  We apply the model relating halo concentration to formation history proposed
by Ludlow et al. to merger trees generated using an algorithm based on
excursion set theory. We find that while the model correctly predicts the
median relation between halo concentration and mass, it underpredicts the
scatter in concentration at fixed mass. Since the same model applied to N-body
merger trees predicts the correct scatter, we postulate that the missing
scatter is due to the lack of any environmental dependence in merger trees
derived from excursion set theory. We show that a simple modification to the
merger tree construction algorithm, which makes merger rates dependent on
environment, can increase the scatter by the required amount, and
simultaneously provide a qualitatively correct correlation between environment
and formation epoch in the excursion set merger trees.
