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Why did some areas near Mount St. Helens recover differently after the 1980 eruption?

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Diagram showing energy flow in a food chain from a tree to a caterpillar, then to a bird, and finally to microbes, with red arrows illustrating energy transfer and heat loss at each stage. — community ecology Energy transfer and heat loss along a food chain.

In community ecology, the varying recovery of areas near Mount St. Helens following the 1980 eruption is primarily attributed to the intensity of the disturbance and the presence of biological "legacies".1

The eruption created a mosaic of impact levels across the landscape. Recovery patterns were dictated by whether an area was effectively sterilized or if it retained biological remnants that could initiate secondary succession.12

Biological Legacies: In areas where organisms survived underground or were protected by patches of snow, recovery began much sooner because existing seeds and organisms were already present to recolonize the site.1
Sterilized Areas: In regions where the disturbance was so intense that no life survived, the ecosystem had to undergo primary succession, a much slower process that relies on the dispersal of species from outside the disturbed zone.12

The speed of recovery in these sterilized patches depends on the dispersal capabilities of colonizing species, such as "fugitive" or "weedy" species, which are specially adapted to invade and thrive in disturbed environments.2