What happens to the fallen leaves in the garden

If you think that it’s not too late to remove the foliage in the spring, and the cold period will quietly lie under the trees and become additional mulch, then you make 5 mistakes at once:

Foliage is a bad material for mulch, because, soaking, the leaves clot and turn into a dense thin layer that does not allow air to flow to the roots and does not allow the ground to evaporate moisture, that is, the foliage left without additional shelter helps the plants to dry out.
On the fallen leaves, insect insects most often lay eggs, and their larvae pupate. With the onset of spring at the first sunbeams, the pests will wake up from sleep, and they will not even have to go far for prey - the native tree or bush is already here, you can begin to grow and reproduce.
In addition, the leaves are often affected by fungal diseases - powdery mildew, anthracnose, gray rot, late blight. The spores of their pathogens winter well and, once in the soil, in the new season they disperse through your garden more actively.
Bacterial diseases are less common in the garden, but their probability cannot be ruled out. Decaying leaves are a wonderful environment for the spread of bacteria, and sunburns on trees in winter become the weak point through which bacteria enter a favorable environment and begin to destroy plants.
The leaves on the lawn are thrown out over the winter, but not alone, but together with the lawn section, on which bald spots appear in the spring. The affected area will not be able to recover by itself, and you will have to first remove the dead grass, and then sow a new one.