Long-term expansion of satellite-measured beaver pond area after boreal forest fire

The engineering activities of beavers have a major influence on hydrology, vegetation, and biodiversity. The availability of preferred broadleaf vegetation forage is an important factor facilitating beaver colonization, and its subsequent depletion may result in the abandonment of beaver ponds. Previous studies have suggested that wildfires can lead to long-term increases in beaver populations by promoting the growth of early successional broadleaf vegetation. However, spatially explicit analyses demonstrating this relationship in the wildfire-modified boreal forest zone are lacking. In this study, we used Landsat satellite data for measuring changes in the extent of beaver ponds to provide an indicator of shifts in beaver activity and populations after fire. A sub-pixel mapping method was used to track annual surface water area in 3597 beaver pond complexes from 1985-2021 within a 4546 km2 region of northwestern Ontario, Canada where wildfires burned 938 km2 since 1985. We found that the surface water in 1390 beaver complexes adjacent to burns initially decreased after fire but then began to recover after four years. After 12 years, fire-impacted beaver ponds exceeded the area that would be expected based on changes in 2207 unimpacted beaver ponds that served as a control. Impacted ponds continued to expand and 34 years after fire had an extent that was 140% larger than control ponds. Annual land cover maps and forest inventory plots indicated that the cover of broadleaf shrub and tree forage adjacent to these expanding ponds was elevated above pre-fire levels for at least three decades following fire, while the cover of non-preferred needleleaf trees was substantially reduced. A similar, post-fire beaver pond expansion can be observed at other locations across Canada where broadleaf tree regeneration is present.