This paper establishes the rate region for a class of source coding function computation setups where sources of information are available at the nodes of a tree and where a function of these sources must be computed at the root. The rate region holds for any function as long as the sources' joint distribution satisfies a certain Markov criterion. This criterion is met, in particular, when the sources are independent. This result recovers the rate regions of several function computation setups. These include the point-to-point communication setting with arbitrary sources, the noiseless multiple access network with "conditionally independent sources," and the cascade network with Markovian sources.