Why are mixed forests healthier than coniferous forests?

The purely coniferous man-made forests of Middle Europe grow mainly spruces for the wood industry. These shallow-rooted plants fall over easily in strong storms and are prone to harmful creatures like the destructive bark beetle. The low level of sunlight in the spruce forest does not allow small shrubs to grow on the forest floor. On the other hand, such an undergrowth is found in plenty in the mixed forests. The deciduous trees in the mixed forests allow sunlight to reach the ground so that water-storing moss and berry shrubs can sprout. This attracts the animals that live on beechnuts, berries, and acorns. The deep-rooted deciduous trees also protect the forest against storms.