Can the Impact of Gravel Roads on Organic Layer Thickness Explain the Distribution of Populus tremuloides along Road Networks in the Boreal Forest of Eastern Canada?

Roads are one of the most widespread forms of natural landscape modification, with numerous ecological impacts on ecosystems [1]. Their construction is accompanied in particular by the creation of edge habitats with environmental characteristics and plant communities that differ from adjacent habitats [2,3]. In forest ecosystems, opening up the canopy through road construction and maintenance modifies habitat conditions by altering wind and light regimes and microclimatic conditions [4,5,6]. These open and frequently disturbed road edge habitats favor the establishment of pioneer plant species, which are intolerant of shade and adapted to disturbance [7]. The use of exogenous materials with properties different from those of existing forest soils as road surfaces also contributes to the alteration of habitat conditions and species composition along roads [8,9,10]. These effects are particularly pronounced in acidic, nutrient-poor environments, where the frequent use of alkaline materials, such as gravel, increases the soil pH and nutrient availability, making conditions unsustainable for acidophilic species [11,12,13]. In addition, these impacts can spread to many adjacent habitats via particle transport by wind, road traffic, and run-off water [14,15].

Although the ecological impacts of roads are well documented, e.g., [16,17,18,19], few studies have examined their impact on native tree species, exceptions being [20,21,22], particularly in the boreal forest [23]. In the Canadian boreal forest, trembling aspen (Populus tremuloides Michx., hereafter referred to as aspen) appears to be particularly effective in using edge habitats [24]. Aspen is the most widely distributed native tree species in North America [25], reflecting its adaptation to a wide variety of environmental conditions. It is also a fast-growing, shade-intolerant pioneer species, capable of establishing massive stands in newly disturbed environments [26,27,28,29] via abundant reproduction by seed and suckering [30,31,32,33]. Aspen also has an excellent dispersal capacity due to its small seeds, which can be carried by the wind over several kilometers [34,35]. According to Andrews [16], species with excellent dispersal abilities and the ability to invade disturbed habitats are “attracted” to edge habitats. In the boreal forest of Eastern Canada, the aspen distribution does indeed appear to be influenced by road edge habitats. For example, east of James Bay (Quebec, Canada), the aspen distribution is concentrated near anthropogenic infrastructures such as roads [36]. South of James Bay in the Clay Belt of Quebec and Ontario, aspen is densely present along the road network and can colonize road edges even when the surrounding habitat matrix is unsustainable for it [37].

In the Clay Belt of Quebec and Ontario, the aspen distribution seems to be partly constrained by edaphic conditions, particularly by the thickness of the organic layer [38,39]. The Clay Belt is a region prone to paludification because the cold climate, flat topography, relatively long fire cycle, and abundance of fine-textured deposits favor the accumulation of a thick layer of organic matter on the forest soils [40]. This accumulation of organic matter affects edaphic conditions, forest productivity, and plant community composition [41,42]. As organic matter has a low thermal conductivity, its accumulation leads to a drop in the soil temperature, which consequently decreases decomposition and nutrient availability [43]. The accumulation of organic matter is also accompanied by a rise in the water table, due to the increased bulk density of the soil, which induces high capillarity and low hydraulic conductivity [44]. The rising water table creates anoxic conditions [45], which reduce the rate of decomposition of organic matter [44] and can lead to tree death. The accumulation of organic matter is also maintained by facilitation processes during the succession of bryophyte communities [46,47]. These changes in edaphic conditions induced by the accumulation of organic matter create unsustainable environments for aspen, which is intolerant to flooded conditions and cold soils [48,49]. The region’s forests thus tend to converge on open peatlands dominated by sphagnum moss (Sphagnum spp.) and black spruce [50], which are more tolerant to these conditions.

The main objective of this study was to evaluate the impact of gravel roads on the organic layer thickness and aspen distribution in a coniferous boreal forest landscape located on the Clay Belt (Quebec, Canada). Despite the strong development of the forest road network since the 1970s in the Province of Quebec, few studies have focussed on their ecological impacts [51]. Between 2017 and 2021, nearly 20,300 new kilometers of forest roads were constructed, reaching a total length of 479,243 km [52,53]. In our study landscape, the road network extends for almost 10,500 km, with approximately 3500 km of this network being gravel roads. Bryophyte-dominated communities, such as those found in the Clay Belt, are known to be particularly susceptible to dust from these roads [15,54] and to edge effects [24,55,56]. At the same time, the edges of these roads can provide a favorable habitat for aspen [57,58]. In a previous study [37], we demonstrated that roads sometimes acted as habitat corridors for aspen, enabling it to establish in unfavorable habitat matrices, generally characterized by the presence of thick organic deposits. In this study, we further assess the relationships between the organic layer thickness, aspen distribution, and the presence of roads by answering the following questions:

• Does the thickness of the organic layer (hereafter OLT) have an impact on the distribution of aspen in the study landscape?

• Do forest roads have an impact on the OLT via the transport of mineral particles from the roads, thus promoting the establishment of aspen?

• Can the impact of roads on OLT explain the distribution of aspen along the road network, and in particular, the role of roads as habitat corridors?

We hypothesize 1. that thick organic deposits >30 cm, cf. [38] limit the presence of aspen in our study landscape; 2. that mineral soil transport from roads reduces the OLT along roads and consequently favours aspen establishment; 3. that the reduction in the OLT along roads explains their role as habitat corridors for aspen.