Added a couple features to the API to allow for turning off checking for air,

deleting nodes surrounding the spawn target during the mapgen code, replacing
the spawn target, and checking the type of node directly below the spawn
target.

API.txt

@@ -250,13 +250,18 @@ here than are available in the ABM-based spawner, as some stuff doesn't make
 sense at map-generation time.
 
 biome = {
-	surface = "string",	-- What node to spawn on.  Must specify one
-				-- node, such as "default:dirt_with_grass".
+	surface = something,	-- What node(s).  May be a string such as
+				-- "default:dirt_with_grass" or a table with
+				-- multiple such entries.
 
 	---- Everything else is optional, but you'll definitely want to use
 	---- some of these other fields to limit where and under what
 	---- conditions the objects are spawned.
 	
+	below_nodes = {table},	-- List of nodes that must be below the target
+				-- node.  Useful in snow biomes to keep
+				-- objects from spawning in snow that's on the
+				-- wrong surface for that object.
 	avoid_nodes = {table},	-- List of nodes or groups to avoid when
 				-- spawning.
 	avoid_radius = num,	-- how much distance to leave between the
@@ -311,6 +316,22 @@ biome = {
 				-- plant (lowest Perlin temperature value).
 	verticals_list = {table}, -- Same as with the spawn_on_surfaces
 				-- function.
+	check_air = bool,	-- Flag to tell the mapgen code to check for
+				-- air above the spawn target.  Defaults to
+				-- true if not explicitly set to false.
+	delete_above = bool,	-- Flag to tell the mapgen code to delete the
+				-- two nodes directly above the spawn target
+				-- just before adding the plant or tree.
+				-- Useful when generating in snow biomes.
+				-- Defaults to false.
+	delete_above_surround = bool, -- Flag to tell the mapgen code to also
+				-- delete the four nodes surrounding the above
+				-- space, and the four nodes above those,
+				-- resulting in a two-node-deep cross-shaped
+				-- empty region above the spawn target.
+				-- Useful when adding trees to snow biomes.
+				-- Defaults to false.
+	spawn_replace_node = bool -- same as with the ABM spawner.
 }
 
 Regarding node_or_function_or_treedef, this must either be table with an