This will change trees into bushes (for finding loot bags). It only changes what you see.
Just add the ID to the tree array.

local treeChange = 3681
local tree_array = {3621, 3622, 3620, 3617, 3619, 3615, 3614 }
local hide_tree = macro(20000, "Hide Trees", "F2", function() end, rightPanel)

onAddThing(function(tile, thing)
    if hide_tree:isOff() then return end
    if not thing:isItem() then return end
    if table.find(tree_array, thing:getId()) then
        thing:setId(treeChange)
        thing:setMarked('#00FF00')
    end
end)

local treeChange = 3681
macro(100, "Hide Trees", "F2", function()
    for i, tile in ipairs(g_map.getTiles(posz())) do
        for u, item in ipairs(tile:getItems()) do
            if (item) then
                tree_array = { 3625, 3621, 3622, 4433 }
                if table.find(tree_array, item:getId()) then
                    item:setId(treeChange)
                end
            end
        end
    end
end)