The fastest ultrasonic sensor I found was 10Hz.  It also seems that multiple hits are required to get a valid range.  That makes the actual Hz 5 or less.  I will assume a tractor speed of 6 mph or 8 ft/sec.  We go faster than that when spreading dry material and slower when spraying.  Eight ft/sec gives one range every 1.6 ft.  A one ft diameter trunk is possible with a very old, generally no longer productive almond tree.  Newly planted trees are in the 1/2 to 3/4 inch range.  The effective diameter of an ultrasonic sensor is a narrow cone and is not going to pick up a target reliably.  That is why I believe ultrasonic sensors are too slow.