  The \emph{metric dimension} $\dim(G)$ of a graph $G$ is the minimum number of
vertices such that every vertex of $G$ is uniquely determined by its vector of
distances to the chosen vertices. The \emph{zero forcing number} $Z(G)$ of a
graph $G$ is the minimum cardinality of a set $S$ of black vertices (whereas
vertices in $V(G)\!\setminus\!S$ are colored white) such that $V(G)$ is turned
black after finitely many applications of "the color-change rule": a white
vertex is converted black if it is the only white neighbor of a black vertex.
We show that $\dim(T) \leq Z(T)$ for a tree $T$, and that $\dim(G) \le Z(G)+1$
if $G$ is a unicyclic graph, along the way, we characterize trees $T$ attaining
$\dim(T)=Z(T)$. For a general graph $G$, we introduce the "cycle rank
conjecture". We conclude with a proof of $\dim(T)-2 \leq \dim(T+e) \le
\dim(T)+1$ for $e \in E(\overline{T})$.
