Introduction

Seismic lines are negatively impacting biodiversity in western Canadian boreal forest and the identification of favorable conditions for natural reforestation is a conservation priority. Many of these lines are in the state of “arrested succession”, with soil compaction and lack of sunlight that generate growth-limiting conditions for a normal pattern of recovery (Filicetti and Nielsen 2018). A key question is whether the interaction with a secondary disturbance, such as forest harvesting, can erase these limiting factors and re-establish favorable conditions for forest regeneration. On the one hand, forest harvesting should lead to successful regeneration of trees through the increase of sunlight, scarification of the soil and reduction of plant competition in the line (Picchio et al., 2020). On the other hand, soil compaction could be reinforced if lines were used as in-block harvest trails, reinforcing the growth-limiting condition for trees. Here I studied whether cut-blocks in mesic upland forest enhance the regeneration of seismic lines and thus “erase” lines from the need for future restoration treatments.

Habitat fragmentation in the boreal forest

The boreal biome represents one of the largest bio-geoclimatic areas in the world, and it contributes to the maintenance of local and global ecosystems with numerous and important ecosystem services (Pan et al., 2011; Steffen et al., 2015). Almost one third of the world’s boreal zones occur in North America with more than 270 million hectares of forest only in Canada (Brandt, 2009). Today, Canada’s boreal forest is threatened by human-driven forest cover changes (Hansen et al., 2013), with habitat loss and fragmentation that are affecting the integrity of ecosystems (Haddad et al., 2015). This phenomenon is especially severe in the Northwest area of the country, where the presence of seismic lines deeply altered the natural landscape (Dabros et al., 2018).

Figure 2: Map of conventional seismic lines in the upper Athabasca oil sands region (BERA study area; ABMI).

Seismic lines

The term “seismic lines” refers to linear corridors of cleared forest (widths between 2-14 m) used to facilitate the exploration of soil sands deposits in the North-West region of Canada’s boreal forest (Dabros et al., 2018). They represent the highest contribution to fragmentation in Canada’s boreal forests (~ 46% of linear features of the entire biome; Pasher et al., 2013) and they can be as dense as 40 km/km² (Dabros et al., 2018). The presence of seismic lines alters the natural behavior of many wildlife species and, due to the creation of natural corridors, it’s causing a population decrease for wildlife population (e.g., decline of the woodland caribou population in Alberta due to the increased predation rates; Schneider et al. 2010). Some cut lines fail to become reforested after they are abandoned (van Rensen et al., 2015) and they require expensive restoration treatments to recover the altered portions of forest (CAD$12 500/km; Filicetti et al. 2020). In response to the arrested succession of seismic lines numerous conservation initiatives have been implemented (e.g., the BERA project), to define a restoration plan for seismic lines and promote a leave-for-natural regeneration of the forest (Government of Alberta 2017).

In the context of Canada’s boreal forest, seismic lines occur in combination with many other disturbances, and understanding how different drivers of ecosystem dynamics interact together is essential to identify which seismic lines will regenerate naturally. The effect of composites disturbances (multiple disturbance events occurring together; Buma 2015) is not just the sum of effects from single factors (Figure 3; Buma 2015) and the complexity emerging from this process can affect the ecosystems resilience in different ways (e.g., as “ecological refugees”; Riva et al., 2020). For instance, it has been shown that wildfires in the boreal forest can result in a positive effect on both biodiversity and tree regeneration in seismic lines in different ecosites (Riva et al., 2020; Filicetti and Nielsen 2022). Here I hypothesize that harvesting practices in mesic upland forests might play an analogous role for the regeneration of seismic lines, with the clearcut that might enhance forest resilience to linear disturbances. Higher sunlight, reduction of plant competition and scarification of the soil that occur in cut-blocks have shown to promote the establishment of early successional regenerating stands (Frey et al., 2003; Picchio et al., 2020), and how this could prevent from the application of active restoration treatments to reach the structural regeneration of lines it's poorly understood.

Figure 3: Schematic representation of the possible effects of composite disturbances (dominant, additive, or Interactive combined with positive, neutral, or negative; Buma 2015).

Objetive

The focus of this study is investigating the composite effect of seismic lines and cut blocks on tree regeneration in the mesic upland boreal forest. More precisely, I evaluated the differences in tree density between 3 different disturbance scenarios (i.e., lines in cut blocks, lines in uncut forest, just cut-blocks), to estimate the effect of clearcutting on the regeneration of seismic lines. Based on the literature I formulated two possible hypotheses that could explain my results (table 1). To quantify how different combinations of disturbances can result in a successful regeneration of the line, I compared the stem density from each disturbance scenario not only with density in the control forest, but also with the Alberta's Regeneration standard. Almost 40% of Canada’s boreal forest is now managed for forestry reasons (Gauthier et al., 2015) and the simultaneous occurrence of seismic lines and cut-blocks is likely one of the main drivers of forest succession inside the line in the mesic upland forest. A cost-effective use of conservation resources is priority and understanding whether or not seismic lines fail to regenerate even after a clear cut occurs, can lead to a more effective targeting of active restoration treatments.

Table 1: Set of hypotheses, description, and prediction tested in this study.

Figure 4a: Example of a full regenerating seismic line inside 10 years old cut-block.

Figure 4b: Example of a seismic line that failed to regenerate inside a cut-block.