Ich habe den Baum nun, wie ich ihn mir vorgestellt habe, nur ist der nicht mittig auf dem Papier. Dafür gibt es bestimmt auch einen Befehl, oder?
\documentclass{report}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture} [xscale=2.25,yscale=2]
%unterste Reihe
\draw[thick] (0.5,0) --(1,1);
\draw[thick] (1,1) --(1.5,0);
\draw[thick] (2.5,0) --(2.5,1);
\draw[thick] (3,0) --(3.5,1);
\draw[thick] (3.5,1) --(4,0);
\draw[thick] (5,0) --(5.5,1);
\draw[thick] (5.5,1) --(6,0);
%links
\draw[thick] (1,1) --(1.5,2);
\draw[thick] (1.5,2) --(2,1);
%mitte
\draw[thick] (2.5,1) --(3.5,2);
\draw[thick] (3.5,2) --(4.5,1);
\draw[thick] (3.5,1) --(3.5,2);
%rechts
\draw[thick] (5.5,1) --(5.5,2);
%oberste Reihe
\draw[thick] (1.5,2) --(3.5,3);
\draw[thick] (3.5,2) --(3.5,3);
\draw[thick] (5.5,2) --(3.5,3);
%Kreise
%Reihe 4
\draw [fill] (0.5,0) circle [radius=0.05];
\draw [fill] (1.5,0) circle [radius=0.05];
\draw [fill] (2.5,0) circle [radius=0.05];
\draw [fill] (3,0) circle [radius=0.05];
\draw [fill] (4,0) circle [radius=0.05];
\draw [fill] (5,0) circle [radius=0.05];
\draw [fill] (6,0) circle [radius=0.05];
%Reihe3
\draw [fill] (1,1) circle [radius=0.05];
\draw [fill] (2,1) circle [radius=0.05];
\draw [fill] (2.5,1) circle [radius=0.05];
\draw [fill] (3.5,1) circle [radius=0.05];
\draw [fill] (4.5,1) circle [radius=0.05];
\draw [fill] (5.5,1) circle [radius=0.05];
%Reihe2
\draw [fill] (1.5,2) circle [radius=0.05];
\draw [fill] (3.5,2) circle [radius=0.05];
\draw [fill] (5.5,2) circle [radius=0.05];
%Reihe1
\draw [fill] (3.5,3) circle [radius=0.05];
%Beschriftungen der Individuen von oben nach unten
\node [above] at (3.5,3) {$(\emptyset)$};
\node [above left] at (1.5,2) {$(1)$};
\node [above left] at (3.5,2) {$(2)$};
\node [above right] at (5.5,2) {$(3)$};
\node [above left] at (1,1) {$(1,1)$};
\node [below left] at (2,1) {$(1,2)$};
\node [above left] at (2.5,1) {$(2,1)$};
\node [above left] at (3.5,1) {$(2,2)$};
\node [above right] at (4.5,1) {$(2,3)$};
\node [above right] at (5.5,1) {$(3,1)$};
\node [below] at (0.5,0) {$(1,1,1)$};
\node [below] at (1.5,0) {$(1,1,2)$};
\node [below] at (2.5,0) {$(2,1,1)$};
\node [below] at (3,0) {$(2,2,1)$};
\node [below] at (4,0) {$(2,2,2)$};
\node [below] at (5,0) {$(2,2,2)$};
\node [below] at (6,0) {$(3,1,1)$};
%Beschriftungen der Hilfslinien
\node [right] at (6.5,0) {$T_3$};
\node [right] at (6.5,1) {$T_2$};
\node [right] at (6.5,2) {$T_1$};
\node [right] at (6.5,3) {$T_0$};
%Hilfslinien
\draw[ultra thin] (0,0) --(6.5,0);
\draw[ultra thin] (0,1) --(6.5,1);
\draw[ultra thin] (0,2) --(6.5,2);
\draw[ultra thin] (0,3) --(6.5,3);
\end{tikzpicture}
\end{document}