Batboard

Which is advisable to use in a rubber and palm plantation with lots of trees.
Mototrbo VHF or UHF band?.
Client is currently on VHF analog with few dead spots.
Will changing VHF analog to UHF Mototrbo be a good idea in terms of better coverage and clarity.

In general, no. The better choice for outdoor use is VHF as far as distance. Now, if the distance isn't all that far, and the dead spots happen to be in buildings, then yes maybe UHF would be a better choice. Is the current VHF analog system simplex, or through a repeater? If it's a repeater, could the location of it be changed to better cover the desired area(s)? Could the height of the antenna be raised in it's current location? How long of a cable run to the antenna? Perhaps the cable could be upgraded to reduce loss. What type of antenna is being used? A different type could potentially work better in your particular situation. There really are a lot of factors to be considered above & beyond basic frequency selection.