Vary sapling species by succession stage for realistic forest growth

Pioneer species (oak, birch) dominate at YOUNG_WOODLAND (intensity ~0.5).
Spruce and dark oak join at mid intensity. At MATURE_FOREST (intensity ~1.0)
the full mix includes cherry and jungle saplings for a diverse canopy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/java/com/livingworld/platform/neoforge/NeoForgeWorldEffectExecutor.java

@@ -131,9 +131,37 @@ public final class NeoForgeWorldEffectExecutor implements WorldEffectConsumer {
             if (!state.is(Blocks.GRASS_BLOCK) && !state.is(Blocks.DIRT)) continue;
             BlockPos above = pos.above();
             if (!level.isLoaded(above) || !level.getBlockState(above).isAir()) continue;
-            level.setBlock(above, Blocks.OAK_SAPLING.defaultBlockState(), Block.UPDATE_ALL);
+            Block sapling = pickSapling(intensity);
+            level.setBlock(above, sapling.defaultBlockState(), Block.UPDATE_ALL);
             LivingWorldLogger.info(DiagnosticCategory.SIMULATION,
-                    "WorldEffect SAPLING_GROWTH_BOOSTED at " + pos);
+                    "WorldEffect SAPLING_GROWTH_BOOSTED (" + sapling + ") at " + pos);
+        }
+    }
+
+    /**
+     * Selects a sapling species weighted by succession intensity.
+     * Low intensity (young woodland, ~0.5): pioneer species — oak and birch dominate.
+     * High intensity (mature forest, ~1.0): diverse canopy — spruce, dark oak, cherry join in.
+     */
+    private Block pickSapling(double intensity) {
+        int r = random.nextInt(100);
+        if (intensity < 0.7) {
+            // Young woodland: pioneer species only
+            return r < 55 ? Blocks.OAK_SAPLING : Blocks.BIRCH_SAPLING;
+        } else if (intensity < 0.9) {
+            // Developing forest: conifers begin to establish
+            if (r < 35) return Blocks.OAK_SAPLING;
+            if (r < 65) return Blocks.BIRCH_SAPLING;
+            if (r < 85) return Blocks.SPRUCE_SAPLING;
+            return Blocks.DARK_OAK_SAPLING;
+        } else {
+            // Mature forest: full species diversity
+            if (r < 25) return Blocks.OAK_SAPLING;
+            if (r < 45) return Blocks.BIRCH_SAPLING;
+            if (r < 62) return Blocks.SPRUCE_SAPLING;
+            if (r < 77) return Blocks.DARK_OAK_SAPLING;
+            if (r < 90) return Blocks.CHERRY_SAPLING;
+            return Blocks.JUNGLE_SAPLING;
         }
     }