spacing between trees in the `forest` package

Question

I'm trying to use the forest package to draw adjacent trees. My problem is that the trees are too close to each other.

\documentclass[border=10pt]{standalone}
\usepackage{forest}
\begin{document} 
\begin{forest}
for tree={grow=north}
[$a+b\cdot c$
    [$\oplus$
        [$b\cdot c$
        ]
        [$a$
        ]
    ]
]
\end{forest}
\begin{forest}
for tree={grow=north}
[$a+b\cdot c$
    [$\oplus$
        [$\otimes$
            [$c$]
            [$b$]
        ]
        [$a$
        ]
    ]
]
\end{forest}
\end{document}

On the other hand the following code doesn't generate two adjacent trees, but only the node a.

\documentclass[border=10pt]{standalone}
\usepackage{forest}
\begin{document}
\begin{forest}
for tree={grow=north}
[a
]
[b
]
\end{forest}
\end{document}

How can I draw adjacent trees?

Answer 1

Only one tree is permitted in any forest, whose name is hopelessly misleading. You can have many trees with Forest, but only one in each forest. That is, a forest of trees requires many forests.

If you really want multiple trees in a forest, you can wrap them in a tree shell with a phantom root.

\documentclass[border=10pt]{standalone}
\usepackage{forest}
\begin{document}
\begin{forest}
  for tree={grow=north}
  [, phantom, fit=band
    [$a+b\cdot c$
      [$\oplus$
          [$b\cdot c$
          ]
          [$a$
          ]
      ]
    ]
    [$a+b\cdot c$
      [$\oplus$
          [$\otimes$
              [$c$]
              [$b$]
          ]
          [$a$
          ]
      ]
    ]
  ]
\end{forest}
\end{document}

I don't know why you would bother, though.