Multiple changes, see below:

Fix replacement of default trees so that it works correctly.  Beeches
now use default tree nodes, but L-systems models.  This redefines the
default tree, wood, leaves, and saplings so that they use moretrees'
textures "Beech" in their descriptions, but only if replacement of
default trees is enabled.

Added backward-compat aliases for this change.

Moved tree replacement mapgen->air aliases to a more logical spot in the
code.

Also, changed out all leaf decay dig_node() calls for nodeupdate(), since the
former calls the latter anyway, and the latter is what does the job.

Increased the palm leaf decay radius to 12.

node_defs.lua

@@ -1,7 +1,6 @@
 moretrees.avoidnodes = {}
 	
 moretrees.treelist = {
-	{"beech",	"Beech Tree"},
 	{"apple_tree",	"Apple Tree"},
 	{"oak",		"Oak Tree",		"acorn",	"Acorn",	{-0.2, -0.5, -0.2, 0.2, 0, 0.2}, 0.8 },
 	{"sequoia",	"Giant Sequoia"},
@@ -201,19 +200,97 @@ if moretrees.enable_redefine_apple then
 	})
 end
 
+if moretrees.enable_replace_default_trees then
+
+	minetest.register_node(":default:tree", {
+		description = "Beech Trunk",
+		tiles = {"moretrees_beech_trunk_top.png", "moretrees_beech_trunk_top.png", "moretrees_beech_trunk.png"},
+		groups = {tree=1,choppy=2,oddly_breakable_by_hand=1,flammable=2},
+		sounds = default.node_sound_wood_defaults(),
+	})
+
+	minetest.register_node(":default:leaves", {
+		description = "Beech Leaves",
+		drawtype = "allfaces_optional",
+		visual_scale = 1.3,
+		tiles = {"moretrees_beech_leaves.png"},
+		paramtype = "light",
+		groups = {snappy=3, leafdecay=3, flammable=2, leaves=1},
+		drop = {
+			max_items = 1,
+			items = {
+				{
+					-- player will get sapling with 1/20 chance
+					items = {'default:sapling'},
+					rarity = 20,
+				},
+				{
+					-- player will get leaves only if he get no saplings,
+					-- this is because max_items is 1
+					items = {'default:leaves'},
+				}
+			}
+		},
+		sounds = default.node_sound_leaves_defaults(),
+	})
+
+	minetest.register_node(":default:wood", {
+		description = "Beech Planks",
+		tiles = {"moretrees_beech_wood.png"},
+		groups = {choppy=2,oddly_breakable_by_hand=2,flammable=3,wood=1},
+		sounds = default.node_sound_wood_defaults(),
+	})
+
+	minetest.register_node(":default:sapling", {
+		description = "Beech Sapling",
+		drawtype = "plantlike",
+		visual_scale = 1.0,
+		tiles = {"moretrees_beech_sapling.png"},
+		inventory_image = "moretrees_beech_sapling.png",
+		wield_image = "moretrees_beech_sapling.png",
+		paramtype = "light",
+		walkable = false,
+		selection_box = {
+			type = "fixed",
+			fixed = {-0.3, -0.5, -0.3, 0.3, 0.35, 0.3}
+		},
+		groups = {snappy=2,dig_immediate=3,flammable=2,attached_node=1},
+		sounds = default.node_sound_defaults(),
+	})
+end
+
+minetest.register_node("moretrees:beech_trunk_sideways", {
+	description = "Sideways Beech Trunk",
+	tiles = {
+		"moretrees_beech_trunk.png^[transformR90",
+		"moretrees_beech_trunk.png^[transformR90",
+		"moretrees_beech_trunk_top.png",
+		"moretrees_beech_trunk_top.png",
+		"moretrees_beech_trunk.png^[transformR90",
+		"moretrees_beech_trunk.png^[transformR90"
+	},
+	is_ground_content = true,
+	groups = {tree=1,snappy=1,choppy=2,oddly_breakable_by_hand=1,flammable=2},
+	sounds = default.node_sound_wood_defaults(),
+	paramtype2 = "facedir",
+})
+
 table.insert(moretrees.avoidnodes, "default:jungletree")
 table.insert(moretrees.avoidnodes, "moretrees:jungletree_trunk")
 table.insert(moretrees.avoidnodes, "moretrees:fir_trunk")
 
--- For compatibility with newly-re-introduced default jungles and related nodes.
+-- For compatibility with old nodes and recently-changed nodes.
+
+minetest.register_alias("moretrees:beech_trunk",  "default:tree")
+minetest.register_alias("moretrees:beech_leaves", "default:leaves")
+minetest.register_alias("moretrees:beech_planks", "default:wood")
+minetest.register_alias("moretrees:beech_sapling", "default:sapling") 
 
 minetest.register_alias("moretrees:jungletree_trunk", "default:jungletree")
 minetest.register_alias("moretrees:jungletree_planks", "default:junglewood")
 minetest.register_alias("moretrees:jungletree_sapling", "default:junglesapling")
 minetest.register_alias("jungletree:sapling", "default:junglesapling")
 
--- These next ones are for the old jungle trees and conifers mods.
-
 minetest.register_alias("jungletree:leaves_green", "moretrees:jungletree_leaves_green")
 minetest.register_alias("jungletree:leaves_red", "moretrees:jungletree_leaves_red")
 minetest.register_alias("jungletree:leaves_yellow", "moretrees:jungletree_leaves_yellow")