TikZ: arrange trees block by block

Question

I use TikZ to create trees. Now I've to arrange four trees in a matrix like this:

Tree A Tree B
Tree C Tree D

I already tried the \matrix command. This works, but it leaves absolutely no space between the trees. I want to have the trees to be seperated by a space, in order to create arrows between the trees. Ideally the four trees are located in four boxes, which all have the same size and the same space between them.

Any suggestions?

Thanks.

Edit: This is what I've come up with:

\documentclass{article}
\usepackage{scalefnt}
\usepackage{tikz}
\usetikzlibrary{arrows,calc}
\usetikzlibrary{decorations.pathmorphing}
\usetikzlibrary{decorations.pathreplacing}
\usetikzlibrary{trees}
\usetikzlibrary{matrix}
\pgfdeclarelayer{background}
\pgfsetlayers{background,main}
\begin{document}
\begin{tikzpicture}
    [
    every node/.style={rectangle, draw, fill=black!10,
      text badly centered, font=\scalefont{0.45}, text width=1.3cm},
    level distance=0.7cm,
    level 1/.style={sibling distance=2cm}
    ]

    \matrix [draw=none, fill=none, column sep=2cm, row sep=1.5cm]
    {
      \path node {Ressourcen} [edge from parent fork down]
      child {node {Zeit}}
      child {node {Umwelt}
        child {node [text width=2.8cm]{Lernumgebung, Personen Material}}
      }
      child {node {Physiologie}};
      &
      \path node {Ziele} [edge from parent fork down]
      child {node {kurzfristig}}
      child {node {langfristig}};\\

      \path node {Volition} [edge from parent fork down]
      child {node {Aufmerksamkeit}}
      child {node {Motivation}}
      child {node {Emotionskontrolle}};
      &
      \node {Lernstrategien};\\
    };
  \end{tikzpicture}
\end{document}

Now it would be nice to have boxes around the trees and to have arrows between the boxes...

Answer 1

Instead of a matrix I used the at syntax to explicitly position the trees; then I used rectangles to draw the frames (basically one rectangle conveniently shifted to guarantee the same size for the boxes):

\documentclass{article}
\usepackage[margin=2.5cm]{geometry}
\usepackage{scalefnt}
\usepackage{tikz}
\usetikzlibrary{trees,arrows,calc}

\begin{document}
\begin{tikzpicture}
    [
    every node/.style={rectangle, draw, fill=black!10,
    text badly centered, font=\scalefont{0.45}, text width=1.3cm},
    level distance=0.7cm,
    level 1/.style={sibling distance=2cm}
    ]

      \node at (0,0) (R) {Ressourcen} [edge from parent fork down]
      child {node (Ze) {Zeit}}
      child {node (U) {Umwelt}
        child {node (L) [text width=2.8cm]{Lernumgebung, Personen Material}}
      }
      child {node (P) {Physiologie}};

      \node at (7,0) (Zi) {Ziele} [edge from parent fork down]
      child {node (k) {kurzfristig}}
      child {node (l) {langfristig}};

      \node at (0,-3.5) (V) {Volition} [edge from parent fork down]
      child {node[text width=1.8cm] (A) {Aufmerksamkeit}}
      child {node (M) {Motivation}}
      child {node[text width=1.9cm] (E) {Emotionskontrolle}};

     \node[text width=1.6cm] at (7,-3.5) (Le) {Lernstrategien};

  \draw (-3.3,-2) rectangle (3.3,0.5); 
  \draw[xshift=7cm] (-3.3,-2) rectangle (3.3,0.5); 
  \draw[yshift=-3cm] (-3.3,-2) rectangle (3.3,0.5); 
  \draw[xshift=7cm,yshift=-3cm] (-3.3,-2) rectangle (3.3,0.5);
  \draw[->] (3.3,-0.75) -- (3.7,-0.75);
  \draw[->] (3.3,-3.75) -- (3.7,-3.75);
\end{tikzpicture}

\end{document}

Answer 2

Here is another less 'manual' approach using a matrix as the awesome positioning library. It is not perfect (due to lack of time). It is left as an 'exercise' to figure out alignment and boxes.

Alternatively, you could create four matrices. This would make boxing and probably alignment trivial; however boxing would be hard as you'd want them to be evenly big.

\documentclass[landscape]{article}
\usepackage{tikz}
\usetikzlibrary{positioning,matrix}
\begin{document}

\begin{tikzpicture}[every node/.style={
    draw,
    fill=black!10,
    text centered,
    text width=2cm,
    inner sep=5pt,
    node distance=1em and 2ex
  },
  ghost/.style={draw=none, fill=none, text width=0em},
  every matrix/.style={fill=none, inner sep=1em}]


  \node[matrix, row sep=2em, column sep=2em] {

    {
      \node (r) {Ressourcen};
      \node [below = of r] (u) {Umwelt};
      \node [left = of u] (p) {Phyilogie} ;
      \node [right = of u] (z)  {Zeit};
      \node [below = of u] (l) {Lernumgebung};
      \foreach \i/\j in {r/p,r/z,r/u,u/l}
      \draw[->] (\i.south) -|+ (0,-.5em) -| (\j.north);
    };
    &
    {
      \node (r) {Ressourcen};
      \node [below = of r,ghost] (u) {};
      \node [left = of u] (p) {Phyilogie} ;
      \node [right = of u] (z)  {Zeit};
      \foreach \i/\j in {r/p,r/z}
      \draw[->] (\i.south) -|+ (0,-.25em) -| (\j.north);
    }\\
    {
      \node (r) {Ressourcen};
      \node [below = of r] (u) {Umwelt};
      \node [left = of u] (p) {Phyilogie} ;
      \node [right = of u] (z)  {Zeit};
      \foreach \i/\j in {r/p,r/z,r/u}
      \draw[->] (\i.south) -|+ (0,-.5em) -| (\j.north);
    }
    &
    {
      \node [below = of u] (l) {Lernumgebung};
    }
    \\
  };
\end{tikzpicture}
\end{document}