diff --git a/simgear/scene/tgdb/TreeBin.cxx b/simgear/scene/tgdb/TreeBin.cxx
index d2cf793c..5687df2a 100644
--- a/simgear/scene/tgdb/TreeBin.cxx
+++ b/simgear/scene/tgdb/TreeBin.cxx
@@ -151,14 +151,26 @@ Geometry* makeSharedTreeGeometry(int numQuads)
     return result;
 }
 
-ref_ptr<Geometry> sharedTreeGeometry;
+static std::mutex static_sharedGeometryMutex;
+static ref_ptr<Geometry> sharedTreeGeometry;
+
+void clearSharedTreeGeometry()
+{
+    std::lock_guard<std::mutex> g(static_sharedGeometryMutex);
+    sharedTreeGeometry = {};
+}
 
 Geometry* createTreeGeometry(float width, float height, int varieties)
 {
-    if (!sharedTreeGeometry)
-        sharedTreeGeometry = makeSharedTreeGeometry(1600);
-    Geometry* quadGeom = simgear::clone(sharedTreeGeometry.get(),
-                                        CopyOp::SHALLOW_COPY);
+    Geometry* quadGeom = nullptr;
+    {
+        std::lock_guard<std::mutex> g(static_sharedGeometryMutex);
+        if (!sharedTreeGeometry)
+            sharedTreeGeometry = makeSharedTreeGeometry(1600);
+        quadGeom = simgear::clone(sharedTreeGeometry.get(),
+                                  CopyOp::SHALLOW_COPY);
+    }
+
     Vec3Array* params = new Vec3Array;
     params->push_back(Vec3(width, height, (float)varieties));
     quadGeom->setNormalArray(params);
diff --git a/simgear/scene/tgdb/TreeBin.hxx b/simgear/scene/tgdb/TreeBin.hxx
index 5f97d4f5..138c2e0b 100644
--- a/simgear/scene/tgdb/TreeBin.hxx
+++ b/simgear/scene/tgdb/TreeBin.hxx
@@ -35,11 +35,14 @@ namespace simgear
 {
 class TreeBin {
 public:
-  struct Tree {
-    SGVec3f position;
-    SGVec3f tnormal;
-    Tree(const SGVec3f& p, const SGVec3f& t) : position(p),tnormal(t)
-    { }
+    ~TreeBin() = default;
+
+    struct Tree {
+        SGVec3f position;
+        SGVec3f tnormal;
+        Tree(const SGVec3f& p, const SGVec3f& t) : position(p), tnormal(t)
+        {
+        }
   };
 
     typedef std::vector<Tree> TreeList;
@@ -55,19 +58,25 @@ public:
     { _trees.push_back(t); }
 
     void insert(const SGVec3f& p, const SGVec3f& tnorm)
-    {insert(Tree(p,tnorm));}
+    {
+        _trees.emplace_back(p, tnorm);
+    }
 
     unsigned getNumTrees() const
     { return _trees.size(); }
+
     const Tree& getTree(unsigned i) const
-    { return _trees[i]; }
+    {
+        assert(i < _trees.size());
+        return _trees.at(i);
+    }
+
     TreeList _trees;
     
-    ~TreeBin() {
-      _trees.clear();
-    }    
 };
 
+void clearSharedTreeGeometry();
+
 typedef std::list<TreeBin*> SGTreeBinList;
 
 osg::Group* createForest(SGTreeBinList& forestList, const osg::Matrix& transform,