diff --git a/_extensions/diagram/diagram.lua b/_extensions/diagram/diagram.lua
index 37f4a62..8cbfd07 100644
--- a/_extensions/diagram/diagram.lua
+++ b/_extensions/diagram/diagram.lua
@@ -291,6 +291,75 @@ local cetz = {
   end,
 }
 
+--- LaTeX template used to compile forest syntax trees.
+--- We provide the \begin{forest} item, too, so that only the [] syntax is needed.
+local forest_template = pandoc.template.compile [[
+\documentclass{standalone}
+\usepackage[linguistics]{forest}
+$for(header-includes)$
+$it$
+$endfor$
+$additional-packages$
+\begin{document}
+\begin{forest}
+$body$
+\end{forest}
+\end{document}
+]]
+
+--- The forest engine is adopted from the TikZ template.
+local forest = {
+  line_comment_start = '%%',
+
+  mime_types = {
+    ['application/pdf'] = true,
+  },
+
+  --- Compile LaTeX with forest syntax trees to an image
+  compile = function (self, src, user_opts)
+    return with_temporary_directory("forest", function (tmpdir)
+      return with_working_directory(tmpdir, function ()
+        -- Define file names:
+        local file_template = "%s/forest-image.%s"
+        local forest_file = file_template:format(tmpdir, "tex")
+        local pdf_file = file_template:format(tmpdir, "pdf")
+
+        -- Treat string values as raw LaTeX
+        local meta = {
+          ['header-includes'] = user_opts['header-includes'],
+          ['additional-packages'] = {pandoc.RawInline(
+            'latex',
+            stringify(user_opts['additional-packages'] or '')
+          )},
+        }
+        local tex_code = pandoc.write(
+          pandoc.Pandoc({pandoc.RawBlock('latex', src)}, meta),
+          'latex',
+          {template = forest_template}
+        )
+        write_file(forest_file, tex_code)
+
+        -- Execute the LaTeX compiler:
+        local success, result = pcall(
+          pipe,
+          self.execpath or 'pdflatex',
+          { '-interaction=nonstopmode', '-output-directory', tmpdir, forest_file },
+          ''
+        )
+        if not success then
+          warn(string.format(
+                 "The call\n%s\nfailed with error code %s. Output:\n%s",
+                 result.command,
+                 result.error_code,
+                 result.output
+          ))
+        end
+        return read_file(pdf_file), 'application/pdf'
+      end)
+    end)
+  end
+}
+
 local default_engines = {
   asymptote = asymptote,
   dot       = graphviz,
@@ -298,6 +367,7 @@ local default_engines = {
   plantuml  = plantuml,
   tikz      = tikz,
   cetz      = cetz,
+  forest    = forest,
 }
 
 --