Comparing the spatial pattern of fire and harvest disturbance in boreal Ontario watersheds

Abstract

Fire and harvest are both major sources of disturbance in Ontario’s boreal forest. They are largely responsible for successional patterns in forest vegetation, and influence regimes of hydrologic change in boreal streams, owing to the relationship between watershed and riparian disturbance and conditions in the stream. These forests and their streams have historically developed under the influence of fire disturbance. However, increased harvest activity and fire suppression have significantly reduced the impact of fire within Ontario’s managed boreal forest. The degree to which harvest activity in these regions results in similar spatial patterns of disturbance within watersheds as fire is unclear. Accordingly, the objectives of this study were to assess if harvest disturbance resulted in a similar extent and landscape pattern of impact as fire, both within boreal watersheds and their riparian forests. In the study’s first chapter fire and harvest were compared within 30 km2 (±20% area) watersheds in the study area. Harvest was the most common of the two, impacting ~30% of the study watersheds during any given period, whereas fire disturbed ~2% of study watersheds. Stark differences were observed between the watershed impacts of the two types. Fire disturbed a greater median percentage of watershed land areas and resulted in a range of impacts, including 100% disturbance. Harvest conversely resulted in lower disturbance percentages, occupying a subset of the variability measured in fire-disturbed watersheds, typically below 20% disturbance. Other contrasts between the types include fire resulting in fewer and more simply shaped patches than harvest, often occurring during a single year of a period compared with multiple years in harvested watersheds. In the study’s second chapter fire and harvest were compared within shoreline riparian buffers, both 30 and 90 m in width around aquatic features. Harvest was the most common for both buffer distances impacting 20% and 25% of the 30 and 90 m buffers respectively, compared with ~1.5% impacted by fire. Fire disturbed a greater percentage of both buffers and resulted in a range of impacts up to 100% buffer area disturbed. Harvest on the other hand resulted in significantly lower disturbance extents, particularly within the 30 m buffer, and only occupied a small subset of the variability resulting from fire, typically <10% area disturbed. Other differences in impact included, more numerous, smaller, and more intricately shaped patches spread over multiple years in harvested buffers over fire. Differences in the watershed chapter indicate that harvest does not provide for a similar extent and landscape pattern of disturbance and as a result, does not likely result in similar regimes of forest succession and stream flow change as fire. In the riparian chapter it is clear that harvest does not provide similar impacts as fire, particularly with widespread usage of reserve forested buffers in Ontario harvest practices. Accordingly harvest is likely not providing for similar successional patterns as fire within riparian forests, and as a result will not provide for flow and stream temperature changes that would naturally occur.