Redstone Dirt

Rannuncarpus on a Redstone Dirt Detector block puts saplings on the other detector

Detector shuts off after the tree breaks and the machine is reset

This farm uses the Redstone Dirt Detector to trigger the gearshift and makes use of the Redstone Dirt Blocks underground to carry the signal

Here is what happens when the tree grows and the signal lights up

Redstone Dirt block carries signal from the Hovering Hourglass into the redstone torch once per minute to pickup saplings and place them near the rannuncarpus

Here is the "wiring". Note the two blocks distance to deal with quasi-connectivity (this is a vanilla bug that is also a feature)

Redstone Dirt blocks as "wiring", triggers pulse mana spreader with a Bore Warp lens to chop the tree

Using just the Powered Redstone Block and a repeater. Because it is strongly powered, you can detect when the tree grows and trigger the gearshift