$p$-elementary non-cyclic subgroups of the Cremona group of the plane

We classify, up to conjugacy, the subgroups of the Cremona group of the plane isomorphic to $(\mathbb{Z}\mathrm{/}p\mathbb{Z}{)}^r$, where $p$ is prime and $r\ge 2$, over an algebraically closed field $\mathbf{k}$ of characteristic not equal to $p$. In particular, we show that $r\le 2$ if $p\ge 5$, $r\le 3$ if $p=3$, and $r\le 4$ if $p=2$. Furthermore, we give an explicit list of representatives via a set of 20 families consisting of subgroups of the de Jonquières group and subgroups of automorphisms of del Pezzo surfaces, and we study the possible conjugacies by birational maps between these families. Finally, we give some results on subgroups of the Cremona group of the plane isomorphic to $(\mathbb{Z}\mathrm{/}p\mathbb{Z}{)}^r$, where $p$ is prime and $r$ is an integer, over an algebraically closed field $\mathbf{k}$ of characteristic equal to $p$.