Getting script to work with multiple models not just a part from one model?

I am not a very good scripter, but I think you should use :GetDescendants('trunk') instead of findfirstchild('trunk'). That way you can group all the trees together and just get the trunk.

Heres a sample code i havent tested it and im not 100% sure it will work sorry:

local trunk = game.workspace.trees:GetDescendants('trunk')

trunk.transparency = 1

Now if you want to get the position of your trunk you could use this, this code could help! note i havent tested it!

local trunk = game.workspace.trees:GetDescendants('trunk')
local trunknum = #trunk


for i = 1,trunknum do
	local num = trunk[i]
	if num.Name == 'trunk' then
	  num.Transparency = 1
	  local trunkposition = num.Position -- there might be something to do with vector3 if it cant print out the position i think so you can also use num.CFrame
		print(trunkposition)
  end
end

If you want it for it to change tranparency within a certain distance i have tested this!

local trunk = game.workspace.trees:GetDescendants('trunk')
local trunknum = #trunk
local distance3 = 50

for i = 1,trunknum do
	wait(5)
	local num = trunk[i]
	if num.Name == 'trunk' then
		num.Transparency = 1
		local trunkposition = num.Position -- there might be something to do with vector3 if it cant print out the position i think so you can also use num.CFrame
		print(trunkposition)
		for i, v in pairs(workspace:GetChildren()) do
			if v:FindFirstChild('Humanoid') then
				if (v.HumanoidRootPart.Position - num.Position).Magnitude > distance3 then
					print("its visible now")
					num.Transparency = 0
				else
					print("Not in range")
					num.Transparency = 1
				end
			end
		end
	end
end