Composite effects of forest harvests and seismic lines influence re-establishment of trees and shrubs in Alberta’s mesic upland boreal forest

Abstract

In the boreal biome, disturbances drive natural vegetation dynamics and forest recovery. However, as the frequency and extent of anthropogenic disturbances increase, there is a potential for composite or synergistic effects to emerge among disturbances, thereby affecting forest regeneration. This is particularly relevant in Alberta's boreal forest, where oil exploration through networks of narrow clearings called seismic lines dissect and fragment forests, but sometimes also overlap with other anthropogenic disturbances. A focus on the restoration of seismic lines without considering these composite disturbances may result in reduced efficacy. One question for seismic line restoration is whether forest harvesting “erases” seismic lines within them by restoring composition and structure, thus precluding the need for further restoration actions. To test this question, we measured responses in the abundance and composition of trees and shrubs at 15 mesic upland forest sites that were recently harvested and contained an existing conventional seismic line. For each site, we compared two pairs of plots on and off seismic lines with respect to inside and outside of forest harvests. We found that harvested seismic lines had ∼ 130 % more woody stems than untreated lines through adjacent mature forests, suggesting that forest harvesting was facilitating the structural recovery of the forest inside the line. Moreover, the simultaneous occurrence of both disturbances led to increases in deciduous tree stems while decreasing shrub abundance, potentially helping to achieve the reforestation goals for seismic lines. Finally, we observed interactive effects between the two disturbances on the relative abundance for ∼ 40 % of the species assemblage. We demonstrate that the spatial overlap of seismic lines and forest harvesting can facilitate restoration of forest structure, but also results in a distinct “composite” legacy signature on species composition. While forest harvesting appeared effective in “erasing” seismic lines without further active restoration efforts, emerging composite effects can significantly influence forest composition, with potential long-term implications for regeneration that require further investigation.