Feature: Alpine tree line in subarctic climate

src/landscape.cpp

@@ -1436,7 +1436,7 @@ static void CreateRivers()
 /**
  * Calculate what height would be needed to cover N% of the landmass.
  *
- * The function allows both snow and desert/tropic line to be calculated. It
+ * The function allows snow, alpine tree line, and desert/tropic line to be calculated. It
  * tries to find the closests height which covers N% of the landmass; it can
  * be below or above it.
  *
@@ -1536,6 +1536,14 @@ static void CalculateSnowLine()
 	_settings_game.game_creation.snow_line_height = std::max(CalculateCoverageLine(_settings_game.game_creation.snow_coverage, 0), 2u);
 }
 
+/**
+ * Calculate the elevation line from which trees do not grow.
+ */
+void CalculateTreeLine()
+{
+	_settings_game.game_creation.tree_line_height = CalculateCoverageLine(_settings_game.game_creation.alpine_coverage, 0);
+}
+
 /**
  * Calculate the line (in height) between desert and tropic.
  * @return The height of the line between desert and tropic.
@@ -1635,6 +1643,7 @@ bool GenerateLandscape(uint8_t mode)
 	switch (_settings_game.game_creation.landscape) {
 		case LT_ARCTIC:
 			CalculateSnowLine();
+			CalculateTreeLine();
 			break;
 
 		case LT_TROPIC: {

src/landscape.h

@@ -32,6 +32,7 @@ uint8_t GetSnowLine();
 uint8_t HighestSnowLine();
 uint8_t LowestSnowLine();
 void ClearSnowLine();
+void CalculateTreeLine();
 
 int GetSlopeZInCorner(Slope tileh, Corner corner);
 std::tuple<Slope, int> GetFoundationSlope(TileIndex tile);

src/lang/english.txt

@@ -1567,6 +1567,10 @@ STR_CONFIG_SETTING_SNOW_COVERAGE                                :Snow coverage:
 STR_CONFIG_SETTING_SNOW_COVERAGE_HELPTEXT                       :Choose the approximate amount of snow on the sub-arctic landscape. Snow also affects industry generation and town growth requirements. Only used during map generation. Sea level and coast tiles never have snow
 STR_CONFIG_SETTING_SNOW_COVERAGE_VALUE                          :{NUM}%
 
+STR_CONFIG_SETTING_ALPINE_COVERAGE                              :Alpine coverage: {STRING2}
+STR_CONFIG_SETTING_ALPINE_COVERAGE_HELPTEXT                     :Choose the approximate amount of alpine terrain on the sub-arctic landscape. Alpine terrain is above the elevation where trees grow
+STR_CONFIG_SETTING_ALPINE_COVERAGE_VALUE                        :{NUM}%
+
 STR_CONFIG_SETTING_DESERT_COVERAGE                              :Desert coverage: {STRING2}
 STR_CONFIG_SETTING_DESERT_COVERAGE_HELPTEXT                     :Choose the approximate amount of desert on the tropical landscape. Desert also affects industry generation and town growth requirements. Only used during map generation
 STR_CONFIG_SETTING_DESERT_COVERAGE_VALUE                        :{NUM}%
@@ -5184,6 +5188,7 @@ STR_ERROR_CAN_T_BUILD_AQUEDUCT_HERE                             :{WHITE}Can't bu
 # Tree related errors
 STR_ERROR_TREE_ALREADY_HERE                                     :{WHITE}... tree already here
 STR_ERROR_TREE_WRONG_TERRAIN_FOR_TREE_TYPE                      :{WHITE}... wrong terrain for tree type
+STR_ERROR_TREE_ABOVE_TREELINE                                   :{WHITE}... can't plant trees above alpine tree line
 STR_ERROR_CAN_T_PLANT_TREE_HERE                                 :{WHITE}Can't plant tree here...
 
 # Bridge related errors

src/settings_gui.cpp

@@ -2255,6 +2255,7 @@ static SettingsContainer &GetSettingsTree()
 			{
 				trees->Add(new SettingEntry("game_creation.tree_placer"));
 				trees->Add(new SettingEntry("construction.extra_tree_placement"));
+				trees->Add(new SettingEntry("game_creation.alpine_coverage"));
 			}
 		}
 

src/settings_type.h

@@ -346,6 +346,8 @@ struct GameCreationSettings {
 	uint8_t oil_refinery_limit;               ///< distance oil refineries allowed from map edge
 	uint8_t snow_line_height;                 ///< the configured snow line height (deduced from "snow_coverage")
 	uint8_t snow_coverage;                    ///< the amount of snow coverage on the map
+	uint8_t tree_line_height;                 ///< the configured tree line height (deduced from "alpine_coverage")
+	uint8_t alpine_coverage;                  ///< the amount of alpine land on the map (no trees at high elevations)
 	uint8_t desert_coverage;                  ///< the amount of desert coverage on the map
 	uint8_t heightmap_height;                 ///< highest mountain for heightmap (towards what it scales)
 	uint8_t tgen_smoothness;                  ///< how rough is the terrain from 0-3

src/table/settings/world_settings.ini

@@ -109,6 +109,19 @@ strhelp  = STR_CONFIG_SETTING_SNOW_COVERAGE_HELPTEXT
 strval   = STR_CONFIG_SETTING_SNOW_COVERAGE_VALUE
 cat      = SC_BASIC
 
+[SDT_VAR]
+var      = game_creation.alpine_coverage
+type     = SLE_UINT8
+def      = DEF_ALPINE_COVERAGE
+min      = 0
+max      = 100
+interval = 10
+str      = STR_CONFIG_SETTING_ALPINE_COVERAGE
+strhelp  = STR_CONFIG_SETTING_ALPINE_COVERAGE_HELPTEXT
+strval   = STR_CONFIG_SETTING_ALPINE_COVERAGE_VALUE
+post_cb  = [](auto) { CalculateTreeLine(); }
+cat      = SC_BASIC
+
 [SDT_VAR]
 var      = game_creation.desert_coverage
 type     = SLE_UINT8

src/tile_type.h

@@ -34,6 +34,7 @@ static const uint DEF_SNOWLINE_HEIGHT = 10;                    ///< Default snow
 static const uint MAX_SNOWLINE_HEIGHT = (MAX_TILE_HEIGHT - 2); ///< Maximum allowed snowline height
 
 static const uint DEF_SNOW_COVERAGE = 40;                      ///< Default snow coverage.
+static const uint DEF_ALPINE_COVERAGE = 0;                     ///< Default alpine coverage.
 static const uint DEF_DESERT_COVERAGE = 50;                    ///< Default desert coverage.
 
 

src/tree_cmd.cpp

@@ -61,6 +61,7 @@ static const uint16_t EDITOR_TREE_DIV = 5;                   ///< Game editor tr
 /**
  * Tests if a tile can be converted to MP_TREES
  * This is true for clear ground without farms or rocks.
+ * This test also considers the tree line in the subarctic climate.
  *
  * @param tile the tile of interest
  * @param allow_desert Allow planting trees on CLEAR_DESERT?
@@ -68,6 +69,9 @@ static const uint16_t EDITOR_TREE_DIV = 5;                   ///< Game editor tr
  */
 static bool CanPlantTreesOnTile(TileIndex tile, bool allow_desert)
 {
+	/* Trees do not grow in alpine terrain, above the tree line. */
+	if (_settings_game.game_creation.landscape == LT_ARCTIC && GetTileZ(tile) >= _settings_game.game_creation.tree_line_height) return false;
+
 	switch (GetTileType(tile)) {
 		case MP_WATER:
 			return !IsBridgeAbove(tile) && IsCoast(tile) && !IsSlopeWithOneCornerRaised(GetTileSlope(tile));
@@ -436,6 +440,12 @@ CommandCost CmdPlantTree(DoCommandFlag flags, TileIndex tile, TileIndex start_ti
 					continue;
 				}
 
+				/* Trees do not grow in alpine terrain, above the tree line. */
+				if (_settings_game.game_creation.landscape == LT_ARCTIC && GetTileZ(current_tile) >= _settings_game.game_creation.tree_line_height) {
+					msg = STR_ERROR_TREE_ABOVE_TREELINE;
+					continue;
+				}
+
 				TreeType treetype = (TreeType)tree_to_plant;
 				/* Be a bit picky about which trees go where. */
 				if (_settings_game.game_creation.landscape == LT_TROPIC && treetype != TREE_INVALID && (