This work presents an approximation algorithm for scheduling the tasks of a parallel application. These tasks are considered as malleable tasks (MT in short), which means that they can be executed on several processors. This model receives recently a lot of attention, due mainly to their practical use for implementing actual parallel applications. Most of the works developed within this model deal with independent MT for which good approximation algorithms have been designed. This work is devoted to the case where MT are linked by precedence relations. We present a 1+epsilon approximation algorithm (for any fixed epsilon) for the specific structure of a tree. This preliminary result should open the way for further investigations concerning arbitrary precedence graphs of MT