Introduction

Gravel highways and roads are commonly constructed in regions underlain by continuous permafrost because they are less costly to construct and easier to maintain than paved surfaces (GNWT, 2012; Walker et al., 1987). These surfaces provide critical transportation corridors between northern communities in Canada and the South. Northern roads are increasingly vulnerable to environmental changes, including changing precipitation regimes and permafrost degradation (Burn and Kokelj, 2009; Moritz et al., 2002; Romanovsky et al., 2010; Tape et al., 2006). Gravel roads alter surface and subsurface conditions, which influence the surface energy balance, near-surface ground thermal properties, and the temperature regime of the underlying and adjacent permafrost. Road embankments modify local hydrology and create a physical barrier that fragments the landscape. During initial construction, the removal of vegetation and establishment of culverts for stream and river crossings alters local hydrology and increases soil erosion (Claridge and Mirza, 1981; Myers-Smith et al., 2006). In permafrost terrain a thaw bulb several meters deep can develop below the toe of the road embankment within a few decades of construction, leading to subsidence of the embankment and adjacent land (Hayley, 2005; Lingnau, 1985). The development and maintenance of gravel roads also requires large quantities of gravel. The extraction of borrow materials from areas of ice-rich permafrost can lead to challenges in the construction and maintenance of some quarries. Roads affect adjacent terrain by increasing dust deposition and runoff, altering soil properties, and impacting plant physiology (Auerbach et al., 1997; Eller, 1977). Roadside gravel deposition, trash, and vehicular pollution are other factors that may affect adjacent vegetation (Walker et al., 1987). Anecdotal evidence suggests that changes in vegetation structure adjacent to many northern roads have been extensive. These changes have the potential to alter ground temperatures adjacent to roadbeds by influencing snowpack (Marsh et al., 2010; Sturm et al., 2001a). Maintenance of permafrost in the road embankment is critical to long-term stability and research is needed to examine the feedbacks between gravel roads, vegetation change, snow accumulation, and near-surface ground thermal regime in the Arctic.

The Dempster Highway is a 740 km gravel road between Dawson City, Yukon, and Inuvik, Northwest Territories. This highway is a critical transportation link between the Beaufort Delta Region and southern Canada. The Dempster Highway begins at 64°N, and traverses boreal spruce forest, woodlands, tundra, and a variety of wetlands before terminating south of the treeline at 68°N (Western Arctic Handbook Society, 2007). In the Peel Plateau region of the Northwest Territories, the Dempster Highway descends 850 m from the Richardson Mountains across a fluvially incised plateau consisting of ice-rich morainal deposits (Fulton, 1995). This area is characterized by spruce forest at lower elevations and dwarf shrub tundra at higher elevations (GNWT, 2007). Residents in nearby communities have expressed concerns about the impacts of the road on vegetation and terrain stability (Gill et al., 2014; Kershner, 2010; Scott, 2011). The Peel Plateau is also an important area for harvesting country foods, including wild berries, and development impacts on the abundance and distribution of berry patches are a concern to Teetl'it Gwich'in harvesters from Fort McPherson, Northwest Territories (Parlee et al., 2005).

Several studies in the western Arctic have investigated the impacts of gravel roads on moist sedge tussock and dwarf shrub tundra (Auerbach et al., 1997; Myers-Smith et al., 2006; Walker and Everett, 1987). However, to date no studies have explored the impact of gravel roads on tall shrub tundra, such as is found along parts of the northern portion of the Dempster Highway. The Dalton Highway in Alaska has been shown to alter acidic and nonacidic tundra plant community composition, decrease species richness and soil moisture, and increase soil pH, bulk density, snow accumulation, and the rate of snowpack melt (Auerbach et al., 1997; Myers-Smith et al., 2006; Walker and Everett, 1987; Walker et al., 1987). The Dalton Highway has also contributed to altered vegetation structure, deeper snowpack, and increased active layer thickness adjacent to the road (Auerbach et al., 1997; Myers-Smith et al., 2006). In the Peel Plateau, the Dempster Highway has likely facilitated similar ecological changes, but the long-term impacts of this road have not been quantitatively investigated (Smith, 2009). Exploring the ecological feedbacks associated with the Dempster Highway in the Peel Plateau will contribute to a more thorough understanding of the regional impact of all-weather roads.

In this study, we used space for time substitution to examine the impacts of 35 years of ongoing use and maintenance of a gravel road on shrub tundra vegetation, soils, and permafrost conditions in the Peel Plateau region. Our primary objective was to examine if the physical disturbance associated with the road and its ongoing use and maintenance, including the effects of dust deposition and altered drainage, have affected vegetation composition and structure, and whether these changes have impacted permafrost conditions. To examine how the effects of the road may be mediated by vegetation type, we compared biotic and abiotic conditions at sites adjacent to the road with undisturbed tundra, and sampled in areas of tall shrub tundra and dwarf shrub tundra. We hypothesized that the effects of the road would be greatest in tall shrub tundra, where feedbacks between tall shrubs and the road amplify the effects on vegetation, soils, and ground temperature underneath tall shrubs.

FIGURE 1.

Quickbird (2008) satellite image of the study area showing sites along the Dempster Highway classified by disturbance and vegetation type. Inset map at bottom left shows the position of the study area in northwestern Canada (red star).

Methods

ABIOTIC RESPONSE VARIABLES

To measure ground temperatures, we installed HOBO data loggers (U23 Pro v2, Onset Computing) under representative vegetation at five roadside tall shrub sites, five control tall shrub sites, and five control dwarf shrub sites. Each data logger was attached to two thermistors (U23-002, Onset Computing) anchored along a PVC tube inserted and positioned at 10 cm and 100 cm below the ground. Temperatures were logged every hour for one year before data collection in August 2012. Thermistors were not installed at roadside dwarf shrub sites.

In March 2012, we measured snowpack thickness at each site along a 75 m snow transect parallel to the highway by inserting a graduated avalanche probe every 5 m. To measure active layer thickness, in late August 2012 we inserted a graduated soil probe to the depth of refusal at 10 random locations along the 200 m transect. In hummocky terrain, active layer measurements were taken in the center of the hummock tops in order to minimize the effects of within-site microtopographic variation. Two measurements of litter and organic layer thickness were also made in the same 10 locations using a metal ruler inserted into the soil to visually demarcate horizons. To measure the supply rate of plant-available nutrients, we used Plant Root Simulator (PRS) nutrient probes (Western Ag Innovations, Saskatoon, Saskatchewan, Canada). PRS probes consist of paired cation and anion exchange resin membranes and provide nutrient supply rates in milliequivalents cm^-2 exchange membrane per days of burial (Western Ag Innovations Inc., 2012). In 2012, we installed 6 pairs of probes at each of our four site types (n = 24) for 30 days (23 July-23 August 2012). In 2011, we deployed probes at four roadside tall shrub sites, three control dwarf shrub sites, and three control tall shrub sites between 22 July and 22 August 2011. All probes were inserted to a depth of 15 cm. To measure soil pH, we collected 100 cm3 organic soil samples from between hummocks at two locations along each transect. In the lab, we mixed 10 mL of soil with 40 mL of deionized water, agitated the solution for three minutes, and left it standing for two hours before measuring the pH.

FIGURE 2.

Plant community composition at four site types: (A) roadside dwarf shrub, (B) roadside tall shrub, (C) control dwarf shrub, and (D) control tall shrub sites.

Results

BIOTIC RESPONSES

The presence of the road had a strong impact on alder abundance and growth. Alder cover was significantly greater at roadside tall shrub sites than at all other site types, including control tall shrub sites (p < 0.001). Other deciduous shrubs (Betula glandulosa, Spiraea beauverdiana, and Salix spp.) were not influenced by the road and had similar abundances across site types. Alders at roadside tall shrub sites were significantly younger, taller, and faster growing compared to controls (Fig. 3). Alders at roadside dwarf shrub sites were slightly younger and shorter, but were faster growing than at roadside tall shrub sites; however, the differences were not significant (Fig. 3). Both roadside site types had a higher proportion of young individuals than control sites, where the population was dominated by individuals >25 yr (Figs. 3 and 4). Alders were not encountered in surveys of control dwarf shrub sites, but recent alder recruitment was evident at dwarf shrub sites adjacent to the road. Comparisons of alder stem counts made using 1972 air photos and 2008 Quickbird satellite images confirm that alder proliferation within 100 m of the road has occurred more rapidly than in undisturbed tundra (Fig. 5).

The plant community at roadside tall shrub sites was distinct from all other site types (Fig. 6, R_ANOSIM values 0.315–0.640, Table 1). These differences were driven primarily by a higher abundance of alders and a lower abundance of acrocarpous mosses, lichens, peat mosses, and dwarf shrubs (Empetrum nigrum, Rhododendron subarcticum, and Vaccinium vitis-idaea) at roadside tall shrub sites (Appendix Table A1). Roadside and control dwarf shrub sites were also distinguished from each other by a lower abundance of lichens, acrocarpous mosses and Petasites spp. at the road, and a higher abundance of E. vaginatum and most dwarf shrubs at the road (Table A1). The only site types with similar community composition were control tall shrub and roadside dwarf shrub sites (Table 1); although there were fewer alders at the latter, both had understory vegetation dominated by sedges and dwarf shrubs. The magnitude of differences in plant community composition among site types is shown visually in an NMDS ordination (Fig. 6). At the road, the abundance of species harvestable for edible berries was lower at tall shrub compared to dwarf shrub sites (Table A1). Away from the road, berry plants were also less abundant at tall shrub sites.

FIGURE 3.

Alder response variables measured in undisturbed tall shrub tundra (control tall shrub) and beside the Dempster Highway (roadside tall shrub), and in dwarf shrub tundra beside the highway (roadside dwarf shrub): (A) average age (years), (B) stem height (cm), (C) radial growth rate (cm/year), and (D) vertical growth rate (cm/year). Alders did not occur in the sampled plots at undisturbed dwarf shrub sites. Bars show means for each site type and error bars are 95% confidence intervals of the mean (untransformed). Bars sharing the same letter are not significantly different (α = 0.05, mixed model and Tukey adjusted least squares means procedure).

TABLE 1

Pairwise comparisons of plant community composition between site types using the ANOSIM procedure. R_ANOSIM values > 0.75 indicate well-separated groups, values between 0.25 and 0.75 describe overlapping but distinguishable groups, and values < 0.25 represent groups that cannot be separated. Readily distinguishable sites are indicated in bold (Clarke and Gorley, 2001).

FIGURE 4.

Alder age distributions at (A) roadside tall shrub tundra sites, (B) roadside dwarf shrub sites, and (C) control tall shrub sites. Alders did not occur in the sampled plots at undisturbed dwarf shrub sites. Bars indicate number of individual alders in each age category.

Discussion

Differences in plant community composition and alder age structure between control and roadside tall shrub sites indicate that the presence of the Dempster Highway has contributed to significant changes in the vegetation beside the road. At tall shrub sites adjacent to the road, young alders formed closed canopies that often exceeded 3 m in height. Differences in alder response variables among sites indicate that environmental changes following road construction facilitated both alder growth and recruitment near the embankment. Alder populations near the road were between two and three times faster-growing than at control sites and were dominated by individuals recruited in the last two decades. Increased deciduous shrub cover and biomass have been observed in other studies of gravel roads in tundra ecosystems (Myers-Smith et al., 2006), but the changes were much less pronounced than the shifts evident next to the Dempster Highway.

At roadside sites where tall shrubs dominated, moss, lichen, and forb cover were significantly reduced, likely in response to increased shading and litter deposition from the closed canopy. Dwarf shrub and forb cover was also reduced at roadside tall shrub sites, including several culturally important berry species (E. nigrum, R. chamaemorus, V. uliginosum, and V. vitis-ideae). Of these, R. chamaemorus (cloudberry) was the only edible berry observed flowering under the alder canopies, but it almost never produced fruits. Dust deposition and increased soil alkalinity likely contributed to reduced cover of mosses, lichens, forbs, and dwarf shrubs (Auerbach et al., 1997; Santelmann and Gorham, 1988; Appendix Table A2). These findings are supported by previous studies on the effects of gravel roads on tundra plant communities, which documented decreases in peat mosses and lichens and an increase in graminoids (Myers-Smith et al., 2006). Even where tall shrubs have not come to dominate the plant community, the road has facilitated vegetation change, promoting sedge growth and reducing the cover of mosses, lichens, and forbs. These differences were likely caused by increased soil alkalinity and shading from a vigorous sedge canopy (Everett, 1980; Farmer, 1993). Similar differences in plant communities were found between tall shrub- and dwarf shrub-dominated sites undisturbed by the road, but the contrasts were smaller because alders did not form closed canopies away from the road.

FIGURE 5.

Vegetation adjacent to the Dempster Highway on the Peel Plateau, Northwest Territories, Canada. Shrub proliferation, particularly of Alnus viridis, since the 1970s has been most extensive adjacent to the road.

FIGURE 6.

Nonmetric multidimensional scaling ordination of plant community composition based on Bray-Curtis similarity matrix. Symbols plotted are individual plots sampled at road-disturbed and undisturbed sites in tall and dwarf shrub dominated tundra. Plots that are in close proximity to each other in this ordination space have similar species composition.

Vegetation change beside the highway was likely facilitated by several processes: microsite disturbance, increased nutrient availability, and elevated moisture. Greater dust deposition beginning within the first few years of road use and increased microbial activity and nutrient mineralization driven by prolonged freezeback beside the embankment likely enhanced deciduous shrub growth (Lantz et al., 2009, 2010; Zimov et al., 1993). Road construction itself may have also promoted shrub recruitment by creating microsites for establishment (Blok et al., 2011; Lantz et al., 2013; Tape et al., 2012), as a swath wider than the current embankment was likely disturbed during construction. Increased snow accumulation and impeded drainage due to presence of the road embankment may have also facilitated vegetation change. Deep snowpack protects shrub vegetation from desiccation by winter winds and, in conjunction with higher soil moisture content adjacent to the road, slows ground heat loss in winter and delays freezeback of the active layer (Zhang and Stamnes, 1998). In summer, warmer soil temperatures likely promoted active layer thaw and increased moisture and nutrient availability. Collectively, these conditions are highly favorable for tall shrub establishment and growth (Lantz et al., 2009; Sturm et al., 2001a; Tape et al., 2012). These effects were likely enhanced over time as terrain adjacent to the road embankment subsided due to thaw of near-surface permafrost (Hayley, 2005).

FIGURE 7.

Median, maximum, and minimum temperatures at (A) 10 cm depth and (B) 100 cm depth from August 2011 to August 2012 at disturbed tall shrub (roadside tall shrub), undisturbed tall shrub (control tall shrub), and undisturbed dwarf shrub (control dwarf shrub) tundra sites. Solid lines show median daily temperature across site type, and upper and lower broken lines show daily maximum and minimum temperatures, respectively.

Canopy-forming alders did not dominate all sites adjacent to the road, suggesting that shrub proliferation is also mediated by existing biophysical conditions. Local differences in soil composition, moisture, slope, aspect, and mechanical disturbance at the time of construction likely also influenced shrub recruitment and growth. This is not surprising, since shrub proliferation in undisturbed environments throughout the Arctic has been heterogeneous (Fraser et al., 2014; Tape et al., 2012). Field studies in other areas that link tall shrub expansion to mesic soil environments (Blok et al., 2011; Lantz et al., 2013; Tape et al., 2012) suggest that heterogeneous changes in soil moisture adjacent to the road may have facilitated shrub proliferation in some areas and not others. Further investigation is required to assess the factors facilitating or constraining shrub proliferation adjacent to the Dempster Highway. Research using historical air photos should be conducted to map vegetation change and examine the relationships between areas of shrub proliferation and landscape-scale variation in biophysical variables.

Our findings indicate that vegetation development adjacent to the highway initiates biotic and abiotic feedbacks that affect ground temperatures. Near-surface ground temperatures beneath areas of tall shrub 15 m from the road embankment were significantly higher than in control areas, and freezeback was delayed more than 3 months. It is known that thawing of near-surface permafrost and terrain subsidence can occur immediately adjacent to road embankments, but terrain more than 10 m away from the toe of the embankment is typically considered unimpacted (Hayley, 2005; Walker and Everett, 1987). Anomalously high ground temperatures and delayed freezeback at our roadside sites were likely caused by the higher latent heat content of wet soils, by a thicker active layer, and by the early and thick accumulation of snow.

FIGURE 8.

Biotic and abiotic response variables measured in undisturbed tall shrub tundra (control tall shrub) and dwarf shrub tundra (control dwarf shrub), and in disturbed tall shrub tundra (roadside tall shrub) and dwarf shrub tundra (roadside dwarf shrub): (A) alder cover (%), (B) snowpack thickness (cm), (C) active layer thickness (cm), (D) litter thickness (cm), (E) organic layer thickness (cm), and (F) soil pH. The solid black line inside the box shows the median, the ends of the box represent the 25th to 75th percentiles, and whiskers show the 10th and 90th percentiles. Boxes sharing the same letter are not significantly different (α = 0.05, mixed model and Tukey adjusted least squares means procedure).

Our conceptual model (Fig. 10), supported by field observations, suggests that the initial thermal disturbance resulting from road construction and increased snow accumulation is accentuated by shrub proliferation. In particular, progressive shrub growth may expand the lateral extent of snow drifting and ground surface warming, further strengthening the feedbacks described above (Fig. 10; Essery and Pomeroy, 2004). Warmer ground temperatures observed beyond subsided areas immediately adjacent to the road suggest that by trapping snow, the proliferation of roadside alders has increased the intensity and spatial extent of the thermal disturbance that would normally be associated with a gravel road in continuous permafrost (Fig. 10). Tall alders trap snow and limit compaction of the snowpack, which insulates the ground and inhibits ground heat loss in winter (Sturm et al., 2001a). The dense alder canopy may have a cooling effect in summer when shading reduces ground temperatures (Fig. 7, part a; Blok et al., 2010; Chapin et al., 2005; Lantz et al., 2013; Sturm et al., 2001a, 2005). However, the shading effect of the alders on ground temperatures is greatly offset by the influence that deep snow has on ground heat loss in winter. The hypothesis that higher ground temperatures are promoted by shrub-snow interactions is supported by our observation of warmer temperatures in control tall shrub tundra compared to control dwarf shrub tundra (Fig. 7). Although we did not find significantly thicker active layers at roadside tall shrub sites, ground temperatures were higher at these sites and thaw depths greater than 120 cm were only observed under roadside shrubs.

FIGURE 9.

Plant-available nutrient supply rate (μg/cm²/d) at undisturbed tall shrub tundra (control tall shrub) and dwarf shrub tundra (control dwarf shrub), and in disturbed tall shrub tundra (roadside tall shrub) and dwarf shrub tundra (roadside dwarf shrub): (A) calcium, (B) magnesium, (C) sulfate, and (D) total nitrogen. The solid black line inside the box shows the median, the ends of the box represent the 25th to 75th percentiles, and whiskers show the 10th and 90th percentiles. Boxes sharing the same letter are not significantly different (α = 0.05, mixed model and Tukey adjusted least squares means procedure).

Shrub proliferation adjacent to the road also created a positive feedback with soil chemistry, as dust and nutrient deposition adjacent to the road were more pronounced where tall shrubs dominated (Fig. 10). Previous research suggests that the dust load is greater closer to the road (Everett, 1980), but taller vegetation likely also traps more dust, leading to greater increases in levels of Ca, Mg, and S at roadside sites covered with tall shrubs. Alder growth is enhanced by increased micronutrients deposited in road dust and runoff (calcium, sulfur, magnesium) and increased pH (Lantz et al., 2009; Sturm et al., 2001b). Since alders have the ability to fix nitrogen, their proliferation also altered available soil nitrogen (Mitchell and Ruess, 2009; Rhoades et al., 2001).

The first order effects of gravel roads on tundra ecosystems are well known: heat transfer to adjacent ground that results in permafrost thaw, altered plant communities, increased soil pH, and microsite disturbance due to road construction and maintenance (Hayley, 2005; Walker and Everett, 1987). Our results show that the conditions generated by the maintenance and use of the road interact with local ecosystem processes to generate feedbacks between biotic and abiotic conditions that enhance dust deposition and snow accumulation and magnify the effects of the road on vegetation communities and substrate properties. In this study, we found that where the road facilitated shrub dominance, taller vegetation enhanced snow accumulation and altered ground temperatures and soil chemistry. In turn, these changes promoted enhanced shrub recruitment and growth (Fig. 10). When vegetation near the road is altered, but does not result in establishment of tall shrubs, disturbance feedbacks are much less pronounced because snow and dust are distributed more sparsely without tall shrubs acting as a windbreak. Tall shrub expansion has clearly been facilitated by the Dempster Highway, and the changes we observed are consistent with other forms of disturbance that also promote shrub growth (Forbes et al., 2001; Johnstone and Kokelj, 2009; Kemper and Macdonald, 2009; Lantz et al., 2009, 2010, 2013; Marsh et al., 2005). The surface of all-weather roads directly impacts a relatively small area, but the extent of hydrological changes, dust redistribution, shrub proliferation, and changes to ground thermal regime extend the effects of roads across a significantly larger area (Walker and Everett, 1987).

The ecological changes we documented, as well as feedbacks with ground thermal conditions, have significant implications for the long-term stability and maintenance of transportation corridors through tundra environments. The stability of northern infrastructure relies on frozen ground in, and adjacent to, the road embankment (Andersland and Ladanyi, 2004; Couture et al., 2000). Growing evidence indicates that anthropogenic and natural disturbances facilitate the establishment and proliferation of tall shrubs (Forbes et al., 2001; Johnstone and Kokelj, 2009; Kemper and Macdonald, 2009; Lantz et al., 2009, 2010, 2013; Marsh et al., 2005). Tall shrub proliferation on disturbances such as drilling mud sumps, roads, and slumps has been shown to contribute to increased snow accumulation, higher ground temperatures, permafrost degradation, and subsidence (Johnstone and Kokelj, 2009; Kanigan and Kokelj, 2008; Kokelj et al., 2010; Lantz and Kokelj, 2008). The implications of ground warming and permafrost thaw adjacent to roads include an increase in ground settlement and loss of ground bearing capacity and soil creep resistance (Couture et al., 2000). To better understand the implications for roadbed stability, our network of ground temperature monitoring sites adjacent to the Dempster Highway can be used to parameterize numerical models that investigate the effects of vegetation, embankment configuration, and snowpack on ground temperatures. Our findings make it clear that the long-term management of gravel roads in the Arctic, including the proposed Mackenzie Valley Highway (GNWT, 2005) and the Inuvik-Tuktoyaktuk Highway, should consider the effects of roads on vegetation, soils, snow accumulation, and ground thermal conditions. Active vegetation management (regular shrub cutting) should be considered as a potential method to maintain permafrost conditions in and around infrastructure constructed in tundra environments. Future work at the sites described here will involve shrub removal experiments to assess the impacts and feasibility of this management strategy.

FIGURE 10.

Cross section of a gravel highway showing the development of ecological feedbacks in tundra ecosystems (A) immediately following gravel road construction, and (B) after 35 years. Construction of the embankment causes hydrological changes (particularly water pooling adjacent to the road) and traps snow, leading to warmer ground temperatures, thicker active layers, and increasing nutrient availability, promoting shrub growth near the embankment. When tall shrubs become dominant, several feedbacks are strengthened: tall shrubs act as a windbreak to increase dust deposition; soil nutrient availability increases; mosses and acidophilous plants are reduced as soil pH and shading increase; and tall shrub growth is promoted through enhanced nutrient availability and reduced competition. Tall shrubs acting as a windbreak may also increase the depth and lateral extent of snow accumulation, insulating the ground and potentially leading to higher ground temperatures and thicker active layers beyond areas immediately adjacent to the road; this in turn promotes tall shrub growth by increasing nutrient availability, soil moisture, and rooting depth. Black text and arrows are processes observed in our study of the Dempster Highway; gray are hypothesized or known to occur in other studies of gravel roads in Arctic tundra.