  In this article, we study the linearity of the minimal free resolution of
powers of facets ideals of simplicial trees. We give a complete
characterization of simplicial trees for which (some) power of its facet ideal
has a linear resolution. We calculate the regularity of the $t$-path ideal of a
perfect rooted tree. We also obtain an upper bound for the regularity of the
$t$-path ideal of a rooted tree. We give a procedure to calculate the
regularity of powers of facet ideals of simplicial trees. As a consequence of
this result, we study the regularity of powers of $t$-path ideals of rooted
trees. We pose a regularity upper bound conjecture for facet ideals of
simplicial trees, which is as follows: if $\Delta$ is a $d$-dimensional
simplicial tree, then $\reg(I(\Delta)^s) \leq (d+1)(s-1)+\reg(I(\Delta))$ for
all $s \geq 1$. We prove this conjecture for some special classes of simplicial
trees.
