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Bagworm caterpillars emerge from their eggs in the spring (around late-May through mid-June) and they begin to feed on foliage. In the beginning, feeding damage will look like etching, but as the caterpillars get larger they will consume entire needles and leaves. Bagworms are voracious caterpillars that can damage trees and shrubs by eating their leaves, buds, foliage, and twigs. Bagworms can damage bark when they build their silk bags and add plant material to them as they grow. The bags can rub against and damage the bark of a tree.

It looks like the organic material under your trees is bits and pieces of dead needles, leaves, twigs and bark.

For more information on bagworms see:

https://extension.umd.edu/resource/bagworms-trees-and-shrubs/

https://www.udel.edu/academics/colleges/canr/cooperative-extension/fact-sheets/bagworms/