diff --git a/backend/alembic/versions/004_add_tree_is_public.py b/backend/alembic/versions/004_add_tree_is_public.py
new file mode 100644
index 00000000..9b1e8d7a
--- /dev/null
+++ b/backend/alembic/versions/004_add_tree_is_public.py
@@ -0,0 +1,32 @@
+"""Add is_public field to trees
+
+Revision ID: 004
+Revises: 003
+Create Date: 2026-02-01
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '004'
+down_revision: Union[str, None] = '003'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # Add is_public column with default False (private by default)
+    op.add_column('trees', sa.Column('is_public', sa.Boolean(), nullable=False, server_default='false'))
+    op.create_index('ix_trees_is_public', 'trees', ['is_public'])
+
+    # Make existing default trees public
+    op.execute("UPDATE trees SET is_public = true WHERE is_default = true")
+
+
+def downgrade() -> None:
+    op.drop_index('ix_trees_is_public', table_name='trees')
+    op.drop_column('trees', 'is_public')
diff --git a/backend/app/api/endpoints/trees.py b/backend/app/api/endpoints/trees.py
index 97947281..2177abea 100644
--- a/backend/app/api/endpoints/trees.py
+++ b/backend/app/api/endpoints/trees.py
@@ -31,11 +31,15 @@ async def list_trees(
     if is_active is not None:
         query = query.where(Tree.is_active == is_active)
 
-    # Only show active trees or trees owned by user (for now)
-    # Later, add team-based filtering
+    # Only show trees user has access to:
+    # - Default/system trees (visible to all)
+    # - Public trees
+    # - User's own trees (public or private)
     query = query.where(
+        Tree.is_active == True,
         or_(
-            Tree.is_active == True,
+            Tree.is_default == True,
+            Tree.is_public == True,
             Tree.author_id == current_user.id
         )
     )
@@ -53,10 +57,15 @@ async def list_categories(
     db: Annotated[AsyncSession, Depends(get_db)],
     current_user: Annotated[User, Depends(get_current_user)]
 ):
-    """List all unique categories."""
+    """List all unique categories from trees the user can access."""
     query = select(Tree.category).where(
         Tree.category.isnot(None),
-        Tree.is_active == True
+        Tree.is_active == True,
+        or_(
+            Tree.is_default == True,
+            Tree.is_public == True,
+            Tree.author_id == current_user.id
+        )
     ).distinct()
     result = await db.execute(query)
     categories = [row[0] for row in result.all() if row[0]]
@@ -77,6 +86,11 @@ async def search_trees(
 
     query = select(Tree).where(
         Tree.is_active == True,
+        or_(
+            Tree.is_default == True,
+            Tree.is_public == True,
+            Tree.author_id == current_user.id
+        ),
         search_vector.op('@@')(search_query)
     ).order_by(
         func.ts_rank(search_vector, search_query).desc()
@@ -103,8 +117,9 @@ async def get_tree(
             detail="Tree not found"
         )
 
-    # Check access: tree must be active OR user is the author
-    if not tree.is_active and tree.author_id != current_user.id:
+    # Check access: tree must be active AND (default OR public OR author)
+    can_access = tree.is_default or tree.is_public or tree.author_id == current_user.id
+    if not tree.is_active or not can_access:
         raise HTTPException(
             status_code=status.HTTP_403_FORBIDDEN,
             detail="You don't have access to this tree"
@@ -130,6 +145,7 @@ async def create_tree(
         tree_structure=tree_data.tree_structure,
         author_id=None if is_default else current_user.id,  # Default trees have no author
         team_id=None if is_default else current_user.team_id,
+        is_public=True if is_default else tree_data.is_public,  # Default trees are always public
         is_default=is_default
     )
     db.add(new_tree)
diff --git a/backend/app/core/config.py b/backend/app/core/config.py
index f830f5a9..a254b10e 100644
--- a/backend/app/core/config.py
+++ b/backend/app/core/config.py
@@ -52,6 +52,7 @@ class Settings(BaseSettings):
     class Config:
         env_file = ".env"
         case_sensitive = True
+        extra = "ignore"  # Ignore extra env vars like DATABASE_URL_SYNC
 
 
 settings = Settings()
diff --git a/backend/app/models/tree.py b/backend/app/models/tree.py
index 48077e35..e5ef4b35 100644
--- a/backend/app/models/tree.py
+++ b/backend/app/models/tree.py
@@ -31,6 +31,7 @@ class Tree(Base):
         index=True
     )
     is_active: Mapped[bool] = mapped_column(Boolean, default=True)
+    is_public: Mapped[bool] = mapped_column(Boolean, default=False, index=True)
     is_default: Mapped[bool] = mapped_column(Boolean, default=False, index=True)
     version: Mapped[int] = mapped_column(Integer, default=1)
     created_at: Mapped[datetime] = mapped_column(
diff --git a/backend/app/schemas/tree.py b/backend/app/schemas/tree.py
index 2d1e98d9..88849aed 100644
--- a/backend/app/schemas/tree.py
+++ b/backend/app/schemas/tree.py
@@ -12,6 +12,7 @@ class TreeBase(BaseModel):
 
 class TreeCreate(TreeBase):
     tree_structure: dict[str, Any] = Field(..., description="The decision tree structure in JSON format")
+    is_public: bool = Field(False, description="Make tree visible to all users")
     is_default: bool = Field(False, description="Mark as a default/system tree (admin only)")
 
 
@@ -20,6 +21,7 @@ class TreeUpdate(BaseModel):
     description: Optional[str] = None
     category: Optional[str] = Field(None, max_length=100)
     tree_structure: Optional[dict[str, Any]] = None
+    is_public: Optional[bool] = None
     is_active: Optional[bool] = None
 
 
@@ -29,6 +31,7 @@ class TreeResponse(TreeBase):
     author_id: Optional[UUID] = None
     team_id: Optional[UUID] = None
     is_active: bool
+    is_public: bool
     is_default: bool
     version: int
     created_at: datetime
@@ -45,6 +48,7 @@ class TreeListResponse(BaseModel):
     description: Optional[str] = None
     category: Optional[str] = None
     is_active: bool
+    is_public: bool
     is_default: bool
     version: int
     usage_count: int
diff --git a/backend/scripts/seed_trees.py b/backend/scripts/seed_trees.py
index e3419c0e..ea9b71fe 100644
--- a/backend/scripts/seed_trees.py
+++ b/backend/scripts/seed_trees.py
@@ -3336,8 +3336,9 @@ async def create_tree(client: httpx.AsyncClient, token: str, tree_data: dict) ->
                 print(f"  [SKIP] Tree '{tree_data['name']}' already exists (ID: {tree['id']})")
                 return None
 
-    # Mark as default/system tree
+    # Mark as default/system tree (public and visible to all)
     tree_data["is_default"] = True
+    tree_data["is_public"] = True
 
     # Create the tree
     response = await client.post(