We investigate pattern avoidance in permutations satisfying some additional restrictions. These are naturally considered in terms of avoiding patterns in linear extensions of certain forest-like partially ordered sets, which we call binary shrub forests. In this context, we enumerate forests avoiding patterns of length three. In four of the five non-equivalent cases, we present explicit enumerations by exhibiting bijections with certain lattice paths bounded above by the line \(y=\ell x\), for some \(\ell\in\mathbb{Q}^+\), one of these being the celebrated Duchon's club paths with \(\ell=2/3\). In the remaining case, we use the machinery of analytic combinatorics to determine the minimal polynomial of its generating function, and deduce its growth rate.

We are grateful to the University of Wisconsin - Eau Claire (UWEC) Department of Mathematics and Office of Research and Sponsored Programs for supporting work done by four of the coauthors at UWEC during the 2014-2015 academic year. The authors also thank Alex Burstein for organizing the Special Session on Patterns in Permutations and Words at the spring 2015 Eastern Sectional Meeting of the American Mathematical Society at Georgetown University, which allowed collaboration on a then-open case in this manuscript.

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