diff --git a/merkletree/collapsed_tree.go b/merkletree/collapsed_tree.go
index 8f4537b..023073b 100644
--- a/merkletree/collapsed_tree.go
+++ b/merkletree/collapsed_tree.go
@@ -47,6 +47,18 @@ func (tree *CollapsedTree) Add(hash Hash) {
 	tree.collapse()
 }
 
+func (tree *CollapsedTree) Append(other *CollapsedTree) error {
+	maxSize := uint64(1) << bits.TrailingZeros64(tree.size)
+	if other.size > maxSize {
+		return fmt.Errorf("tree of size %d is too large to append to a tree of size %d (maximum size is %d)", other.size, tree.size, maxSize)
+	}
+
+	tree.nodes = append(tree.nodes, other.nodes...)
+	tree.size += other.size
+	tree.collapse()
+	return nil
+}
+
 func (tree *CollapsedTree) collapse() {
 	numNodes := calculateNumNodes(tree.size)
 	for len(tree.nodes) > numNodes {