The first design uses a Observer to detect the top of the tree trunk; it's big, resource intensive for what it does (3 r-pistons & 6 repeaters), ugly (IMO), and I still have concerns about it glitching
It does, however, solve the problem of observer/piston interaction creating a clock — something that seems to happen (apparently at random) with my attempts to build 1-wide tilable pumpkn/melon farms…
Easily adaptable to a 2x2 tree, although I don't see why anyone would want to use this detector in preference to the second….
My version of an old conventional design. (I was far from the first to use this method, but don't know who originated it.)
Also, an adaptation to 2x2 trees – which is not something I've seen before.
This needs RS-torch x3, Repeater x1, and some blocks and RS-dust for the detector and power line;
R-Piston x4, and some blocks and RS-dust for the first tree (top ONLY).
Extending the farm (top ONLY) requires: R-Piston x4/additional tree as well as more solid blocks and RS-dust (and possibly Repeaters)
The bottom of each tree requires R-Piston x4, as well as whatever solid blocks, RS-dust (and possibly Repeaters) are needed to connect to the pulse-shortener