pebesma et al. 2023). In total, we classified 30 image dates to develop a final gradient of snow disappearance for the divide (see Appendix A for a more detailed workflow and Fig. 5 for an illustration of the steps). Although we do not concentrate on specific dates of snow loss in any one year, our model emphasizes areas that are more consistent in holding late snow versus not, which tends to be consistent across years in alpine ecosystems (Erickson et al. 2005). Late snow beds in swales can persist until late July in some years (2022 & 2024), although in other years with poor snowfall and warm spring and early summer weather, snow may last until late June (2023). Because dense tree cover obscures snow cover detection with remote sensing, we limited our analyses to sites (plots) with less than 30% tree canopy cover (n = 338), as canopy cover below 30% did not affect snow persistence values.

Measures of Local Terrain Features

Cardinal Divide has distinct directional slopes, tree lines, and swales that affect snow accumulation and persistence (Fig. 2). We quantified these features for each sample plot using a LiDAR-derived 1-m Digital Elevation Model (DEM) from Altalis (Government of Alberta) and subsequently using this model to derive surface curvature, slope, and aspect (Fig. 2a). Terrain curvature, modelled in ESRI ArcGIS Pro using the Curvature script in the Spatial Analyst Tool which expressed the relationship between concave (positive) and convex (negative) slopes. As such, the values of curvature are unitless and represent standard deviation from the mean of the elevation/slope metric. Comparisons between curvature and snow are shown in Fig. 2a and 2b, illustrating the strong relationship between them. Slope and aspect for each quadrat were calculated with a handheld compass at the center of each quadrat and used to calculate a Solar Severity Index (SSI) (Nielsen and Haney 1996). SSI shows the relationship between slope steepness and a ‘folded’ southwest aspect to emphasize the effects of cumulative daytime heating. The index ranges from − 1 (north-east 45˚ slope) to 1 (south-west 45˚ slope), with 0 representing either flat slopes (0˚) or north-west/south-east aspects (Fig. 2c).

Quadrat-Level Tree Canopy and Soil Depth Measures

Tree canopy cover was measured over each quadrat at waist height using a spherical densitometer (Lemon 1956). However, we excluded sites within forests (> 30% canopy cover) in our analysis, as our interest was in the alpine plant community and because estimating snow persistence from remote sensing is limited in areas of higher tree cover. Thus, only plots with densiometer canopy readings of 30% or less were retained. These types of sites were characterized as being krummholz-like patches of trees. Finally, we estimated the maximum soil depth (cm) within each quadrat by probing the ground surface with an 18-gauge metal prong and recording the deepest measure. Soil depth affects, among other things, the presence and cover of taller shrub species and thus is included here.

Statistical Analyses

We used Structural Equation Models (SEMs) to identify and quantify the causal relationships among our set of interrelated variables (Laughlin 2014, Moura et al. 2016). SEMs distinguish between direct and indirect effects, thereby creating a network of relationships among variables that are assumed to be correlated. Here, we used a Piecewise Structural Equation Model (Lefcheck 2016). We express our causal network as a set of linked hypotheses, as follows:

H₁

Snow features can be explained by slope and aspect, which differentiate between cooler and warmer slopes, curvature that captures local terrain depressions accumulating more snow in winter, and canopy shade, which moderates solar exposure and spring melt. These factors influence the distribution and persistence of snow.

H₂

The effect of snow on shrubs will depend on their growth traits (tall versus dwarf) and leaf habits (deciduous versus evergreen). This variation arises from differences in their tolerance to the length of the growing season, with some shrubs better adapted to shorter or longer snow cover durations.

H₃

Deciduous shrubs, which benefit from earlier snowmelt and are often more competitive in areas of early snowmelt, are expected to have an indirect negative effect on herbaceous composition through their interaction with evergreen species. Evergreen shrubs, which tolerate, or even favour, prolonged snow conditions, may be indirectly influenced by the competitive presence of deciduous shrubs.

H₄

Taller shrubs are predicted to negatively affect dwarf shrubs by overshadowing and outcompeting them for resources. Additionally, this competitive dynamic is expected to indirectly affect herbaceous plant composition by altering the competitive balance among different shrubs and herb species. Further, we expect deeper soils to support taller shrubs more strongly, as they have greater rooting depth requirements.

H₅

Each shrub growth form (based on growth traits and leaf habits) will influence herbaceous composition differently. This is due to the varying cover strategies employed by each group, which affect the composition and abundance of herbaceous plants in their vicinity.

H₆

Persistent snow is expected to reduce growing season length, and shade is predicted to limit light availability, both of which may negatively affect herbaceous composition. In contrast, warmer solar exposure is anticipated to promote herbaceous growth. These effects vary through the indirect pathways mediated by shrubs, as the interaction between shrubs and environmental factors (snow, shade, solar exposure) should influence resource availability and conditions for herbaceous growth.

To improve model fit, we tested for missing pathways and standardized coefficients for ease of variable comparisons. Specifically, four piecewise SEMs were fitted to compare potential network causalities between terrain features that affect snow persistence, shrub cover (among growth forms), and herbaceous cover/richness. We first fit separate pathway diagrams for different groups of variables, based on a priori knowledge of interactions among shrub growth forms (see Fig. 6 in Appendix B). We used the Akaike Information Criterion (AIC) to compare support among hypothesized models and assessed model fit using Shipley’s test of direct separation, which tests the probability that no pathways are missing from the hypothesized framework (Shipley 2000). Models were rejected if Fisher’s C statistic and χ² tests were p < 0.05.

We then used linear models (R Core Team, 2024) to test: (1) the effects of terrain (solar severity index, curvature, canopy cover) on snow persistence, (2) the effect of snow and terrain on (a) shrub composition, (b) tall/dwarf shrub composition, (c) evergreen/deciduous shrub composition, and (d) combination of size and leaf habits of shrubs; and (3) the effects of terrain, snow, and shrubs growth forms (a-d) on herbaceous cover and richness. Although SEMs allow for multiple response variables, our primary final interest was herbaceous cover and richness. Herbaceous richness was normally distributed; thus, models were tested under Gaussian and Poisson distributions, with the Gaussian distribution being the most supported and with more homogenous residuals. All model variables are reported in standardized units. For a summary of responses and predictors, including mean and standard deviations, see Table 3 in Appendix B. For the most supported model, we estimated the effects of the indirect pathways on herbaceous abundance/richness by multiplying the standardized effect of the variable and the mediator, and of the mediator and herbaceous richness. We plot model predictions across the full gradient of snow and shrub cover between growth/phenology traits to better understand our results, highlighting the combined effects among variables on herbaceous abundance/richness.

General Patterns in Plant Composition

We identified 101 vascular plant species across 338 quadrats, of which 74 were herbs, 16 shrubs, eight graminoids, and three tree species (Table 2 in Appendix B). Herbs were present in 95% of quadrats, averaging 30 ±1% cover when present, with an average richness of 6 (±2) species. Shrubs were present in 78% of the quadrats, averaging 57 ±2% cover when present, and an average richness of 3 ±1 species (Table 4 in Appendix B). There were seven tall/deciduous, two tall/evergreen, three dwarf deciduous, and three dwarf/evergreen shrubs (Table 5 in Appendix B). Tall evergreen species had the largest average cover when present (46 ±3%), but the lowest prevalence (31%), while dwarf evergreen species had the second highest average cover (28 ±1%) and the highest prevalence of shrubs (75%). Tall deciduous shrubs had a higher prevalence than dwarf shrubs (73% to 57%), but similar average cover (Table 6 in Appendix B).

Structural Equation Model (SEM) of the Alpine System

Model Selection of SEM and Model Fit

For both herbaceous cover and richness, our most supported model was one with both size and leaf habit growth forms of shrubs (Fig. 6d and Tables 7 & 8 in Appendix B). The goodness of fit for the herbaceous cover SEM was Fischer’s C = 32.18 (p = 0.187), and for the herbaceous richness SEM was Fischer’s C = 35.71 (p = 0.218). Pathways between terrain, snow, and shrubs remained the same for both models (Fig. 3). Models explained 29% of the variation in snow persistence, 8% in tall deciduous cover, 30% in tall evergreen shrub cover, and 35% in dwarf evergreen shrub cover. None of our variables were significantly related to dwarf deciduous shrub cover (Tables 9 & 10 in Appendix B). The final SEM models explained 33% variation of herbaceous cover (Fig. 3a) and 17% variation of herbaceous richness (Fig. 3b).

Effects of Terrain Features on Snow Persistence

The average snow persistence across all sites was 48 ±1% from mid-March to mid-July, with the model values ranging from 10 to 90% snow cover. Surface curvature and solar exposure significantly affected snow persistence patterns across sites (quadrats). Concave curvatures increased snow persistence by 37% per standardized unit increase in curvature, while cooler and steeper slopes increased snow persistence by 38% per standardized unit.

Effects of Terrain and Snow on Shrub Abundance by Growth Forms

Shrub growth forms responded differently to snow and terrain features (Fig. 3a; Tables 9 & 10 in Appendix B). Tall deciduous shrubs were influenced by soil depth, with cover increasing by 28% per standardized unit increase in soil depth. Similarly, tall evergreen shrubs increased by 29% per standardized unit increase in soil depth. However, the cover of tall evergreen shrubs also increased by 40% per standardized unit increase in snow persistence. In contrast, tall deciduous shrub cover was negatively affected by snow cover, decreasing by 16% per standardized unit increase in snow cover. Dwarf evergreen shrubs were negatively affected by snow (37% per standardized unit increase), as was the cover of tall evergreen shrubs (17% per standardized unit increase), tall deciduous shrubs (16% per standardized unit increase), and dwarf deciduous shrubs (16% per standardized unit increase). Convex slopes were the only factor positively influencing dwarf evergreen shrubs, increasing their cover by 10% per standardized unit increase in convexity. Finally, dwarf deciduous shrub cover was not affected by any of the variables measured.

Direct Pathways of Terrain, Snow, and Shrubs on Herbaceous Cover and Richness

Snow and soil depth positively influenced herbaceous cover, with increases of 18% per standardized unit for snow and 24% per standardized unit for soil depth. Tall evergreen shrub cover had the most adverse effect on herbaceous cover, reducing herbaceous cover by 45% per standardized unit change in tall evergreen shrub cover, followed by dwarf deciduous and evergreen shrub cover, which decreased cover of the herbaceous community by 29% and 27% per standardized unit of tall evergreen shrub cover, respectively. Tall deciduous shrub cover had the lowest adverse effect on herbaceous cover, with an 11% decrease per standardized unit. Terrain features affected herbaceous cover with solar severity and surface curvature, reducing herbaceous cover by 11% per standardized unit on warm slopes (with increased cover on cool slopes) and by 16% per standardized unit on convex slopes (with increased cover on concave slopes). Detailed pathways and standardized coefficients are shown in Fig. 3, while the contrasting effects of snow persistence (positive) and shrub cover (negative) are illustrated in Fig. 4a-d.

Snow persistence and soil depth both had positive effects on herbaceous richness, increasing it by 0.33 species per standardized unit for snow persistence and 0.15 species per standardized unit of soil depth. Cover of tall evergreen shrub had the most substantial effect on herbaceous richness, reducing herbaceous richness by 0.35 species per standardized unit of tall evergreen shrub cover. Tall deciduous and dwarf evergreen shrub cover had more minor negative effects, reducing herbaceous richness by 0.12 and 0.15 species per standardized unit, respectively. Dwarf evergreen shrub cover did not significantly affect herbaceous richness. Surface terrain also played a role, with concave slopes decreasing herbaceous richness by 0.12 species per standardized unit of concavity, whereas convex slopes increased it. Pathways and standardized coefficients for herbaceous richness are shown in Fig. 3b, and the relationships between types of shrubs and snow on herbaceous richness are detailed in Fig. 4e-g. Tall deciduous shrub cover experienced the largest decrease in total effect size, with indirect pathways through tall and dwarf evergreen shrubs nullifying its direct effect and rendering it neutral with respect to herbaceous cover. Total direct and indirect effects for each variable on herbaceous cover and richness are presented in Table 1 and visualized in Fig. 8 in Appendix B.

Table 1 Direct, indirect, and total effects for pathway variables affecting herbaceous cover and richness at Cardinal Divide, Alberta, Canada. Bolded text is the total effect size of the predictor.

Predictor	Pathway to herbaceous cover	Effect
Surface Curvature	Direct	-0.16
	Indirect: Snow	-0.07
	Indirect: Dwarf/Evergreen Shrubs	-0.03
	Total	-0.26
Solar Severity	Direct	-0.11
	Indirect: Snow	-0.07
	Total	-0.18
Soil Depth	Direct	0.24
	Indirect: Tall/Evergreen Shrubs	-0.13
	Indirect: Tall/Deciduous Shrubs	-0.03
	Total	0.08
Snow Persistence	Direct	0.17
	Indirect: Dwarf/Evergreen Shrubs	0.10
	Indirect: Tall/Evergreen Shrubs	-0.18
	Total	0.09
Tall/Deciduous Shrubs	Direct:	-0.11
	Indirect: Tall/Evergreen Shrubs	0.07
	Indirect: Dwarf/Evergreen Shrubs	0.04
	Total	0
Tall/Evergreen Shrubs	Direct	-0.45
	Indirect: Dwarf/Evergreen Shrubs	0.05
	Total	-0.40
Dwarf/Deciduous Shrubs	Direct	-0.29
	Indirect: Dwarf/Evergreen Shrubs	0.04
	Total	-0.25
Dwarf/Evergreen Shrubs	Direct	-0.27

Predictor	Pathway to herbaceous richness	Effect
Surface Curvature	Direct	-0.12
	Indirect: Snow	-0.13
	Total	-0.25
Solar Severity	Indirect: Snow	-0.13
Soil Depth	Direct	0.15
	Indirect: Tall/Deciduous Shrub	-0.03
	Indirect: Tall/Evergreen Shrubs	-0.10
	Total	0.02
Snow	Direct	0.34
	Indirect: Tall/Evergreen Shrubs	-0.14
	Total	0.20
Tall/Deciduous Shrubs	Direct	-0.12
	Indirect: Tall/Evergreen Shrubs	0.06
	Total	-0.06
Tall/Evergreen Shrubs	Direct	-0.35
Dwarf/Deciduous Shrubs	Direct	-0.15

General Findings

We found that herbaceous alpine communities at Cardinal Divide in Alberta, Canada, were negatively affected by shrub cover, and positively influenced by snow and terrain features that supported higher snow persistence and deeper soils. On average, herbaceous cover was lower than shrub cover, although herbaceous richness was higher. Given the relatively low average richness of six species per 0.25 m², the effects of our predictors on herbaceous richness, though seemingly small, are still considerable.

Effect of Shrubs on Alpine Herbaceous Communities

As hypothesized, deciduous shrubs negatively affected their evergreen shrub counterparts, supporting previous observations (Dobbert et al. 2021). We also found that tall deciduous and evergreen shrubs negatively affected dwarf evergreen shrubs (Tremblay et al. 2012), but not deciduous shrubs. Further, the dwarf-deciduous shrubs were not significantly affected by any of our predictors. This was surprising given the moderate occurrence and relatively high coverage of dwarf-deciduous shrubs in the study area, which led us to assume that snow and tall shrubs negatively affect dwarf-deciduous shrubs (Press et al. 1998, Saarinen et al. 2016, Boscutti et al. 2018). As anticipated, tall deciduous shrub cover was positively influenced by soil depth. Unexpectedly, no relationship was observed between snow and tall deciduous shrubs, in contrast to studies indicating mutual effects in which snow provides protection and moisture, and shrubs offer shade that reduces snowmelt time (Hallinger et al. 2010, Wheeler et al. 2016). Both deciduous shrub forms had similar direct effects on herbaceous cover and richness. However, because of indirect pathways through both forms of evergreen shrubs, tall deciduous shrubs did not affect herbaceous cover and had a reduced effect on herbaceous richness compared to dwarf deciduous shrubs. Tall evergreen shrubs, although less prevalent among sites, had the highest cover when present and exerted the most substantial adverse effects on both herbaceous cover and richness. Tall evergreen shrubs were positively associated with areas of higher snow persistence and deeper soils, as found in previous studies (Mallik et al. 2011, Christiansen et al. 2018), and were negatively associated with tall deciduous shrubs, consistent with our hypothesis and other work (Dobbert et al. 2021). This resulted in tall evergreen shrubs reducing the positive effects of snow and soil depth on herbaceous composition, thereby mediating indirect pathways. The direct effect of tall evergreen shrubs on herbaceous cover and richness was attenuated by indirect pathways involving dwarf evergreen shrubs. Dwarf evergreen shrubs, which were the most frequently encountered shrub and the second most abundant, were negatively affected by all shrub types, snow, and surface curvature, establishing them as primary mediators of indirect effects. Despite their significant mediating role, dwarf evergreen shrubs did not affect herbaceous richness, which in turn limited the number of indirect pathways that influenced it. Nevertheless, dwarf evergreen shrubs had the second-largest adverse effect on herbaceous cover, underscoring their coexistence with numerous small, growing herbaceous species (Wookey et al. 1995).

Influence of Snow Persistence on the Alpine System

As hypothesized, snow was positively related to swale-like features on cooler, north-facing slopes, supporting the observation that snow accumulates in these sites (Erickson et al. 2005, Björk and Molau 2007, DeBeer and Pomeroy 2017). Snow was positively associated with tall evergreen shrubs and negatively related to dwarf evergreen shrubs, reinforcing the idea of the relationships between snow and shrub heights and leaf habits (Wheeler et al. 2014, Saarinen et al. 2016). In contrast to other studies, snow did not affect the local abundance of dwarf deciduous shrubs (Saarinen et al. 2016, Gamm et al. 2018, Dobbert et al. 2021). This may be due to the high prevalence of evergreen species in our system or to differences in snow disappearance rates relative to snow persistence. In our study, areas of short snow persistence often emerged just one week after complete snow cover, presumably due to high wind exposure (Dadic et al. 2010). Rapid changes in snow cover, including winter snow loss, should affect the limitations on deciduous shrubs.

Snow was positively associated with both herbaceous cover and richness. This contradicts some prior studies (Alatalo et al. 2014, Good et al. 2019, Cheng et al. 2023), but aligns with recent research indicating shifts in community composition along snow gradients (Verrall et al. 2023, Morgan and Walker 2023). While we cannot confirm whether our system has undergone compositional shifts over time, the persistence of snow until late July in some years underscores changes in melt timing observed at other sites (Inouye 2020). Earlier melt times appear to alter the trade-off between growing season length, frost protection, and moisture reserves provided by the presence and/or persistence of snow (Kudo and Ito 1992, Wheeler et al. 2014, Conner et al. 2016). Despite the positive effect of snow on herbaceous cover and richness, this relationship was diminished by the indirect effects of tall evergreen shrubs. For instance, the negative indirect pathway through tall evergreen shrubs slightly outweighed the positive direct effect of snow on herbaceous cover. Without the positive indirect pathway through dwarf evergreen shrubs, the overall positive effect of snow on herbaceous composition could be negated, potentially becoming negative. Our findings suggest that areas with both high snow persistence and tall evergreen shrubs can have the lowest herbaceous cover and richness. These insights would not have been intuitive without the indirect pathways provided by structural equation models (Wang et al. 2019). Traditional approaches may have led us to expect areas of high snow persistence and tall evergreen species would be associated with moderate herb cover and richness. In contrast, the lowest herbaceous cover and richness would occur in areas with very low snow persistence and very high shrub cover (as shown in Fig. 3a). However, the causal pathways revealed that (1) high cover of tall evergreen shrubs is related to high snow cover, making the scenario of lowest herb richness unlikely, and (2) the positive association between snow and tall evergreen shrubs creates an adverse indirect effect on herbaceous cover that outweighs the positive direct effect of snow. Consequently, areas with high snow persistence and tall evergreen shrub cover are more likely to exhibit the lowest herbaceous cover and potentially richness.

Effects of Terrain on Snow Persistence and the Structuring of Alpine Plant Communities

Our study revealed that concave terrain surfaces, such as swales on the Lower Divide, and cool-facing slopes positively influenced herbaceous composition. The persistence of snow may protect against frost (Körner 2003a). Swales collect runoff from snowmelt, particularly at their base, thus providing greater soil moisture (DeBeer and Pomeroy 2010, Abudu et al. 2016). The cool, moist conditions of these swales should benefit herbaceous composition, as some herbs are sensitive to intense heat and drought (Notarnicola et al. 2021). North-facing slopes intuitively support higher herbaceous cover due to reduced solar exposure. While our proxy for solar exposure did not directly affect herbaceous richness, it positively influenced richness through an indirect pathway involving snow. Unexpectedly, our proxy for solar exposure did not affect any of the shrub groups. Surface curvature and soil depth outperformed solar exposure in the models, and these are indirectly related to solar exposure. Instead, dwarf evergreen shrubs were positively associated with convex curvatures, which correlated with reduced snow persistence. Our hypothesis that soil depth positively affects tall-shrub and herbaceous composition was supported. Still, soil depth did not influence dwarf-species cover, which typically have shorter and/or lateral root systems, whereas taller shrubs and many herbs are deep-rooted and thus prefer deeper soils. However, our results indicated that the negative indirect pathways through tall deciduous and evergreen shrubs nearly offset the positive direct effect of soil depth on herbaceous cover and richness.

Implications for Climate Change

Ongoing changes in snow dynamics and shrub expansion have significant implications for alpine ecosystems (Wipf et al. 2009, Ballantyne and Pickering 2015). As temperatures rise and snow patterns shift, plant community composition is likely to change further. For instance, tall evergreen shrubs, such as heather species in krummholz habitat with late snowbeds, are unlikely to expand their habitat due to their association (protection) with later snow melt (Christiansen et al. 2018, Dobbert et al. 2021). Conversely, some dwarf shrub species appear to be threatened by warming temperatures in some places, yet are exhibiting growth and expansion in others (Press et al. 1998, Saarinen et al. 2016). In alpine systems, which are high-stress environments with relatively few species, even minor shifts in plant composition can have major consequences for the plant community (Myers-Smith et al. 2011).

Herbaceous communities appear to face the greatest threat from climate change. With earlier snowmelt, some herbaceous species are shifting their ranges to deeper snow patches, which offer trade-offs among a longer growing season, frost protection, and moisture reserves during prolonged droughts (Verrall et al. 2023, Morgan and Walker 2023). Although shrubs often outcompete herbs, some shrubs can promote herbaceous cover and richness (Ballantyne and Pickering 2015, Wang et al. 2019). Tall deciduous shrubs, with their erect stems and canopy shade, may provide refugia-like conditions that protect herbs from excessive heat, keep soils cool, reduce evapotranspiration, and slow snowmelt, thereby maintaining higher moisture content (Weinig 2000, Myers-Smith and Hik 2013, Sholto-Douglas et al. 2017). Increased shrub cover, especially deciduous shrubs, has been linked to changes in surface albedo, soil composition, and reduced herbaceous diversity (Sturm et al. 2005, Myers-Smith and Hik 2013, Sholto-Douglas et al. 2017). Long-term monitoring will be essential for understanding and potentially mitigating the effects of climate change on alpine biodiversity (Scharnagl et al. 2019, Zong et al. 2022).

While our study provides valuable insights, it has limitations. Deciduous shrubs do not appear to be affected by terrain features or snow in our site, suggesting that other factors, such as soil characteristics or surface temperatures (MacDonald et al. 2025), might be more influential (Francon et al. 2021). Variability in annual snowmelt patterns that we didn’t measure may be important (Abudu et al. 2016, Verrall et al. 2023). Future research should focus on long-term studies to capture temporal changes in snow dynamics, soil properties, and temperature, and examine their effects on plant communities, particularly deciduous shrubs (Sholto-Douglas et al., 2017). Experimental approaches could clarify the causal relationships between snow, shrubs, and herbs, enhancing our understanding of these complex interactions (Bell and Bliss 1977, Pardee et al. 2018). Despite these limitations, our findings illustrate critical linkages (pathways) among terrain, snow, shrubs, and herbs, thereby providing a better understanding of the complex ecological processes at play in alpine ecosystems (Körner 2003b).

Our study underscores the complex, synergistic relationships between terrain features, snow dynamics, shrub growth forms, and herbaceous communities. By employing Structural Equation Models (SEM), we uncovered a set of indirect pathways that influence the direct effects of these factors on herbaceous alpine communities, revealing the complexity of abiotic-to-biotic relationships. SEMs provide a valuable approach for elucidating complex direct and indirect ecological relationships. Our findings highlight the importance of alpine shrubs in shaping alpine community composition. While evergreen shrubs negatively affect herbaceous cover and richness, our results suggest that deciduous shrub cover and snow moderate their influence. Finally, despite the perceived threat posed by deciduous shrubs to alpine plant diversity (Sholto-Douglas et al. 2017), deciduous shrubs in our site had minimal effects on herbaceous cover and richness when present alongside evergreen shrubs. Recognizing these nuances is crucial for understanding community dynamics and prioritizing future research.

Disclosure Statement

The authors have no competing interests to declare that are relevant to the content of this article.

Funding

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery Grant #RGPIN-2019-06040 to Scott E. Nielsen) and by the Alberta Conservation Association (Grant #ACABIO 1265-1/1202).

Author Contribution

Z.J.M. and S.E.N. conceptualized and designed the study. Z.J.M. led the analysis and first manuscript draft with support from S.E.N.S.E.N. led in project administration, supervision, and funding.

Acknowledgement

We acknowledge the local Indigenous communities and Treaty 6 First Nations for supporting research on their traditional lands near Cardinal Divide, Whitehorse Provincial Park, Alberta. We thank Whitehorse Creek Provincial Park for permission to conduct research following the research permit #23-395. We thank Alexa MacDonald and Natalya Klutz for their assistance with data collection.

Data Availability

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

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Direct and indirect effects of terrain, snow, and shrubs on the structure of an alpine herbaceous community in the Canadian Rocky Mountains

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Abstract

Figures

Introduction

Methods

Study Area

Study Design

Sampling Methods

Snow Persistence

Measures of Local Terrain Features

Quadrat-Level Tree Canopy and Soil Depth Measures

Statistical Analyses

Results

Discussion

General Findings

Effect of Shrubs on Alpine Herbaceous Communities

Influence of Snow Persistence on the Alpine System

Effects of Terrain on Snow Persistence and the Structuring of Alpine Plant Communities

Implications for Climate Change

Conclusion

Declarations

Funding

Author Contribution

Acknowledgement

Data Availability

References

Additional Declarations

Status:

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