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