diff --git a/src/tree_cmd.cpp b/src/tree_cmd.cpp
--- a/src/tree_cmd.cpp
+++ b/src/tree_cmd.cpp
@@ -516,7 +516,7 @@ static void DrawTile_Trees(TileInfo *ti)
 }
 
 
-static uint GetSlopePixelZ_Trees(TileIndex tile, uint x, uint y)
+static int GetSlopePixelZ_Trees(TileIndex tile, uint x, uint y)
 {
 	int z;
 	Slope tileh = GetTilePixelSlope(tile, &z);
@@ -776,7 +776,7 @@ void InitializeTrees()
 	_trees_tick_ctr = 0;
 }
 
-static CommandCost TerraformTile_Trees(TileIndex tile, DoCommandFlag flags, uint z_new, Slope tileh_new)
+static CommandCost TerraformTile_Trees(TileIndex tile, DoCommandFlag flags, int z_new, Slope tileh_new)
 {
 	return DoCommand(tile, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
 }