Before revision:  Let f_d(n) denote the minimum number N of points such that every n-vertex tree with maximum degree d\in\{3,4\ admits an L-shaped embedding in every point set of size N, where no two points have the same abscissa or ordinate.
After revision:  There has been considerable work on establishing upper bounds on the minimum cardinality such that any point set of that size admits an L-shaped embedding in every point set of size N, where no two points have the same abscissa or ordinate.