Apples and pears have starches that convert to sugar after being picked. Stonefruit do not- they usually soften so you can call this ripening. Many new breeds of stonefruit get up plenty of sugar while still firm to improve storage life- this seems a primary quest of the Zeiger work and other programs seem to be achieving the same goal.

Older varieties of plums that I grow have to be soft on the tree to achieve maximum sugar and be what I consider "tree ripe". Even small growers do not find it practical to try to market such soft fruit as it can almost burst from its own weight.

I have to harvest my plums well before they are ripe or the racoons & squirrels eat them all. Unfortunately, I haven't had good luck w/ ripening them off the tree. They are much sweeter if ripened on the tree.
If anyone has found a solution to either the racoon/squirrel issue or the ripening of the tree issue, I'd love to hear it.

Here is a link that might be useful: trailsnet

I had a peach tree for 9 years. I too had a squirrel problem eating the fruit before it ripened just for the nut. I completely covered the branches with old bed sheets tied at the bottom and used clothes pins to close any openings. My tree was a dwarf tree so it only took 3 sheets and maybe a pillow case for a smaller branch. Although the squirrels could have ripped through the sheets, they did not. Out of sight out of mind!

Anytime you cover your fruits...it should be done before ripe or before critters take snacks. Plums...pick when ripe, if necessary for firmer fruits for transport..pick only a couple of days before.

That is just not true. Plums can complete their ripening cycle off the tree. Just lay out on a counter, or on a mat in the garage, or some put in paper sacks. Keep an eye on them if in the garage, as they ripen quickly.