diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 21e72ea..70e6d9d 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -183,6 +183,15 @@ public sealed class GameWindow : IDisposable
     private static readonly bool s_retailCloseDegrades =
         string.Equals(Environment.GetEnvironmentVariable("ACDREAM_RETAIL_CLOSE_DEGRADES"), "1", StringComparison.Ordinal);
 
+    // Issue #48 diagnostic — dump per-scenery-spawn placement evidence
+    // (rendered gfx id, sample source physics-vs-bilinear, ground/baseLoc/finalZ,
+    // mesh vertex Z range, DIDDegrade slot 0). One log line per spawn lets
+    // the user identify a floating tree by its world coordinates and tell
+    // whether the cause is BaseLoc.Z addition (H1), bilinear-fallback drift
+    // (H2), or DIDDegrade selection (H3). Diagnostic-first per CLAUDE.md.
+    private static readonly bool s_dumpSceneryZ =
+        string.Equals(Environment.GetEnvironmentVariable("ACDREAM_DUMP_SCENERY_Z"), "1", StringComparison.Ordinal);
+
     /// <summary>
     /// Issue #47 humanoid-setup detector. Matches Aluvian Male
     /// (<c>0x02000001</c>) and the 34-part heritage sibling setups
@@ -4639,10 +4648,63 @@ public sealed class GameWindow : IDisposable
             // fall back to the local bilinear sample.
             var worldPx = localX + lbOffset.X;
             var worldPy = localY + lbOffset.Y;
-            float groundZ = _physicsEngine.SampleTerrainZ(worldPx, worldPy)
+            float? maybePhysicsZ = _physicsEngine.SampleTerrainZ(worldPx, worldPy);
+            float groundZ = maybePhysicsZ
                 ?? SampleTerrainZ(lb.Heightmap, _heightTable, localX, localY);
             float finalZ = groundZ + spawn.LocalPosition.Z;
 
+            // Issue #48 diagnostic. One log line per (spawn, rendered-mesh)
+            // disambiguates H1 (BaseLoc.Z / mesh-zMin per-species), H2
+            // (physics-vs-bilinear sampler drift), and H3 (DIDDegrade slot 0).
+            // User identifies a floating tree visually, finds the matching
+            // line by world coords + gfx id, the data picks the hypothesis.
+            if (s_dumpSceneryZ)
+            {
+                string source = maybePhysicsZ.HasValue ? "physics" : "bilinear";
+                foreach (var mr in meshRefs)
+                {
+                    var dgfx = _dats.Get<DatReaderWriter.DBObjs.GfxObj>(mr.GfxObjId);
+                    if (dgfx is null) continue;
+
+                    float zMin = float.PositiveInfinity, zMax = float.NegativeInfinity;
+                    foreach (var v in dgfx.VertexArray.Vertices.Values)
+                    {
+                        if (v.Origin.Z < zMin) zMin = v.Origin.Z;
+                        if (v.Origin.Z > zMax) zMax = v.Origin.Z;
+                    }
+                    if (float.IsPositiveInfinity(zMin)) { zMin = 0f; zMax = 0f; }
+
+                    bool hasDD = dgfx.Flags.HasFlag(DatReaderWriter.Enums.GfxObjFlags.HasDIDDegrade);
+                    string ddInfo = string.Empty;
+                    if (hasDD && dgfx.DIDDegrade != 0)
+                    {
+                        var ddi = _dats.Get<DatReaderWriter.DBObjs.GfxObjDegradeInfo>(dgfx.DIDDegrade);
+                        if (ddi is not null && ddi.Degrades.Count > 0)
+                        {
+                            uint slot0Id = (uint)ddi.Degrades[0].Id;
+                            float slot0Min = 0f;
+                            var slot0Gfx = _dats.Get<DatReaderWriter.DBObjs.GfxObj>(slot0Id);
+                            if (slot0Gfx is not null && slot0Gfx.VertexArray.Vertices.Count > 0)
+                            {
+                                slot0Min = float.PositiveInfinity;
+                                foreach (var v in slot0Gfx.VertexArray.Vertices.Values)
+                                    if (v.Origin.Z < slot0Min) slot0Min = v.Origin.Z;
+                                if (float.IsPositiveInfinity(slot0Min)) slot0Min = 0f;
+                            }
+                            ddInfo = $" deg[0]=0x{slot0Id:X8} deg[0]ZMin={slot0Min:F3}";
+                        }
+                    }
+
+                    Console.WriteLine(
+                        $"[scenery-z] lb=0x{lb.LandblockId:X8} root=0x{spawn.ObjectId:X8} gfx=0x{mr.GfxObjId:X8}" +
+                        $" source={source}" +
+                        $" world=({worldPx:F2},{worldPy:F2}) localXY=({localX:F2},{localY:F2})" +
+                        $" groundZ={groundZ:F3} BaseLoc.Z={spawn.LocalPosition.Z:F3} finalZ={finalZ:F3}" +
+                        $" zRange=[{zMin:F3}..{zMax:F3}] zSpan={zMax - zMin:F3}" +
+                        $" hasDIDDegrade={hasDD}{ddInfo}");
+                }
+            }
+
             var hydrated = new AcDream.Core.World.WorldEntity
             {
                 Id = sceneryIdBase + localIndex++,