I agree with rainbowgardener -- I don't think you have thought this through.
If you wrap the entire trees with clear plastic, you would create not only humidity but heat inside from the sunlight.
Soft mulberry fruit will quickly spoil and turn into red-brown-black goo. Buzzing with flies, wasps, fruit-flies, and other insects. Mold and rot will quickly follow, which will be swimming with maggots. It will be much messier job to try to clean all that up later.
Besides, don't you have birds and animals eating the berries, too?
Mulberries have tendency to fall off the branches as soon as they are close enough to ripe. When the branches are disturbed/shaken, more will drop. When birds and animals are trying to eat them, they disturb the branches, if they touch berries in trying to eat them, the berries drop. In the mean time, birds poop.
Unless you are diligent, what you get on anything you lay under the trees are mixture of good fruits with wormy fruits, unripe fruits, and birdpoop. Also, mulberry tree develops dead branches and twigs that break off easily. Those come falling down as well.
If you are seriously trying to think of ways to collect the good fruits, only way is to either pick the good fruits ONLY from the branches very carefully, or lay a clean tarp/sheet under the trees and pick the good fruits directly from the branches while the tarp.sheet saves what you accidentally drop. THEN pick up the good berries from the tarp.sheet. THEN Give the tree and branches a good shake. You can sort through the good and the bad that fall.
You need to then remove the tarp and clean it until next time you want to harvest. If you leave the tarp on the ground, then it will be covered with junk in less than a day.
NOW, as far as simply keeping the berries from falling on the pavement, don't try to cover the entire tree, but think "umbrella" or "roof" --my best recommendations would be
(1) to stretch an awning or tunnel for people to walk under if that's all you need
(2) or -- stretch an awning that spans the full area under the tree that is sloped so fruits that fall will roll/slide off one edge
(3) CRAZY IDEA -- *somehow* devise an upside down umbrella kind of a cone or spiderweb shaped awning around the trunk of the tree, stretched all the way to the dripline and higher on the outer rim than the inner rim. Make it so there is a gap in the center that goes around the trunk so all the fruits and debris that fall will slide down to the center and pile up around the base of the trunk.
...letting the berries fall to the ground in a grassy area is by far the EASIEST disposal method. Then you won't even see them and the natural air/ground crew can take them away or eat or decompose them in situ, and the residue will feed the tree for another crop of bountiful fruits next year.
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